Is Carbohydrate Loading Still Relevant For Endurance Sports?

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Runners accept that carbo loading to be a norm prior to a race. Exercise physiologists have long told us that our muscles primarily use muscle glycogen (what the carbohydrate we eat turns into) during high intensity exercise. Hence we are advised to eat large amounts of carbohydrate before and during exercise.

However, 3 famous exercise scientists have come forward and suggested this is not totally logical. Since most if not all elite (and recreational) athletes spend most of their time training at submaximal levels (and not at high intensities) is it necessary to always eat a high carbohydrate diet?

The authors suggest that there is a lack of evidence surrounding low carb diets and its relationship to performance in the endurance exercise. Out of only 11 published studies done previously, 9 of them actually suggests that the subjects perform better or at least the same on a low carb diet compared to a high carb diet.

As a matter of fact only one study out of the 11 used subjects that were used to a low carb diet. Why is this important? Well just like a runner who have just changed shoes from a "regular cushioned" pair to a thinner soled pair will need time to adapt, a low carb eater will need time to adapt if he/ she has been used to a typical high carb diet (used by most runners).

As humans store limited amounts muscle glycogen in the body, this glycogen is mostly depleted after 20 miles of running. Hence runners often "hit the wall" after 20 miles into the marathon. Our bodies however store large amounts of fat in the body, if we can utilize the fat stores we have, we would be able to run two marathons back to back.

One of the authors (Phinney) in the current paper published a previous study in which cyclists after being on a low carb diet for three weeks used significantly more fat than they had on a typical high carb diet. Moreover, they also showed a four fold reduction in the use of muscle glycogen. This is remarkable as it was thought to very difficult (or even impossible) to produce energy purely from utilizing body fat (and not using muscle glycogen) at such high intensities of exercise.

Based on the Paleo outlook, the authors suggested that the subjects were able to have instant access to their fat reserves at all times since they live and train on low blood sugar levels (based on their low carb diet). This metabolism is similar to our ancestors in the prehistoric (or caveman era) where they were predatory hunters.

Since there is much variation among athletes, there are many unanswered questions regarding low carb diets especially since there are so many more studies done on high carb diets.

We await more research on the effectiveness on low carb diets since there may be long term health consequences of regularly eating a high carbohydrate diet since there are increasing numbers of recreational athletes who are insulin resistant (IR 4-6) and those eating too much refined sugar from high carbohydrate diets may be at risk of developing Type II Diabetes Mellitus.

Reference

Nokes T, Volek JS and Phinney SD. (2014). Low-carbohydrate Diets For Athletes: What Evidence? BJSM. doi: 10. 1136/bjsports-2014-093824.

Phinney SD et al (1983). The Human Metabolic Response To Chronic Ketosis Without Caloric Restriction: Preservation Of Submaximal exercise Capability With Reduced Carbohydrate Oxidation. Metabolism. Aug 32(8): 768-776.

Low carb diet

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