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Across the English Channel, Crawford was also conducting

experiments on the heat of combustion in animals. Crawford

noted that a given portion of pure air is “altered” by the respira-

tion of an animal and that the extent of this alteration is nearly

equal to that produced by combustion of an amount of wax or

charcoal that used the same volume of oxygen during combus-

tion. That is, the amount of heat produced per unit of oxygen

consumed is nearly the same for animal catabolism as it is for the

combustion of inanimate material (23). Lavoisier further con-

cluded that a flame and an animal both consume oxygen, which

combines with organic substance to release water and carbon

dioxide.Thus,Lavoisier andCrawfordshowedthat fromapurely

thermodynamic point of view, a calorie is indeed a calorie.

METABOLIZABLE ENERGY

Thehuman body, however, isnot a perfect engine,and thus the

thermodynamics may not be so pure. It is now known that the

energy liberated from the combustion of a food is not identical to

the energy available to the body from consumption of that food.

This is the concept of “metabolizable energy,” or the difference

between the gross energy (as measured by bomb calorimetry) of

consumed food and the energy contained in feces and urine (also

measured by bomb calorimetry) (24). The systematic investigation

of the gross energy content of food and of the availability of that

energy can be credited to Rubner in Germany and to Atwater in the

United States. Both scientists’ work is described in detail by Wid-

dowson (25). Using a bomb calorimeter, Rubner measured the

heats of combustion of many different proteins, fats, and carbohy-

drates found in individual foods. He thus determined the energy

density of dietary fat to be 9.3 kcal/g on the basis of the mean

combustion values for olive oil (9.384 kcal/g), animal fat (9.372

kcal/g), and butterfat (9.179 kcal/g). The energy density of dietary

carbohydrate (specifically of starch and sugar in a mixed diet) was

determined to be 4.1 kcal/g on the basis of the average combustion

values for glucose (3.692 kcal/g), lactose (3.877 kcal/g), sucrose

(3.959 kcal/g), and starch (4.116 kcal/g), which were weighted for

their average contribution to a mixed diet. Rubner, however, made

no allowance for fecal losses in deriving his calorie-conversion

factors for fat and carbohydrate. He did, however, conclude that the

heat of combustion of protein in a bomb calorimeter is higher than

the energy value available to the host because the body oxidizes

protein only to urea, creatinine, uric acid, and other nitrogenous end

products, which can themselves be further oxidized in a bomb cal-

orimeter. Fromurinary and fecal combustion in one subject, Rubner

determined that the loss of energy from the nitrogenous substances

in urine and feces totaled 23% of energy intake, 16.3% from meat

sources and 6.9% from vegetable sources. Thus, meat and vegeta-

ble protein differed in their metabolizable energy densities: the

former provided 4.23 kcal/g and the latter provided 4.30 kcal/g

(after correction for the heat of combustion of nitrogenous end

products in urine and losses of nitrogen in feces). Assuming that

60% of dietary protein was from animal sources and 40% from

vegetable sources and recognizing that the energy content of wheat

and rye protein (“the most important sources of vegetable protein”)

was overestimated by 7.9% because of the higher nitrogen content

in wheat and rye protein than in animal protein, Rubner suggested

that 4.1 kcal/g be used as an average factor for determining the

energy content of dietary protein. Thus, Rubner showed that a cal-

orie is a calorie; however, he also showed that the human body

cannot extract all the calories liberated from combustion of a food

and that macronutrients differ according to their chemical compo-

sition in the number of calories per unit of weight.

With Bryant, Atwater extended Rubner’s work by studying

the availability of the other macronutrients. Data from human

digestion experiments were combined with other data in the

literature to devise “coefficients of availability” (defined as in-

take minus fecal excretion divided by intake) for protein, fat, and

carbohydrate. Atwater and Bryant applied these coefficients of

availability to the heat of combustion of “mixed” diets that were

typicalofthe time(consistingof foods suchas beef, butter,ginger

snaps, parched cereal, rye bread, baked beans, and canned pears)

and were consumed by 3 adult male subjects. The foods con-

sumed, as well as the subjects’ urine and feces, were collected

and analyzed for nitrogen and fat content; thedifference between

total organic matter and the sum of protein and fat was taken to

represent carbohydrate. An additional correction was made for

protein: for each gram of nitrogen in urine, there was sufficient

unoxidized matter to yield an average of 7.9 kcal, or 1.25 kcal/g

absorbed protein (7.9 divided by 6.25). Thus, after correction

forthecoefficient ofavailability,1.25 kcal/g wassubtractedfrom

the heat of combustion of protein. The calculated availability of

the mixed diets agreed closely with the actual availability as

found by experiment.

The energy values obtained by Atwater and Bryant’s experi-

ments, to which we refer today as the Atwater factors, are pre-

sented in Table 2. The metabolizable energy values in the right

column, ie, 4, 9, and 4 kcal/g protein, fat, and carbohydrate,

respectively, are more appropriately known as the Atwater gen-

eral factors for metabolizable energy and were proposed for

application to mixed diets of similar composition tothose used in

Atwater’s experiments. With the use of the Atwater general fac-

tors,metabolizable energy iscalculated as 4.0Pѿ 9.0F ѿ 4.0TC,

where P is protein (P ҃ 6.25 ҂ nitrogen; in g), F is fat (in g), and

TC is total carbohydrate (in g, calculated by dry weight differ-

ence). Not only have these factors been applied to the total

amounts of protein, fat, and carbohydrate in a mixed diet, as

Atwater and Bryant had intended, but they have also been used,

and continue to be used, in assessing the energy value of indi-

vidual foods.

At first glance, calculated metabolizable energy would appear

to be equivalent to measured metabolizable energy. The work of

Atwater and Bryant, however, clearly showed that these factors

were average values. Although the general factors could be used

to calculate the metabolizable energy of a mixed diet, they were

inerror to some degreefor almost any particularsingle food item.

This error results from differences in chemical structure that can

alterthegross energyperunit weightbyup toseveralpercent and,

to a slightly larger degree, from differences in availability. Thus,

TABLE 2

Atwater factors for heat of combustion, coefficient of availability, and

“available energy” for nutrients in a mixed diet

Macronutrient

Heat of

combustion

Coefficient of

availability Available energy

kcal/g % kcal/g total nutrients

Protein 5.65 92 4.0

Fat 9.40 95 8.9

Carbohydrate 4.10 97 4.0

Corrected for unoxidized material in the urine, ie, (5.65 kcal/g ҂

0.923) Ҁ 1.25 kcal/g.

IS A CALORIE A CALORIE? 901S

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