In the early 1970s carbohydrates (CHOs) were believed to induce overconsumption and excess fat deposition. They are now perceived to protect against these consequences. This paper evaluates the evidence for this change of interpretation by considering (1) the energy content of CHOs, (2) the energetic efficiency with which they are handled by the body, and (3) their effects on appetite, relative to other macronutrients. CHOs are the least energy-dense macronutrients (delta Hc) and exhibit the greatest variability in digestibility. Doubling the usual levels of nonstarch polysaccharides (fiber) may decrease digestibility of a Western diet by 5%. De novo lipogenesis from dietary CHO is energetically inefficient but very limited on Western diets, which are relatively high in fat. There appears to be a hierarchy (protein > CHO > fat) in the extent to which the stores of the macronutrients are autoregulated by oxidation. Excess CHO intake tends to promote storage (but not de novo synthesis) of fat. The thermogenic effects of CHO are therefore relatively limited on Western diets. Per MJ of energy ingested, macronutrients differentially affect satiety (protein > CHO > fat) under conditions where fat is disproportionately energy dense. Isoenergetically dense loads of fat and CHO exert less pronounced differences on satiety. Under some conditions HC diets promote excess energy intakes. There is little evidence that a CHO-rich diet, or one with intense sweeteners, promotes spontaneous weight loss.