Starch, sugars, and triglycerides provide the bulk of dietary energy. To preserve homeostasis, most of the glucose and fat absorbed must be stored to be mobilized later at rates appropriate to bring about the oxidation of a fuel mix matching on average the macronutrient distribution in the diet. The body's glycogen stores are so small that regulatory mechanisms capable of efficiently adjusting carbohydrate oxidation to carbohydrate intake have developed through evolution. Fat oxidation is regulated primarily by events pertaining to the body's carbohydrate economy, rather than by fat intake. Adjustment of fat oxidation to intake occurs because cumulative errors in the fat balance lead over time to changes in adipose tissue mass, which can substantially alter free fatty acid concentration, insulin sensitivity, and fat oxidation. Fat intake and habitual glycogen concentrations are important in determining how fat one has to be to oxidize as much fat as one eats.