Measure percent dietary fat as a function of body fat. The fatter people consume a greater percentage of their total calories as fat. And so on. Finally, ask the subjects in the isocaloric studies how they feel. Eskimos prove that a very low carbohydrate diet can be healthful.
Anaerobic metabolism is primarily carbohydrate.
Swimming muscles use carbohydrate disproportionately to running or cycling muscles. Swimmers are much more carbohydrate depleted than are runners after workouts of comparable duration and intensity. This may be one reason why it is harder to lose weight with swimming than with running. A high carb snack post-workout will also help prevent muscle breakdown to liberate protein for conversion to glucose through gluconeogenesis to replete carbohydrate stores. Because aerobic carbohydrate metabolism consumes less oxygen than aerobic fat metabolism, a carbohydrate burning muscle will perform at a higher level than a fat burning muscle.