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although a calorie is still a calorie from a thermodynamic point of

view, calculations of the metabolizable energy of a diet that are

obtained by using the Atwater general factors are not exact and

could thus introduce an error in the calculated metabolizable

energy content of a particular food or diet. In recognition of this,

amodificationof thegeneralfactors, the Atwaterspecificfactors,

was devised in the mid-1950s for specific classes of foods to

account for the differences in the average digestibility of differ-

ent food groups and thus reduce this potential for error (26).

The Atwater general factors, however, continue to be com-

monly used. In 1970 Southgate and Durnin (27) tested the At-

water general factors and determined that they were still valid,

with one exception. Large amounts of unavailable dietary car-

bohydrate resulted in increased excretion of fecal fat, nitrogen,

and energy, and these findings were subsequently confirmed by

other researchers (28–31). Thus, Southgate and Durnin (27)

found that the Atwater protein and fat factors overestimate the

energy derived from these constituents. Others have since found

that the Atwater general factors overestimate the measured me-

tabolizableenergyof mixeddiets, especiallythosehigh indietary

fiber, by a mean (앐SD) of 6.7 앐 4.4% (range: 1.2–18.1%)

(28–30, 32–35). The reasons hypothesized to explain the effect

of dietary fiber on metabolizable energy are many. Dietary fiber

may decrease the transit time of food in the intestine (resulting in

less time for digestion and absorption), increase bulk and water-

holding capacity (reducing the rate of diffusion of digestion

productstoward theintestinal mucosalsurface forabsorption), or

cause mechanical erosion of the mucosal surface (leading to

increased endogenous material) (29, 36). Wisker and Feldheim

(28) also note that in contrast with the energy content of protein

and fat, the energy content of dietary fiber is liberated by fer-

mentation. Thus, factors affecting the microbial degradation of

dietary fiber—the chemical structure of nonstarch polysaccha-

rides, the solubility and degree of lignification of the fiber compo-

nents, and physiologic factors such as the composition of the colon

microflora and the transit time—may affect metabolizable energy

(28). This may be the reason why the Atwater general factors were

found to overestimate measured metabolizable energy to a greater

extent for diets high in nonavailable fiber than for diets high in

available fiber (overestimations of 7.0% and 2.6%, respectively;

P쏝 0.05) (35). Together,findings fromthe abovestudies showthat

not all dietary carbohydrates provide 4 kcal/g.

The differences between the general Atwater factors, the spe-

cific Atwater factors, and true metabolizable energy might ex-

plain some of the difference in weight loss observed after con-

sumption of 2 diets with different fiber content. For example,

Miles et al (29) carefully determined the metabolizableenergy of

2 diets, one with 16 g fiber and one with 37 g fiber (Table 3). If

the energy intakes in this study were extrapolated downward to

2 weight-loss diets each providing 1500 metabolizable kcal/d,

one high and the other low in fiber, the Atwater general and

specific factors would overestimate the measured metabolizable

energy of the high-fiber diet by 120 and 102 kcal, respectively.

Similarly, for the low-fiber diet, the Atwater general factors

would overestimate metabolizable energy intake by 60 kcal, and

the Atwater specific factors by 78 kcal. Thus, if a weight-loss

study is performed and the energy intake of the 2 diets is calcu-

lated on the basis of the macronutrient content of the diets and the

Atwater general factors, the 2 diets would differ in measured

metabolizable energy by 앒60 kcal/d (120 Ҁ 60). If one assumes

that weight loss averages 80% fat by weight, then this error could

account for a difference in weight loss of 0.008 kg/d, or 0.6 kg over

12 wk. If the energy intakes were calculated by using the Atwater

specific factors or tabulatedfood valuesfrom theUS Department of

Agriculture Agriculture Handbook no. 8 (37), which are based on

theAtwaterspecificvalues,thenthe2diets woulddifferby24kcal/d

(102Ҁ 78),andtheweight-loss effectwouldbe0.003 kg/d,or0.3kg

over 12 wk. This error, however, does not bring into question the

thermodynamics of a calorie being a calorie, but it does point to the

limits of our ability to determine the exact metabolizable energy

intake from a given diet.

ENERGY EXPENDITURE

A second potential mechanism through which diets differing

in macronutrient composition can produce differences in energy

balance and hence weight loss is a change in energy expenditure.

For example, if a particular diet were to increase energy expen-

diture relative to another diet, then for the same energy intake,

energy balance would be more negative for the former diet, and

weight loss would probably be greater. Although the most impor-

tant consideration of energy expenditure with regard to energy bal-

ance is total energy expenditure, it is also helpful to look at the

components of energy expenditure, ie, resting metabolic rate, the

TABLE 3

Comparison of gross energy and measured and calculated metabolizable energy between 2 diets with different fiber content that were fed to 12 healthy,

free-living men for 5 wk

High-fiber diet

Low-fiber diet

Value

Difference from measured

metabolizable energy Value

Difference from measured

metabolizable energy

kcal/d kcal (%) kcal/d kcal (%)

Gross energy 3069 앐 448

360 (13.3) 3032 앐 490 249 (8.9)

Measured metabolizable energy 2709 앐 402 2783 앐 461

Metabolizable energy (Atwater general factors) 2925 앐 427 216 (8.0) 2894 앐 468 111 (4.0)

Metabolizable energy (Atwater specific factors) 2892 앐 422 183 (6.8) 2927 앐 5472 144 (5.2)

Adapted from reference 29.

Containing 37 g fiber and 14% of energy from protein, 33% from fat, and 53% from carbohydrate.

Containing 16 g fiber and 14% of energy from protein, 36% from fat, and 50% from carbohydrate.

x៮ 앐 SD (all such values).

902S BUCHHOLZ AND SCHOELLER

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