Martial Art Institute

Author Wai-Po Tang, 1996. Webpublish Dec 2001.

Nutrition - Eating for Sports

Carbohydrate

Scandinavian researchers in 1930’s found that larger amount of carbohydrate consumption by endurance athletes can prolong exercise intensity than if the athletes adopted normal diet. Firstly, the subjects were measured on the ergometer at a constant exercise rate of 70% VO2 Max. and exhaustion set in after two hours. However, when the same subjects consumed larger amount of carbohydrate several days prior to the test, the subjects were able to continue on the ergometer for twice as long. This is conclusive that endurance athletes would benefit from carbohydrate for energy. Carbohydrate is stored as glycogen in the liver and the muscles.

The body’s Carbohydrate store is minimal holding only 600-800kcal/2500-3400kJ from the normal diet. An athlete running at a marathon pace would use up 600-800kcal between 70-80 minutes and the performance would be severely affected if energy foods are not replenished. The body can also use fats stored under the skin for energy combined with glycogen thus the glycogen reserves would last considerably longer. Endurance training helps the body to adjust to using fats efficiently for energy and sparing the glycogen reserves.

Maintaining glycogen reserves

There are two main factors affecting the rate of glycogen replenishment within the skeletal muscles. When muscles are completely depleted of glycogen after prolonged exercise, it may take 48 hours or more to replenish again. Heavy training sessions such as speedwork, weight training, hill running or competition would take much longer to restore the normal level of glycogen. The rigours of a marathon would take as much as seven days or more to re-fuel and the level of glycogen remains at a low level unless carbohydrate foods are consumed.

Most athletes in Western countries consume around 250-400g/9-14oz of carbohydrate per day. This would suggest that many athletes are failing to refuel their body stores of carbohydrate sufficiently within 24 hours. Eventually over a period of seven days the glycogen level is reduced to a level where training sessions is greatly affected. A diet containing 500g of carbohydrate per day is considered to be high and necessary for endurance sports.

Foods rich in Carbohydrate

The best form of carbohydrate is starchy foods rich in unrefined complex carbohydrate. These are: whole grain cereals and cereal products (wholemeal or whole wheat bread, granola or muesli etc), fresh or dried fruit, fresh or frozen vegetables (particularly root vegetables), beans, peas, and lentils. Not only are these high fibre foods, high in carbohydrate, but also in fibre, protein, vitamins and minerals.

Carbohydrate content of selected foods (g per 100g and average values)

Examples from: The Composition of Foods, (5th Edition), (and supplements), McCance & Widdowson, Royal Society of Chemistry and MAFF, 1991.

^ Editor’s note: Total Carbohydrate is a sum of starch and total sugars, expressed as their monosaccharide equivalent. It does not include fibre. * Englyst Method.

Body temperature during exercise

During exercise, only 20-25% of stored energy are used for mechanical work and the rest of the energy is released heat when the muscles contract. As heat production increases, the body starts to sweat to maintain body temperature in order to prevent excessive rise (hyperthermial). For every 1 litre/1.75 pints of sweat that evaporates, some 600 kcal/2500kJ of heat energy may be released from the body. It is possible to lose as much as 2 litres/3.5 pints of sweat per hour during prolonged exercise in a hot environment. Sweat is simply a dilute version of blood when sweating is prolonged or pronounced, the body loses both water and electrolytes. Electrolytes (salts dissolved in the body’s fluid) are lost from the body at a much slower rate than water and this does not present an immediate problem. It does not appear to be necessary to replace these electrolytes during exercise. If anything, the concentration of the major electrolytes in plasma tends to increase. However, water loss will cause serious problems if no attempts are made to replace the lost fluid. Losses of fluid corresponding to as little as 2% of body weight can seriously impair the capacity to perform muscular work.

As sweating continues, the water portion of the blood decreases, reducing the volume of blood available to the circulation and making it more difficult to satisfy the energy demands of the muscle and to transfer heat to the environment via the skin.

Thirst in itself is a very poor indicator of the need to start taking fluid. By the time the athlete feels thirsty, the losses of fluid during exercise are irreplaceable. Therefore taking regular small amount of water throughout the training session will prevent dehydration.

Preparation for Competition

The most important nutritional consideration is ensuring that you start competition fully recovered from the rigours of training with at least normal glycogen stores. As training will result in substantially lowered glycogen stores, the first step is to reduce the volume of training over the week preceding competition. This, combined with a healthy diet containing adequate amounts of carbohydrate, should ensure that normal glycogen stores are achieved in three to four days. Consumption of a high carbohydrate diet can, however, result in significantly greater than normal glycogen stores: this provides the foundation for a popular dietary manoeuvre called carbohydrate-loading.

One of the main disadvantages of traditional carbohydrate-loading regimen is the lack of energy during heavy training sessions while taking low carbohydrate diets. A recent study measured the muscle glycogen levels in well-trained runners after they had completed three different types of race preparation:

The results were clear that reducing mileage, while consuming a normal diet indicated higher glycogen levels. Therefore it is important to reduce the volume of training over the final week before competition and ensure adequate rest and a good basic diet. Gradual reduction in training combined with carbohydrate restriction and then a high carbohydrate diet provides 2-3 times greater level of glycogen stores than normal.

During Competition

On the day of competition, the most important thing to remember is never try anything new. Instead, follow a simple, sound nutritional routine: if you are competing in the morning, just eat a light carbohydrate rich breakfast with plenty of fluids: cereal with milk, toast and jam, peanut butter or honey, or baked beans on toast are all ideal pre-event meals. Do not take large quantities of sugar, confectionery or honey - the complex carbohydrates are best. Avoid those foods you know that will upset your stomach.

Recovery Post Competition