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Due to circulating levels of glucose and the storage capacity of the body, taking in nutrition during events shorter than an hour or so is generally accepted to be unnecessary, although interestingly Geoffry Kamworor consumed some Maurten on his way to smashing the half marathon world record (58:01) in Copenhagen earlier this year. Is this merely for publicity for the company (of which there is no shortage given it has been used by the vast majority of major marathon winners in recent years, as well as by Eliud Kipchoge in the Ineos 1:59 Challenge), or is there more to the story? 

Replenishing the calories that are being burnt during exercise is clearly important to maintain consistent output, but how much do we need to be putting in? Running at a reasonably high intensity (such as the pace in a marathon or half marathon, or competing in a longer event such as an Ironman or ultra) will burn through upwards of 1000 calories per hour, which equates to roughly 250 grams of carbohydrate – or 15 bananas!

Fat oxidation will also be contributing to energy production, the proportion of energy substrate which fat supplies is inversely proportional to exercise intensity. As intensity increases more glucose is used in the place of fat, a proxy measurement for this is the respiratory exchange ratio, which is a ratio of the volume of carbon dioxide released to the amount of oxygen used during exercise. Sitting at slightly below threshold pace fat oxidation will be providing roughly 50% of substrate. However, that still means that, ignoring glycogen stores, you want to be taking in 125 grams of carbohydrate per hour just to replace what is being used. Importantly, endurance training prior to an event increases the mitochondria content in adipose tissue, essentially allowing greater energy production from fat sources over carbs. Efficacy of fat oxidation can further be enhanced through training in a fasted state. Ketogenic diets rely almost completely on fat oxidation for energy production, however we will save rabbit hole for another day. Needless to say, none of the top marathon athletes are adhering to such dietary regimes, but I digress.

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