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LETTERS TO THE EDITOR |
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 (
: –19 ± 54 kcal/d). However, as clearly stated in our review, when the protein intake was not held constant but rather increased from 15% to 30–35% of energy, 24-h energy expenditure did increase. We determined, however, that the increase would be only 
41 kcal/d for a 1500-kcal/d energy-restricted diet. This would only increase weight loss by 0.04 kg/wk, or 0.44 kg over a 12-wk course of weight-loss treatment. It should be noted that this is less than the 95 kcal/d calculated theoretically by Feinman and Fine, and it has the advantage of being based on experimental data. Thus, we do not disagree with Feinman and Fine from the perspective of pure thermodynamics; in fact, we presented evidence at the whole-body level that supports their point. However, we found the experimentally measured differences in 24-h energy expenditure, between subjects who followed a high-protein diet compared with those who followed a high-carbohydrate diet, to be too small to satisfactorily account for the differences in weight loss observed after 12 wk of treatment with these 2 diets. Thus, this is not a plausible mechanism to account for the observed increased weight loss. The experimental data on energy expenditure provide evidence of only a minimal metabolic advantage for low-carbohydrate diets.
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
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