http://www.nutritionj.com/cont­ent/3/1/9

Abstract

The principle of "a calorie is a calorie," that weight change in
hypocaloric diets is independent of macronutrient composition, is
widely held in the popular and technical literature, and is frequently
justified by appeal to the laws of thermodynamics. We review here some
aspects of thermodynamics that bear on weight loss and the effect of
macronutrient composition. The focus is the so-called metabolic
advantage in low-carbohydrate diets - greater weight loss compared to
isocaloric diets of different composition. Two laws of thermodynamics
are relevant to the systems considered in nutrition and, whereas the
first law is a conservation (of energy) law, the second is a
dissipation law: something (negative entropy) is lost and therefore
balance is not to be expected in diet interventions. Here, we propose
that a misunderstanding of the second law accounts for the controversy
about the role of macronutrient effect on weight loss and we review
some aspects of elementary thermodynamics. We use data in the
literature to show that thermogenesis is sufficient to predict
metabolic advantage. Whereas homeostasis ensures balance under many
conditions, as a general principle, "a calorie is a calorie" violates
the second law of thermodynamics.

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TC