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1104

TI lE AMERICAN JOURNAL O F (L IN ICA L NUTR IT IO NVo l. 20 , N o . 10 , October, 1967 , pp . 1104-1112

Pr in ted in U.S .A .

Fa t, C arbohydra te , S a lt,and W eigh t L oss

WALTER M . BO RTZ , M .D .,2 A RLEN E W ROLDSON , M S.,3 PA ULA M O RR IS , B .5 ., M .P .H .,4AN D BELA I5SEKU TZ , JR ., M .D .

I N THE PROM OT ION of e ffectiv e w e igh t lo ssth e equ iv a len cy of hypocalo n ic d ie ts

e itlie t- low in carb ohydrate o r low in fatha s been an issim e of conce rn to th e pu b licas w ell as to its a ttend in g m edica l p ro fes-s ion . W ith a reg u lar ity tha t is now nearly1 )led ic tab le , new w eigh t-red itc ing reg i-m en s ap pea r, each prom is ing prev iouslyuno l)ta inab le succes s. T hese d ie ts usu allyc ite , freq uen ti) in accurate ly , so ine sci-em stif ic ra tion a le fo r th eir ju stif ica tion .F tm ndam em ita lly , the issue reso lves in to thepropo sed! g rea te r e ffectiveness o f a d ie t h ighin fa t and low in carbo hyd ra te as opposedto o tlie i d iets o f calo ric eq u iv alence . AH urn be t- o f w orkers addressed them selvesto th is qu es tion ea rlie r , bu t the re appea rsno genem -a l consen sus o n th e w o rk to da te( 1-9 ). T hose lio have repo rted no d iffe r-ence in th e rate o f w e igh t lo s s on iso -ca lo r ic fa t and ca rbo hydra te subs titu tionco n c l imd ed tha t any d ifferences ob se rv ed b yo th e rs w ei-e d im e to the ca rb ohydrate - in -du ced re ten tion o f f lu id (4 , 7 -9 ). T heo ppo site v iew po in t den ie s th is p ossib ility(3 , 5 , 6 ). In spectio n o f th e p ro toco ls o fthese p e rtinen t stud ies h as ind icated ava riab ility in salt in tak e in th e ex pe ri-m en ts . Th e pre sen t stu d ie s w ere design edto exp lom -e the p oss ib ility th at the d if fe r-en ce in con clu sion s is d ue to the e ffec t o f

F rom th e D iv ision o f R e sear ch , La nkem au H o s-pita l , P hilade lp hia, P en nsy lv an ia .

2 R esearch A ssoc ia te , D iv ision of R e sea rch , an dA ssis tan t, D epartm ent of M edic ine. R esearch As -s i s tant , D iv isio n of R ese arch . R esearch A ssistam it ,D iv is ion of R e sea rch . H ead , D cp artm nent ofPh io 1og 1)iv is io ii o f R es ea rc h.

the lev el o f sa lt in the d ie t. In th is w orkisoca lon ic subs titu tion of fa t and carb o-hyd ra te w as p erfo rm ed in b o th the ab-sence and the presen ce of add ed sod iuman d potas sium , an d the e ffec ts o f eachsub sti tittion on th e ra te o f w eig h t lo ss ,n i trogen equ ilib rium , sod ium an d po ta s-sium ba lan ce, and setum lip ids w ere ob-served .

EX P ER IMEN TA L

Th e sub jects w ere 10 ob ese ind iv idua ls (1 9-60 yea rs o ld) . E a ch w as ov erw eig h t by an ysta nda rd and free from am iy co nsp lic atin g d is-ea se . P lsys ic al exans ina tion a nd thy ro id -func -tion te sts show ed no abnorm alities aside f romthe ob es ity . T h e sub jects w e re co nfin ed to tiseM eta l)o lic W a rd ex cep t for tw o 2 -hr pa sse s p erw eek . T h ey were perm itted ad lib itum ac tiv ityits the s le tabo lic % T ard , bu t w ere proh ib itedfrom u engag ing in an y a(Id itiona l phy sic al ex e r -tion . M otiva tiom s Wd S h igh throug hout.

L iqu id fo rm nula d iets w ere used (1 0). U pona(llis iss iofl to th e w ard , the sub jects w e re p lac edon a stab iliz ation d ie t w ith the in ten t of e stab -l ish ing c alo ric requ irem nent fo r w eig h t m ain-tem sance. In bo th the sta b ilizat iom s and ex per i-m em stal p eriods a ll d ie ts con tained 40 g ofpro teim i. Fat wa s added iii the stab iliz ationp er io d to p rov ide 35% of the ca lo r ie s . T herem ainder o f th e ca lo ries w as m ssade up fromcarbo hydra te . Its sonse of the stud ies, fo llow ingwe ig h t stab iliz ation an im ii tial 800 -cal d ie tperiod wa s included im s wls ich the fa t and ca r-bohydrate w ere redu ce! in p ropor tion to theleve l in the s tab iliza tion d ie t. T h e purp ose o ftls is w as to p revem it tlse rap id in itia l w e ig lst lo ssseen im pon ins titu tiom s o f any hypoca lo ric d ie tf rom interfering w ith the ob serva tion of anysepara te e ffec t o f fa t o r ca rbo lsydra te subs titu -

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N um bers rep resen t the m eans o f com parab leb lo od va lu es fo r a ll sub jec ts an d th eir stand arder rors.

Fa t, C a rbohydra te, S alt, and W eigh t L oss 1105

tio n. F ollo w in g th e e arly contro l pe rio ds, thesub jec ts w ere p l:tce (l on d ie ts in w hich 80% ofth e to tal calo ries w ere com sip ris ed o f ca rbo liy -d rate o r fat to th e com plete exc lus ion o f theo the r. T lsese d ieta ry pe riod s w e re 2 4 ( lay s. Insom e experim en ts no sod ium o r p ota ssi im isi w asadded , am id chcm ss ical a isa lys is o f these c !ie tss h ow ed app rox im siate ly 100 m ug of so d ium and180 m ug of p o ta ssium per to ta l (!a ily feed ing . Ino th er stud ies 4 g of sod ium chlo rid e and 3 g of ch lo ride w ere ad ded . T he ingred ien tsused w ere purified p ow dered case im i, sucrose ,corm is tarch , corn o il, am id b utter. Th ey w erean a ly zed ch em siica lly fo r confirm ation of theca lcu la ted pro te in con ten t. T ise d ie ts w ere ac -c ep tab le to the su l)je cts , w ho w ere given th reeeq u a l feed ing s s1 )re d tls rou ghou t Use clay .

E ach si tb je ct w as w eig h& ! daily 1)e forc thef ir st feed in g . E v ery 6 days d uring tlse ex peri-m ent b loo d w as co llec ted fo r de te rns ina tion s o ffree fatty acids (1 1 ), trig ly cerides (12 ), andcholestero l (1 3). C orrespom sd ing co llec tions o fim rim ie % ere tak em s an d itr inary n itro gen (1 4),sodiim m ss (1 5), and po tas sium (16) w ere m ea sured .Iii im istam ices w here salts w ere added to the d iets ,da i ly u rin ary co lle ctions w ere de term in ed forsoc ! i um u am id po tass ium m easu re issem st at th set im e o f d ie tary chan ge. C rea tin ine w as alsode term im secl a s a che ck on th e con sp leten ess ofco llectiom s (17). A t imitervals during the exp er i-nsem it, oxyg en com isum ption and carb om s d iox ideprod uc tio n w ere m easured an d RQ valu esc!e term n im sed (1 8).

RESULTS

F ig ures 1 -5 show th e w eigh t respo nse tova rian ce in the d ie ta ry com ponen ts in theabsence of an y ad ded so d ium or p o ta s-s i um . Chang e in the d ie t w as m ade as in -d ica ted in th e figu re s. I t is seen th at a sid efrom ran dom varia tion , w e igh t lo ss w aslinea i w ith tim e w ith re sp ec t to iso -calo ric s itb stitu tio n of fa t fo r ca rboh ydra tean d v ice versa . E xc lu d ing th e early rap idw eigh t lo ss o f th e firs t h ypoca lon ic period ,the m ean da ily w eigh t lo ss fo r a ll fa t d ie tperiods wa s 0 .59 lb ., w h ile th at fo r theca rboh yc !rate pe rio d w as 0 .55 lb .

Supp lied as C asec by M ead Johnson Labora-to rie s, Ev an sv il le , Ind ian a.

TABLE IEffect of D ieta ry Fat-C a rbo hydra te

In terchange on Se rumL ip id L eve ls

FFA ,m E q/liter

T riglycerides,m g/lot) m l

C holesterol,mg/ tOO ml

Stab ih iza- 0 .66 0 . 7 1 6 3 3 . 1 1 7 9 3 . 9t ion

M ix ed 800 0 .86 0 .3 138 2 .6 1 77 2 .9cal

H igh fa t 1 .1 9 0 .4 10 8 1 .1 1 56 2 .1H igh carbo - 0 .83 0.4 16 1 4 .0 173 4.0

hydrateR estabiliza- 0 .72 0 .6 1 56 4 .3 16 7 3.1

tion

Figures 6 -1 1 show the w eigh t resp onsesto isoca lo r ic fat and ca rb ohydra te su bstitu -tion in the p re sence of an ad ded 4 g o f so -d ium ch lo ride and 3 g of p o ta ssirmmch lo ride . F igu re 6 show s the resp onse inpat ien t C C , w ho also is rep re sen ted inF ig . 5 . It is seen tha t the re spon se in th isf igu re an d th e o th ers w as d iffe ren t fromtho se in w hich no sa lts w ere ad ded . U ponthe in trod uc tion of 1 60 g of ca rbo hyd ra tein to the d iet, a w eig h t p la tea it genera llyoccurred . T h is is pa rticu lar ly ev id en t inF igs. 6- 8 in w h ich the h ig h -fa t p e riod fo l-low ed the in itia l m ix ed d ie t 800-ca lper iod . That th is w eig h t p la teau w as self-lim ited , how eve r, w as ev id en t from theob se rv atio n th at in a ll sub jects w e igh tlo ss resum ed once aga in in the la te r phaseo f th e ca rboh ydra te feed ing pe rio d . A s theloss recu rred , it ap pea red to be a t ap -prox im ate ly the sam e ra te as th at w hen onthe fat, a s n o ted in the ex pe rim en ts w ith -ou t sa lt added . C onv erse ly , in th e p re s-ence o f add ed d ie tary sa lt, w hen fa t w asre in troc !uced and carboh yd ra te w ith -d raw n , a pe riod of m ore rap id w e igh t lo s sw as ob served , w h ich then le ssened un tilth e ra te o f loss w as as no ted in the ea rlierfeed ing pe riod .

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07140

160040

1106

SubJ L RCalories 3 0 0 0 4

C HO 44& 6 0F A T 1 1 7 0

PR O 40 40240

230

220

210

200

non

71

40

2 ) 4 2 C o I o n e s 9 , O H2 6 9 C HO 44 81 0 1 F A T 1 1 740 P R O 40

180

ITO

S60

- 50 , , , , , , ,0 0 20

160

40

30 40 50 60

Doy s

426log

- -V.

0 20 30 40 50 60 70 80 90Days

.. - 80 060 0 600 71 0

40 40 40

07140

5380

0 160 6071 0 0

40 40 40

19 0

80

C C

C alories 4 -C R 0 0

D iet F A T 7 1PR O 40

56 0

55 0

54 0

53 0

52 0

6 510

50 0

49 0

48 0

47 0

46 0

45 0

26 b 20 30 40 50

Days60 70 80 90

0740

160 00 71

40 40 5 Ocys

0 0 20 3 0 40 50 60 70 80Days3

Further light wa s shed on these obser-vations upon analy sis o f the sodium andpotass ium ex cretion data. A s w ould beantic ipated, the sodium- plus po tass ium-free die ts w ere accompanied by neg lig ibleurinary excre tion o f these catio ns. Figures12-17 show the results o f the urinary cx-

Fmcs . 1 -5 . D aily w e ight lo ss oh 800 -c al die ts w ith-om it a d d ed Na C l o r K C 1 .

cre tions w hen dietary sodium and potas-s iitm w ere included.

Figures 12-14 show the urinary balance swhen the high-fat period fo llow ed themixed 80 0 -cal period. In each ins tancetlse ie w as an im iitial sodium lo ss that w asvaiiable in duration. In Fig s. 15 -17 thehigh-fat die t perio d is charac terized by asodium diure sis . In addition the initial800 -cal m ixed die tary period show ed auniform sodiitm lo ss. \V hetlser this w asdue to calorie res tric tion per se or to areduction, but not an e lim ination, in car-

B ortz e t al.

SabI SRCones 2800

CR0FA T

PR O23 0

220

210

200

C a lo rie s 2 5 00 C H O 3 6 6

Diet FA T 97 P R O 4 0

26 0

25 0

240

230

220

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N

5 O H

Caloses 3 6 0 C

DA t

32 5

31 5

30 5

29 5

s285

27 5

26 5

1

9 Da y s

90 too ic

S u b s CCC a lo rie s 5 00 0

C R 0 773 90 0 60 0D iet FAT 94 3 1 71 0 71qm PR O 40 40 40 40 4067 5

6 1 5

60 5

59 5

58 5

57 5

565

55 5

54 5 , -.-- -,---- . .0 0 2 0 3 0 4 0 5 0 6 0 7 0 9 0

6 Days

C A o r e s 3 0 0 0 - - -- - -- - -- - 80 0 - -- - -- - -- - -- - -- -

9,, PR o 40

325

3,5 N

l0 0

Calor ies 270 0 800 FCR0 3 9 9 9 0 0 6 0

Diet FAT 05 31 71 0a 40 40 40 4 0

27 0

26 0

2 5 0

240

230

22 0 ....,0 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0

10 Days

5o5 NC alories 4000

. C H O 60D,::,t F AT 56PRO 40

410

400

39 0

38 0

37 0

36 0

35 0

34 0

11

- - - -8O0--- - - - - - -90 60 031 0 714040401

08 Ct O 20 30 40Days

FIG s. 6-11. D aily w eight loss on 800-cal diets wi t had ded N aC 1 and K C 1 .

8 Days

Fat, C arbo hy drate , S alt, and W e igh t L o ss 11 07

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S1411 CCC alories 5000 800

Slob b lued H igh F ot H igh C H O H igh Fat

12 00,5 5 Ocy s

SoN C CC iries

qN m

3000Stab, h 6 d

::k

80 0H i qi t P o t H igh C H O H igh

h-P 2600

F at R e sio b

-100

l o : }

S c c

Calories 3000 4 8fl0-S ob M a tS H i g h CR 0 Hi h Fo t p 2 6 0 0H i g h C HO Resiob

I I-

m(q #{163}.

00

IOOF i-200

50

K ,(q J

16-

6 20 40 60 80Day s13

S ob i J PC a l o r i e s

-

T T T.T T

l

00a 20 40

2700 4 800 800S tab M ixed H igh Fat H,gh C R 0 Restob

60Days

80 I#{212}O 20

H ig h F ol

U T I-- - I

1 4 Days

U0 20 40 60 8 0

17 Days

F ios. 12-17 . E ffect of dietary fat and carbohydrate interchange on u r inary n itro gen , sod ium , and po-ta s sium . The X axis represents the equilibrium betw een intake and u rin ary ex cretio n.

1108

NCqm

Si 41 A NCa l o r i e s 4000 4- 800 -

S lob M ixed H igh C H O

0 gin

00

N a mnE q

-100

50K

rnfq 0

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Fa t, C a rb ohydrate , S a lt, an d W eigh t L oss 1109

bohydra te ca lo rie s is un clea r from th esestud ie s . C onv erse ly , in a ll cases in w h ichth e h ig h -ca rboh ydra te d ie t fo llow ed th eh igh -fat d ie t, sod ium re ten tion w as ap -paren t .

N et p o tassium excre tion tended topa ralle l u r ina ry sod ium exc re tion , p ar-ticu la rly in th e p eriod im m edia tely fo l-low ing th e chang e in d ie t. U rine vo lum epa ra lle led u rina ry so d ium ex cre tion . C lo sean aly sis o f the w eig h t, so d ium , and w a te r-exc re tion d ata in d icate s , how eve r , tha t th eab ove -ou tlin ed chang es no ted in d ieta ryfat an d ca rb ohydrate substitu tion a re o ftenon ly tem porary phenom ena . T en denc iestow ard w e igh t stab iliz atio n and sod iumreten tio n w ere no ted in th e late r phases o fthe fa t-feed ing pe rio ds in th e absen ce o fd ieta ry ca rbo hydra te . C onve rse ly , w e ig h tand u rin ary so d ium lo ss gene rally ap -p ea red in the la te r pa rts o f th e ca rbo -h ydra te-feed in g pe rio ds. I t seem s like ly ,th ere fo re , th at seve ra l se rvom ech an ism sa re op era tive w h ich appear to seek aso d ium or ex tracellu lar f lu id vo lum e equ i-lib r ium , and tha t d ie tary in f luences o n th isa re rela tive ly se lf-lim ited in dura tion .

A na lys is o f the n itrogen exc re tion da taind ica ted the tend en cy tow ard n itrog enre ten tion ev en on m arked calo ric - an dp ro tein -re stric ted d ie ts (F igs. 1 8 -22 ) . T h isw as p articu la rly appa ren t in the ca rboh y-dra te -d ie ta ry period s b itt w as also seen in thefa t p eriods . N itrogen exc re tio n seem ed topa ra lle l ro ug h ly sod ium and po ta ssiumex cre tion . T he re a lso app ea red to be a co r-re la tion b etw een the tim e tha t ke tonu riaclea red and the m ark ed n itrogen con se rv a-tion s no ted o n th e carbo hyd ra te fo rm ula .The 1)to te in ba lances seem ed to d ep end onthe carbohyd ra te co n ten t o f th e d ie t an dw e t-c ind ependen t o f the inc lusion o fso dbum and po tass ium itt the d ie t.

Se r tmm lip id s likew ise show ed va riance7 Observ an ce o f all R Q valu es show s li ttle if any

v aria nce betw een the d ie tary reg im en s an d ind icatesnearly exc lus ive o xida tion of fa t a s th e en erg y source.

w ith the isoca lo r ic su bstitu tio n . T h is w asob se rv ab le by a drop in the free fa tty ac idsan d a rise in the tr ig lyce rid es on p re sen ta -tio n of the ca rbo hyd ra te , no fa t, d ie ts .S erum cho le ste ro l seem ed low er on the fatd iets , poss ib ly du e to the co rn o il in thesefo rmulas.

A dd ition al s tud ie s to be repo rted late r ,us in g fo rm ulas w ith 8 0 g o f p ro tein bu tw ith o the rw ise s im ila r p ro to co ls , con firmth ese da ta .

DISCUSSIONThese stud ies sup po rt the fund am en-

ta lis t co ncep t o f the equ iv alency of food -s tu ffs as sources o f energ y . M o re spec ifi-ca lly , th ey deny the m e tabo lic su pe rio rityo f a low -ca rb ohydrate d iet in the p rom o-tion of w eig h t lo s s .

In 1 932 , L yon an d D un lop reported in astudy in vo lv ing 3 5 sub jec ts tha t w e igh tlo ss w as les s w h en ca rb ohydrate w as sub -stitu ted fo r fa t (1 ) . In 1951 , P enn in g tonprop osed a h igh -fa t d ie t as a m ore ef-fec tive w e igh t-lo s ing reg im en (3 , 19 ). H ea ttrib u ted th e advan tag e to the insu linstim u la tio n and co nsequ en t inc rea sedfa tty acid syn thesis ob se rv ed on ca rbo -hy drate d ie ts . In ad d ition , h e felt tha tth ere ex isted in ob esity an ab norm a lity inp yru va te ox id atio n w hich in tu rn led toan inh ib itio n in fat u tiliz a tion . A s fa r a s isk now n , th is hy po th es is has neve r beencon firm ed . In 1 955 , \\T e rn er rep orted ac lin ical s tudy in w hich su b jec ts o n P enn ing -to n s ow n d iet lo s t w e igh t at a ra te equ iv -alen t to an isocalo ric on e co n ta in ing aneq u iva len t am oun t o f ca rboh ydra te (4 ) .

K ekw ick and P aw an h ave rep eated lyrepo rted th e adv an tag e of a h igh -fat d iet inthe p ro duc tion o f w e igh t lo ss . The iro p in io n is b ased ! o n a n um ber o f stud ie sbo th in the labo ra to ry and in th e m e ta -bo lic w ard (5 , 6 , 20 , 21 ). In a s tudy in -vo lv ing ca rbo n b alance in m ice they re -po rted th at fa t feed in g pro vok ed w as te o fcalo rie s v ia ke tone loss in th e u rin e tha t

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S u b j L .RCa l o r i e s

Ngm

30000 800 02100Stab H igh C H O H igh F at H igh C H O R estob

2L I.

-4o 20 40 60 80

18 Day s

SubI S 80Ca lo rie s 3 00 0 4 800 2800

Slob H i g h Fat H igh C H O R eslob

21 Days

Sutm SR.Ca l o r i e s 2 8 0 0

Stab.

N A L - -0

192 0 4 0 6 0 8 0

Da y sI # { 2 1 2 } O Days

am Day s

1110 B ortz et a!.

4 800 P2500Hig h CHO Hig h F a t Hig h CHO Re siob.

I.

% as hot I)1)alent 015 carbolsydm ate feeding(20 ) . In the 1)tesetit study, ketonuria w asapl)alent onl on the fat diets, bitt even atits lssO St Islarked level it w as insignificantlien otssidetcd as a sottrce of calories.Q m iam iti tative analyses of ilrinary ketone-hod!)- contem st carried out in periods of m ax-im sl kctom ium iia show ec! the presence of only})1oxim siate1y 2 g of acetoacetate cx-(-reted P 2- I hr.

K insell C t a!. (9), hoivever, w ere criticalof the hullltn expeiim ents of K ekw ick andPaw am i for the shoi-tness of their periods ofobservation, 5-9 days. The form er reported! study of four patients w hich ind!icated no

S ob 1 C CC alories 800

H igh Fat H igh C R 0 H igh C R 0 H igh F at

1 1 ( 5 . 1 8-22. E fF ect of (Iictarv f at am id car l) oh vd rat einterchange 013 urinaiv nitrog..-n. .Fhe X axis repre-SC1)tS I he equilihm iunl betw een nitrogen intake am idii i- i mia mv (.X(IttiOli

(hifference ill eight loss 1sen fat w asStlJ)StitiltC (! for car l)ol)y(llate and vice v ersaon h1yO C a1O lic c!iets (9). These w orkers(hi(I hot 51)CC ifV the exact L l55O tlt1t O f SO -(1iL1 151 ill their diets. R ussell and Pilkingtonet a!. (7, 8) have ieported! sttm dies in w hichthe ifltlO(Itlctiots of carbohy(lrate resultedill a Period of eight stabilitatiom i or evenof eighit gain. This w as self-lim ited, lll)W -ever, in 1l cases. Their diets containedal)I)lecial)le sodium aiid at-c therefore com -paral)le to Ivirt of the l)leSC Ist study. Thew eighs t-loss curves iecotded in thei t peisiesem isblc closely otit ow m s its the sodiuni-containim sg i-egim ens.

O n the othei hand, in the absence ofa!ded salt, iseiglit loss appeals to be con-stant regatc!less of the nature of the c!ietarycom sstituem sts. It is interestiisg to observethat one ieducing diet of som e renow n isthe rice-frim it diet w hicls of coim m se is m searlyentit-ely call)olsyd!rate. T he soditm m con-tent of this diet is vei-y low , how ever.

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Fa t, C a rbohydra te, S alt, and W eigh t Lo ss 1111

Tha t th e w eigh t p la teau of ca rboh ydra tefeed ing is the resu lt o f sod ium and flu idre ten tion is seen by th e so d ium -exc re tionda ta o f th is paper. T he fina l p ro of o f th isp ro posa l d epends on m easu rem en t o f to ta lbod!y s a te r U nfor tu na te ly , p resen t tech -n iques a re s till n o t su ff icien tly p rec ise toenab le m easurem en t o f th is com partm en ta t the neces sai-y leve l o f d isc r im ina tion .The sod ium -re ta in ing p o ten tia l o f ca rbo-hyd ia te is c lea rly show n. It is in te restingto no te , how ever, tha t even on our carbo-hydrate -con ta in ing d ie ts the predom in an tm etabo lite is still fa t, in th is case of end og-eno im s orig in . I t w ou ld be of in te t-est tode tet-m in e w he the r the re is a c ritic a l d ie-ta ry leve l o f carbohydra te w here sod iumre ten tion b ecom es m an ife st, an d a lsow h eth er th is lev el co rre la tes w ith any o th erobservab le m etab o lic p aram ete r, i.e ., he -pa tic g lucose ou tp u t, ren al g luco se con -sumption , etc.

Th e sp ec if ic m ech an ism o f th is carbo -hydrate e ffec t on sod ium re ten tio n is a tl)Iesen t um sknow n. C lsanges in ke to nuria ,to ta l u rina ry ac id , a ldo ste rone secre tion ,an d rena l tub u la r so litte lo ad have b eenpropo sed by prev iou s stud ies as p oss ib lem echan ism s to exp la in the chan ges in so -d ium and w ater excre tion fo llow ing carbo-hydrate feed in g or ca rbohydra te w ith -d raw al. N o s ing le hypo thes is fits the da taav ailab le . T he ro le o f so d ium as an ob li-ga te ca tion cover fo r u r in ary ke toacid sp roduced on the fa t d iet o r du ring sta rva-t ion is no t sub sta n tia ted by lack of tim eco inc idence o f these chang es im pon re -p res -en ta tion of ca rbo hydrate (22, 23). Simi-lar ly , co rrec tio n w ith so d ium bicarbon a teof the ac ido sis o f starva tion d id n o t d im -m ate the sod lium loss (24). O ther s tu d iesind ica te !)a la lle lis ID , h ow eve r, be tw een so -d ium loss an d ke tone an d titra tab le ac idexc re tion (25 ).

A ld !os teron e as a p ossib le con tro l fac to rhas been cons id ered in th is connec tion .O ur find ing o f rou gh ly pa ralle l so d iumand po tass ium excre tion speaks aga ins t

such a co n tro l. S p irono lac to ne a lso hasb een fo und no t to d im in ish the sod ium -re ta in ing e ffec t o f ca rb ohydrate (26 ).

A nother possib le m echan ism pro posedfo r the an tina triu re tic e ffec t o f ca rboh y-cira te feed in g is the d ecrease in ov era llso lu te exc retio n fo llow in g ca rb ohydra tein gestion . Su ch a decrease in so lu te loadm ay resu lt in a low er sod ium excre tion .T h e fa ilu re o f u rea in fus ions to e ffec t asod ium exc re tion du rin g ca rb oh y-d ra te feed in g seem s to ru le ou t th is hy -po thesis (27 ).

T he re fo re , to da te the re is n o h ypo thesisw hich su its a ll observ ed even ts. O ther andpossib ly m ultip le fac to rs are invo lved .U ltim ate ly som e effec t o f g lu co se on them em brane transport o f sod ium is p rob-ab ly o pera tiv e.

U nfortuna te ly , m any d ie ts p ro po sed toth e pu b lic lay cla im to the sc ien tific va lid -ity len t them by s tu d ies such as those c itedabov e. T he pre sen t w ork co nfirm s ea rlie rrep o rts , how ev er , th at the on ly m e tab o licadvan tag e o ffered b y a low -ca rb ohydrated ie t is its dehy dra tin g po ten tia l. U pon re -p re sen ta tio n o f ca rb ohydrate in th e p res -ence o f sod ium , th is w a ter d eb t m ust berep a id . O n the basis o f th is w ork it is fe ltby the au thors th at a m ixed d ie t is one inwhich flu id sh ifts an d th e ir co nsequ en tperiods of confu sion and a lte rna ting de-ligh t and despa ir are o ffse t, an d is the re-fo re one lik ely to be of m os t su sta inedbene fit to the o bese ind iv idu al.

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