A well-trained athlete can achieve about 33% efficiency in terms of calories used (not 'consumed' - we're talking about energy coming from glycogen and fat, in

A well-trained athlete can achieve about 33% efficiency in terms of calories used (not 'consumed' - we're talking about energy coming from glycogen and fat, in addition to whatever is eaten. The human digestive system can only process a couple hundred calories per hour before it starts diverting blood flow to the digestive system, which impairs athletic performance significantly.)