When an athlete is nearing the end of a race ad her cells are low on oxygen?

When an athlete is nearing the end of a race and her cells are low on oxygen, it is likely to be fermentation. When cells have used up all their oxygen they ferment glucose into lactic acid. Fermentation is a metabolic process that consumes sugar in the absence of oxygen

Lots of differences, but here's one: Carbohydrates are more rapidly transformed by the body into energy than lipids are. As a result, carbohydrates have a more pronounced impact on insulin levels than lipids do.