Carbohydrate Recommendations During Exercise
Substantial evidence supports the benefit of consuming carbohydrate during exercise of specified duration and intensity. Interestingly, this benefit is realized through multiple mechanisms in the body.
Historically, research looking at carbohydrate intake during exercise has been based on the assumption that consuming carbohydrate increases glucose availability, allowing athletes to maintain an exercise intensity for a longer duration, which ultimately may result in enhanced performance. While there is validity to this statement, recent research has identified independent mechanisms by which carbohydrate impacts performance. For exercise lasting more than 2 hours, exogenous carbohydrate provision prevents hypoglycemia, supports carbohydrate oxidation, and thus allows athletes to exercise longer or harder or both (Jeukendrup 2014).
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However, when carbohydrate is provided during exercise shorter than 2 hours, it is not the increased glucose availability that seems to enhance performance since muscle glycogen stores are not the limiting factor. Rather, the effect is seen on the CNS. Specifically, when carbohydrate-containing foods are consumed, receptors in the mouth sense the presence of carbohydrate, resulting in a sense of well-being that allows the athlete to train harder (Burke et al. 2011). This is supported by research that demonstrates improved performance with administration of a carbohydrate-containing mouth rinse versus a placebo iJeukendrup 2014) where carbohydrate is not actually consumed, but its presence is merely sensed in the mouth. While the exact mechanisms are not fully understood, research has clearly shown a performance benefit at s horter-duration, higher-intensity exercise (about 1 hour) when carbohydrate is detected in the mouth but is not necessarily digested (Jeukendrup 2014).
The recommended amount of carbohydrate to be consumed during exercise depends upon duration and intensity. For exercise lasting 45 to 75 minutes, only small amounts of carbohydrate are needed and even a mouth rinse with carbohydrate is sufficient (see Table 4.2); the exception would be if an athletes has not eaten for 5 to 6 hours. In this case, their need for carbohydrate can be as great as 30 to 60 g/hr. For exercise lasting 1 to 2 hours, 30 g/hr supports optimal performance. At longer-duration exercise, exogenous carbohydrate becomes an important energy source and there seems to be a dose-dependent relationship. With exercise
Keep in mind that exercise intensity is still an important consideration. Tour de France cyclists riding for many hours at high intensities are oxidizing glucose at high rates and thus should be consuming the upper end of the range. The 5-hour marathon runner may be exercising at a more moderate intensity and with this lower oxidation rate may only need 60 g/hr, for example. See Table 4.3 for a list of the carbohydrate content of common foods consumed during exercise.
The type of carbohydrate may have significant implications as well. The previous belief that maximal carbohydrate oxidation rate was 60 g/hr was based upon research investigating glucose. This is because glucose metabolism is limited by intestinal absorption of glucose. Glucose relies