Basics in Sports Nutrition

Patricia A. Deuster

Stacey A. Zeno

Selasi Attipoe

The opinions and assertions expressed herein are those of the authors and should not be construed as reflecting those of the Uniformed Services University of the Health Sciences or the Department of Defense.

INTRODUCTION

This chapter focuses on the basic concepts of nutrition and sports. Topics include basics in energy metabolism, nutritional needs of athletes, hydration and fluid replacement, nutrient timing, selected nutritional issues, and information on ergogenic agents and nutritional supplements, to include sports drinks, bars, and energy drinks.

BASICS IN ENERGY METABOLISM

General Energy Needs

Energy balance is critical for all athletes and demands matching energy intake to total energy expenditure (TEE).
TEE is the sum of resting energy expenditure (REE), physical activity, and the energy used to digest foods (thermic effect of foods).

REE can easily be estimated for men and women as follows (26):

Men:	9.99 × Weight (kg) + 6.25 × Height (cm) − 4.92 × Age (yrs) + 5, or 4.54 × Weight (lbs) + 15.875 × Height (inches) − 4.92 × Age (yrs) + 5
Women:	9.99 Weight (kg) + 6.25 × Height (cm) − 4.92 × Age (yrs) − 161, or 4.54 × Weight (lbs) + 15.875 × Height (inches) − 4.92 × Age (yrs) − 161

Physical activity levels (PAL) vary, but a factor can be applied to REE to estimate TEE. Table 11.1 presents suggested ranges for PAL factors. However, TEE may be overestimated in the low range of activity.

The Institute of Medicine (IOM) (41) also proposes formulas for TEE as follows:

Men:

662 − 9.53 × Age (yrs) + PAL × [15.91 × Weight (kg) + 539.6 × Height (m)]

662 − 9.53 × Age (yrs) + PAL × [7.232 × Weight (lbs) + 13.71 × Height (inches)]

Women:

354 − 6.9 × Age (yrs) + PAL × [9.36 × Weight (kg) + 726 × Height (m)]

354 − 6.9 × Age (yrs) + PAL × [4.255 × Weight (lbs) + 18.44 × Height (inches)]

Comparison of the two formulas at the same PAL values reveals that the TEE estimated by the IOM formula is higher by 600-1,200 kcal · d⁻¹ for both men and women.
Failure to maintain energy balance can result in loss of muscle mass; menstrual disturbances; compromised bone density; increased risk of fatigue, injury, and illness; and overtraining (63).

Energy Sources and Stores

The universal source of metabolic fuel for energy is adenosine triphosphate (ATP) and the fuel sources to produce ATP are carbohydrates (CHO), fats (fatty acids), protein (amino acids), and alcohol. One gram of CHO (glucose, glycogen, etc.) yields 4 kcal; 1 g of protein yields 4 kcal; 1 g of fat provides 9 kcal; and 1 g of alcohol yields 7.4 kcal. These fuel sources are converted into energy.
ATP, glucose, and free fatty acids are stored in various amounts: ATP storage is sufficient for only 5-7 seconds of activity.
Glycogen, the storage form of CHO and a polymer of glucose, is found in both liver and muscle tissue.
Triglycerides, the storage form of lipids, consist of three fatty acids and a glycerol backbone.
Energy yields for glycogen (4 kcal · g⁻¹) in liver and skeletal muscle and triglycerides (9 kcal · g⁻¹) in skeletal muscle and adipose tissues are shown in Table 11.2.
Storing 1 g of glycogen requires at 2.7-4 g of water; thus, the energy yield is less than half of what would be expected: If 120 g of glycogen were stored, at least 80 g would be water, and the energy yield would be 160 rather than 480 kcal (69).
The breakdown of glycogen to form glucose for energy is called glycogenolysis.
The process of converting amino acids or lactate into glucose is called gluconeogenesis.

Energy Systems

Figure 11.1 presents the three different energy systems used in muscular activity, and Table 11.3 presents the length of time exercise can be maintained for these energy systems.

Table 11.1 Suggested Ranges for Physical Activity Factors Based on Activity

Level of Physical Activity	PAL
Sedentary	1.2
Minimal regular physical activity	1.4-1.5
Some regular activity	1.6-1.7
Moderate physical activity	1.8-1.9
Strenuous physical activity	2.0-2.4
PAL, physical activity level.

Table 11.2 Energy Stores for a 60-kg Person with 15% Body Fat

Energy Source	Energy (g)	Energy (kcal)
Liver and muscle glycogen	400-750	500-1,000
Glucose in body fluids	15-20	60-80
Total Carbohydrate Stores	415-770	560-1,080
Subcutaneous adipose triglycerides	9,000	81,000
Intramuscular triglycerides	150	1,350
Total Triglyceride Stores	9,150	82,350
Total Energy Stores	9,565-9,920	82,910-83,430

Figure 11.1: A schematic of the energy systems in skeletal muscle: immediate, short term, and long term. Glc, glucose; FFA, free fatty acid; IMM, inner mitochondrial membrane; OMM, outer mitochondrial membrane; SM, sarcolemma. Designed by Mark Roseman, PhD.

Table 11.3 Comparison of the Maximal Rates of Power and Length of Time Exercise Can be Maintained for the Immediate, Short-Term, and Long-term Energy Systems

Energy Systems	Mole of ATP/min	Time to Fatigue
Immediate: phosphagen (phosphocreatine and ATP)	4	5-10 seconds
Short term: glycolytic (glycogen-lactic acid)	2.5	1.0-1.6 minutes
Long term: aerobic (glucose, fatty acids)	1	Unlimited time
ATP, adenosine triphosphate.

The immediate, or phosphagen, system consists of ATP and creatine phosphate (PC or phosphocreatine) and allows for very short bursts of maximal power. It does not rely on oxygen.
The short-term, or glycogen-lactic acid, system consists of glucose entering the glycolytic pathway and subsequent oxidation of pyruvate to lactate. The energy formed allows for only 1-1.6 minutes of maximal muscle activity at a reduced power output as compared to the phosphagen system. This often can happen in the absence of oxygen, but lactate can be both formed and used under fully aerobic conditions (10).
The aerobic, or long-term, system involves glucose entering the glycolytic pathway through to pyruvate and its subsequent oxidation to acetyl-coenzyme A (acetyl-CoA) for entry into the tricarboxylic acid (TCA) cycle. Free fatty acids (FFAs) from triglycerides can be broken down into two-carbon acetyl fragments to form acetyl-CoA. This process, known as β-oxidation, can only occur under aerobic conditions (34).
Amino acids can enter the TCA cycle after deamination/transamination and conversion into acetyl-CoA and/or acetoacetate. NADH and FADH₂, the reduced coenzymes formed during glycolysis and the TCA cycle, are shuttled to the electron transport chain within the mitochondria to be oxidized to NAD and FAD, respectively. ATP is regenerated from adenosine diphosphate (ADP) with 2.5 ATP per NADH and 1.5 ATP per FADH₂. The O₂ used for oxidation is tightly linked to aerobic ATP production.
One molecule of glucose going through the glycolytic pathway to lactate yields 2 ATP, whereas one molecule of glucose going through the aerobic pathway yields 30 ATP.

Determinants of Fuel Utilization

The primary determinant of substrate preference is exercise intensity. The higher the intensity, the greater is the demand on the immediate and short-term glycolytic energy system. Low-intensity exercise uses the aerobic system almost exclusively.
The crossover point for metabolism is the power output (hard to maximal) at which an individual switches from a predominant reliance on FFA to a primary reliance on CHO utilization (glycogen; blood glucose; and blood, muscle, and liver lactate). Moreover, further increases in power elicit relative increments in CHO utilization and relative decrements in lipid oxidation (11).
Use of FFA increases in proportion to the energy demand until the exercise intensity approaches 60% of [V with dot above]O_2max, after which glucose becomes the dominant fuel source (35,65,66). Figure 11.2 shows the contributions of fat and CHO as substrate for various exercise intensities.
Trained athletes are able to oxidize fatty acids as fuels at higher exercise intensities than untrained persons.
During exercise and recovery from the exercise at altitude, CHO utilization is increased compared with conditions at sea level (45).

NUTRITIONAL REQUIREMENTS OF ATHLETES

Most athletes can meet their nutritional needs by consuming a well-balanced diet. Athletes have increased energy needs, and if these needs are met by consuming a balanced diet, the nutritional requirement should be met as well.

Carbohydrates

CHO intake is necessary to maintain blood glucose levels during exercise and restore muscle glycogen after exercise. Although the total intake will depend on TEE, intakes of 6-10 g · kg⁻¹ body wt · d⁻¹ (˜2.7-4.5 g · lb⁻¹ body wt · d⁻¹), depending on energy needs, are currently recommended. This equates to 420-700 g · d⁻¹ for a 70-kg (154-lb) person (63).

Figure 11.2: The relative contributions of fat and carbohydrate (CHO) to energy at rest and during exercise at 25%, 65%, and 85% of maximal aerobic capacity.
The percentage of kilocalories in the diet from CHO should range between 55% and 70% of total energy intake. For a 70-kg person who requires 3,750 kcal · d⁻¹, 2,000-2700 kcal should come from CHO, which would equate to 500-675 g of CHO·d⁻¹.

Protein

Approximately 10%-35% of the total energy should come from protein, depending on the actual energy intake.
Protein needs of athletes are somewhat higher than for sedentary populations. The recommended range of intake for strength athletes is 1.2-1.7 g · kg⁻¹ body wt · d⁻¹ (0.5-0.8 g · lb⁻¹ body wt · d⁻¹). This equates to a protein intake of 84-120 g · d⁻¹ for a 70-kg (154-lb) person, which would be 9%-13% of the total kilocalories, if 3,750 kcal · d⁻¹ were being consumed.
The recommended protein intake for endurance athletes is 1.2-1.4 g · kg⁻¹ body wt · d⁻¹ (0.55-0.64 g · lb⁻¹ body wt · d⁻¹) (63). This equates to a protein intake of 84-98 g · d⁻¹ for a 70-kg (154-lb) person, which would be 336-392 kcal or 9%-10.5% of the energy, if total energy were 3,750 kcal.
Essential amino acids (EAA) are not synthesized by the body and must be provided in the diet; they include leucine, isoleucine, valine, histidine, lysine, methionine, phenylalanine, threonine, and tryptophan (9,27).
The requirement for protein can be met through diet alone, without using protein powders or amino acid supplements (63).
When energy intake is low, it is important to consume the recommended amount of protein: Protein may contribute up to 35% of the total energy.

Fat

Dietary fat intake should usually provide no more than 30% of the total energy. For example, a 70-kg (154-lb) person who consumes 3,750 kcal · d⁻¹ would ingest ˜125 g of fat (or 1,125 kcal from fat).
Endurance athletes in training may decrease their intake of fat to only 20%-25% of total energy so they can consume adequate amounts of CHO.
Ingestion of a high-fat, low-CHO diet is associated with lower resting muscle glycogen content and higher rates of fat oxidation during exercise as compared to a CHO-rich diet (18,32,33,34).

Vitamins and Minerals

Active individuals should consume adequate vitamins and minerals to achieve the most recent Dietary Reference Intakes (DRIs) based on their life stage, gender, and activity level.
Athletes who perspire heavily or engage in physical activity in hot conditions may be prone to increased losses of minerals in their sweat.

HYDRATION AND FLUID REPLACEMENT

Water is the most important nutrient for regulating hydration status in individuals.
Water losses during exercise occur primarily through sweat and can be considerable during prolonged exercise, particularly in warm or hot weather. Electrolyte losses in sweat can also be substantial.
Sweat rate is influenced by ambient temperature, humidity, exercise intensity, training status, and rate of fluid intake.
Fluid replacement is important for sustaining exercise performance and the current American College of Sports Medicine^® (ACSM) guidelines note that the goal of fluid replacement is to prevent dehydration in excess of 2% weight loss and extreme changes in electrolyte balance to avoid performance decrements (67).
ACSM recommends that people take personal responsibility for sustaining their hydration and develop fluid replacement programs to suit their individual needs (67).

Effects of Dehydration

Dehydration (loss of body fluids) in excess of > 2% of body mass (> 1.4 L of water for a 70-kg person) can impair aerobic exercise performance, especially when the ambient temperature is warm.
Dehydration between 3% and 5% of body mass (2.1-3.5 L of water for a 70-kg person) does not appear to compromise either strength or anaerobic performance (67).
The greater the level of dehydration, the greater is the physiologic strain and degradation of aerobic exercise performance.
Dehydration can increase the perceived effort of the task, degrade the ability to perform optimally, impair balance control, and is a risk factor for exertional heat illness, including heat stroke.

Monitoring Hydration

The most effective way to evaluate the adequacy of a hydration program is to measure body mass before and after to quantify weight loss.
Nude body mass should be measured before and after exercise. If body mass loss is greater than 2%, the individual is drinking too little; if body mass is greater than preexercise, the individual is drinking too much.
Drinking in excess of sweat rate should be avoided.

Hydration Recommendations

Fluid intake prior to exercise is necessary to increase the likelihood of starting the activity well hydrated. Consuming 400-600 mL (14-22 oz) of fluid 2 hours prior to exercise is recommended (63,67).
Fluid intake during exercise is necessary to replace the fluid lost through sweat; fluid intake should approximate fluid losses. The practical recommendation is to consume 150-350 mL (5-12 oz) of water every 15-20 minutes of exercise if the activity lasts less than 1 hour (13).
When exercise lasts more than 1 hour, a beverage containing 6%-8% CHO (glucose, sucrose, fructose, glucose polymers, and the like) and/or electrolytes can be beneficial (63,67,72). This amount of CHO with the addition of electrolytes ensures maximal stimulation of fluid absorption because of increased palatability and promotes gastric emptying.
Restoration of hydration status can be accomplished by consuming regular foods and beverages.
With excessive dehydration, persons should ingest approximately 1.5 L of fluid for each kilogram of body weight lost, and the fluid should contain more sodium than usual: A concentration of approximately 50 mmol · L⁻¹ or 1 g of sodium per 1 L of fluid will stimulate thirst and fluid retention to enhance a more rapid and complete recovery (13,54,56,67,68).