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the Atwater factors are average values for a mixed diet, and

individual nutrients do deviate from the mean by a few percent.

These effects, however, should not be interpreted as a thermo-

dynamic advantage of one diet over another. The difference in

energy can be totally explained by the increase in fecal energy,

and the reality is that the difference is actually an error in calcu-

lating the metabolizable energy of the diets.

Of course, the increased energy expenditure associated with

increased protein intake also does not violate the laws of ther-

modynamics, because the energy is conserved. It does, however,

come close to the spirit of the argument that a calorie is not a

calorie, because feeding diets that induce a difference in energy

expenditurecanintroduce adifferenceinenergy balanceandthus

a difference in weight loss. Of the macronutrients, only protein

hasbeen found to have thiseffect, but the magnitude ofthis effect

is small and perhaps accounts for a 0.8-kg difference in weight

loss between diet treatments over 12 wk. This difference in pre-

dictedweight losscould only accountfor one-thirdof the average

greater weight loss of 2.5 kg reported for a 12-wk high-protein

and/or low-carbohydrate weight-loss diet and thus should not be

taken as evidence that a calorie is not a calorie.

OTHER EXPLANATIONS FOR DIFFERENCES IN

WEIGHT LOSS

If a calorie is a calorie, then what other factors could account

for the reported differences in weight loss between either high-

proteinor low-carbohydrate dietsand low-fatdiets? One obvious

explanation is a difference in the composition of the weight loss.

A greater loss of solids or water from fat-free mass for one

treatment than for a second treatment would result in greater

weight loss with the former treatment. One particular factor for

a low-carbohydrate diet is, of course, the loss of glycogen stores

and associated water, which can be as great as 2 kg (51). In this

regard,note that the2 short studies (3 and 4 wk;references 14 and

17 in Table 1) of weight loss with a high-protein and/or low-

carbohydrate diet found an 앒2-kg greater weight loss with the

high-protein and/or low-carbohydrate diet than with the high-

carbohydrate and/or low-fat diet, which is comparable to the

average difference in weight loss in the 10–12-wk studies. This

suggests that the difference in weight loss is an early event and is

not one that increases with time; thus, this difference is more

consistent with a rapid loss of extra water than with a loss of fat

mass. Furthermore, the one 12-mo study (10) reported that the

difference in weight loss between the 2 diets decreased after

3 mo, which is when the participants should have been adding

back some carbohydrate to the low-carbohydrate diet (1) and

would have been expected to regain the weight lost due to gly-

cogen and water loss. Three studies (11, 12, 15), however, did

measurechangesin bodycomposition after욷10wk oftreatment,

and the composition of the weight lost with the high-protein

and/or low-carbohydrate diet was quantitatively similar to that of

the weight lost with the control diet, which reduces the likelihood

that the difference in weight loss typically reported is simply

water weight.

Participants with lower initial relative fatness lose more fat-

free mass per unit of weight loss than do those with higher initial

relative fatness (19), and men may lose more fat-free mass per

unit of weight loss than do women (52). These differences could

confound weight-loss results if the 2 diet treatment groups in a

study are not well matched. Because fat loss and preservation of

fat-free mass are important goals in the treatment of obesity

through weight loss, future comparisons between weight-loss

treatments should include a measure of change in body compo-

sition to provide more specific information about the quality of

the weight loss.

Another important consideration in clinical trials comparing

weight-loss treatments is the accuracy of the participants’ energy

intake data. For studies in which intake is prescribed and actual

intake is assumed to equal the prescription or in which intake is

calculated from self-reported diaries, actual intakes are almost

certainly higher than prescribed or reported intakes (53–57) be-

cause participants frequently underreport their energy intake.

Evenif mealsare provided, noncompliancecan occur and dietary

intakes are likely to be higher than prescribed. However, this

higher intake may not be apparent from the diet records because

of underreporting and the tendency to report an intake similar to

the intake prescription (53, 56).

Finally, there is no reason to speculate that underreporting is

any greater for one diet treatment than for another unless there is

a difference in satiety between the meals. Preliminary evidence,

which was comprehensively reviewed by Yao and Roberts (58)

and Eisenstein et al (48), indicates that both protein and a low

energy density of the diet increase satiety. Thus, subjects who

consume a prescribed diet high in protein or fiber may be more

compliant with the diet than are subjects who consume other

diets, but this speculation has been confirmed by only one long-

term study to date. Skov et al (11) instructed 2 groups of subjects

to consume ad libitum amounts of a diet high in protein (25% of

energy from protein and 46% from carbohydrate) or high in

carbohydrate (12% of energy from protein and 59% from carbo-

hydrate) for 6 mo. The subjects who consumed the high-protein

diet reported that they consumed 앒22% fewer calories and sub-

sequently lost 2.7 kg more than did those who consumed the

high-carbohydrate diet, which led the authors to conclude that

protein had a higher satiating effect than did carbohydrate. Fur-

ther research into the dietary factors that increase satiety and

decrease energy intake in the long term is recommended.

CONCLUSION

We conclude that a calorie is a calorie. From a purely thermo-

dynamic point of view, this is clear because the human body or,

indeed, any living organism cannot create or destroy energy but

can only convert energy from one form to another. In comparing

energy balance between dietary treatments, however, it must be

remembered that the units of dietary energy are metabolizable

energy and not gross energy. This is perhaps unfortunate because

metabolizable energy is much more difficult to determine than is

gross energy, because the Atwater factors used in calculating

metabolizable energy are not exact. As such, our food tables are

not perfect, and small errors are associated with their use.

In addition, we concede that the substitution of one macronu-

trient for another has been shown in some studies to have a

statistically significant effect on the expenditure half of the en-

ergy balance equation. This has been observed most often for

high-protein diets. Evidence indicates, however, that the differ-

encein energy expenditureis smalland canpotentiallyaccount for

less than one-third of the differences in weight loss that have been

reported between high-protein or low-carbohydrate diets and high-

carbohydrate or low-fat diets. As such, a calorie is a calorie. Further

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