Title: Athletics and Nutrition
Key words: Diet, macronutrients, micronutrients, aerobic exercise, anaerobic exercise,
Date: July 2000
Category: 12. Sports
Author: Dr M Draper
Athletics and Nutrition
All athletes want to improve their performance, and many serious athletes spend hours each week in coaching and training sessions. However, the extra benefits to be gained from a good diet are often overlooked. This can be due to a variety of reasons:
Nutrition for Athletics is designed to help fill some of the gaps in athletes' current knowledge. It aims to show athletes of all performance levels that the foods they eat, and when they eat them, can make a real difference in training and competition.
Nutrition for Athletics is based on the conclusion of the International Scientific Consensus Conference on Current Issues on Nutrition in Athletics held in Monaco in February 1995. We gratefully acknowledge the contribution of the conference participants as the expert scientific sources for this article.
MEETING MACRONUTRIENTS NEEDS
Athletes' energy needs
Top class athletes undergoing strenuous training can have daily energy expenditures 2-3 times greater than those of untrained individuals. Training can use up as much as 40% of an athlete's total daily energy expenditure. Energy demands in competition can also be very high: Events such as the decathlon and marathon are obvious examples, but large energy costs are also incurred in many events where a prolonged warm-up and warm-down are involved. To avoid too much weight loss and its negative consequences, and to sustain effective training, athletes must match energy output with increased energy intake.
It is essential not only to get enough energy, but to get it from the right sources. This means understanding how different sources of energy are used by the body
Energy is provided mainly by carbohydrate and fat.
During low intensity exercise the body uses oxygen to burn up both carbohydrate and fat to provide energy for the muscles to work. This method of energy production is known as aerobic metabolism. At this stage, fat accounts for more than half of energy production.
As exercise becomes more intense, the body utilises mainly carbohydrate, and relies less on aerobic metabolism. This is known as anaerobic metabolism.
There is a small store of creatine phosphate which can be broken down, but most of the anaerobic energy comes from converting carbohydrate to lactic acid.
The maximum oxygen uptake level
As exercise intensity increases further, the point comes where the body cannot take up any more oxygen; this is known as the maximum oxygen uptake level. Beyond this point, for example, during rapid sprints, energy must be produced anaerobically, and only carbohydrate can be tilised.
Training enables the muscles to take up more oxygen from the blood. Raising the maximum oxygen uptake level means more energy can be produced aerobically.
ENERGY VALUES: 1 g carbohydrate = 4 Calories
( 4 kcal or kj )
Carbohydrate, whether from sugars of starches, is broken down to glucose in the body and is then stored as glycogen. Some is stored in the liver until it is needed to raise the blood glucose level and to supply the brain with glucose; the majority is stored in the muscles themselves.
Heavy exercise makes great demands on the body's carbohydrate stores and glycogen depletion leads to fatigue. Consuming foods which contain carbohydrates restores the body's glycogen: the body cannot convert fat to carbohydrate.
Good sources of carbohydrate are sugars, fruit, rice, bread, pasta, potatoes, breakfast cereals, confectionery, cakes and soft drinks.
Many foods which contain carbohydrates are relatively 'bulky' in comparison with foods which are high in fat. Therefore, eating a high carbohydrate diet helps athletes not only meet their energy needs but also avoid unwanted weight gain.
Athletes should consume more carbohydrate than the 50% of daily energy intake recommended for less active people. Carbohydrate is especially important in endurance events. When the race distance is shorter, muscle glycogen stores are less critical, but performance is impaired if the muscle glycogen level is low. A high carbohydrate diet is therefore advised for these events too. High muscle glycogen levels are important for sprint training, when many sprints are carried out with short recovery times.
Carbohydrate loading can be achieved simply by gradually decreasing the amount of training during the week before competition and increasing the carbohydrate content of the diet to about 70% of daily energy intake during the three days before competition.
Consuming even 60% or 70% of energy as carbohydrates might still be insufficient where athletes have very restricted energy intakes. For this reason it is also helpful to set targets in absolute terms per unit of body weight (i.e. grams of carbohydrate/kg body weight).
Dietary surveys of athletes show that many do not meet carbohydrate intake goals. One reason is that they may rely too much on bulky fibre-rich carbohydrates which can promote satiety before carbohydrate/energy needs are met.
Some athletes fail to take advantage of high-sugar foods or drinks as a more compact carbohydrate and energy source; in fact some athletes mistakenly regard such foods as 'unhealthy'. Yet studies have shown that carbohydrate loading with confectionery products, rice and pasta are equally effective in improving endurance running capacity.
Recovery from exercise
The most immediate nutritional priority, after prolonged heavy exercise, is rehydration, closely followed by restoration of the body's carbohydrate stores. The most rapid rate of muscle and liver glycogen resynthesis occurs immediately after prolonged heavy exercise when carbohydrate stores are low.
An immediate post-exercise carbohydrate intake of between 0.7 and 1.5g/kg/body weight in the first half hour is generally recommended.
To achieve the best recovery within 24 hours, a total carbohydrate intake of about 9-10 g/kg/body weight is necessary.
Eating high or moderate glycaemic index carbohydrates during recovery is more effective in replacing muscle glycogen stores than eating the same quantity of low glycaemic index foods.
PRACTICAL DIETARY STRATEGIES
Energy values: 1 g fat = 9 Calories ( 9 kcal or 37 kj )
Surveys show that athletes tend to have diets which are too high in fat, making it difficult for them to consume the recommended amounts of carbohydrate. However, reductions in fat intake to very low levels are not recommended, not only because of the important role that fat metabolism plays in energy production, but also because fats contribute to the general health of athletes.
Sources of fat are oils, butter, margarine and other spreads, meat fat, whole milk and cream, fatty/oily fish and nuts.
Essential fatty acids, found in fish oils and in some vegetable oils, must be part of any diet, whereas saturated fat, found mainly in animal fats, should be restricted to no more than 10% of daily energy intake.
Fat is stored mainly in the adipose tissues and some is stored in muscle cells. Endurance training increases the capacity for fat metabolism in the muscles, so that fat metabolism will cover a greater proportion of the energy production of athletes during exercise than in the case for untrained people.
High fat diets may also increase fat metabolism in some situations, but such diets are not generally recommended.
Energy values: 1 g protein = 4 Calories ( 4 kcal or 17 kj )
High protein diets have long been associated with training for fitness due to the mistaken belief that this leads to greater strength because muscle itself is protein.
Available evidence suggests that the daily protein requirements of athletes are indeed greater than the daily 0.8 g/kg/ body weight/day recommended by the WHO for 'normal people'.
These greater intakes can usually be achieved by increasing overall energy intake. It is not necessary to eat large amounts of high protein each day.
Good sources of protein are meat, milk, cheese, yoghurt, poultry, fish, eggs, nuts and legumes (pulses).
It is commonly believed that supplementing athletes' diets with amino acids, such as arginine and ornithine, will stimulate an increased release of growth hormone. However, since even brief periods of high intensity exercise will produce a significant increase in the concentration of circulating growth hormone, the use of such supplements is unnecessary.
Meeting micronutrient needs
Vitamins and minerals play an important role in energy metabolism; deficiency of one or several micronutrients can impair exercise capacity. Vitamins and minerals are found in a wide range of foods and intakes are positively associated with energy intake so that deficiencies are rare in people who eat large amounts of food.
Good sources are fruit, vegetables, nuts, fish, meat, eggs, dairy products and cereals.
Studies show that exercise increases the formation of free radicals (compounds which damage cells through oxidation ). With training there is an increase in the antioxidant defence system, but it is not known whether this is sufficient to keep up with the increase in free radicals. Certainly, 'week-end warriors' who do not have an enhanced antioxidant defence system may be more susceptible to free radical damage.
Because of the lack of full understanding of antioxidants and exercise, and the belief that high doses are not harmful, many athletes have begun taking antioxidant supplements as an 'insurance policy'. However, extra antioxidants could be harmful.
Although some experts feel that athletes should take antioxidant supplements, this will remain controversial until more data are available.
Adequate calcium intake is needed to prevent bone mineral loss and reduce the risk of osteoporosis later in life. Inadequate calcium intake can result from restricted energy intake, dietary extremism and fad diets, vegan eating patterns, and where athletes are intolerant to dairy products.
Calcium in the diet
Dairy products are the best source of calcium and provide 60-70% of the calcium intake in typical Western diets. Calcium in vegetables and high fibre foods may not be as easily absorbed as that in milk.
Supplementation of calcium well above the RDA is not recommended because high levels can inhibit the absorption of iron, zinc and other essential minerals. Individuals predisposed to kidney stones should also avoid high calcium intakes.
Inadequate intake can result from a restricted energy intake, dietary extremism and fad diets, vegetarian diets (especially poorly chosen ones), and unvaried very high carbohydrate eating.
Studies have shown that some middle and long distance runners, especially female distance runners, may be iron depleted. However, the number of athletes who are iron depleted is no more than that found in the general population, where iron deficiency among women is also high. Athletes who fall into high risk categories should have their non-haem iron status regularly monitored.
Iron in the diet
Iron deficiency in most athletes can be prevented by eating more liver, lean red meat or dark chicken meat, and by replacing coffee and tea drunk at meat-free meals with a vitamin C drink to enhance iron absorption. Eating a source of animal protein with vegetables also increases iron absorption.
Iron supplements should be recommended only when dietary intervention fails. Supplements should not be recommended indiscriminately since excessive iron can inhibit the absorption of zinc. Those with very high iron stores may also be at increased risk of cancer, stroke and coronary heart disease.
Laboratory studies have shown that moderate iron depletion is not associated with reduced exercise performance, and iron supplementation does not improve performance in individuals with non-anaemic deficiency. In contrast, there is much anecdotal and clinical evidence that iron deficiency does have a negative impact on training and recovery.
Other micronutrients and supplements
There is no clear evidence that athletes are deficient in chromium, zinc, phosphate or magnesium. Further studies are needed on intake and on loss during exercise.
Likewise, no clear performance benefits have been shown for mineral supplements, although research studies may not actually be able to detect the small changes that would be critical to competitive performance.
Any mineral loss during exercise can be made up[ by eating a diet containing mineral-rich foods.
PRACTICAL DIETARY STRATEGIES
Athletes have tried almost every nutrient possible, ranging from Amino Acids to Zinc, as well as numerous alleged ergogenic aids, such as ginseng and royal jelly, in attempts to enhance physical performance. Caffeine, creatine and sodium bicarbonate are examples of permitted substances which may have beneficial effects, but there are potentially negative effects as well.
The most important effect of caffeine may be in mobilising fatty acids, which can then be used as a fuel, sparing the glycogen stores. This may benefit the endurance athlete, but the diuretic effect of caffeine may be harmful, especially in the heat.
Caffeine is also a stimulant, and may help power events, while the diuretic property will lead to a loss of weight which could be beneficial in certain events, for example high jumping, long jumping and pole-vaulting.
Caffeine in high doses is not permitted and anyone exceeding the permitted dose is liable to disqualification.
Recent reports suggest that creatine levels in skeletal muscle can be increases, and performance of high intensity exercise enhanced, following a period of creatine supplementation. However, neither endurance exercise performance nor maximal oxygen uptake appear to be enhanced.
Although creatine is normally present in the diet ( in meat and fish ), the amounts (about 1 gram per day) are much less than the 20g/day which has been shown to enhance performance when taken for 5-6 days.
Sodium bicarbonate is an alkaline salt found naturally in the body. Sodium bicarbonate in the blood is referred to as the alkaline reserve. The alkaline reserve is responsible for buffering lactic acid which builds up in the muscles during intense exercise and is a key factor in fatigue.
Numerous studies have shown that sodium bicarbonate supplements can delay the onset of fatigue.A dosage of 300mg/kg body weight taken about 2-3 hours before exercise appears to be both effective and medically safe.
Although most egogenic aids have not been shown to enhance physical performance in well-trained, well-nourished athletes, there is evidence that certain substances do bring about ergogenic benefits. Additional research is needed to evaluate their potential for enhancing performance in specific athletics events.
Too much training can lead to prolonged fatigue and reduced performance. Strenuous exercise can damage muscle leading to muscle pain, soreness, stiffness, reduced range of motion and loss of strength that can last 5-10 days.
Mood disturbances are also key. Self recorded 'profiles of mood scores' (POMS) of promising athletes (as training begins) show high scores for vigour and low scores for tension, depression and stess often emerge.
It has been known for over a century that psychological factors can affect exercise performance, yet 'central fatigue' has often only been recognised when no physical dysfunction can be found. Today more interest is being shown in this area in the light of emerging evidence.
The effects of serotonin
This evidence shows that the concentration of the neurotransmitter serotonin (5-HT) increases in parts of the brain during prolonged exercise. Experiments using drugs to alter 5-HT levels have demonstrated a positive relationship between 5-HT levels and fatigue.
Attempts to delay 5-HT synthesis nutritionally have involved both amino acid and carbohydrate supplementation. Trials using amino acid supplements have had somewhat mixed results, including negative side effects. Carbohydrate feeding trials have resulted in beneficial effects of carbohydrate on muscular fatigue. There is also evidence of some beneficial effect on central fatigue.
The best treatment for overtraining is prevention. Mood (POMS), fatigue and soreness should be regularly monitored. Athletes may also benefit from keeping logs on training and diet, and a diary on their thoughts and feelings. Sleep is important, 'time outs' are healing, and athletes should rest when sick. Coaches can tailor training, stay close to their athletes, help make goals realistic, provide variety, praise gains, tolerate plateau's and help athletes find balance and fun. In young athletes, 'HEADS' questions ( stress from Home, Education, Employment, Activities, Drugs, Depression, Sport or Sex ) may also pinpoint causes of overtraining. Good nutrition is also essential, including a high carbohydrate diet.
Severe dehydration is potentially fatal. Exercising whilst dehydrated causes body temperature to rise quickly and this can lead to heat stroke. Even when the weather is cold, significant sweat loss will occur leading to a degree of dehydration.
SWEATING & DEHYDRATION
The major electrolyte in sweat is sodium, with smaller amounts of potassium and magnesium also present. Loss of substantial amounts of sweat will inevitably reduce the body's reserve of these electrolytes. However, except where losses are very high, replacement during exercise is not a priority, and even then only sodium may need to be replaced.
After exercise rehydration can only be achieved if the sodium lost in sweat is replaced as well as the water (see later). Sodium also helps to maintain the thirst stimulus. Potassium may also be helpful in promoting rehydration.
Adding carbohydrate to drinks is a useful way of increasing fuel supplies, although high concentrations can delay gastric emptying and so delay water absorption from the intestine. Carbohydrate depletion results in fatigue and reduced performance, but is not life threatening. The composition of drinks must therefore be influenced by the relative importance of the need to supply fuel and water. This in turn depends on the individual athlete and the ambient temperature and humidity. In hot weather, provision of water is the first priority and carbohydrate concentration should be lower.
In spite of the clear evidence for the negative effects of dehydration, most athletes still do not drink enough to match fluid losses during exercise. Athletes must make a conscious effort to increase fluid intake before, during and after exercise. Providing them with more palatable drinks might also help.
Establishing a pattern of fluid intake should be part of training. This will allow athletes to develop personal fluid replenishment strategies as well as getting them used to the sensation of running with fluid in the stomach. This is especially important for those athletes who live and train in cold climates so they can cope with the increased fluid intake necessary in hot weather competitions.
Athletes from temperate climates can adapt to the higher temperatures and humidity found in hotter climates. Adaptation begins within a few days, and the major changes are largely complete within the first 6-8 days of exposure. These changes include reduced heart rate and core body temperature during exercise and exercise becoming easier. Full adaptation may take 14 days or more.
Acclimatisation does not reduce the need to replace fluids during exercise. Heat acclimatisation will actually increase the requirement for fluid replacement because of the earlier onset of sweating and greater sweat rates.
Salt tablets are seldom, if ever, necessary, but some extra salting of food at the table may be appropriate in the early stages of acclimatisation if sweat losses are very large.
PRACTICAL DIETARY STRATEGIES
To increase fluid intake
CHILDREN AND FEMALE NEEDS
Children have more difficulty than adults in adjusting to hotter climates (particularly if air temperature is 40oC or more). When competing in warmer climates, their training volume and intensity during the first 7-10 days should be reduced even more than that of adult athletes.
Extra care must be taken to avoid dehydration in young athletes. Proper education of children and their parents can be effective. A simple practical instruction is to advise the young athlete to drink periodically 'until you are not thirsty any more, and then another few gulps'. This means half a cup beyond thirst for children under 10 years and a full cup for an older child or adolescent. Again it is important to provide palatable drinks.
Growing children and adolescents also have increased calcium requirements. They therefore need to ensure adequate energy and calcium intakes.
Many female athletes fail to take in sufficient energy to match their energy output. This leads to less body fat, and can also lead to menstrual disturbances, loss of performance and inadequate nutrient intake. Inappropriate eating habits among some female athletes are a cause of concern.
Amenorrheic or post-menopausal women and women who are pregnant or breast-feeding have increased calcium requirements. Nutritional factors, as well as monthly menstrual flow, put female athletes, especially distance runners, at particular risk of iron-deficiency anaemia.
Female athletes should therefore ensure adequate energy, iron and calcium intakes. Being away from home provides a number of challenges to sound nutrition practices including:
To eat well while travelling
An organised and assertive nutrition plan may assist athletes, teams and coaches to eat well while travelling. This should include: