Metabolic

Cellular recovery after exercise

Exercise increases the generation of oxygen free radicals and lipid peroxidation which have harmful cellular effects. The rate of oxygen consumption and the presence of cellular antioxidant systems influence the magnitude of cellular damage occurring as a result of exercise.

Free radicals generated during exercise may arise from three potential sources:

The mitochondria from which oxygen radicals have escaped.
The capillary endothelium through hypoxia.
Inflammatory cells mobilized from tissue damage.

Skeletal muscle has adapted to protect against further cell injury following exercise. Normal cellular adaptation occurs in response to an appropriate stimulus, and ceases once the need for adaptation has ceased. These adaptations include:

Biochemical changes such as an increase in antioxidant enzymes and production of heat shock proteins.
Response to increased work demands by changing their size (atrophy or hypertrophy), number (hyperplasia), and form (metaplasia).

Exercise, repair, and recovery

Exercise training reduces the susceptibility of muscles to further damage by increasing the activity of antioxidant enzymes, such as superoxide dismutase, catalase, and glutathione peroxidase. The ability of cells to respond to stress by increasing the content of cytoprotective proteins is blunted over time by the ageing process.

Physical damage to muscle cells from repeated muscular contractions (particularly eccentric contractions) is repaired in the following sequence:

During days 2-4, there is invasion by phagocytic cells and prominent degeneration of cellular structures.
During days 4-6, regeneration of muscle begins by the activation and migration of satellite cells. These satellite cells migrate into the damaged area, differentiate to form myoblasts, and fuse to form multinucleated myotubes, which develop into mature skeletal muscle.
Rapid repair of plasma membrane disruption is essential to cell survival and involves a complex and active cell response that includes membrane fusion and cytoskeletal activation. Tissues, such as cardiac and skeletal muscle, adapt to a disruption injury by hypertrophy. Cells adapt by increasing the efficiency of their re-healing response.

Bodyweight

Total energy intake must be raised to meet the increased energy expended during training. Maintenance of energy balance can be assessed by monitoring bodyweight, body composition, and food intake.

Weight gain

For those athletes who wish to gain bodyweight, caloric intake should exceed energy expenditure. Increased muscle mass, from physical training, can lead to increased bodyweight even with a reduction in body fat percentage.

Weight loss

For those athletes who wish to lose weight, caloric intake should not exceed energy expenditure. The caloric deficit will dictate the amount of weight that can be lost. To maximize the loss of fat and minimize the loss of lean tissue, weight loss should be limited to 500-1000g per week. This would require an energy restriction of 2-4MJ per day. The body, however, acts as a homeostatically regulated system so that attempts to lose weight by calorie restriction or increased energy expenditure will usually generate hunger and the desire to eat more rather than weight loss. The most effective weight loss is achieved by reducing the carbohydrate content of the diet by increasing fat and protein intake. This reduces circulating insulin concentrations which regulate the body fat mass by stimulating fat and glucose uptake and storage by insulin sensitive fat cells.

The use of diuretics and laxatives should be discouraged.

Fluid status will affect bodyweight. Volume depletion will present as ‘weight loss’, while hyper-hydration can present with bodyweight gain. These fluid gains and losses are transient and can be detrimental to health if performed for the wrong reasons (i.e. ‘making weight’ in sports which require weight classes for competition).

Body mass index

BMI is generally used as a descriptive tool for assessing health risks. BMI can be calculated with imperial or metric units:

Weight in pounds/[(height in inches)²] × 703;

Weight in kg/[(height in m)²].

Energy requirements

The main component of daily energy turnover in an average person is the basal metabolic rate (BMR).

Basal metabolic rate (BMR) represents the minimum amount of energy expenditure needed for ongoing processes in the body in the resting state, when no food is digested and no energy is needed for temperature regulation. The most variable component of daily energy turnover is the energy expenditure for activity (EEA) which can range between 15-25% of the BMR in moderately active persons.

For sports such as gymnastics, dancing, diving, and running in which a lean physique is desired, athletes will routinely restrict their caloric intake or chose a diet with a higher protein/fat content with fewer calories to achieve a leaner body composition. Caloric restriction reduces the metabolic rate and may lead to conditions such as menstrual dysfunction (amenorrhoea), iron deficiency (anaemia), and a decrease in bone density (osteoporosis/osteopenia); complications not present when a higher fat/protein diet is ingested.

A chronic negative energy balance will result in a loss of fat free (muscle) mass as well as a loss of body fat. Lethargy from the excessive loss of lean body mass and depletion of glycogen stores generally limits performance and the ability to train properly, making an athlete more susceptible to illness and injury. These complications are not seen when fat loss is achieved by modifying the macro-content balance by reducing the carbohydrate intake.

An increase in lean body mass will increase energy requirements for basal activities and vice versa. Athletes who sustain hard and vigorous activity for prolonged periods of time must supplement their caloric needs by ingesting more energy dense foodstuffs to sustain exercise and match energy demands during exercise. While increased energy consumption can be achieved by ingesting primarily more carbohydrate-rich solid food or liquid carbohydrate formulas, increases in fat and protein ingestion are also part of a healthy mixed diet.

The higher the intensity of the exercise and the more muscle groups that are activated, the more the energy requirements of that activity are increased.

Food and exercise

Physically active individuals must meet their energy requirements by ingesting a variety of foodstuffs both: before, during, and after exercise.

For most individuals the consumption of a normal mixed diet consisting of 50% carbohydrate and 10-30% fat, with the difference made up with protein, is adequate. Athletes who are unable to maintain the desired fat mass on this diet should reduce their carbohydrate intake by eating proportionately more fat and protein.

The only reason to consume protein or fat several hours before exercise or exercise performance is to provide satiety, which can influence performance by promoting a sense of well-being. If carbohydrate stores are adequate, the choice of food before exercise should be based on the past experience of the athlete in so much as that the food choice should minimize hunger yet not interfere with the exercise mode and duration.

Habituation to a high-fat diet decreases the amount of muscle glycogen used during exercise by increasing the body’s ability to use and mobilize fat. The increased ability to oxidize fat for fuel may enable an athlete to continue exercising for longer periods at intensities of 70% of VO₂ max or less which may be beneficial in events lasting days or weeks. However, habituation to a high-fat diet is associated with a reduced performance ability during high intensity exercise performance, such as when attempting to elevate power output during hill climbing in endurance cycling or running events.

Carbohydrate needs

The CHO needs of individuals vary depending on their mass and the level of physical activity. Larger and more active individuals will require generally more energy and, therefore, more CHO. However, in the normal exercising population the CHO needs will be met by consuming a normal mixed diet as described above. Athletes with insulin sensitive fat cells will have greater difficulty maintaining an ideal weight if their diets contain excessive amounts of carbohydrate.

Protein needs

The protein needs of individuals also vary with mass and activity level, but are also generally met when one is consuming a normal mixed diet. Although the recommended daily allowance (RDA) for protein is 0.8g/kg of body mass, highly active endurance athletes may require up to 1g per kg of body mass, while those striving to build large amounts of muscle mass or to lose body fat may require in excess of 1.5g/kg of body mass. Usually, these requirements are adequately covered by the increased energy intake stimulated by physical activity, and it is often unnecessary to increase actively the protein content of the diet by eating selectively protein-rich foodstuffs. Protein is the macronutrient that most affects satiety so that a higher protein intake promotes fat loss, in part by reducing calorie intake, but also by reducing elevated circulating insulin concentrations produced by a higher carbohydrate intake.

Pre-event diet

Carbohydrate loading has been shown to increase muscle glycogen content before exercise and delay the time at which low muscle glycogen concentrations are reached. The main effect of this practice is to decrease the amount of fat oxidized during exercise. Carbohydrate loading is achieved by eating a high carbohydrate diet (75-90%) for 3 days prior to the competitive event. To achieve this, athletes should consume approximately 8g CHO or more per kg of body mass.

Pre-event meal

The pre-event diet aims to optimize muscle glycogen and liver glycogen stores that maintain blood glucose levels during exercise. The ingestion of carbohydrate before an event will stimulate carbohydrate oxidation and inhibit fat oxidation. Evidence suggests that performance is improved when 200-300g CHO is consumed 3—4h before prolonged exercise, compared to when no food is ingested.

Immediately prior to the event

Foods that are consumed immediately prior to an event should be low in fat, protein, and fibre, and should not cause GI distress.

Foods ingested 4-6h before an event should be of a low to moderate glycaemic index to minimize the insulin response. If the muscles are not fully stocked with glycogen (because of recent exercise or a low CHO diet), however, then foods with a moderate to high glycaemic index are preferred prior to an athletic event.

Food supplements

Numerous well-controlled studies have concluded that individuals eating a well-balanced diet do not need to supplement their diet with vitamins, minerals, or trace elements when undertaking an exercise programme. The ingestion of these supplements on physical performance has not been clearly shown to have benefits.

Vitamins

Vitamins are divided into water- and fat-soluble categories. Water-soluble vitamins such as thiamin, riboflavin, B-6, niacin, pantothenic acid, biotin, and vitamin C are involved in mitochondrial metabolism. Folate and vitamin B-12 are primarily involved in DNA synthesis and RBC development. Fat-soluble vitamins include vitamins A, K, E, and D with vitamin E being the most widely studied as an ergogenic aid. Vitamin E has antioxidant properties, as do vitamins C, A, and beta carotene. There is a linear relationship between energy intake and vitamin intake, thus vitamin intake should exceed the RDA provided that a varied diet is consumed. Mega doses of both fat and water-soluble vitamins have been shown to have toxic effects.

Minerals

Minerals are divided into macrominerals and microminerals (trace minerals). Macrominerals include calcium, magnesium, phosphorous, sulphur, potassium, sodium, and chloride with calcium, phosphorus, and calcium each constituting 0.01% of total bodyweight. Trace minerals include iron,
zinc, copper, selenium, chromium, iodine, fluorine, manganese, molybdenum, nickel, silicon, vanadium, arsenic, and cobalt with each constituting less than 0.001% of total bodyweight. Iron, zinc, copper, selenium, and chromium have been proposed to enhance physical performance and may improve physical performance if an athlete is deficient in that certain mineral, or if an increased level of this mineral would boost the body’s natural response to enhance performance.

Amino acids, electrolytes, and herbal supplements have not been shown to improve physical performance in well-controlled scientific studies.

Iron

Iron is a necessary component of haemoglobin and myoglobin, and facilitates the transport of oxygen through the bloodstream as well as the transfer of electrons in the electron transport chain system. 60-70% of iron is found in haemoglobin with the remainder found in bone marrow, muscle, liver, and spleen.

Organ meats, black strap molasses, clams, oysters, dried legumes, nuts and seeds, red meats, and dark leafy vegetables are good exogenous sources of iron. Symptoms of an iron deficiency include generalized fatigue and anaemia. Liver damage can occur from iron excess.

Iron deficiency anaemia in athletes may result from a poor diet, excess blood loss (through menstruation in females), footstrike haemolysis, and through sweat loss. Although numerous studies have documented that athletes, particularly endurance athletes, are iron depleted, the degree and percentage of iron depletion is similar between athletes and non-athletes.

Pseudoanaemia occurs in athletes secondary to an increase in plasma volume, which occurs as an adaptation to training. This increase in plasma volume ‘dilutes’ an otherwise normal red blood cell count into falsely low levels. Hence, this is an artificial lowering of haemoglobin rather than a true anaemic response, which is a common laboratory finding in endurance athletes.

The recommended daily allowance for iron intake for males is 10-12mg/day and 15mg/day for females.

Calcium

Calcium is required for the formation and maintenance of hard bones and for the conduction of nerve impulses. Calcium activates enzymes responsible for the transmission of membrane potentials as well as for muscle contraction. 99% of calcium is found in the skeleton with the remainder found in extracellular fluid, intracellular structures, and cell membranes.

Dairy products, sardines, clams, oysters, turnips, broccoli, and legumes are good exogenous sources of calcium. Osteoporosis and fractures can occur from a deficiency in calcium intake over decades. Constipation, kidney stones, and chelation with antibiotics and other nutrients (such as iron and zinc) can occur with calcium excess.

Weight-bearing exercise has been shown to increase bone density, especially when undertaken at critical growth periods (8-14yr). Oestrogen has been shown to reduce urinary calcium excretion, increase intestinal absorption of calcium and increase the secretion of calcitonin which reduces bone resorption. Calcium also can be lost through sweat, which may increase an athlete’s total daily requirement if exercise is regularly performed in hot and humid environments.

The recommended daily allowance for calcium is 800-1200mg/day for both men and women.

Creatine

Creatine is a naturally occurring compound in the body, specifically in the muscles.

Creatine supplementation is widely practised by professional and recreational athletes. The benefits and side effects are varied and debatable. Purported benefits include increased mass, improved rate of recovery after exercise, and increased power or speed. Side effects include cramping and water retention.

Although much research has been devoted to creatine and its effects, the real value of creatine supplementation remains uncertain. Untrained persons undertaking a weight training programme for the first time appear to benefit the most. Elite athletes who eat diets with adequate protein intake may benefit less.

Recovery after exercise

Recovery after exercise is most important if another bout of physical activity will shortly follow. Full recovery after exercise is also important in reducing the risk of overtraining, although the training volumes required to produce overtraining are usually achievable only by highly trained individuals.

Recovery is also an important part of adapting to exercise training, because the physical adaptations that are stimulated by exercise training occur during the rest and recovery phase.

Recovery appears to be aided by the ingestion of carbohydrate- and protein-rich foodstuffs within 30-60min after the termination of exercise.

Exercise and the environment

Different environmental conditions will affect exercise tolerance and capacity in different ways. There are three overriding environmental factors to which athletes may be exposed in either training or racing, or both—heat, cold, and reduced oxygen content in the inspired air as occurs at increasing altitude.

In order to survive, humans require to regulate their body temperatures at between 35 and 41°C, and to maintain the partial pressure of oxygen in their blood in excess of about 40mmHg.

Environmental conditions, in particular the environmental temperature, the wind speed, and the water content of the air (humidity) determine the rate at which heat is lost from the body. The normal average human skin temperature is 33°C. At any lower environmental temperature, heat will be lost from the skin to the environment in the process of heat conduction. The rate at which this heat will be lost by conduction from the body will, in turn, be determined by the magnitude of the temperature gradient—the steeper the gradient, the greater the heat loss—and the rapidity with which the cooler air in contact with the skin is replaced by colder air. Continual replacement of warmed air by cooler air causes loss of heat from the body, by means of convection. Convective heat loss rises as an exponential function of the speed at which air courses across the body, in effect the prevailing wind speed. With high humidity, sweat loss by evaporation is reduced.

Body temperature

At rest, humans regulate their body temperatures within a narrow range of between 36.5 and 37.5°C. During exercise, this safe thermoregulatory range is increased to up to 41.5°C. Heat-acclimatized human athletes have a superior capacity to exercise without apparent distress even up to body temperatures of 41.5°C.

The human body temperature represents a balance between the rate of heat production by, and heat loss from the body. Hence, changes in the rates of either heat production or heat loss, or more commonly both, determine whether an abnormal rise in body temperature (hyperthermia leading to heat stroke) or an excessive fall (hypothermia) is likely to develop and under what conditions. The principal physiological challenge that athletes face during exercise is how to lose the excess body heat produced by muscle contraction. The control of body temperature during exercise is, however, homeostatically-regulated so that the exercise behaviour is modified to ensure a safe body temperature. Thus athletes slow down when exercising in the heat; this reduces their rates of heat production when environmental conditions become too severe.

Hypothermia

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