As we have discussed at length in previous blogs, and in our courses, carbohydrates and fats provide the energy to fuel metabolism during prolonged, endurance exercise (3). The carbohydrate energy we have stored in our muscles and liver as glycogen is limited, and can become depleted to low concentrations during long-duration exercises like Ironman triathlon (1, 6). In contrast, our fat energy stores, whilst metabolised more slowly, are effectively limitless in the context of exercise. For example, given that 1 g of fat provides ~9.75 kcal of energy (4), it can be estimated that a lean 70-kg individual with 10% body fat has ~68,250 kcal of endogenous fat energy. Theoretically at least, this is enough energy to complete more than six full-distance Ironman triathlons (5).

Accordingly, exercise physiologists have conducted huge amounts of research into the factors that influence the rates at which we metabolise fats and carbohydrates during exercise. This research has been particularly...