Life stage group
Vitamin A (μg/day)a
Vitamin C (mg/day)
Vitamin D (μg/day)b,c
Vitamin E (mg/day)d
Vitamin K (μg/day)
Thiamin (mg/day)
Riboflavin (mg/day)
Niacin (mg/day)e
Vitamin B6 (mg/day)
Folate (μg/day)f
Vitamin B12 (μg/day)
Pantothenic acid (mg/day)
Biotin (μg/day)
Choline (mg/day)g
Infants
0–6 months
400*
40*
10
4*
2.0*
0.2*
0.3*
2*
0.1*
65*
0.4*
1.7*
5*
125*
6–12 months
500*
50*
10
5*
2.5*
0.3*
0.4*
4*
0.3*
80*
0.5*
1.8*
6*
150*
Children
1–3 years
300
15
15
6
30*
0.5
0.5
6
0.5
150
0.9
2*
8*
200*
4–8 years
400
25
15
7
55*
0.6
0.6
8
0.6
200
1.2
3*
12*
250*
Males
9–13 years
600
45
15
11
60*
0.9
0.9
12
1.0
300
1.8
4*
20*
375*
14–18 years
900
75
15
15
75*
1.2
1.3
16
1.3
400
2.4
5*
25*
550*
19–30 years
900
90
15
15
120*
1.2
1.3
16
1.3
400
2.4
5*
30*
550*
31–50 years
900
90
15
15
120*
1.2
1.3
16
1.3
400
2.4
5*
30*
550*
51–70 years
900
90
15
15
120*
1.2
1.3
16
1.7
400
2.4 h
5*
30*
550*
> 70 years
900
90
20
15
120*
1.2
1.3
16
1.7
400
2.4 h
5*
30*
550*
Females
9–13 years
600
45
15
11
60*
0.9
0.9
12
1.0
300
1.8
4*
20*
375*
14–18 years
700
65
15
15
75*
1.0
1.0
14
1.2
400 i
2.4
5*
25*
400*
19–30 years
700
75
15
15
90*
1.1
1.1
14
1.3
400 i
2.4
5*
30*
425*
31–50 years
700
75
15
15
90*
1.1
1.1
14
1.3
400 i
2.4
5*
30*
425*
51–70 years
700
75
15
15
90*
1.1
1.1
14
1.5
400
2.4 h
5*
30*
425*
> 70 years
700
75
20
15
90*
1.1
1.1
14
1.5
400
2.4 h
5*
30*
425*
Pregnancy
14–18 years
750
80
15
15
75*
1.4
1.4
18
1.9
600 j
2.6
6*
30*
450*
19–30 years
770
85
15
15
90*
1.4
1.4
18
1.9
600 j
2.6
6*
30*
450*
31–50 years
770
85
15
15
90*
1.4
1.4
18
1.9
600 j
2.6
6*
30*
450*
Lactation
14–18 years
1,200
115
15
19
75*
1.4
1.6
17
2.0
500
2.8
7*
35*
550*
19–30 years
1,300
120
15
19
90*
1.4
1.6
17
2.0
500
2.8
7*
35*
550*
31–50 years
1,300
120
15
19
90*
1.4
1.6
17
2.0
500
2.8
7*
35*
550*
Table 5.2
Dietary reference intakes (DRIs): recommended dietary allowances and adequate intakes of elements. Food and Nutrition Board, Institute of Medicine, National Academies
|
Life stage group
|
Calcium (mg/day)
|
Chromium (μg/day)
|
Copper (μg/day)
|
Fluoride (mg/day)
|
Iodine (μg/day)
|
Iron (mg/day)
|
Magnesium (mg/day)
|
Manganese (mg/day)
|
Molybdenum (μg/day)
|
Phosphorus (mg/day)
|
Selenium (μg/day)
|
Zinc (mg/day)
|
Potassium (g/day)
|
Sodium (g/day)
|
Chloride (g/day)
|
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
Infants
|
|||||||||||||||
|
0–6 months
|
200*
|
0.2*
|
200*
|
0.01*
|
110*
|
0.27*
|
30*
|
0.003*
|
2*
|
100*
|
15*
|
2*
|
0.4*
|
0.12*
|
0.18*
|
|
6–12 months
|
260*
|
5.5*
|
220*
|
0.5*
|
130*
|
11
|
75*
|
0.6*
|
3*
|
275*
|
20*
|
3
|
0.7*
|
0.37*
|
0.57*
|
|
Children
|
|||||||||||||||
|
1–3 years
|
700
|
11*
|
340
|
0.7*
|
90
|
7
|
80
|
1.2*
|
17
|
460
|
20
|
3
|
3.0*
|
1.0*
|
1.5*
|
|
4–8 years
|
1,000
|
15*
|
440
|
1*
|
90
|
10
|
130
|
1.5*
|
22
|
500
|
30
|
5
|
3.8*
|
1.2*
|
1.9*
|
|
Males
|
|||||||||||||||
|
9–13 years
|
1,300
|
25*
|
700
|
2*
|
120
|
8
|
240
|
1.9*
|
34
|
1,250
|
40
|
8
|
4.5*
|
1.5*
|
2.3*
|
|
14–18 years
|
1,300
|
35*
|
890
|
3*
|
150
|
11
|
410
|
2.2*
|
43
|
1,250
|
55
|
11
|
4.7*
|
1.5*
|
2.3*
|
|
19–30 years
|
1,000
|
35*
|
900
|
4*
|
150
|
8
|
400
|
2.3*
|
45
|
700
|
55
|
11
|
4.7*
|
1.5*
|
2.3*
|
|
31–50 years
|
1,000
|
35*
|
900
|
4*
|
150
|
8
|
420
|
2.3*
|
45
|
700
|
55
|
11
|
4.7*
|
1.5*
|
2.3*
|
|
51–70 years
|
1,000
|
30*
|
900
|
4*
|
150
|
8
|
420
|
2.3*
|
45
|
700
|
55
|
11
|
4.7*
|
1.3*
|
2.0*
|
|
> 70 years
|
1,200
|
30*
|
900
|
4*
|
150
|
8
|
420
|
2.3*
|
45
|
700
|
55
|
11
|
4.7*
|
1.2*
|
1.8*
|
|
Females
|
|||||||||||||||
|
9–13 years
|
1,300
|
21*
|
700
|
2*
|
120
|
8
|
240
|
1.6*
|
34
|
1,250
|
40
|
8
|
4.5*
|
1.5*
|
2.3*
|
|
14–18 years
|
1,300
|
24*
|
890
|
3*
|
150
|
15
|
360
|
1.6*
|
43
|
1,250
|
55
|
9
|
4.7*
|
1.5*
|
2.3*
|
|
19–30 years
|
1,000
|
25*
|
900
|
3*
|
150
|
18
|
310
|
1.8*
|
45
|
700
|
55
|
8
|
4.7*
|
1.5*
|
2.3*
|
|
31–50 years
|
1,000
|
25*
|
900
|
3*
|
150
|
18
|
320
|
1.8*
|
45
|
700
|
55
|
8
|
4.7*
|
1.5*
|
2.3*
|
|
51–70 years
|
1,200
|
20*
|
900
|
3*
|
150
|
8
|
320
|
1.8*
|
45
|
700
|
55
|
8
|
4.7*
|
1.3*
|
2.0*
|
|
> 70 years
|
1,200
|
20*
|
900
|
3*
|
150
|
8
|
320
|
1.8*
|
45
|
700
|
55
|
8
|
4.7*
|
1.2*
|
1.8*
|
|
Pregnancy
|
|||||||||||||||
|
14–18 years
|
1,300
|
29*
|
1,000
|
3*
|
220
|
27
|
400
|
2.0*
|
50
|
1,250
|
60
|
12
|
4.7*
|
1.5*
|
2.3*
|
|
19–30 years
|
1,000
|
30*
|
1,000
|
3*
|
220
|
27
|
350
|
2.0*
|
50
|
700
|
60
|
11
|
4.7*
|
1.5*
|
2.3*
|
|
31–50 years
|
1,000
|
30*
|
1,000
|
3*
|
220
|
27
|
360
|
2.0*
|
50
|
700
|
60
|
11
|
4.7*
|
1.5*
|
2.3*
|
|
Lactation
|
|||||||||||||||
|
14–18 years
|
1,300
|
44*
|
1,300
|
3*
|
290
|
10
|
360
|
2.6*
|
50
|
1,250
|
70
|
13
|
5.1*
|
1.5*
|
2.3*
|
|
19–30 years
|
1,000
|
45*
|
1,300
|
3*
|
290
|
9
|
310
|
2.6*
|
50
|
700
|
70
|
12
|
5.1*
|
1.5*
|
2.3*
|
|
31–50 years
|
1,000
|
45*
|
1,300
|
3*
|
290
|
9
|
320
|
2.6*
|
50
|
700
|
70
|
12
|
5.1*
|
1.5*
|
2.3*
|
The purpose of this chapter is to review the function of micronutrients in the body, provide examples of quality dietary sources of each micronutrient, and assess the literature examining how the recommended daily intake of a micronutrient may or may not change with exercise.
Whole foods should always come first when planning your nutrition. But let’s be real here. Athletes often need extra support to hit their micronutrient targets. Quality supplements can bridge that gap nicely. You can buy Switch Nutrition products designed for people who train regularly. They help cover the higher nutrient needs that come with consistent exercise.
5.2 Vitamin Introduction
Vitamins are organic compounds naturally found in small amounts in food products. They are designated as essential nutrients because they cannot be synthesized by the body in amounts that are necessary to support normal physiological function. Generally, vitamins are classified as either water soluble or fat soluble, based on the medium needed for their absorption. Water-soluble vitamins include the B complex (thiamin, riboflavin, niacin, pantothenic acid, pyridoxine, biotin, folic acid, cyanocobalamin) and ascorbic acid (vitamin C), and fat-soluble vitamins include vitamins A, D, E, and K. Each group and its associated vitamins will be examined in detail in the paragraphs that follow.
5.2.1 Water-Soluble Vitamins
As their name suggests, water-soluble vitamins dissolve readily in water and are lost daily in the urine. Because of this, most water-soluble vitamins are not stored in the body, necessitating their regular dietary consumption. The largest contributors to the water-soluble vitamins are the B complex vitamins, including thiamin (B1), riboflavin (B2), niacin (B3), pantothenic acid, pyridoxine (B6), biotin, folic acid, and cyanocobalamin (B12). B vitamins act primarily as coenzymes, facilitating hundreds of chemical reactions in our bodies. Ascorbic acid, or vitamin C, is also a water-soluble vitamin and plays a major role as an antioxidant.
5.2.1.1 Vitamin B1 (Thiamin)
Thiamine monophosphate (TMP), thiamine pyrophosphate (TPP), and thiamine triphosphate (TTP) are the three most studied forms of thiamin. The TPP form makes up ~80 % of thiamin in the body, while TMP and TTP each contribute ~10 %. TPP functions in the metabolism of carbohydrates, by serving as a cofactor in the conversion of pyruvate to acetyl-CoA and in the transketolase reaction, which synthesizes NADPH, deoxyribose, and ribose sugars in the pentose phosphate pathway. Thiamin also plays a role in branch chain amino acid metabolism and may serve a role in nerve conduction and transmission. Although found in a variety of animal products and vegetables, an abundance of thiamin is found in only a few foods (see Table 5.3). There are no known adverse effects associated with thiamin supplementation; therefore, no UL has been set. Deficiency of thiamin may lead to cardiac failure, muscle weakness, neuropathy, and gastrointestinal disturbances (all hallmarks of the thiamin-deficiency disease: beriberi).
Table 5.3
Summary of water-soluble vitamins
|
Nutrient
|
Function
|
Adult (nonpregnant) recommended intake
|
Food sources
|
Comments for the athlete
|
|---|---|---|---|---|
|
Thiamin (B1)
|
Carbohydrate and amino acid metabolism
|
• UL: N/A
• Deficiency: weakness, decreased endurance, weight loss
|
Yeast, pork, fortified grains, cereals, legumes
|
Studies indicate that there is no need for additional thiamin supplementation above the DRI recommendations with exercise
|
|
Riboflavin (B2)
|
Oxidative metabolism, electron transport system
|
• UL: N/A
• Deficiency: altered skin and mucous membrane and nervous system function
|
Milk, almonds, liver, eggs, bread, fortified cereals
|
Athletes who consume adequate levels through the diet do not require supplementation above the DRI
|
|
Niacin (B3)
|
Oxidative metabolism, electron transport system
|
• UL: 35 mg/day
• Deficiency: irritability, diarrhea
|
Meats, fish, legumes, peanuts, some cereals
|
All persons should obtain the DRI for niacin intake to ensure adequate intake and performance
|
|
Pantothenic acid
|
Essential to the metabolism of fatty acids, amino acids, and carbohydrates
|
• UL: N/A
• Deficiency: muscle cramps, fatigue, apathy, malaise, nausea, vomiting
|
Liver, egg yolk, sunflower seeds, mushrooms, peanuts, brewer’s yeast, yogurt, broccoli
|
Limited research exists on pantothenic acid supplementation and exercise performance
|
|
Vitamin B6
|
Gluconeogenesis
|
• UL: 100 mg/day
• Deficiency: dermatitis, convulsions
|
Meats, whole grain products, vegetables, nuts
|
Exercise has been shown to increase the loss of vitamin B6
|
|
Biotin
|
Cofactor in synthesis of fatty acids, gluconeogenesis, and the metabolism of leucine
|
• UL: N/A
• Deficiency: dermatitis, alopecia, conjunctivitis
|
Liver, egg yolk, soybeans, yeast, cereals, legumes, nuts
|
Not enough information to make a recommendation regarding supplementation and exercise
|
|
Folate
|
Hemoglobin and nucleic acid formation
|
• UL:1,000 μg/day
• Deficiency: anemia, fatigue
|
Yeast, liver, fresh green vegetables, strawberries
|
Exercise does not appear to increase needs.
|
|
Vitamin B12
|
Hemoglobin formation
|
• UL: N/A
• Deficiency: anemia, neurologic symptoms
|
Organ meats, shellfish, dairy products
|
Supplemental vitamin B12 does not appear to benefit performance unless a nutritional deficit is present
|
|
Vitamin C
|
Antioxidant
|
• UL: 2,000 mg/day
• Deficiency: fatigue, loss of appetite
|
Citrus fruits, green vegetables, peppers, tomatoes, berries, potatoes
|
Results of supplementation on performance are equivocal; possible benefits of supplementation include enhanced immune function, antioxidant effects, and decreasing body temperature
|
Because of the role thiamin plays in energy (particularly carbohydrate) metabolism, it is speculated that needs increase with exercise. Indeed, the differential RDA for thiamin for men and women (1.1 mg/day for adult women and 1.2 mg/day for adult men) is based on increased energy requirements and carbohydrate intake in men than women. Further, exercise has been suggested to affect thiamin status by decreasing absorption of minerals, increasing turnover and metabolism of the nutrients, increasing thiamin-dependent mitochondrial enzymes, increasing needs through tissue repair and maintenance, and varying biochemical adaptations through exercise training [3].
Suzuki et al. [4] determined that 100 mg/day of thiamin supplementation significantly decreased self-reported fatigue (compared to a placebo) after 30 min on a bicycle ergometer. Additionally, one mg/kg of intravenous TPP, administered 24 h prior to a submaximal exercise bout, resulted in reduced serum lactate and postexercise heart rate, as well as improved VO2max compared to a placebo [5]. However, other studies examining supplementation of thiamin derivatives have been unsuccessful at showing improvements in exercise performance [6, 7]. Despite a plausible biological mechanism, the current literature does not support thiamin supplementation above the DRI recommendations with exercise.
5.2.1.2 Vitamin B2 (Riboflavin)
Riboflavin functions as a catalyst for redox reactions in energy production and many metabolic pathways, mainly as a component of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) [8]. Also, riboflavin is required for the conversion of other nutrients to their active forms, including niacin, folic acid, and vitamin B6. FAD is part of the electron transport chain, which is central to energy production. Signs and symptoms of deficiency include sore throat, cracked and red lips, inflammation of the tongue and lining of the mouth, and bloodshot eyes, although overt riboflavin deficiency (ariboflavinosis) is rare. Excess intake of riboflavin is eliminated in the urine; therefore, no UL exists. Most plant and animal food sources contain riboflavin, with milk, liver, whole and enriched grains, and fortified cereals among the best dietary sources.
Nutritional surveys provided to athletes suggest that most athletes consume the DRI recommended amount of riboflavin [9, 10]; however, whether current recommendations are adequate for athletes has been debated. Although conflicting data exists as to whether exercise affects the biochemical status of riboflavin in the blood [3, 11], potentially due to increased losses [3, 12], supplementation does not appear to improve physical fitness if baseline values are within the normal range [13, 14]. If riboflavin deficiency is present, however, limited data suggest that supplementation may improve fitness performance [13]. The current consensus is that individuals who consume adequate levels through the diet do not require supplementation above the DRI.
5.2.1.3 Vitamin B3 (Niacin)
Niacin is a water-soluble vitamin whose derivatives, such as NADH, NAD, NAD+, and NADP, play vital roles in substrate metabolism. There is growing evidence that NAD+ plays an important role in mitochondrial function and energy metabolism, calcium homeostasis, and inflammation [15]. Deficiencies of this vitamin can result in a condition known as pellagra, a disease characterized by scaly skin sores, diarrhea, inflamed mucous membranes, mental confusion, delusions, and ultimately death. Although pellagra has almost disappeared from industrialized countries, it is still common in regions that subsist primarily on a corn-based diet, as corn is a very poor source of niacin. (It is now known that treating corn products in an alkali bath, typically limewater, will increase the bioavailability of niacin.)
Although critical to the oxidation of fuel sources, and thus exercise metabolism, studies assessing the role of niacin on metabolic responses during acute exercise are limited. With small doses of niacin (<1 g/day), supplementation has been shown to improve agility [16], but not endurance [17]. Large doses of niacin (typically supplemented in the form of nicotinic acid) have been shown to adversely affect plasma free fatty acid concentrations during exercise [18, 19], and thus, it is hypothesized that large doses may negatively impact lower-intensity exercise. However, to date, large niacin supplementation doses have only been shown to decrease performance when carbohydrate availability is low [20, 21]. Surveys of athletic populations show that niacin consumption is adequate, with only athletes participating in weight-restrictive behaviors falling below recommended levels [9, 10]. At present, it is recommended that all persons obtain the DRI for niacin intake to ensure adequate intake and performance.
5.2.1.4 Vitamin B5 (Pantothenic Acid)
Pantothenic acid performs multiple roles in cellular metabolism and regulation as an integral part of two acylation factors: coenzyme A (CoA) and acyl carrier protein (ACP). In these forms, pantothenic acid is essential to the metabolism of fatty acids, amino acids, and carbohydrates, as well as the synthesis of cholesterol, steroid hormones, vitamin A, and vitamin D [22]. Although human studies looking at the effect of pantothenic acid and exercise performance are limited, two studies suggest that supplementation with pantothenic acid (or vitamin B5 derivative) does not alter exercise metabolism or exercise performance [23, 24].
5.2.1.5 Vitamin B6 (Pyridoxine and Related Compounds)
Vitamin B6 collectively refers to all biologically active forms of vitamin B6 (including pyridoxal, pyridoxamine, and pyridoxine) although the metabolically active form of the vitamin is pyridoxal phosphate or PLP. B6 is involved in many different cellular processes including gluconeogenesis, niacin formation, lipid metabolism, erythrocyte function and metabolism, and hormone modulation [22]. All persons, including athletes, consume adequate amounts of this vitamin [10, 25]. Vitamin B6 is widely distributed in foods (see Table 5.3 to view foods with the greatest concentrations of vitamin B6) and commonly found in multivitamin and mineral supplements.
Because exercise has been shown to increase the loss of vitamin B6 as 4-pyridoxic acid [26] and because PLP acts as a cofactor in both gluconeogenesis and glycogenolysis, it has been postulated that supplementation of the vitamin may increase exercise performance. Studies show that time to exhaustion does not improve following B6 supplementation [27, 28], and thus supplementation above the DRI does not appear to improve performance.
5.2.1.6 Vitamin B7 (Biotin)
Biotin, also known as vitamin H, plays an important role in the catalysis of many essential metabolic reactions, including the synthesis of fatty acids, gluconeogenesis, and metabolism of leucine. To date no studies have been conducted looking at the role of biotin in exercise performance in humans.
5.2.1.7 Folic Acid
Named for the abundance of the vitamin in green, leafy vegetables (or foliage), folic acid plays several important roles in energy metabolism. Folic acid is the synthetic form of folate and is needed for DNA production and erythropoiesis. Deficiencies, common among athletes [10, 26, 29], can cause errors in cellular replication, particularly affecting the red blood cells; as a result of folate deficiency, megaloblastic anemia can occur. The DRI for folate is 400 μg/day for women and men. Due to its prominent role in cellular growth and differentiation, this value increases to 600 and 500 μg/day during pregnancy and lactation, respectively [2]. Folate is ubiquitous in nature and found in most all natural foods. However, the vitamin is highly susceptible to oxidative damage, and thus the folate content of foods is easily destroyed by heat.
Due to its role in erythrocyte production, it is not surprising that researchers question whether folic acid supplementation can increase athletic performance. Results of one study showed that while folate supplementation in an athletic population did significantly increase circulating levels of serum folate, this increase did not translate into increased performance [30]. The authors of this study speculate that changes in circulating concentrations of folate may not reflect changes in cellular folate status; thus, over-supplementation cannot be justified. At present recommended intake of folate follows the DRI for normal individuals.
5.2.1.8 Vitamin B12 (Cyanocobalamin)
Cyanocobalamin, or vitamin B12, is unlike other B vitamins in that plants do not provide it and the body is capable of storing it in the liver. Vitamin B12 is involved in fat and carbohydrate metabolism, as well as protein synthesis. Additionally, vitamin B12 is responsible for the conversion of homocysteine to methionine, and deficiencies in the vitamin have been linked to hyperhomocysteinemia, an independent risk factor for cardiovascular disease [31]. In nature, this vitamin is synthesized by microorganisms, and thus it is not found in plant foods, except when they are contaminated by microorganisms. Small amounts of vitamin B12 are found in legumes, which contain microorganisms, and may provide the only dietary source of vitamin B12 for vegans.
5.2.1.9 Vitamin C (Ascorbic Acid)
The functions of vitamin C are based primarily on its biological reductant capabilities. As such, vitamin C is involved in collagen formation, cortisol synthesis, neurotransmitter synthesis, and iron absorption. Vitamin C has also been shown to promote resistance to infection through the immunologic activity of leukocytes, production of interferon, process of inflammatory reaction, and/or integrity of the mucous membranes [22]. When dietary intake of ascorbic acid is insufficient, a set of conditions occur (e.g., malaise, lethargy, petechiae, gum disease, poor wound healing) that are collectively known as scurvy.
Important to the athlete, vitamin C has certain biological functions that can influence physical performance. Due to its requirement in the synthesis of carnitine (the enzyme responsible for the transport of long-chain fatty acids into the mitochondria), it is thought to play a major role in lipid energy availability. Additionally, it may act as an antioxidant, along with vitamins A and E, preventing cellular damage caused by free radical intermediates. In general, intakes of 0.2–1.0 g/day reduce oxidative stress, but do not improve athletic performance [34]. Large doses (>1.0 g/day) may possibly impair athletic performance through reduced mitochondrial biogenesis or alterations in vascular function [34].
5.2.2 Fat-Soluble Vitamins
The fat-soluble vitamins include vitamins A, D, E, and K. Vitamins A and E function as antioxidants, and vitamins D and K play a role in bone metabolism. Because fat-soluble vitamins are stored for extended periods when consumed in excess, they create a greater risk for toxicity than water-soluble vitamins. Disease states affecting the absorption or storage of fat could cause deficiency of these vitamins.
5.2.2.1 Vitamin A
The roles of vitamin A within the body are vast, including immune function, vision, growth, and gene expression. Vitamin A comprises a group of compounds, including retinol, retinal, retinoic acid, or retinyl ester. Provitamin A carotenoids (i.e., α-carotene and β-carotene) are precursors to retinol, one of the most active forms of vitamin A. Some provitamin A carotenoids have been found to have antioxidant activity, with β-carotene suggested to be the primary anticancer agent in fruits and vegetables [35].
Dietary carotenoids are consumed primarily through oils and brightly colored fruits and vegetables, whereas preformed vitamin A is found only in animal products. The RDA requirements for vitamin A are expressed in retinol activity equivalents (RAE). A UL of preformed vitamin A has been recommended because retinol is stored and metabolized in the liver; however, the liver has a protective mechanism for reducing vitamin A metabolites by excreting the metabolites in bile. Adverse effects associated with an overdose of vitamin A include acute effects such as vertigo, blurred vision, and nausea, as well as chronic effects such as bone loss and liver abnormalities [36].
To date, there is very little research examining the effects of β-carotene supplementation alone on muscular strength or endurance. Although there is potential for vitamin A supplementation to decrease oxidative stresses from exercise, research is limited, predominantly because vitamins C and E have greater antioxidant capabilities. Further, evidence of vitamin A deficiency in athletic individuals is lacking, likely because body storage is appreciable [37]. Due to the fat solubility properties, supplementation of β-carotene is not recommended.
5.2.2.2 Vitamin D (Calciferol)
Although many forms of calciferol (vitamin D) exist, the two main forms are ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3). Vitamin D2 is produced from ergosterol in the diet, while vitamin D3 is synthesized by ultraviolet radiation from a precursor of cholesterol in the skin. Both forms are biologically inert and must be converted to the biologically active form, 1,25-dihydroxyvitamin D (1,25 (OH)2D). The major source of vitamin D for humans is sunlight. This source is especially important because few dietary sources contain vitamin D naturally (see Table 5.4). In the United States, many dairy foods (i.e., milk and cheese) are fortified with vitamin D to avoid deficiency in latitudes where exposure to sunlight is limited during the winter months.
Table 5.4
Summary of fat-soluble vitamins
|
Nutrient
|
Function
|
Adult (nonpregnant) recommended intake
|
Food sources
|
Comments for the athlete
|
|---|---|---|---|---|
|
Vitamin A
|
Vision, immune response, epithelial cell growth and repair
|
• UL: 3,000 μg preformed vitamin A/day
• Deficiency: dry skin, dry hair, broken fingernails, susceptibility to infections
|
Broccoli, squash, sweet potatoes, pumpkin, cantaloupe, liver, milk, eggs
|
Supplementation of β-carotene is not recommended
|
|
Vitamin D
|
Bone remodeling and maintaining serum calcium and phosphorus concentrations
|
• UL: 100 μg/day
• Deficiency: osteomalacia, osteoporosis, heart disease, hypertension
|
Natural sources: fatty fish, egg yolks
Fortified sources: milk, cereals
|
May influence bone mineralization and help prevent fractures
|
|
Vitamin E
|
Antioxidant
|
• UL: 1,000 mg α-tocopherol/day
• Deficiency: retinopathy, neuropathy, and myopathy
|
Vegetable oils, unprocessed cereal grains, green leafy vegetables, nuts
|
Antioxidant properties may be beneficial in decreasing oxidative stress during exercise bouts
|
|
Vitamin K
|
Essential for normal blood clotting
|
• UL: N/A
• Deficiency: increase in blood clotting time and decrease in bone mineral density
|
Green leafy vegetables, cereal, organ meats, dairy products, eggs
|
Supplementation may be needed for formation of bone
|
Excess amounts of vitamin D (hypervitaminosis D) can lead to high blood pressure, bone loss, and kidney damage. These effects are thought to be caused, for the most part, by hypercalcemia that may occur with hypervitaminosis D [38]. This is because the function of vitamin D is to maintain serum calcium and phosphorus levels within the body by enhancing their absorption from the gastrointestinal tract and promoting their release from the bones. Active vitamin D provides such a role by working in combination with the parathyroid hormone (PTH) to mobilize calcium and, indirectly, phosphorus from the bone to maintain serum concentrations as needed.
Studies suggest that serum 25-hydroxyvitamin (25(OH)D) concentrations <50 nmol/L is common among athletes [39–41], with serum profiles often worse depending upon winter season, unfavorable latitude of training, and indoor training routine [42]. Decreased vitamin D is associated with increased inflammation in athletes [43]. In subjects with elevated baseline inflammation, the addition of vitamin D supplementation to exercise has been shown to reduce inflammation beyond that of exercise alone [44]. Further, insufficient vitamin D may increase risk for injuries, such as stress fractures, which are quite common among athletes. In older adults, increasing serum 25(OH)D levels from 53 to 74 nmol/L by supplementation reduced fracture risk by 33 % over 5 years [45]. In a large sample of active female military recruits, supplementation with 800 IU/day of vitamin D for 8 weeks resulted in 20 % lower incidence of stress fractures than the placebo group [46].
With regard to athletic and muscular performance, studies examining the effects of vitamin D supplementation in young athletes are sparse. A recent study examining the effects of 5,000 IU/day of vitamin D3 supplementation for 8 weeks showed that supplementation resulted in improved 10 m sprint time and vertical jump height [39]. In older adults (>50 years of age), beneficial effects of vitamin D treatment have been observed on muscle function, including muscle strength [47, 48] and physical function [49, 50], when vitamin D status is low prior to supplementation. However, the beneficial effect of supplementation on physical function is not universally observed. A recent review showed that over half of the previously published papers examining the effects of vitamin D supplementation on muscle function show a lack of an effect versus beneficial results [51].
In conclusion, and much like the other micronutrients, while randomized controlled trial evidence is not strong enough to suggest vitamin D as an ergogenic aid, insufficient levels of vitamin D can contribute to decreased athletic performance, potentially by increasing the risk of debilitating stress fractures. Thus, obtaining optimal 25(OH)D levels (i.e., ≥50 ng/mL) through diet and UV exposure is recommended.
5.2.2.3 Vitamin E
Unlike the other fat-soluble vitamins, vitamin E has no specific metabolic function. Instead, its major function is as an antioxidant of polyunsaturated fatty acids, preventing free radical damage in biological membranes caused by lipid peroxidation. Because vitamin E is absorbed similarly to dietary fat, changes in pancreatic function, chylomicron transport of lipids, and bile production are known to impair vitamin E absorption [52]. The majority of vitamin E is stored in the adipose tissue, while smaller amounts are stored in the heart, liver, lungs, brain, muscles, and adrenal glands. Eight naturally occurring isomers of vitamin E exist; however, only the α-tocopherol form is maintained in human plasma [53]. Adverse effects of vitamin E deficiency include retinopathy, neuropathy, and myopathy; however, deficiency occurs rarely in humans. There are no adverse effects noted with ingestion of naturally occurring vitamin E in foods. As a nutritional supplement, side effects include fatigue, gastrointestinal disturbances, and altered lipid concentrations. Nutritional supplements contain either a natural or synthetic form of α-tocopherol.
Studies examining the effects of vitamin E supplementation on oxidative stress and lipid peroxidation during exercise are numerous. Compared to a placebo, supplementation of 800 mg/day α-tocopherol for 2 months [54] resulted in improvements in antioxidant potential 1.5 h after completing a triathlon. However, the large dose also appeared to cause significant increases in oxidative stress markers and did not improve performance time. Conversely, free radical production was reduced in basketball players following lower dose supplementation (1 month of 200 mg/day of α-tocopherol) [55]. Regarding performance, no difference in aerobic work capacity was observed following supplementation with a low (35 days of 268 mg/day of vitamin E [56]) or high dose (4 weeks of daily 1,200 IU of α-tocopherol [57]) of vitamin E. Additionally, supplementation with 400–1,000 IU of vitamin E does not appear to reduce biochemical indices of muscle damage following acute, strenuous exercise [58, 59]. Interestingly, data from a recent randomized controlled trial suggests that combined vitamin E and C supplementation (which often occurs together) may hamper beneficial cellular adaptations to exercise [60], with authors interpreting results to advocate caution when considering antioxidant supplementation combined with endurance exercise. Further research should be aimed at determining specific recommendation regarding vitamin E supplementation on exercise performance, oxidative stress, and muscle damage.
5.2.2.4 Vitamin K
Two forms of vitamin K naturally exist: phylloquinones (vitamin K1), produced by plants, and menaquinones (vitamin K2), produced by bacteria in the large intestine. The predominant forms of intake in the diet are phylloquinones, from green leafy vegetables. Vitamin K functions as a critical cofactor of γ-carboxylase, an essential posttranslational modification required for the functional activity of coagulation proteins such as prothrombin. Deficiency of vitamin K leads to changes in blood clotting (increased prothrombin time) and a decrease in bone mineral density (increase in plasma under-γ-carboxylated osteocalcin) [61].
Although no studies on the effects of vitamin K supplementation and performance exist, supplementation benefits on bone mass have been studied. Vitamin K1 (10 mg/day) supplementation in younger (20–30 years of age), female endurance athletes showed no effect on the rate of bone loss following 2 years of supplementation [62]. It was determined that females beginning endurance training at younger ages were at risk for higher amounts of bone loss than those that began their training at a later age; however, both groups had a relatively high rate of bone loss when compared to standards for females of the same age. In postmenopausal elite female athletes, 1 month of K1 supplementation (10 mg/day) resulted in increased bone formation marker and decreased bone resorption marker production [63].
5.2.3 Vitamins and Exercise Summary
Because both fat- and water-soluble vitamins are essential to human physiological function, examining their effect on exercise, both on athletic performance and deficiency avoidance, is popular in sport nutrition research. In summary, the grouping of “B vitamins” has two major functions directly related to exercise. Thiamin, riboflavin, vitamin B6, niacin, pantothenic acid, and biotin are involved in energy production during exercise, whereas folate and vitamin B12 are required for the production of red blood cells, protein synthesis, and tissue repair and maintenance. Vitamin C may play a role in improved immune function and may indirectly benefit athletic performance. Although it is thought that exercise may slightly increase the need for these vitamins, demands can usually be met by the increased energy intakes required for physically active persons to maintain energy balance. The two major functions of the fat-soluble vitamins include the antioxidant activity of vitamins A and E and bone formation of vitamins D and K.
In general, benefits of vitamin supplementation in regard to increased exercise needs or improved athletic performance are assumed inconclusive, unless stated otherwise. More research is needed before micronutrient supplementation above the DRI should be recommended to athletes, either as a requirement for increased needs during exercise or as an ergogenic aid.
Sidebar 5.1 Multivitamin and Mineral Supplementation: To Supplement or Not?
Current recommendations from the Academy of Nutrition and Dietetics suggest consuming a wide variety of foods to avoid chronic disease and micronutrient deficiency. Although it is ideal to consume nutrients through a balanced diet, over-the-counter multivitamin and mineral supplements may contribute to total nutrient intake, especially if dietary intake is inadequate. The percentage of US adults who used at least one multivitamin and mineral supplement increased from 30 % in 1988–1994 to 39 % in 2003–2006, with use more common among women than men [64]. Lun et al. [65] recently reported that multivitamin and mineral use in high-performance athletes is 16 %, with 87 % of taking ≥3 dietary supplements (e.g., combination of multivitamin and mineral supplements, sports drinks, sports bars, protein powder, and meal-replacement products). In general, supplementation to an athlete on a well-balanced diet has not been shown to improve performance. Eight weeks of supplementation with a liquid multivitamin and mineral supplement in resistance-trained men did not appear to improve short-duration anaerobic exercise performance compared to a placebo [66]. In addition, no apparent difference in endurance performance was observed in athletes who regularly consume multivitamin and mineral supplements versus those who do not [67]. Research to evaluate whether supplementation with megadoses of multivitamin and mineral improves performance is necessary. However, because supplements are concentrated sources of nutrients and dietary supplements may be marketed without providing evidence of safety or efficacy, it is important for health-care professionals to monitor for excess nutrient intake in those choosing to consume a multivitamin and mineral supplement.
5.3 Mineral Introduction
Dietary minerals are chemical agents required by living organisms to maintain physical health. Like vitamins, minerals also regulate macronutrient use and are classified as either macrominerals or microminerals/trace elements, depending on the daily amount needed. Additionally, minerals play various roles involved in enzyme regulation, maintenance of acid-base balance, nerve and function, and cellular growth. Because many of these processes are heightened during exercise, the field of exercise nutrition has sought to explore the relationship between different mineral needs and physical activity. Such findings, along with the general function, effects of deficiency or over-supplementation, and recommended intake levels, are the focus of this section.
5.3.1 Macrominerals
Macrominerals are required in amounts greater than 100 mg/day and include calcium, phosphorus, magnesium, sulfur, potassium, sodium, and chloride. A comprehensive review of the above listed macrominerals and their role in physical performance can be found in Table 5.5.
Table 5.5
Summary of minerals
|
Nutrient
|
Function
|
Adult (nonpregnant) recommended intake
|
Food sources
|
Comments for the athlete
|
|---|---|---|---|---|
|
Calcium
|
Structure of the teeth and bone, vascular and muscle contractions, blood coagulation, nerve transmission
|
• UL:
2,500 g/day in adults ≤50 years
2,000 g/day in adults >50 years
• Deficiency: improper bone mineralization, tetany, muscle pain and spasms
|
Dairy products, pinto and black beans, spinach, fortified cereal, orange juice
|
Possible effects of calcium supplementation on body weight and sweat losses during exercise; however, current recommendation is DRI
|
|
Phosphorus
|
Essential for strong bones and teeth and energy metabolism
|
• UL:
4 g/day in adults ≤70 years
3 g/day in adults >70 years
• Deficiency: anorexia, muscle weakness, bone pain, rickets, confusion
|
Milk, carbonated cola drinks, eggs, whole wheat bread, almonds, lentils, some fish
|
Phosphate loading may increase exercise performance; however, supplementation is potentially harmful
|
|
Magnesium
|
Energy metabolism, neuromuscular coordination, bone mineralization
|
• UL: 350 mg/day
• Deficiency: hypocalcemia, tetany, tremors, muscular weakness, confusion
|
Wheat flour, artichokes, pumpkin seeds, almonds, tuna
|
Although exercise may increase needs, current recommendation is DRI
|
|
Sulfate
|
Protein synthesis and formation of disulfide bridges
|
• UL: N/A
• Deficiency: stunted growth
|
Meat, poultry, fish, eggs, dried beans, broccoli, cauliflower
|
Although exercise may increase needs, current recommendation is DRI
|
|
Potassium
|
Water balance, acid-base balance, electrical potential gradients across membranes
|
• UL: N/A
• Deficiency: muscle weakness, myalgia, increased risk of hyponatremia
|
Tomatoes, orange juice, beans, raisins, potatoes, grapefruit
|
Exercise does not appear to increase needs
|
|
Sodium
|
Maintain extracellular volume and plasma osmolality
|
• UL: 2.3 g/day
• Deficiency: hyponatremia, muscle cramps, overhydration, hypotension
|
Processed and cured meats and cheeses, frozen meals
|
Ultra-endurance athletes and those with occupational physical activity and heat exposure may benefit from supplementation
|
|
Chloride
|
Same as sodium
|
• UL: 3.6 g/day
• Deficiency: overhydration, hypotension, muscle cramps
|
Similar to sodium-containing foods
|
Similar to sodium
|
|
Iron
|
Transportation of oxygen in the body
|
• UL: 45 mg/day
• Deficiency: fatigue, lack of stamina, breathlessness, headaches, insomnia
|
Lean red meats, seafood, beans, leafy green vegetables, molasses
|
May have beneficial effects on physical performance in those who are iron deficient
|
|
Zinc
|
Aids in wound healing and is a vital component of many enzymatic reactions
|
• UL: 40 mg/day
• Deficiency: altered taste, hair loss, diarrhea, fatigue, delayed wound healing
|
Oysters, wheat germ, ground beef, liver, ricotta cheese
|
Evidence supporting zinc supplementation in athletes has been equivocal
|
|
Chromium
|
Involved in carbohydrate, protein, and lipid metabolism and facilitates the action of insulin
|
• UL: N/A
• Deficiency: weight loss, peripheral neuropathy, impaired glucose utilization, and increased insulin requirements
|
Eggs, liver, oysters, wheat germ, spinach, broccoli, apples, bananas
|
Studies suggest that chromium supplementation benefits may only occur in individuals with impaired chromium concentrations
|
|
Boron
|
Function unknown—proposed functions include metabolism of vitamin D, macromineral metabolism, and immune function
|
• UL: 20 mg/day
• Deficiency: proposed effects include decreased bone density, mineral metabolism, and cognitive function
|
Grapes, leafy vegetables, nuts, grains, apples, raisins
|
Boron supplementation does not appear to effect physical performance
|
|
Copper
|
Enzyme catalyst, enhances iron absorption, antioxidant
|
• UL: 10,000 μg/day
• Deficiency: leukopenia, fatigue, hair loss, anorexia, diarrhea
|
Shellfish, finfish, beef, table salt, coffee
|
No known benefits of supplementation on performance
|
|
Fluoride
|
Mineralized bones and teeth
|
• UL: 10 mg/day
• Deficiency: dental caries weakened bone
|
Fluorinated water, tea, fish, legumes, potatoes
|
No known benefits of supplementation on performance; however, suboptimal intake may affect bone mineral density
|
|
Iodine
|
Essential in thyroid hormone function
|
• UL: 1,100 μg/day
• Deficiency: goiter, reduced mental function, hypothyroidism
|
Iodized table salt, seafood, kelp, dairy
|
No known benefits of supplementation on performance; however, suboptimal thyroid hormone concentrations may affect performance
|
|
Manganese
|
Antioxidant, bone formation, metabolism of amino acids, lipids, and carbohydrates
|
• UL: 11 mg/day
• Deficiency: decreased growth, impaired glucose tolerance, dermatitis
|
Nuts, leafy vegetables, whole grains, pineapple, teas
|
No known benefits of supplementation on performance
|
|
Molybdenum
|
Enzymatic cofactor
|
• UL: 2,000 μg/day
• Deficiency: headache, night blindness, tachycardia, tachypnea
|
Carrots, cabbage, legumes, nuts
|
No known benefits of supplementation on performance
|
|
Selenium
|
Defends against oxidative stress
|
• UL: 400 μg/day
• Deficiency: cardiomyopathy muscular weakness, pain
|
Brazil nuts, seafood, fish and shellfish, meats, garlic, eggs
|
No known benefits of supplementation on performance; however, may be beneficial due to antioxidant effects. Toxic if consumed in excess
|
|
Vanadium
|
Stimulates cell proliferation and differentiation, regulates phosphate-dependent enzymes, insulin-mimetic activity
|
• UL: 1.8 mg/day
• Deficiency: heart and kidney disease, reproductive disorders
|
Black pepper, beer, wine, mushrooms, sweeteners, grains
|
No known benefits of supplementation on performance
|
5.3.1.1 Calcium
Calcium is the most abundant mineral in the body, totaling approximately 1–2 % of body weight. Ninety-nine percent of calcium in the body is found in the structure of the teeth and bones. The remaining 1 %, found in the blood, muscle, extracellular fluid, and other tissues, functions in different roles throughout the body, such as in vascular and muscle contractions, blood coagulation, and nerve transmission [68]. At low calcium concentration, absorption depends on the activation of vitamin D; however, passive diffusion becomes more common at higher concentrations [68]. In addition to vitamin D, calcitonin and parathyroid hormone (PTH) are two hormones that regulate serum calcium concentrations. Calcitonin and PTH increase when blood calcium concentrations drop, causing calcium to be released from bone, reabsorbed in the kidneys, and absorbed in the intestines. A high-protein diet, as well as foods containing sodium, phytates, fiber, oxalic acid, and caffeine, may decrease the bioavailability and absorption of dietary calcium.
The two main calcium compounds found in supplements are calcium citrate and calcium carbonate. Calcium carbonate supplements typically contain 40 % calcium, while calcium citrate supplements contain 21 % calcium; therefore, more calcium citrate must be taken to equal a similar amount of calcium available in calcium carbonate [69]. Amounts less than 500 mg/day of calcium generally are recommended because absorption decreases as the amount of calcium in the supplement increases. Calcium citrate supplements are typically better absorbed in individuals with decreased stomach acid [69], usually a result of taking the supplement with food. When adolescent females were supplemented with 670 mg/day of a calcium citrate malate supplement (mean daily intake approximately 1,500 mg/day) for 7 years, supplementation positively influenced gain of bone mass throughout the bone-modeling phase of the pubertal growth spurt, which is when requirements of calcium are determined to be the highest [70]. By the beginning of young adulthood, the only significant findings were seen in those of tall subjects, suggesting that calcium requirements vary with skeletal size. Positive effects of calcium supplementation were seen at all skeletal regions examined during the young adulthood assessment period.
In a large placebo-controlled trial of 1,000 mg calcium carbonate plus 400 IU vitamin D3, examination of physical performance and self-reported exercise measures after 1, 2, and 4 years did not result in improvements in subjective or objective physical function [71]. However, the effects of calcium supplementation on physical performance are lacking. Possible effects of supplementation on body weight and sweat losses warrant review, especially since low calcium intake is well documented in athletes [72–74]. Martin et al. found that 400 mg/day of calcium carbonate supplementation can correct negative calcium balance attributable to low calcium dietary intake and additional dermal losses following a 1-h strenuous exercise session [75]. Elevated urinary calcium losses also are observed following high-impact and resistance training exercise program [76]. Thus, calcium supplementation may benefit those involved in high-intensity sports, as well as those with weight restrictions. Overall, current research does not support the need for calcium supplementation above the DRI; however, more research is required.
5.3.1.2 Phosphorus
Phosphorus is essential for all living cells as a component in phospholipid membranes, as well as in nucleic acids and nucleotides. Eighty-five percent of total body phosphorus is found in the bone, and the remaining 15 % is found in the soft tissues. The form of phosphorus most commonly found in nature is phosphate; however, while stored in the bone, the main form is hydroxyapatite crystals. Although extremely rare, because phosphorus is well dispersed throughout plant and animal foods, deficiency of serum phosphorus may lead to anorexia, muscle weakness, bone pain, rickets, confusion, and death. High intake of phosphorus may reduce serum calcium concentrations and reduce the formation of active vitamin D, leading to an increase of PTH. Elevated PTH is associated with increased bone loss to maintain serum calcium concentrations. Along with its adverse effects on the bone, overconsumption of phosphorus also may cause calcification of soft tissues, especially the kidney.
Athletes often consume excess phosphorus due to “phosphate loading.” The aim of the supplementation is to improve tissue oxidation by increasing erythrocyte 2,3-diphosphoglycerate concentrations. Phosphate loading may result in improved athletic performance in endurance athletes by improving oxygen release in tissues. For ergogenic purposes, sodium phosphate typically is supplemented orally in capsule form, at a dose of 3–5 g/day for a period of between 3 and 6 days [77]. Numerous studies have shown positive effects of phosphate loading on peak power output, increased anaerobic threshold, and improved cardiovascular responses in trained athletes [78–80]. Before phosphorus supplementation is recommended among athletes; however, effects on performance and bone mineralization must be evaluated.
5.3.1.3 Magnesium
As a required cofactor for over 300 enzymatic reactions, magnesium plays an important role in aerobic and anaerobic energy generation. Other functions of magnesium include immune function, neuromuscular coordination, and bone mineralization. Magnesium is important in vitamin D absorption and metabolism. It also plays a structural role in the body. Fifty to sixty percent of body magnesium is stored in the bones, and the parathyroid hormone is dependent upon magnesium for regulation of calcium in the bone. Also, magnesium is required for regulating the outward movement of potassium from myocardial cells and the intracellular concentration of calcium during muscle contractions [68]. Serum magnesium depletion may lead to hypocalcemia, tetany, tremors, muscular weakness, and confusion.
Several studies report that athletes may be deficient in magnesium [74, 81, 82], and exercise is believed to increase magnesium requirements by as much has 10–20 % [83]. Because of its role in immune function, low magnesium status could contribute to the depressed immunological changes observed after strenuous exercise, which may be sufficient to lead to an infection, particularly upper respiratory tract infections [84]. Mooren et al. [85] observed that 2 months of magnesium supplementation was unable to prevent exhaustive exercise-induced alterations in immune cell function in athletes with balanced magnesium status. Marginal magnesium deficiency also resulted in increased peak oxygen uptake and peak heart rate following submaximal exercise compared to those with adequate magnesium intake [86]. Recently, Kass et al. found that supplementation with 300 mg of magnesium oxide resulted in reduced resting and recovery of systolic blood pressure following aerobic and resistance exercise, but did not have an effect on athletic performance [87]. Overall, studies do not support the need for supplementation of physically active individuals, with adequate magnesium status, to improve performance.
5.3.1.4 Sulfur
The mineral, sulfur, is a major constituent of three amino acids: cystine, cysteine, and methionine. Additionally, sulfur is involved in protein synthesis, as it is responsible for the formation of disulfide bridges, a necessary component of the tertiary structure of proteins. Dietary sources of sulfur include meat, poultry, fish, eggs, dried beans, broccoli, and cauliflower.
Current research studying the effect of sulfur on athletic performance is limited to amino acids containing the mineral. While a DRI is currently not available for sulfur, at present there is no literature to suggest that athletes need to consume higher amounts than the average person.
5.3.1.5 Potassium
As an electrolyte, potassium plays a major role in electrical and cellular body functions. Along with sodium and chloride, potassium is involved in maintaining water balance and distribution, osmotic equilibrium, acid-base balance, and electrical potential gradients across membranes [88]. Because nerve and muscle cells have the highest gradients of bodily cells, potassium plays a major role in nerve and muscle function.
Due to its role in muscle function, several studies have been conducted looking at the relationship between potassium and exercise performance. Prolonged exhaustive exercise has been shown to impair potassium transport processes in exercising muscle [89]. This impairment can lead to a rise in extracellular potassium concentration in the skeletal muscle, which is thought to play an important role in the development of fatigue during intense exercise [90].
Although few studies have been conducted looking at the effects of potassium supplementation on exercise performance, an interesting study suggests that potassium phosphate supplementation may mediate perceived and physiological exertion [91]. In a double-blind, placebo-controlled study, eight highly trained endurance runners were asked to provide a rating of perceived exertion (RPE) during maximal graded exercise tests. Results showed that overall RPE was lower with supplementation, thus encouraging prolonged activity; however, no group differences were observed in maximal physiological response. Additional studies are warranted before exercise-specific recommendations can be made.
5.3.1.6 Sodium and Chloride
The cation, sodium, and the anion, chloride, are normally found together in most foods as sodium chloride, also known as salt, with the highest concentrations found in prepared, cured, or pickled food products. In the body, sodium and chloride are required to maintain extracellular volume and plasma osmolality. Healthy adults should consume 1.5 g of sodium and 2.3 g of chloride each day, or 3.8 g of salt, to replace the amount lost in sweat [92]. Sweat is produced by our bodies as a by-product of thermoregulation. Should the capacity for sweat production be hindered, a rise in core temperature and resultant heat illness could result. For the athlete, conditions such as extreme heat or exercise intensity can significantly elevate sweat losses above what is considered normal, and resultant dietary adjustments must be made.
Given the critical need to maintain fluid homeostasis, the American College of Sports Medicine has put forth guidelines for proper hydration [93] (see Table 5.6). Inclusion of sodium chloride in rehydration beverages has been shown to reduce urinary water loss, leading to a more rapid recovery of fluid balance, with some experts now recommending sodium concentrations of 20–50 mmol/L and an osmolality between 200 and 330 mOsm/kg water in glucose-electrolyte beverages consumed during physical activity [94, 95]. Fruits, vegetables, and other high-moisture foods also make an important contribution to total fluid intake. Evidence suggests that humans receive 20–25 % of their daily water intake from foods and that recovery from exercise and heat exposure is improved when food is ingested before consuming water after exercise [96].
Table 5.6
Hydration guidelines for exercise
|
Pre-exercise
|
During exercise
|
Postexercise
|
|---|---|---|
|
• ~4 h before exercise, consume 5–7 mL/kg of a 20–50 mEq Na+ solution
• If urine is dark, drink another 3–5 mL/kg of a 20–50 mEq Na+ solution ~2 h before exercise
|
• Consume 0.4–0.8 L/h
• If prolonged, solution should contain:
– 5–10 % carbohydrate solution
– 20–30 mEq/L Na+
– 2–5 mEq/L K+
|
• If time permits, consumption of normal meals and beverages will restore euhydration
• If a more rapid recovery is required, consume ~1.5 L fluid per kg body weight lost
|
Recently, there have been reports of hyponatremia among individuals who tend to over-ingest water during exercise lasting more than 4 h in length [97]. Additionally, lower plasma sodium and development of exercise-associated hyponatremia may be attributed to pituitary secretion of vasopressin, an impaired mobilization of osmotically inactive sodium stores, and/or an inappropriate inactivation of osmotically active sodium. For ultra-endurance athletes, inclusion of sodium chloride in the fluid replacement beverage is often suggested as a potential means of reducing risk of hyponatremia. Although hyponatremia is not likely to be a major risk factor for the general population, ultra-endurance athletes and people with occupational physical activity and heat exposure may benefit from these recommendations [98].
5.3.2 Microminerals/Trace Elements
Microminerals, or trace elements, include iron, zinc, copper, selenium, iodine, fluoride, chromium, manganese, molybdenum, boron, and vanadium. In general, these elements are required in amounts less than 100 mg/day. Although 14 trace minerals have been identified as essential for life, there is sufficient information on only four, as related to physical performance. The following section will provide detailed information on four of these trace elements, with Table 5.5 summarizing recommended dietary intake, food sources, and functional role in the body for all other microminerals.
5.3.2.1 Iron
Dietary iron is a constituent of hemoglobin, myoglobin, cytochromes, and iron-containing enzymes. As such, iron plays a fundamental role in the transport of oxygen in the body, and adequate stores are necessary for optimal athletic performance. Dietary iron can be obtained through quality food sources, as well as obtained from foods prepared in cast iron cookware. Furthermore, the bioavailability of iron in certain foods (particularly vegetables) can be increased by the addition of an acid (i.e., vitamin C) during preparation.
Iron deficiency is the most common nutritional disorder disease [99], with iron status negatively altered in many populations of chronically exercising individuals [100]. Despite claims of blood loss as a result of foot striking, gastritis, and menstruation, as well as the pseudo-anemia caused by an increase in plasma volume during exercise, the true cause of anemia in athletes often can be attributed to a diet inadequate in iron [101]; thus, efforts should be placed on the improvement of dietary quality. If left untreated, an iron deficiency can cause anemia, a condition where hemoglobin cannot be formed. Iron deficiency without anemia is found in 29 % of female and 4 % of male recreationally active subjects [102] and, depending on the sport surveyed, can increase to a prevalence of 80 % in elite athletes [103]. In general, reductions in tissue oxidative capacity hinder endurance and energetic efficiency, which translates into impaired athletic performance. Numerous studies have shown the negative impact of iron deficiency anemia on work output and physical performance [104, 105], with supplementation in deficient individuals shown to improve athletic performance [106, 107].
Because some individuals carry a gene for increased iron absorption, or hemochromatosis, over-supplementation is not advised. Iron is a very powerful oxidant and is toxic at high concentrations; it is for this reason that iron supplementation should only be reserved for those individuals who are deficient. Moreover, even in individuals without hemochromatosis, iron supplementation can cause side effects, usually stomach upset such as nausea, vomiting, diarrhea, dark stools, or constipation. In general, female athletes, vegetarians, and endurance athletes are considered at greater risk for iron deficiency than the typical athlete; however, proper diagnosis of the condition by assessing ferritin levels in the blood by a medical provider is necessary before supplementation should be considered.
Sidebar 5.2 The Vegetarian Athlete
In a recent poll, 4 % of US adults were found to be vegetarian [108]. While this percentage may seem small, in actuality it translates to over nine million people! With the prevalence at only 1 % in 1997 [109], the trend of vegetarianism shows no sign of stopping.
According to the Vegetarian Resource Group, vegetarian diets can be classified into four major groups:
-
Vegans: Do not eat meat, fish, or poultry. Additionally, do not use other animal products and by-products such as eggs, dairy products, honey, leather, fur, silk, wool, cosmetics, and soaps derived from animal products.
-
Lacto-vegetarians: Do not eat meat, fish, poultry, or eggs; do consume dairy products.
-
Ovo-vegetarians: Do not eat meat, fish, poultry, or dairy; do consume egg products.
-
Lacto-ovo vegetarians: Do not eat meat, fish, or poultry; do consume dairy and eggs.
Additionally, some persons may self-describe themselves as vegetarians if they are occasional meat eaters who predominately practice a vegetarian diet .
In addition to the numerous health benefits associated with a vegetarian diet [110], the high-carbohydrate nature of a vegetarian diet can be beneficial for the athlete during heavy training when maximizing body glycogen stores is a must. Although the benefits to following a vegetarian diet are numerous, appropriate nutrition education and planning are necessary to ensure that dietary needs are being met. Certain nutrients are either not present or are not as easily absorbed in plant products as they are in animal products. Specifically, vegetarians need to be mindful of their intake of iron, calcium, vitamin B12, and vitamin D, as good sources of these nutrients are mostly of animal origin. Listed below in Table 5.7 are vegetarian-friendly food sources of the nutrients that are most likely to be lacking in a vegetarian diet.
Table 5.7
Good food sources of specific micronutrients for the vegetarian
|
Nutrient
|
Good food choices
|
|---|---|
|
Iron
|
Legumes, leafy green and root vegetables, prune juice, tahini, and dried fruits
|
|
Zinc
|
Grains, legumes, and nuts
|
|
Calcium
|
Calcium-set tofu, fortified beverages (orange juice, soy milk), kale, collard, mustard greens, tahini, and blackstrap molasses
|
|
Vitamin D
|
Fortified foods (soy and rice milk). Sun exposure (~10–15 min 2–3 times per week)
|
|
Vitamin B12
|
Fortified foods (cereal, soy, and dairy products) and meat analogues
|
5.3.2.2 Zinc
The mineral zinc primarily serves a structural role in thousands of proteins. Additionally, zinc is also involved as a cofactor in many enzyme reactions and plays a vital role in tissue repair. As with many other nutrients, it has been suggested that athletes generally consume less zinc than the RDA [112, 113]. In athletes, zinc deficiency can lead to anorexia, significant loss in body weight, latent fatigue with decreased endurance, and a risk of osteoporosis [114]. Zinc depletion can reduce total work capacity of the skeletal muscle [115]; however, exercise has not been shown to cause significant losses in the athlete when dietary zinc intake is sufficient [116].
To date, evidence supporting zinc supplementation in athletes has been equivocal. Two recent studies showed that zinc supplementation in athletes resulted in higher antioxidant [117] and greater inflammatory responses [118] than in non-supplemented athletes. Kilic et al. reported that 4 weeks of zinc supplementation positively affected hematological parameters in athletes [119]; yet Lukaski et al. found that neither zinc supplementation nor a restricted zinc intake was found to have any effect of maximal oxygen uptake over a 4-month period [120].
Although the ergogenic potential for zinc supplementation is debatable, the effects of over-supplementation are not. In the body, an intake of zinc greater than 50 mg/day has been shown to inhibit copper bioavailability [121]. Additionally, zinc intake ten times greater than the RDA has been shown to decrease immune function, reduce HDL cholesterol, and increase LDL cholesterol [122]. For these reasons, zinc supplements exceeding 15 mg/day are not recommended.
5.3.2.3 Chromium
The two most common forms of chromium are chromium III and chromium VI, with chromium III being the form most often found in foods because of its greater stability. Chromium VI is recognized as carcinogenic if inhaled or ingested, whereas chromium III is important in carbohydrate, lipid, and protein metabolism. Chromium helps facilitate the action of insulin, ultimately increasing insulin sensitivity and decreasing the need for insulin. Chromium is well dispersed throughout many food sources. Side effects associated with chromium deficiency include weight loss, peripheral neuropathy, impaired glucose utilization, and increased insulin requirements. Although nephritis, hepatic dysfunction, carcinogens, and rhabdomyolysis (extreme skeletal muscle damage) are possible effects of high chromium intake, at present, no upper limit for chromium has been set.
Because of chromium’s role in energy metabolism, numerous studies examining the effects of chromium supplementation and exercise have been performed. Volek et al. determined that 11 μmol chromium III supplementation had no effects on glycogen synthesis during recovery from high-intensity cycle ergometry in overweight adult males on a high-carbohydrate diet [123]. Likewise, no differences in strength gains in older adults during twice-weekly resistance training for 12 weeks were observed with 924 μg chromium/d as chromium III versus a placebo [124]. Multiple studies have hypothesized that benefits of supplementation may only occur in individuals with impaired chromium concentrations [125, 126]. Further, chromium supplementation has been suggested to dispose an individual to iron deficiency, depending on the dose and duration of chromium supplementation [125].
5.3.2.4 Boron
The physiological role of boron in the body is not clearly understood. Proposed functions include metabolism of vitamin D, macromineral metabolism, and immune function. Due to a lack of evidence surrounding boron, no DRI has been established.
Some data suggest that boron may play an ergogenic role in athletic performance by increasing the concentration of plasma steroid hormones [127]. Additionally, Meacham et al. have conducted two studies to determine if supplementing 3 mg/day of boron versus a placebo in athletic versus sedentary participants has an effect on minerals, namely, phosphorus, magnesium, and calcium, affecting bone mineral density (BMD). The first study found that athletes supplemented with boron had lower serum magnesium concentrations than the sedentary subjects, but no differences were seen among activity groups receiving the placebo [128]. Plasma calcium did not differ between any groups, and serum phosphorus concentrations were significantly lower than baseline values among all groups. The second study not only looked at blood mineral concentrations, but also BMD using a dual-photon absorptiometer [129]. Boron supplementation did not appear to influence BMD; however, serum calcium and magnesium increased and phosphorus decreased over time in all subjects. Serum phosphorus concentrations were significantly lower in boron-supplemented subjects, with sedentary levels lower than active individuals. Athletic subjects supplemented with boron had lower serum magnesium levels than sedentary individuals. Due to varied findings on serum mineral concentrations with boron supplementation, more research should be conducted to determine effects on BMD.
5.3.2.5 Other Minerals
Little research exists on exercise and the following minerals: copper, fluoride, iodine, manganese, molybdenum, selenium, and vanadium. Functions, DRIs, known effects of exercise, and food sources of each nutrient may be found in Table 5.5. Exercise does not appear to increase needs above the DRIs, nor is there conclusive evidence recommending the use of supplementation for increased athletic performance.
5.3.3 Minerals and Exercise Summary
Athletes should consume a balanced diet in an attempt to obtain adequate amounts of minerals necessary for optimal performance. Mineral supplementation may be recommended in those who do not consume a balanced diet. Research has consistently found iron and calcium to be consumed in low amounts by athletes. During strenuous activity or exercise in a hot environment, elevated sweat losses may result in increased dietary requirements of sodium and chloride. Mineral deficiencies, especially iron and chromium, may lead to performance impairment, while deficiencies in calcium, magnesium, and phosphorus may decrease bone health. Overall, when well-nourished athletes are supplemented with minerals, including calcium, magnesium, iron, zinc, copper, and selenium, no improvements in athletic performance have been found. Phosphorus is the lone mineral in which multiple studies have shown that supplementation may improve performance in athletes without deficiency. However, due to adverse effects with over-supplementation and the need for further controlled research, current recommended intake remains the DRI.
5.4 Chapter Summary
In conclusion, the micronutrient needs of the athlete do not appear to differ from that of a healthy individual; that is, the athlete may refer to appropriate DRI tables to gauge nutrient needs. Generally, when dietary intake is adequate, supplementation is unnecessary. If dietary intake is inadequate (such as the case of strict vegans and intake of vitamin B12), or increased losses through sweat occur, supplementation may be warranted; however, care should be taken not to exceed the upper limit of the specific micronutrients in question.
5.5 Practical Application
For the professional working with the athlete, proper assessment should be made of caloric intake and expenditure prior to dietary prescription recommendations. This involves evaluation of current dietary habits (i.e., analysis of a 4-day food record or 24-h dietary recall) and exercise status (i.e., type, frequency, duration, and intensity of exercise). Additionally, age and gender as well as environmental factors (i.e., temperature and terrain) should also be considered when making dietary recommendations. Professionals should emphasize consuming a well-balanced diet before recommending supplementation. Special attention should be given to female and vegetarian athletes who may present with low calcium and iron levels. Tables 5.3 through 5.5 serve as a quick reference for recommended intakes and good dietary sources of specific micronutrients, as well as the role each plays in physical performance.
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