energyintakerequiredtomaintainbw

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35 0 Am J C /in Nu ir l99 2 ;55 :350 -5 . P rin ted in U SA . 1992 A m erican Soc ie ty fo r C lin ica l N utrition

Ene rgy in ta ke requ ired to m a in ta in bo dy w e igh t is no ta ffe c ted by w ide va r ia tio n in d ie t com pos itio n13R udo lp h L L eib e l, Ju les H irsch , B u rto n E Ap p e l, a n d G reg g C C heca n i

ABSTRACT D ie ts rich in fa t m ay p rom ote ob esity by lead -ing to a grea ter depos ition o f ad ipose -tis sue trig lycerides thando isoene rge tic d ie ts w ith less fa t. T h is poss ib ility w as ex am inedby a re trospec tive ana lysis o f the ene rgy n eed s o f 1 6 hum ansub jects (1 3 adu lts , 3 ch ild ren ) fed liqu id d ie ts o fp rec ise ly kn ow ncom po sitio n w ith w ide ly v ar ied fa t co n ten t, fo r 15 -56 d (33 2d , i SE ). Su b jec ts lived in a m e tabo lic w ard and rece ived flu idfo rm ula s w ith d iffe ren t fa t and ca rboh ydra te con ten t, ph ysica lac tiv ity w as kep t cons tan t, and p rec iseda ta w ere ava ilab le o nene rg y in take and d aily body weigh t. Isoene rge tic fo rm u las con -tam ed v ar io us percen tag es ofca rboh ydra te as ce re loselow , 1 5% ;in te rm ed iate , 40% or 45% ; h igh , 7 5% , 8 0% , o r 85% ), a cons tan t15% of energ y as pro te in (as m ilk pro te in ), and the ba lance ofene rg y as fat (as co rn o il) . E v en w ith ex trem e ch an ges in the fa t-carboh ydra te ra tio (fa t energ y v ar iedfrom 0% to 70% of to talin tak e), the re w as no de tectab le ev iden ce ofsign itlcan t varia tionin energ y need as aun c tion of p ercen tag e fa t in tak e . Am JC liii N u ir l992 ;55 :350-5 .

KEY W ORDS O besity , d ie tary fa t, d ie tary ca rboh ydra te,d ie t com positio n , en ergy requ irem en ts

In troduct ion

S ix ty years ago , LH N ewburgh and h is co lleag ues exam inedthe po ssib ility tha t so -ca lled end ogenous o besity m igh t be theresult o f spec ial m e tabo lic fac to rs u n rela ted to en ergy in tak e o rphys ical ac tiv ity (I) . T hey found no ev iden ce fo r such pure lyendogenous ob esity and a lso dem on stra ted tha t the lo ng-termeffect o f an y d ie t onbody weigh t is re la ted on ly to the to ta lene rgy con ten t o f the d iet. O the r fea tu re s o f the d iet such ascarbohydra te o r fa t con ten t d id no t, in the long run , have co n-sequ en tia l ef fects o n body weigh t .

In recen t yea rs the ad ve rse e ffec ts o f h ig h -fa t d ie ts on healthhave been em phasized , an d th e poss ib ility th a t a h igh -fa t in tak em ay be accom pan ied by som e specia l in v ivo econom y ofnergym etab o lism has led to sp ecu la tion on the ro le o fd ie t com positio nin the pro duc tio n of ob es ity . V ariou s inves tig ato rs have repor teda p ositive co rre latio n be tw een body fa t and the pe rcen tage offa t in iso en erge tic d ie ts fed to ro den ts (2 , 3) and hum ans (4 , 5 )and a lack of e ffect o f d ie ta ry fa t o n resp ira to ry qu o tien t (RQ )in hum an su b jects s tud ied fo r 9 or 24 h afte r the feed ing of fa t(6 , 7 ). A dd ition ally , 24 -h en ergy b alance in aespiratory ch amberis repor ted to be due a lm os t exc lusive ly to d ifferen ces in fa tba lan ce an d unre la ted to carbohyd ra te o r p ro te in ba lance (8 ).

O ne group o f in vestiga to rs co nc lud ed tha t fat in take m ay p laya ro le in ob es ity tha t is ind ep en den t o f ene rg y in take (4 ) .

Th e u ltim a te tes t o f the va lid ity o f su ch a propo sition is thelon g-term effec t on m e tabo lic eff iciency o f p ertu rba tion s o f d ie tcom pos ition . U sin g th e techn ique of ca lo r ic titrim etry (9 ) , iw hich a fo rm u la d ie t o fspec ified com po sitio n is fed ov er a periodo fw eek s u nde r c ircum stances o fcon tro lled ph ysica l activ ity , w eexam ined the effec t on m e tabo lic effic iency (as ref lected bychan ges in body w eigh t) o fex trem e ch an ges in the com positionofo therw ise isoene rge tic d ie ts . T h is tech n iqu e w as m ade po ssib leby the ca refu l co llec tion ofda ta o n the effec ts o ffo rm ula feed ing ,in stud ies d one o ver m any years by EH A hrens and h is co lleaguesa t th e H osp ita l o f th e R ocke fe lle r U n iv ersity ( 10 , 1 1 ).

Sub jec ts and m eth odsThe reco rd s o f a ll sub jec ts stu d ied b y the L ip id Labora to ry

ofthe R o ckefe lle r U n iv ersity H o sp ita l b etw een 1955 and 1965w ho w ere fed liqu id -fo rm u la d iets o fva rio us ca rb oh ydrate (CHO )and fat com po sition w ere rev iew ed . T hese d ie ts w ere p repa redin the research k itchen of the R ockefe lle r U nivers ity H osp ita l.P ro te in (2 0 .9 Id /g ) , w h ich represen ted a cons tan t 1 5% of energ y ,w as de riv ed from m ilk p ro te in . F at (37 .7 kJ/g )as from corno il. CH O (16 .7 kJ /g ) w as supp lied as cere lose , a hydra ted fo rmof g lucose . C oe ffic ien ts o f d igestib ility w ere used to d e term inethe fin al av a ilab le ene rgy va lu e o fthe co nstitu en ts o fthe fo rm ula(12 ): p ro te in , 0 .92 ; fa t, 0 .95 ; and carbohydra te , 0 .98 . D ata rg ard in g bom b calo rim etry o f these fo rm ula s a re no t av ailab le .Bom b ca lo rim etry pe rfo rm ed on e igh t b atch es o fs im ila r fo rm u lap repa red in the re search k itchen of the R ocke felle r U n iv ersityH osp ital be tw een A p ril 198 8 an d Ju ly 1 990 had a coe ff ic ien t ovaria tion fo r Id /g of 1 .9% . Su ch fo rm ula show s a 0 .2% correspondence be tw een bom b ca lo rim etr ic an d ca lcu la ted ene rgycon ten t. In stud ies d es igned to exam ine the e ffec ts o f d ie ta rylip id typ e an d qu an tity on lipop ro tein m e tab o lism , the pe r-cen tage of CHO in the d ie t w as system a tically a lte red b y iso -

I F rom the L aborato ry o f H um an B ehav io r an d M etab o lism and th ePew Cente r o fN utritio na l E xcellence , R ockefe lle r U nivers ity , N ew Y ork .

2 Supported in pa rt bygran ts DK 30583 and RROO 102 from th e N a-tiona l In stitu tes o f H ea lth .

3 A ddress rep rin t req uests to RL Leib el, L ab ora to ry of H um an B e-hav io r an d M etab o lism , R ockefe lle r U nivers ity , 123 0 Y ork A venue , N ewY ork , N Y 10021 .

Rece iv ed O ctob er 25 , 1990 .A ccep ted fo r p ub lication M arch 6 , 1991 .


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D IE T C O M PO SITIO N A N D E N ER G Y R EQ U IR E M EN TS 3 5 1TA B LE IA nthropom etry and diet characteristics of adults

ID Sex A geM eanw eight W T IN TS H eight B M It

D ayson diet

D iet

PercentC H O

Percentfat Energy

C O R R E CId

y k g k g cm d % % Id k J1 M 56 76.01

7 6 . 1 075.597 6 . 1 9

165-

27.9-

243 2

457 5

401 0

100671 0 0 6 7

90711 0 2 4 2

2 M 43 75.287 5 . 4 2

75.417 5 . 6 8

167.5-

26.8-

432 8

458 5

400

99379 9 9 1

101131 0 5 1 9

3 M 5 5 9 6 . 2 19 5 . 9 5

9 5 . 4 69 6 . 2 1

1 7 3. 5-

3 2. 0-

2 31 8

1 58 5

7 00

1 3 8 6 21 3 8 6 2

1 2 0 1 61 4 6 5 2

4 M 5 3 5 9 . 4 25 8 . 8 1

5 9 . 0 35 8 . 1 9

1 5 7. 5-

2 4. 0-

3 944

1 575

7 010

9 1009 1 0 0

86908 2 8 0

5 M 4 1 6 2 . 2 66 4 . 0 2

6 2 . 3 16 3 . 8 7

1 6 4. 5-

2 3. 0-

1 53 9

1 54 5

7 04 0

1 0 3 3 01 0 3 3 0

1 0 3 3 01 0 0 9 6

6 M 5 4 6 5 . 9 36 6. 1 1

6 6 . 2 56 6 . 1 3

1 7 8-

2 0. 8-

3 43 8

4 58 5

4 00

1 0 2 0 11 0 2 0 1

1 0 7 2 81 0 2 0 0

7 M 4 9 6 3 . 8 06 4 . 1 3

6 3 . 9 56 3 . 9 2

1 6 4-

2 3. 7-

2 54 2

4 58 5

4 00

9 4 1 49 4 1 4

9 7 6 59 1 2 1

8 M 4 1 7 0 . 7 27 1 . 0 4

7 0 . 8 77 1 . 0 9

1 7 0-

2 4. 5-

2 24 2

4 58 5

4 00

1 0 5 9 01 0 5 9 0

1 0 5 9 01 0 5 9 0

9 M 5 7 6 1 . 5 86 1 . 5 6

6 0 . 9 86 1 . 7 9

1 7 5-

2 0. 1-

5 24 7

4 58 5

4 00

1 0 3 3 09 2 8 4

9 6 5 79 5 7 7

1 0 M 6 4 7 0 . 2 47 0 . 5 5

7 0 . 6 87 0 . 7 8

1 6 5-

2 5. 8-

2 518

1 58 5

7 00

8 4 9 88 4 9 8

9 4 9 39 2 0 1

1 1 F 4 9 4 5 . 7 046.95

4 5 . 4 246.92

1 6 3. 5-

1 7. 1-

2 81 8

4 58 5

4 00

7 4 5 26 9 2 9

6 8 9 56 9 2 9

1 2 F 6 4 5 2 . 6 45 2 . 6 8

5 2 . 8 45 2 . 3 8

1 5 6-

2 1. 6-

5 634

1 575

7 010

7 3 2 27322

7 4 6 86824

1 3 F 5 7 4 9. 0 74 8. 5 5

4 8 . 9 84 8 . 5 7

1 5 5. 5-

2 0. 3-

1 52 8

1 58 5

7 00

7 3 2 27 7 0 7

7 3 2 27 7 0 7

* T im e zero intercept of regression line of w eight vs days.t B ody m ass index, in kg/rn.t Id corrected for any w eight change during the period of form ula feeding.

energetic replacem ent w ith fat. These form ula diets of variedcom position w ere fed in random order. V itam in and m ineralsupplem ents w ere given daily in addition to 2 g N aC I. A ll subjectslived in the C linical R esearch C enter (R ockefeller U niversityH ospital); physical activity w as constant at activities of dailyliving w ith no additional exercise. The form ulas w ere alw ays fedin quantities sufficient to m aintain a constant bodyeight asnearly as possible. Interim adjustm ents in the quantity of form u law ere m ade to achieve this goal. W eights (to nearest 0. 1 kg) w ereobtained each m orning w ith the subject clothed in underw ear.T hough stools w ere not collected from these subjects, sim ilarsubjects on sim ilar diets did not show differences in the fractionof energy intake excreted in the stool (E H A hrens, personalcom m unication, 1991).

The records of all patients studied from 1955 to 1965 w eresearched for consecutive periods in w hich patients w ere fed for-m ula diets of differing C H O -to-fat ratios. For inclusion in thisanalysis, each period of form ula feeding had to be2 w k andthe subject had to have rem ained w eight stable (to w ithin 1 kg)w ithin that period. R easons for rejection of records w ere lackof feeding of tw o different form ulas or the absence of feedingperiods of 2 w k. A total of 47 patient records w ere review edto obtain 16 (13 adults and 3 children) fitting the above criteria.

T he daily w eights of each subject w ere exam ined by regressionanalysis ofw eight vs tim e. W e found no evidence ofearly changesin slope for these functions as the 2-w k period proceeded, sug-gesting that acute changes in w ater excretion and retention w erenot occurring w ith changes in diet com position. T his is m ostlikely the result of feeding exactly 2 g N aC I/d in all dietary pe-riods. Therefore, all days on a given diet w ere included in theseanalyses. The slope of each w eight-vs-tim e plotas exam inedfor significant deviation from zero. If the value of this rela-tionship w as < 0.05, the energy requirem ent for the individualw as corrected for this intercurrent change in w eight. In m akingthis correction w e assigned the conservative (high) value of 29.3Id/g to the w eight change. T he corrected daily energy require-m ent w as calculated as:C O R R EC K J = Id form ula - 29.3

x (regression slope in gram s per day).T he tim e zero intercepts (W T IN T ) of these plots are given inTable 1. T he clinical characteristics and diet specifications foreach adult subject are given in T able 1.

M any ofthese subjects w ere fed a no-fat diet for several w eeksat a tim e. A t the tim e these studies w ere done, it w as uncertain


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352 LE IB EL E T A Lw hether essential fatty acid (E FA ) deficiency w ould occur inadult hum ans. O nly one subject (subject 6, T able 1) developedsym ptom s possibly related to E FA deficiency (ie, m ild eczem arelieved by the addition of fat to the diet).

This study w as approved by the Institutional R eview B oardof the R o ckefeller U n iversity.

Energy requirem ents of each subject w ere expressed both ab-solutely and corrected for significant slope ofthe w eight-vs-tim eplots. In addition each subjects requirem ents w ere expressedper unit of body surface area (13) orer am ount ofbodyw eight#{176}73.oth body surface area and w e ight# {1 76}prov ide goodindirect estim a tes of m e tabolic m ass (14). Statistical analysesw ere perform ed on a V AX PD P 1 1/780 com puter runningUNIXBMDP (1 5) and UNIXSTA T (16) program s.

R esults

AdultsSix subjects had been m aintained on low C H O form ula, eight

on interm ediate C H O , and 12 on high C H O . T he m ean periodon each diet w as 32 2 d (i SE ). A plot of the daily w eightis show n in a typical patient (subject 12) w horeceived a 10% -fat diet for 5 w k follow ed by a 70% -fat diet for 8 w k (Fig 1). Thesm all corrections, calculated as described, are show n in Table1. The absence of effect ofdiet com position on energy require-m ents to m aintain w eight are show n in T able 2. This table pro-vides sum m ary data on the w eight-m aintenance energy require-m ents of the 13 adult subjects studied on tw o diets of differingpercent carbohydrate. R egression plots of diet vs various bodysize indexes for the three diet com positions are show n in Figure2. This figure exam ines the relationship ofeasured energy in-take (Id) and corrected energy intake (C O RR EC Id)egressedon surface area (SA ) and also on w eight#{176}75% ,J1O .l5). A lthoughenergy requirem ents w ere highly significantly related toodysize indexes, analysis of variance (A N O V A ) of regression coef-ficients over groups did not indicate any significant betw een-group difference, regardless of theody size index used in theabscissa (Table 3). Five of the subjects w ho had received bothvery-low -C H O (1 5% ) and very-high-C H O (75-85% ) diets w ereseparately exam ined. Paired t tests of the energy requirem ents(C O R R EC Id) of these subjects on the tw o diets indicated nodifference due to diet com position [difference of m eans (low- high) = -334.7 Id; t = -0.558; P = 0.6 1]. Sim ilarly, a com -parison (paired t test) ofenergy requirem ents in the sev en su bjectsw ho received both m oderate-C H O (45%) and very-high-CH O(75, 85% ) diets indicated no difference due to diet com position

T A B L E 2A n thropornetry and energy requirem ents of 13 adult subjectson various carbohydrate diets

I n t e r me di a t eLow C H O

( n=6)C H O

( n = 8 )H igh C H O( n = 1 2)

W eight (kg) 65.0 6.9 64.4 3.4 65.7 3.9En e r g y ( I d ) 9406 1004 9791 356 9401 527k J CORREC 9 2 1 7 7 3 2 9 6 1 1 4 3 1 9 4 8 5 6 0 7A ge(y) 55.7 3.5 48.7 2.3 53.5 2.1D ays on diet (d) 28.8 6.5 33.4 3.8 32.4 3.1W t#{176}75kg) 22.8 1.8 22.9 0.9 23.0 1.0E n er g y / S A

( U/ r n ) 5 5 1 9 3 0 5 5 6 1 1 1 1 3 5 4 2 2 1 5 1E ne r g y / wt # { 1 7 6 } 7

(U/IO .7i) 4 1 0 17 427 8 410 8C O R R E C ICJ/SA

(kY/rn) 5 4 3 9 1 8 0 5506 1 6 3 5 4 4 8 180C O R R E C 0 7

(/lO.7S) 406 13 418 13 410 8* i S E. T hirteen subjects w ere studied onw o diets, hence a total

of 26 feeding periods. T he characteristics of all subjects exam ined oneach diet (low , interm e diate, or high C H O ) are given.

[difference of m eans (low - high) = -51.5 Id; I -0.2236; P= 0. 8305) .

B ecause the no-fat diet m ight have induced subclinical E FAdeficiency, a separate analysis w as done ofthe nine subjects w horeceived a0-fat diet vs the 17 dietary periods of those on 10-70% fat intake. L inear-regression analysis of energy and C O R-R E C Id vs SA and .7S w as perform ed for these tw o groups,and A N O V A of regression coefficients over groups show ed nosignificant differences betw een lines of regression for no-fat andfat-containing diets (Table 4). Thus, there w as no aberration ofenergy requirem ent in those receiving a no-fat diet.

ChildrenT able 5 show s data for the three children studied. A lthough

energy need for w eight m aintenance w as 30-40% higher (kukg#{176})han that for adults, no significant effect of dietom po-sition on energy requirem ents iseen in children, as is the casew ith adults.

D iscussionT he energy content ofoodstuffs is conventionally determ ined

by m easuring heat released by the com plete com bustion of the

D a y s

1 O % F a t 70% F a t SFIG 1. A l3-w k study ofsubject 12, firstn 10% (75% C H O ) ofenergy intake as fat and then on 70% (15% C H O)

ofenergy intake as fat. D uring both periods, 7322 U per day w ereed. A ctual Id com b usted w ere corrected, asescribedin Subjects and M ethods, to 7468 U and824 Id per day, respectively. It is notable that there w as only 1 kg of w eightvariation during the entire study.


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0

c DO0

1 6 0 0 0

1 4 0 0 0-) 1 2 0 0 0

1 0 0 0 00U 8000

6000

01 4 0 0 0

12000

-) 1 0 0 0 0

8000

6 0 0 0

1 4 0 0 0

1 2 0 0 0

. 1 0 0 0 0

8000

6 0 0 0

1 .4 1 .6 1 .8 2 .0 2 .2 2 .4SA

0

C

.0

1 .4 1 .6 1 .8 2 .0 2 .2 2 .4SA

01 6 0 0 0

1 4 0 0 0

1 2 0 0 0

1 0 0 0 0

8 0 0 0

6 0 0 0

C.)w0U

15 20 25 30 35w 0.75

15 20 25 3 0 35

D IET COM POSIT ION A ND ENERGY REQU IREM ENTS

& L OW C HO= 1 5 %. IN T E R M E D IA T E C HO = 4 0% o r 4 5 %0 HIGH CHO = 75% , 80% or 85%

c

FIG 2. Regressi on analy ses of 24-hr energy i ntakes vs ei ther body surf ace area (SA ) orody we igh t#{176 } (WT#{ 176 }75 )orl iqui d-f ormula di ets of di f f erent CHO content.n = 6, 8, and 12, respecti vel y , f or l ow -CHO, i ntermediate, and high-CH O diets. A ll subjects ar e ad ul ts.

353

speci f i c components of the diet. A recent analy si s of the energyi n f oodstuf f s showed that conventi onal f ood fats have a heat ofcombusti on of 37.690-40.091 Id/g. T he metabol izabl e energy(corrected for urea) of conventional f ood protei n v aries f romI 7.43 1 to 2 11 50 I d/g. CHOs vary less: starches, 1 7.48 1 Id/g;glucose, 1 .560 I d/g; and sucrose, 16.481 kJ/g (17). A n anal ysi s

T A B L E 3Regressi on equati ons (y= r nx + b) f or the plots in Figure 2

Plot Equati on r P(cx)A

L ow CHO y = 9699x - 6950 0.92 0.009Intermediate CHO y = 699 1x - 2393 0.88 0.005H ighCHO y=9025x-6146 0.93 0.000

B L ow CHO y = 7243x - 2992 0.94 0.004Intermediate CHO y = 7979x - 4297 0.83 0.0 16High CHO y = 9443x - 8962 0.96 0.000

CL ow CHO y=5l5x -2330 0.92 0.010Intermediate CHO y 310x - 2720 0.79 0.020High CHO y = 473x - 1477 0.92 0.000

DL ow CHO y = 38 1 x - 590 0.93 0.008Intermediate CHO y = 339x - 1883 0.72 0.050H ig h CH O y = 556x - 3343 0.95 0.000

of the energy value of f oodstuf f s as used to cal cul ate total energyexpendi ture by indi rect cal orimetry may be in error by5% ofthe true value even under the most careful ci rcumstances (171 8). T here could be even greater error i f ,ecause of di f ferencesi n the metabol ic handl i ng of di f f erent f oods, the actual contri -buti on to system ic f uel ox i dation is not di rectl y predi ctedenergy rel ease as measured in v i tro. Speci f i cal ly , the A TP rqui red for the acti vati on of am ino acids preparatory to thei ri ncorporation i nto proteins and the ready interconversionglucose to and f rom gl ycogen v ia energy - requi r i ng steps couldreduce the actual energy value of these substrates i n terms

TA BL E 4For each of the four pl ots, the separate regressi ons for each of thethree level s of CHO f eeding exam ined for di f f erences i n regressi oncoef f i ci ents by A NOV A*

Plot A NOV A P (a )A : I d vs SA F14,01 = 0.464 0.761B : CORREC U vs SA F14201 = 1 1 77 0.351C: Id vs O.75 F14,,1 = 0.830 0.522D : CORREC Id vsO.73 F14201 = 1.236 0.328

* There w as no signi f i cant correl ati on betw een the occasi onal smal lshi fts i n appar ent body energy stores (I d - CORREC U ) and ei ther SA(r = -0.027, P = 0.89) or O.75 (r = 0.15, P = 0.94).


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354 L EIB EL ET A LTA B LE 5A nthropom etry and diet characteristics of children

ID Sex A geM eanw eight W T IN T H eight B M I

D ayson diet

Di e t

PercentC H O

Percentfat Energy

C O R R E CkJ

y k g k g cm d % % k .J1 4 M 9 1 9 . 9 1

2 0 . 7 71 9 . 6 62 0 . 3 7

1 1 5-

1 5. 0-

3 64 1

4 08 0

4 00

6 1 4 66 1 4 6

5 7 6 65 5 9 0

1 5 M 1 0 2 0 . 8 02 1 . 4 4

2 0 . 5 92 1 . 1 8

1 15 . 5-

1 5. 7-

4 03 0

4 08 0

4 00

5 6 2 35 6 2 3

5 3 3 05 1 2 5

1 6 F 8 2 6 . 1 925.88

2 6 . 1 025.91

1 3 7-

2 0. 0-

4 223

4 080

4 00

6 5 9 06590

6 5 9 06590

net energy provision for in vivo chem ical respiration. C onversely,the presum ably low er rate and energy cost of hydrolysis andreesterification offree fatty acids m ight result in greater efficiencyofhandling ofdietary fats and hence an apparent relative increasein net in vivo generation of A TP. D ifferences in the therm iceffect of feeding w hen C H O s are com pared w ith protein or fatis yet an additional cause for potential system atic differences inthe energy value of foods in vivo as com pared w ith laboratorym easures of heat of com bustion.

Short-term studies (24-48 h) in hum ans suggest that a sub-stantial (l00 Id) dietary fat supplem ent doesot im m ediatelyprom ote the use offat as a m etabolic fuel (7). O ur results suggestthat this phenom enon does not operate in a m ore extended tim efram e ( 8 w k) to produce preferential storage of dietary fat inadult hum ans. This point w as in fact m ade by Schutz et al (7),w ho em phasized that hum an subjects achieve w eight equilibriumon high-fat diets despite the operation of these short-term phe-nom ena. O ther investigators, studying subjects over shorter tim eperiods than w e used here, reported no effect on various aspectsofenergy expenditure (24-h expenditure, resting m etabolic rate)of high- vs low -C H O diets (19-23). O n the other hand, Prew ittet al (24) recently com pared the effects on energy need and bodycom position of sw itching 18 outpatient fem ale subjects (B M I18-44) from a 37% fat diet (1 4 w k) to a 21.4% fat diet (1 20w k). O n average, w hile ingesting the low er fat diet, these subjectsrequired an average of 10.5% m ore calories and lost 2.8% (1 1.3%ofbody fat) despite efforts to feed sufficient calories to m aintainw eight. D ifferences in physical activity and noncom pliance onthe low er-fat diet m ay account for som e of the observed differ-ences.

Forbes (25) recently review ed a series of studies in w hich hu-m an subjects w ere overfed for 2- 1 2 w k and found that althoughobese individuals appear to deposit a larger proportion of excessenergy intake as fat, the energy cost ofw eight gain in either leanor obese subjects w as not significantly influenced by the com -position of the excess energy fed.

The lack of agreem ent betw een our data and those obtainedin rodents (2, 3) m ay be because rodents are grow ing continu-ously, introducing a possible effect ofgrow th itselfon the responseto diet com position. A gainst this explanation is our finding oflack of diet-com position effect on energy requirem ents in threechildren in this study (Table 5). These rodents w ere also fed adlibitum (perm itting w eight gain), w hereas our hum an subjectsintake w as designed to m aintain body w eight.

A lthough the num ber ofsubjects studied w as relatively sm all,each individual w as studied for extended periods of tim e (15-56 d, i 33 d) on tw o diets. C om parable studies concludingdifference in energy requirem ents on diets of differing C H Ocontent w ere conducted for a m axim um of 6 d1 9-22). Theextended period of our diets adds considerable pow er to thdesign of this study, as dem onstrated by the follow ing calcula-tions: our assignm ent ofa value of29.3 kJ/g to any intercurrentw eight changes is extrem ely conservative, a m ore likely figurebeing I 3- 1 7 Id/g. N onetheless, by using the figure of 29.3 Idg, the confidence interval (level 0.95) around the slope of w eightvs days (8.2 g/d) and the total energy ingested by each adultsubject m inus the 95% confidence lim its (range) ofthe differencesin energy requirem ents on diets ofdiffering com position equals 7853 Id/32 days w hich equals 2.7% of total kilojoulesgested. Thus, despite the relatively sm all num ber of subjectsstudied, w hen the large num ber of total kilojoules fed to eachsubject during each diet period are taken into account, it is unlikely that w e failed to detect a5% difference in energy effi-ciency betw een diets. H ow ever, very sm all differences in thefficiency of energy m etabolism integrated over prolonged periods oftim e can theoretically lead to clinically significant shiftsin body com position. For exam ple, 2% greater m etabolic effciency of one isoenergetic diet vs another w ould (assum ing10 460 Id/d intake and 29.3 Id/g w eight gain) lead to a 3.2w eight gain in 1 y.

A form al analysis ofthese data can be m ade. H ow ever, suchanalysis is very sensitive to sam ple size and cannot readilym ade to account for the special pow er of this study in the largenum ber ofdays for w hich each subject w as m aintained on eachdiet. W ith this caveat in m ind, the /3 (type II) error can betim ated in tw o w ays. 1) The variance about the slope of theregression equation relating C O RR EC Id to surface area (plotB , Fig 2) m ay be used to estim ate the likelihood of m issing differences ofany desired m agnitude betw een energy requirem entson form ulas of different com position. A ssum ing, for exam ple,a 10% decrease in energy efficiency (C O R R EC Id) conferred bthe high-C H O (75% or 85% ) form ula (n = 12 adult studies) andvariance proportional to that for the high-C H O form ula, thprobability ofa /3 error (acceptance of a false null hypothesis)P = 0.1 1 (2 6). 2) A direct com p arison betw een the distributionsofC O RR E C Id for the seven adult subjects studied on both 45%(interm ediate) and 75% or 85% (high) C H O gives paired= -0.2236, P(a) = 0 .8304 , and S D ofd ifferences = 607 Id. For


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D IET C O M PO SIT IO N A N D E N ER G Y R EQ U IR E M E N TS 355Pu. = 0.05, the betw een-diet difference is 912 Id (one tail). Thatis, the probability of a difference betw een the tw o diets as largeas 9 12 kJ/d escaping detection is 0.05. This difference represents.5% of total energy intake per day. For reasons discussedabove, such calculations overstate the probable error in this study.

Sim ilar results m ight not have been obtained in a group ofobese individuals or lean individuals susceptible to obesity. Therem ay be an interaction of diet com position and predispositionto obesity, w ith higher dietary fat content facilitating the expres-sion ofa m ore efficient m etabolic phenotype (27). In this regardit is of interest to note that the three subjects (subjects 1-3 inT able 1 with highest B M Is had low er C O RR E C Id on the higher-fat diets, w hereas all but one of the other adult subjects hadequal or higher C O R R EC U on the higher-fat diets. It is alsopossible that fats putative enhanced efficiency as a m etabolicsubstrate is seen m ainly in circum stances ofpositive energy bal-a n c e .

Finally, the physical activity of som e of our subjects w assom ew hat dim inished by their restriction to a m etabolic w ard.There m ay be interactions betw een physical activity and dietcom position that predispose to fat accum ulation (28); such in-teractions w ere not exam ined in this study.

It has been noted that obese individuals have a preference forhigh fat-foods (29). This preference, based on palatability, m ayfavor obesity by increasing total caloric intake. H ow ever, fromour results here and w ith the caveats m entioned above, such fatpreference probably does not provide obese individuals w ith anintrinsically m ore-efficient fuel source. V ariations in fat intakefrom 0% to 70% oftotal energy under conditions ofequal energyintake produced no significant changes in bodyeight over pe-nods of observation averaging 33 d. 0

These studies could not have been perform ed w ithout the generoushelp and advice of D r EH A hrens. Tw o anonym ous review ers providedhelpful comm ents.

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