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Carbohydrates are top priority for athletes aiming for peak performance. Intake is carefully managed before, during and after exercise. However, the optimal amount, type and timing of carbohydrate to maximise performance is an on-going area of research.
The GI provides a ranking of CHO foods based on the measured post-prandial blood glucose response compared with that of a reference food (either glucose or white bread). This indicates the overall rate of digestion and absorption of the CHO in a food. Foods with a high GI (>70) are generally digested and absorbed quickly, whereas foods with a low GI (here.
If the GI of CHO influences the rate at which CHO are digested and absorbed to be used by the body, it seems plausible that altering the GI of the foods and fluids before, during and after exercise will influence sport performance. However, despite more than 20 years of research there is still much debate1.
The purpose of the pre-event meal is to top up muscle and liver glycogen stores from CHO, to be used as muscle fuel during exercise. Currently there are no universal performance benefits from manipulating the GI of the pre-exercise meal. There are many reasons for this including; the psychological value of ingesting familiar foods that are ‘tried and true’; gastrointestinal comfort; food availability; and individual athlete likes and dislikes. Furthermore, when carbohydrate is consumed during exercise the effects of the pre-exercise CHO intake are significantly reduced. However, when CHO cannot be consumed during exercise, low GI foods prior to exercise may be of benefit to help sustain the energy release .
Consuming CHO during exercise is generally only warranted when exercise is longer than 90 minutes or of a sustained high intensity for more than 60 minutes. Despite the faster digestion and absorption rates of high GI foods, there is little research into the performance benefits of consuming them during exercise compared with lower GI alternatives. High GI foods are generally easily consumed, less bulky and thus less likely to cause gastrointestinal discomfort during exercise. For these reasons, together with the speed at which they are digested and absorbed, sports Dietitians and nutritionists recommend the use of high GI CHO during exercise when warranted1.
Recovery from exercise is a critical issue for athletes, especially those who undertake multiple training sessions each day. During the first phase of recovery (the two hours immediately following glycogen depleting exercise) the uptake of glucose to form glycogen occurs independently of insulin, so there is no difference between low and high GI food during this window. After this, the GI can influence muscle glycogen with lower levels seen with low GI diets. This makes higher GI foods post exercise a better option for most athletes to ensure adequate muscle glycogen is restored before the next exercise session1.
The GI, along with the glycaemic load, is one of the many tools available to sports Dietitians and nutritionists to manipulate dietary intake to maximise an athlete’s performance, recovery and overall health and wellness.
By Kath Fouhy, NZ Registered Dietitian and Sports Nutrition Specialist
[Kath Fouhy is currently a Senior Performance Nutritionist with High Performance Sport New Zealand]
1. Donaldson, C. Int. J, Sport. Nutr. Exerc Metab. 2010; 20, 154-165.