Updated 26 February 2014

GI and sports nutrition

The use of a tool such as the GI to determine what types of food can enhance endurance or produce top results, is highly topical.


With South African athletes competing all over the world and many people involved in intensive exercise programmes, the use of a tool such as the GI to determine what types of food can enhance endurance or produce top results, is highly topical.

Timing is important
The timing of carbohydrate intake, the type of carbohydrate and also the amount of carbohydrate ingested by sportsmen and women are all important factors that can affect performance.

a) Pre-exercise

In periods between events, athletes should basically carbo-load using low-GI foods and drinks.

Immediately pre-exercise
Generally a high-GI diet is recommended during the immediate pre-exercise period. This would mean that athletes should eat foods with a high-GI just before they exercise, e.g. brown, white and wholewheat bread, maize meal, cornflakes, white rice, oats, sugar, jam, potato and energy drinks.

Athletes with hypoglycaemia
Research has however identified a small sub-group of athletes who develop hypoglycaemia (low blood sugar levels) during intensive exercise if they eat high-GI pre-exercise foods. If you are such an athlete, then in your case, it is recommended that you should eat low-GI foods in amounts of up to 70 g immediately before you exercise.

Endurance athletes
The general recommendation that athletes should eat high-GI foods before exercise may also not hold true for athletes who participate in events that require great endurance. Such athletes may also benefit from eating low-GI foods before exercising (e.g. dried or baked beans, peas and lentils, fat-free yoghurt, fruit bars, milk, fresh and dried fruit, pasta made from Durum wheat, cooked Bulgur wheat, high-fibre bran, wholewheat Pronutro (new formulation), samp, pumpernickel bread, and low-fat ice cream.

The reason for this recommendation is that the use of low-GI foods ensures that carbohydrate oxidation is much lower, while free fatty acid oxidation is much higher during exercise, which translates into a sustained release of energy.

b) During exercise
Research indicates that carbohydrate intake during exercise improves performance, decreases stress and prevents post-event respiratory infections. Basically athletes need 1-1,1 g of carbohydrate per kg body weight during exercise. A 60 kg athlete should, therefore, ingest 60-66 g of carbohydrate during a bout of intensive exercise.

Fructose (a low-GI food) is a favourite source of carbohydrate used by many athletes. It must, however, be kept in mind that fructose can cause gastrointestinal symptoms and distress which could have a negative effect on performance. If you do use fructose, make sure that it does not exceed 20 percent of the food or drink you ingest.

Other foods with a low-GI are not recommended during exercise as they may ferment in the digestive tract and lower the amount of energy that is readily available.

c) Post-exercise, recovery period
Carbohydrates can play an important role in the post-exercise period when an athlete recovers from strenuous physical exertion. During this period, the vital body glycogen stores are re-synthesised.

The recovery period can be divided into short-term (the first eight hours) and long-term (20 hours) periods. Generally, it is recommended that athletes stock up on high-GI foods during the 24 hours following exercise (see above for typical high-GI foods). Low-GI foods result in slower glycogen replenishment and should be restricted for 24 hours.

In addition to carbohydrates, athletes can also increase their protein intake post-exercise to boost whole body protein synthesis. As a rule of thumb, you can use 2 g of protein for every 3 g of carbohydrate in the post-exercise period to ensure that you synthesise both glycogen and muscle protein while recovering.

Five stages
Present-day research indicates that the majority of athletes can use the following nutritional guidelines for planning their food and beverage intake around an event:


  • Pre-exercise - carbo-load with low-GI foods
  • Immediately pre-exercise - have a high-GI snack or drink(remember that this does not apply to those athletes who develop hypoglycaemia during events and athletes who participate in ultra-endurance exercise)
  • During exercise - use high-GI snacks and drinks
  • First 24-hours after exercise - use high-GI foods and drinks and increase protein intake
  • Post-exercise - back to carbo-loading with low-GI foods

By applying this relatively simple manipulation of the GI, athletes can improve their performance, endurance and rate of recovery.

Next week we will have a look at the use of the GI in the prevention and treatment of degenerative diseases, such as diabetes, insulin resistance and obesity. – (Dr I.V. van Heerden, DietDoc)

Any questions? Ask DietDoc


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