Reply to AH Manninen and to RD Feinman and EJ Fine

Department of Nutritional Sciences
University of Wisconsin-Madison
1415 Linden Drive
Madison, WI 53706

Dear Sir:

We thank Manninen and Feinman and Fine for their interest in our review, "Is a Calorie a Calorie?" (1). These 2 letters correctly point out that there are indeed some differences between the energetics of human metabolism and the measures of heat release of nutrients in a bomb calorimeter. We agree with the known concept that the metabolic route through which carbon flows to carbon dioxide, the concentrations of substrates, as well as entropy can all slightly alter the efficiency of ATP production in humans (2). This concept, however, does not automatically mean that these differences constitute a quantitatively plausible mechanism that would explain the differences in weight loss observed with a high-protein, energy-restricted diet relative to a low-fat, energy-restricted diet.

Rather than relying on a theoretical treatment of metabolic efficiencies, as did Feinman and Fine, we reviewed studies in which known experimental diets were fed to subjects under laboratory conditions to test whether energy expenditure was actually higher with a low-carbohydrate diet than with a high-fat diet. In studies in which protein intake was held constant and fat was substituted for carbohydrate, the difference in 24-h energy expenditure between the high-carbohydrate and high-fat diets was not different from zero ( We do apologize for not having cited Feinman and Fine's detailed and well-written review, but the journal in which it was published appears to be new and thus is not yet indexed in common biomedical databases.

REFERENCES

1. Buchholz AC, Schoeller DA. Is a calorie a calorie? Am J Clin Nutr 2004;79(suppl):899S-906S.
2. Flatt JP. The biochemistry of energy expenditure. In: Bray G, ed. Recent advances in obesity research. II. Proceedings of the 2nd International Congress on Obesity. Westport, CT: Technomic Publishing Co, 1978:211-27.