Isocaloric diets: effects of dietary changes12

158 Am J C/in Nuir l987;45: 158-63. Printed in USA. 1987 American Society for Clinical Nutrition

Gilbert A Leveille, Phi)3 and Paul F Cloutier, PhD

Discussion

The laws of thermodynamics apply to ani-mal energy balance just as they apply to non-living systems. The chemical energy in foodis converted to work, heat, or stored energy,predominantly as fat, by the bodys metabolicprocesses. Figure 1(1-3) depicts a partitioningof energy intake that has evolved over time(2,4,5). It is important to recognize, however,that this model has limitations, the primaryshortcoming being the inability to accounteasily for metabolic adaptations.

Gross energy is the total amount of energyin food. It can be measured using bomb cal-orimetry and includes both digestible andnondigestible (ie, crude fiber) sources. Digest-ible energy is not the total energy available tothe animal because it still includes incom-pletely oxidized products, such as urea ex-creted in urine and possibly ketones excretedin urine and expired air.

Metabolizable energy is the energy directlyavailable to the animal. Some of it is lost asheat in what has been referred to as specificdynamic action (SDA) or heat increment offoods. Essentially, this heat is considered wasteheat although it may contribute to themaintenance of body temperature. The re-maining energy is net energy; it is used formaintenance of basal body functions, forphysical activity resulting in the performanceof work, and for productive processes includ-ing growth, pregnancy, and lactation.

Around the turn of the century, Atwater andBryant measured caloric value of various foodsand diets consumed by humans with diets thatwere popular in the late 19th century. Fromtheir calculations, the Atwater factors, stillcommonly used today, were derived. A sum-mary of these data is shown in Table 1 (6).These values, usually expressed as kcal/g, areestimates of metabolizable energy; they areuseful for estimating caloric intake and for

planning diets. (For research purposes it mustbe clear that the Atwater factors are roundedvalues.)

The discussion thus far would suggest thatenergy intake can be conveniently partitionedand that the energy value of foods is a constant.Unfortunately, while it is a fact that the ab-solute energy content of a food is essentiallyconstant, the efficiency with which it is usedvaries and isinfluenced by numerous factors.

In the experimental design of animal stud-ies, it is important to recognize the complex-ities associated with comparing diets. Becauseof the greater caloric density of fat comparedto carbohydrate or protein (9 kcal/g vs 4kcal/g), alteringthe percentage of fat in a dietchanges the ratio of nutrients to energy. Anincrease in percentage of One component, eg,fat, necessitates a decrease in one or more ofthe other components, eg, carbohydrateand/or protein. A change in the relative pro-portion of protein, carbohydrate, or fat mayinfluence eating patterns. For example, ratsfed diets with low or inadequate protein tendto eat more in an apparent effort to obtainsufficient protein. Thus, studies involving di-etary manipulations often require the use ofpair-feeding. If control of caloric intake and/ormeal pattern is considered necessary, yoke-feeding may be used. In this procedure, whenthe animal fed ad libitum presses a lever toobtain food, the pair-fed partner simulta-neously receives the same amount of food (7).

Forbes and colleagues (8, 9) published sev-eral papers in 1946 on the relationship betweenthe level of fat in the diet of adult rats and the

From Nutrition and Health Sciences, General FoodsCorporation, White Plains, NY.

2Adcfr reprint requests to: Paul F Cloutier, PhD,General Foods Corporation, 250 North Street, WhitePlains, NY 10625.

3Present address: Nabisco Brands Inc., TechnologyCenter, P.O. Box 1943, East Hanover, NJ 07936-1943.

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VFecal energy

BLE ENERGY

GROSS ENERGY

VDIGEST]

VMETABOLfl

VNET ENERGY

VUrinary energy

A8LE ENERGY

V

Heat includingSpecific dynamic action

or

Heat increment of foodsMaintenanceWorkProductive Functions

FIG 1.Classificationof energy in food (adapted fromref 1-3).

100

f

I:25

I 1 1

2 5 10 30

Percent of Fat in Diet

FIG 2. Efficiencyof use and heat production of foodenergy in dietswith increasing percentage of fatcontentand constant gross energy, protein,and essentialnutrients(based on data from ref8, 9).

a low-protein diet when the content of dietaryfat was maintained at a constant level and lev-els of protein varied from 5% to 25%. The an-imals had free access to food and water. Ab-solute food consumption, ie, g/rat, was notsignificantly different between the 5% and 25%groups. Yet total body weight gain and bodyfat were higher in the 25% group (Table 2).

Humans appear to respond in much thesame way as rats to changes in dietary com-position. Danforth (11) noted that lean sub-jects gained weight relatively easily whenoverfed fat but not when fed a mixed diet ofcarbohydrate and fat. In an earlier study byMiller and Mumford (12), researchers ob-served that students overfed a high-protein dietgained more weight than a group of studentsfed a low-protein diet with a similar numberof calories. The weight gain for both diets wasless than the predicted value.

TABLE 2Final body weight and body fat of rats fed ad libitumlow- and high-protein diets of equal caloric contents

TABLE 1Basis for Atwater factors*

Protein in diet Body wt Body fat

% g %

5 397 1625 487 24

* Based on ref(l0).

ISOCALORIC DIETS 159

* Based on original data of Atwater and Bryant (6).

efficiency of use of dietary energy. In thosestudies dietary fat varied from 2% to 30%,while intake of gross energy, protein, and es-sential nutrients remained constant. Re-searchers observed that as the dietary fat con-tent increased, relative heat production de-creased. Thus, with increasing fat intake theapparent efficiency of food use increased, al-lowing for more energy to be stored in the an-imal carcass (see Fig 2). In essence, more en-ergy is stored as fat in the body as the pro-portion of fat to carbohydrate increases in thediet.

Increasing the protein level of the diet at theexpense of carbohydrate also results in changesin apparent energy efficiency. Donald et al (10)observed that adult rats fed a high-protein dietgained more weight than a similar group fed

Grossenergy

Digestibleenergy

Metabolizableenergy

Atwaterfactors

Protein (kcal/g) 5.65 5.20 4.00 4Carbohydrate

(kcal/g) 4.15 4.00 4.00 4Fats (kcal/g) 9.40 8.90 8.90 9

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FATTY ACIDS

CO2 +

ENERGY

PROTEIN CAOHYDRATE FAT(Triglycerides

IGLU ,SE

J ii __Glucogenic PYRIATE IKI > GLYCEROLAMINOACIDS

Ketogenic 5 ACE L CoA [ > FATTY ACIDS

#{231}LE

CO2 + H20

ENERGY

FIG 3. Metabolic pathways with intake of excess pro-

160 LEVEILLE AND CLOUTIER

tein.

Level of dietary energy intake has beenshown to influence the use of dietary proteinin humans (13). Inadequate energy intake re-duces the apparent efficiency of nitrogen use.On the other hand, excess energy intake mayincrease retention of nitrogen. Most studies inhumans indicate that the effects of energy andprotein intakes on fat metabolism are inter-related (14-16). For example, in studies ofisocaloric exchange of fat for carbohydrate, theefficiency of the use of dietary protein was im-proved on diets in which more energy was de-rived from carbohydrates than from fats.This protein-sparing effect was more notice-able under situations where energy intakeswere marginal and individuals were losingweight (17).

At least part of the observed differences inefficiency between fat, carbohydrate, and pro-tein can be explained on theoretical grounds.Figures 3,4, and 5 illustrate schematically themetabolic fate of protein, carbohydrate, andfat, respectively. For example, as depicted inFigure 3, protein that is not needed to replen-ish body protein is degraded to amino acids,which are deaminated and oxidized to provideenergy or are converted to fat. The energy cost

PROTEIN CARBOHYDRATE FAT Trig1ycerjj

i r AJ GLrOSEGlucogenic >P9LJVATE < 5 GLYCEROL

AMINO I IACIDS v

Ketogenic 1< >ACETYL C0A< >FATTY ACIDS

KREBS

CYCLE

V

+

FIG 4. Metabolic pathways with intake of excess car-bohydrate.

for the conversion of amino acids to fat is high,-24%. In the case of dietary carbohydrate (Fig4), complete oxidation would produce essen-

El

CO2 + H2O

+

ENERGY

FIGS. Metabolic pathways with intake of excess fat astriglycerides.

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a)

Ca)C

L.a)C

RMR

70

BMR

TEE

ISOCALORIC DIETS 161

tially 100% of available energy as heat andchemical energy stored as ATP. Conversionof glucose to fat can occur at a cost of -28%of available energy. On the other hand, if car-bohydrate is stored as glycogen, only --5% ofavailable energy is lost. Finally, as shown inFigureS, dietary fat can be catabolized to yieldenergy or, if not immediately used, can beconverted into triglycerides and deposited intoadipose tissue at a minimal energy cost of only-7% (18).

Energy expenditure is normally divided intoseveral components as shown in Figure 6(11,19, 20). The resting metabolic rate (RMR) isthe energy expended for essential body func-tions. The basal metabolic rate (BMR), themore familiar term, is measured early in themorning upon wakening after a 12-18-h fastand before any physical activity. It is a littlelower than the RMR. RMR typically accountsfor 60-75% of the total daily energy expen-diture and varies with age, sex, and body com-position. RMR appears to be similar in bothlean and obese individuals when expressed on

RMR

I-.

60

wQ.)