Exercise
What is it?
People who exercise regularly are healthier than those who don't. They are less likely to develop diseases such as high blood pressure, heart disease, diabetes and cancer. They usually live longer, look and feel better and younger throughout life, and maintain their independence for longer.
Regular exercise is a vital part of life for many people. Some people include moderate physical activity such as walking in their daily routines while there are others, such as elite athletes, whose lives revolve around training and competition. From the recreational exerciser to the professional athlete, good nutrition is vital for optimal performance through its effects on the functioning of organs, muscles and other tissues. A poor diet combined with increased requirements can impair performance and general health.
Aerobic and anaerobic exercise
There are two basic different types of exercise, aerobic and anaerobic. Aerobic means 'with oxygen' and is any kind of exercise which forces the heart and lungs to work harder to supply the oxygen to the muscles. Aerobic exercise burns calories, strengthens and conditions the heart and lungs and improves general fitness and stamina, improves mental ability, and significantly lowers disease risk. Aerobic exercise includes activities like running, cycling, swimming, fast walking, aerobics classes and dancing.
Anaerobic means 'without oxygen' and is any exercise that requires short bursts of power, such as sprinting or lifting weights. Someone who is exercising anaerobically is using energy sources stored in the muscle rather than oxygen from the air. Because this energy supply is limited, anaerobic exercise can be sustained only for short periods of time. In reality, most exercise is a combination of aerobic and anaerobic exercise. The amount of each is dependent on how hard and fast the exercise proceeds.
Nutrition and exercise
Regular exercise increase demands for calories and most nutrients. It also improves metabolism and elimination. Good nutrition helps to improve performance, prevent bone and muscle breakdown, prevent injuries and, for competitive athletes, can make the difference between winning and losing.
A daily diet built around high levels of complex carbohydrates, moderately low fat levels and moderate protein intake is the best approach for most athletes. It is also important to obtain vitamins and minerals in amounts at least as high as the RDAs. A diet that consists of a wide variety of wholesome, minimally processed foods, fortified foods and, in some cases, supplements is essential to meet these goals.
Body mass and composition
Some people exercise to lose weight and stay healthy while others work out for weight gain. Most athletes avoid high fat diets and strive for low body fat percentages, as body fat does not contribute to athletic performance. Those who exercise tend to have more muscle and less body fat than those who don't.
Water and electrolyte balance
Dehydration can reduce athletic performance and lead to fatigue, and it is vital for those who are exercising to drink enough water. This is particularly important during long endurance events. A general rule is to drink twice as much water as is necessary to quench thirst. Water is also essential for regulating body temperature. Exercise also increases needs for the electrolytes, sodium, chloride and potassium. Magnesium and calcium needs may also be greater. Supplements may be useful during strenuous exercise although sodium supplements are rarely necessary.
Energy
Several factors influence the amount of energy an athlete needs. These include the type, intensity and frequency of training as well as the size, age, sex and genetic make-up of the person. A middle-aged male marathon runner has different requirements to a young female gymnast. The main sources of energy for an athlete are carbohydrates and fats.
In an athletic event, carbohydrates are the initial fuel source. The energy for short bursts of activity all comes from carbohydrates. For longer events the energy comes from both carbohydrates and fats, with the proportions depending on the intensity of the exercise, pre-event diet and the level of training of the athlete.
Protein
Adequate, good quality protein is necessary to preserve lean body mass and maximize athletic performance. Protein is essential for growth, repair and maintenance of body tissues. A typical Western diet provides more than enough protein for athletes and exercisers. Recommended protein intake for adults is estimated to be 0.8 g protein per kg of body weight per day, although some experts feel that the needs for athletes may be higher, with endurance athletes needing up to 1.2 to 1.4 g per kg per day. Athletes who are involved in very intense strength training or endurance sports may need up to 1.4 to 1.8 g per kg of body weight per day, an amount which can easily be met by diet. Those who wish to gain weight also have slightly higher protein requirements. In such cases it may be better to take amino acid supplements rather than increase intake of flesh foods. Many athletes eat two to three times the amount of protein they need. This can put stress on the kidneys and increase the risk of dehydration.
Carbohydrates
Carbohydrate-rich foods are the best fuel source for athletes. Experts recommend that 55 to 65 per cent of an athlete's calories come from carbohydrates. Most of this carbohydrate should be in the form of complex carbohydrate, which is the type found in foods such as whole grain breads, cereals, cooked dried beans and peas, potatoes and corn. Fruits and vegetables are important sources of simple carbohydrates.
The body stores limited amounts of carbohydrates as glycogen. The amount of carbohydrate in the diet is directly related to glycogen storage and athletic performance. Daily high intensity training and endurance events deplete body stores of glycogen and high glycogen stores mean that an athlete can tolerate repeated training and endurance exercise. Physical training enables athletes to store more glycogen and use its limited supply sparingly. About 1800 to 2000 calories are available from glycogen stores and this is usually enough for about 90 minutes of continuous exercise at maximal aerobic pace.
When an event lasts more than one hour an athlete may benefit from consuming carbohydrates during exercise. Drinks, such as diluted fruit juices or sports drinks, which contain less than 24 g of carbohydrate per cup may be the best form for this. It is important to eat a high carbohydrate snack after an exercise session to replace muscle glycogen stores.
Carbohydrate loading
Adequate carbohydrate is essential for athletes to replace glycogen stores depleted by training and competition. This glycogen depletion has led to the practice of carbohydrate loading, which benefits athletes who are involved in training or competition for more than 90 minutes. There are several forms of carbohydrate loading; a common method involves eating a diet that consists of about 60 to 70 per cent carbohydrate about 72 hours before a competition. If muscles are not damaged, 24 to 36 hours of rest before the event will allow maximum glycogen storage.
Fat
Fats are the body's other major energy source. They are twice as dense in calories as carbohydrates. However, fats cannot be used exclusively as an energy source and a small amount of carbohydrate must always be available. Aerobic training increases the ability of the body to use fat as an energy source. Fat is a major source of fuel for exercising muscles but they only store a small amount. When this is used up, fat is taken from body stores.
Body fat stores are more than adequate to provide extra energy and it is not necessary to get large amounts of fat in the diet. Most athletes eat moderately low fat diets. Health authorities recommend keeping fat intake below 30 per cent of total calories as this has been shown to be beneficial in protecting against various diseases.
Oxygen metabolism and energy production
Several nutrients are vital for the efficient burning of fuel with oxygen and the production of energy. A good diet provides optimal amounts of these nutrients, which include iron, magnesium and other trace elements.
Bone structure
A high density bone structure is vital for optimal athletic performance and the avoidance of injuries such as shin splints. Nutrients such as calcium and vitamin D are vital for healthy bones.
Vitamins, minerals and exercise
Most nutritional studies indicate that vitamin and mineral requirements of athletes are not greater than those of people who don't exercise. Some types of exercise may lead to losses of certain nutrients but these extra requirements can often be met by the extra food intake that is necessary to meet energy requirements.
Supplements
Athletes tend to take nutritional supplements more often than the general population and some may take very high doses. Many athletes believe that the requirements for sport are too high or that their diet is too poor to meet their vitamin and mineral needs. However, most studies also show that blood levels of vitamins and minerals are similar in athletes and nonathletes, suggesting that exercise does not deplete body stores. There are a few possible exceptions, for example, iron in vegetarian athletes.
Many athletes believe that certain supplements can enhance sports performance. Advertising claims for such supplements are often impressive but the vast majority of such supplements are either untested or have failed to show results in the tests that have been done. Megadoses of vitamins are often used by athletes to enhance performance. There is little evidence that they do, unless there are deficiencies. For many athletes, a balanced supplement which contains vitamins and minerals at the RDA level may be useful as nutritional insurance. Supplements are not a substitute for a healthy diet.
Due to increased losses of minerals in sweat and urine, there is a potential increased need for minerals in athletes. Some studies report that mineral intakes among athletes are inadequate, especially among those who are attempting to lose weight for competition. However, most athletes do seem to eat adequate amounts. It is important to eat minerals in balanced ratios. An excess of one mineral can have adverse effects on others in the body. An athlete who eats fortified cereals, sports bars and mineral supplements may be getting excessive doses of certain minerals.
Antioxidants
High intensity exercise may cause excess free radicals to be produced in the cells. The damage from these increase a person's susceptibility to cancer, heart disease, cataracts, premature aging, decreased immunity and other diseases. (See page 417 for more information.)
Free radical production increases as the body's oxygen consumption increases, particularly in the muscle fibers and in the mitochondria, the energy centers of the cell. During exercise, the blood is diverted away from organs not actively involved in the exercise process and then flows back when the exercise is completed. This reperfusion may also cause the production of free radicals, a process that can also occur in muscles during exhaustive exercise. A growing amount of evidence indicates that free radicals also play an important role in causing skeletal muscle damage and inflammation after strenuous exercise. The antioxidant vitamins, C, E and beta carotene, and minerals such as selenium may protect against this free radical damage, giving protection against disease and, possibly, faster recovery from damaging exercise.
Strenuous exercise also leads to a lowering of immune function, putting endurance athletes at increased risk for upper respiratory tract infections during periods of heavy training and after events.1 This is likely to be due to oxidative damage and antioxidant supplements may be useful in helping to prevent such damage.
Vitamin C
As an antioxidant vitamin C may help to prevent exercise-induced oxidative damage. In a 1997 study, researchers examined the effects of supplements on oxidative stress in athletes. They found that exercise-induced oxidative stress was highest when those involved in the study did not supplement with vitamin C.2
Vitamin C supplements may be beneficial for those who exercise heavily and have problems with frequent upper respiratory tract infections. Three placebo-controlled studies have examined the effect of vitamin C supplementation on common cold occurrence in people under acute physical stress. In one study, the subjects were school children at a skiing camp in the Swiss Alps, in another they were military troops training in Northern Canada, and in the third they were participants in a 90 km running race. In each of the three studies, the results showed a considerable reduction in common cold incidence in the group supplemented with vitamin C at levels of 600 mg to 1000 mg per day.3
Vitamin E
There is some evidence that vitamin E may decrease muscle fatigue and improve endurance performance. In a 1998 study, researchers investigated the effects of high intensity resistance exercise on free radical production. They also assessed the effects of vitamin E supplementation on free radical formation or muscle membrane disruption. They divided 12 weight-trained males into two groups. The supplement group received 804 mg (1200 IU) once a day for a period of two weeks and the other group received a placebo. The results showed that high intensity resistance exercise increased free radical production and that vitamin E supplementation decreased muscle membrane disruption.4
B vitamins
B vitamins are essential for the conversion of carbohydrates to energy, and as calorie and carbohydrate intakes increase, the requirement for B vitamins also increases. However, this increase will usually be covered by the increased food intake if nutrient-dense foods such as whole grain breads and cereals are eaten. There is little evidence that intake of B vitamin supplements improves athletic performance.
Thiamin
Thiamin supplements may be helpful in preventing or accelerating recovery from exercise-induced fatigue. In a study done in 1996, researchers assessed the effects of 100 mg per day of thiamin in 16 male athletes. The athletes exercised on bicycles and changes in blood, heart and lung functioning were measured. In the thiamin supplement group, changes in blood glucose were suppressed and the athletes felt less fatigued.5
Vitamin B6
Exercisers and athletes often have low vitamin B6 levels. Exercise causes pyridoxine blood levels to increase during an exercise session, possibly because pyridoxine-dependent enzymes are released from muscle storage or the vitamin is transferred from the liver to the muscles. There is no evidence that vitamin B6 supplements enhance performance.
Pantothenic acid
Pantothenic acid is essential for normal adrenal gland function and the production of cortisone and other adrenal gland hormones. These hormones play an essential part in the body's reaction to stress. Pantothenic acid has been reported to improve athletic performance by improving aerobic capacity and endurance performance, although results of studies have been mixed.
Calcium
Female athletes often have menstrual irregularities, which are due to a disruption of the normal estrogen cycle. Lack of estrogen can lead to a loss of bone mineral density. If this is combined with a diet low in calcium, there is an increased risk of developing stress fractures such as shin splints, weak bones, poor bone healing and eventually osteoporosis. Calcium supplements in doses of around 1000 to 1500 mg per day may be beneficial for such women. Increased loss of calcium can also occur during periods of inactivity.
Recent research suggests that male athletes who train intensively may also lose enough calcium from their bones to increase their chances of stress fractures. In a 1996 study, researchers at the University of Memphis measured the calcium levels of basketball players and found that after intense exercise the players lost an average of 422 mg of calcium in sweat during each session compared with a loss of 40 to 144 mg per day during mild activity. The researchers also found that over ten months, the players' total body bone mineral content declined by 6 per cent. When the players received 2 g of calcium supplements the next season, their lean body mass increased and average bone mineral content increased by 5 per cent.6
Chromium
Strenuous exercise may increase chromium excretion. Limited research suggests that chromium supplements, in the form of chromium picolinate may cause weight loss, reduce fat and increase muscle mass. Because of these reports, chromium picolinate supplements have become very popular, particularly among athletes. Most of the research into performance-enhancing effects has been conducted by manufacturers of supplements and there is very little independent research to corroborate the findings.
Iron
Heavy exercise may lead to iron deficiency. Distance runners are at particular risk as running appears to lead to a decrease in body stores of iron. Symptoms of iron deficiency in athletes include reduction in exercise time, increased heart rate, decreased oxygen consumption and increased blood lactic acid. A deficiency may result from increased metabolic requirements, increased red blood cell breakdown and increased iron losses in sweat. However, unless a person is iron-deficient, supplements do not appear to improve athletic performance.
Athletes at risk of iron deficiency include menstruating females; adolescent males whose iron requirements are high due to growth needs; vegetarians who do not eat red meat (the richest source of easily absorbed heme iron); and endurance athletes who may lose a greater amount than usual through blood losses or sweat.
Sports anemia
Sports anemia is often used to describe a low hemoglobin condition that is relatively common at the beginning of training. After adaptation, the anemia seems to subside. It may be due to inadequate dietary intake of iron or the use of protein for tasks other than red blood cell production during the early training stages. Iron intake of athletes needs to be carefully monitored.
Some experts believe that there is a possibility that exercise may have beneficial effects in reducing the risk of heart disease and cancer by lowering body stores of iron which can cause oxidative damage. (See page 257 for more information.)
Magnesium
Strenuous exercise alters magnesium concentrations in muscle and blood. Endurance exercise is particularly likely to do this as higher levels of hormones such as adrenaline and noradrenaline increase urinary loss of magnesium. Stressful conditions such as intense training and competition may also increase the need for magnesium.
In a 1998 study, researchers from the University of Texas examined body magnesium concentrations in 26 marathon runners during an endurance run. They found that levels in the muscles and urine dropped significantly, possibly putting the athletes at risk of decreased performance and muscle cramps.7
Athletes on calorie restricted diets may be at particular risk of magnesium deficiency and amounts higher than the RDA may be necessary for those who exercise intensively, possibly in doses as high as 6 mg per kg of body weight. Supplements may help to increase bone mineral density in women athletes who have menstrual irregularities and are at risk of osteoporosis. They may also improve muscle strength in athletes.
Magnesium deficiency decreases energy efficiency and research has shown that people who are deficient in magnesium may use more energy during exercise. In a recent study, USDA researchers investigated the amount of oxygen needed by healthy women over 50 to perform a certain amount of low intensity work on an exercise bicycle. When their dietary magnesium was inadequate (150 mg daily) they used 10 to15 per cent more oxygen to perform the work and their heart rates climbed by about ten beats per minute. The results suggest that magnesium deficiency is associated with increased physiological demands to do the same amount of work as when magnesium is adequate.
Zinc
Athletes can be deficient in zinc as increased needs and increased sweating and urinary excretion may lead to greater requirements. Zinc deficiency can affect energy production, tissue repair and resistance to infection.
Other nutrients
Coenzyme Q10
Due to its role in energy production, coenzyme Q10 has also been used to enhance athletic performance. Because exercise increases the risk of oxidative damage, coenzyme Q10 as an antioxidant may have a role to play in protection from such damage. In a 1997 study done in Finland, the effects of coenzyme Q10 supplements were studied in a double-blind cross-over study of 25 cross-country skiers. The results showed that all measured indexes of physical performance improved significantly. Ninety-four per cent of the athletes felt that the supplements had been beneficial in improving their performance and recovery time, whereas only 33 per cent of those in the placebo group did.8