Is a calorie a calorie?
1– 4
Andrea C Buchholz and Dale A Schoeller
ABSTRACT
The aim of this review was to evaluate data regarding potential
thermodynamic mechanisms for increased rates of weight loss in
subjects consuming diets high in protein and/or low in carbohydrate.
Studies that compared weight loss and energy expenditure in adults
consuming diets high in protein and/or low in carbohydrate with
thosein adults consuming dietslow in fatwere reviewed. Inaddition,
studies that measured the metabolizable energy of proteins, fats, and
carbohydrates were reviewed. Diets high in protein and/or low in car-
bohydrate produced an 앒2.5-kg greater weight loss after 12 wk of
treatment. Neither macronutrient-specific differences in the availability
of dietary energy nor changes in energy expenditure could explain these
differences in weight loss. Thermodynamics dictate that a calorie is a
calorie regardless of the macronutrient composition of the diet. Further
research on differences in the composition of weight loss and on the
influenceofsatietyoncompliancewithenergy-restricteddietsisneeded
to explain the observed increase in weight loss with diets high in protein
and/or low in carbohydrate. Am J Clin Nutr 2004;79(suppl):
899S–906S.
KEY WORDS Weight loss, energy metabolism, protein, At-
kins diet
INTRODUCTION
High-protein diets, low-carbohydrate diets, and combined
high-protein and low-carbohydrate diets have been highly pop-
ularized, and this is particularly true of the Atkins diet (1). This
is notan entirely new phenomenon because both the high-protein
diet and the protein-sparing modified fast have been popular in
the not-too-distant past (2–9). As a consequence of this more
recent interest, however, several controlled trials were per-
formed to test the efficacy of these diets (10–18). Despite the
initial skepticism of many investigators, these recent studies
found that high-protein and/or low-carbohydrate diets do yield
greater weight losses after 3-6 mo of treatment than do low-fat
diets (10–13).
We identified 9 studies of free-living adults in which weight
losses in subjects consuming diets high in protein and/or low in
carbohydrate were compared with those in subjects consuming
diets high in carbohydrate and/or low in fat (Table 1). Of these
studies, 6 lasted 욷12 wk. On average, consumption of the high-
protein and/or low-carbohydrate diets resulted in 12-wk and
24-wk weight losses that were 2.5 앐 1.8 (x 앐 SD) and 4.0 앐
0.4 kg greater, respectively, than those that resulted from con-
sumption of the high-carbohydrate and/or low-fat control diets.
If these weight losses are assumed to have the typical composi-
tion of 80% fat and 20% fat-free mass (19), then this difference
in weight can be estimated to reflect a 19 500–31 300-kcal dif-
ference in energy balance, or 186–233 kcal/d. These findings,
however, are enigmatic because the energy intakes of the treat-
mentgroups inmost studieswere similar.This hascaused several
investigators to ask whether a calorie is indeed a calorie or
whether a calorie isdependent onthe macronutrientcomposition
of the diet. We herein review the possible explanations for this
difference in energy balance.
THERMODYNAMICS
A calorie, by its simplest definition, is a unit of energy and is
equivalent to 4.184 absolute J. In the popular press and in the
labeling of food products in the United States, a food calorie
actually refers to a kilocalorie, or 1000 cal. That is, 1 food cal
equals 1 kcal, or the amount of energy needed to raise 1 kg water
from 15 to 16 °C.
From a thermodynamic viewpoint, a calorie is of course a
calorie.The firstlaw ofthermodynamics statesthat energycan be
neither created nor destroyed, but only transformed. Thus, the
human body is constantly transforming energy—in this case,
kilocalories—by combusting foodstuffs to produce heat. Al-
though this concept is widely held today, our knowledge of life
as a combustion process is limited to the last 2–3 centuries and
arose from a very old and fundamental question. Because hu-
mans and animals are warm and animal heat is the essence of
being alive, that question, as variously phrased, was, What is the
innate fire, the vital force, animal heat (20)? The Greek philos-
ophers Plato, Aristotle, and Hippocrates and the Roman physi-
cian Galen thought that the innate fire was in the heart and that it
was somehow related to food, but the scientific answer to this
question arose, in part, only in the latter half of the 18th century
from the work of Lavoisier in France (21, 22). Lavoisier’s ex-
periments involved the first-ever animal calorimeter, a device
used to measure heat production. The outer shell of the calorim-
eter was packed with snow, which melted to maintain a constant
1
From the Department of Nutritional Sciences, University of Wisconsin–
Madison.
2
Presented at the ASNS/ASCN Public Information Committee Sympo-
sium “The Weight Debate: Balancing Food Composition and Physical Ac-
tivity,” held in Washington, DC, at Experimental Biology 2004, April 19,
2004.
3
Supported in part by NIH grant DK30031 (to DAS) and research funds
from the Institute for Molecular Virology at the University of Wisconsin–
Madison (to ACB).
4
Reprints not available. Address correspondence to DA Schoeller, De-
partment of Nutritional Sciences, University of Wisconsin–Madison, 1415
899SAm J Clin Nutr 2004;79(suppl):899S–906S. Printed in USA. © 2004 American Society for Clinical Nutrition
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