Am J Clin Nutr 1978 Cummings S21 9

7/27/2019 Am J Clin Nutr 1978 Cummings S21 9

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T he m er ica n J ou rna l o f C lin ica l N u tr itio n 31 : O TO ER 1978 , pp . S 2l-S2 9 . P rin ted in U .S .A . S2 1

Nu tr itio na l im p lica tio ns o f d ie ta ry fib e rJ o h n H C u m m in g s 2 M S c M B M R C P

A BSTR A CT W hen dieta ry f ibe r in take s are increased b y su pplem enting d iets w ith b ran an dwhole w heat p rod uc ts, then feca l fa t, n itrogen , en erg y , an d m ineral e xc retion ris e . T hese change ssugg es t th at fibe r m ay be alte ring no rm al d ig est ive and ab sorp tiv e fu nction . R ecen t stu d ie s havecon firm ed th is and have also shown th at fib er o f d ifferen t com po sition and from con tra stin gsources pro du ces d if feren t phy sio log ica l ef fects. T he g el-fo rm in g poly sac cha ride s su ch as gua r gumand pectin a lte r th e pat tern of g luco se ab sorp tio n and are h ypo ch oles tero lem ic : fiber f rom cerealsis no t hy pocho lestero lem ic b ut ex erts a p ron ounced e ffec t o n the la rge gu t. D ie tary f ibe r is larg elyd ig este d in the co lo n b y the m icro flo ra and so in flu enc es co lo n ic fu nction , fe cal w eig h t. an dcom po sit ion . Th e s ign ific an ce o f the ch ang es in fa t, n itrogen , and energy o utp u t rem ain s to b eeva lua ted , bu t the im pa irm ent o f m in era l ab so rp tion-par ticu lar ly of calc ium , zinc, and iron -byf ibe r g iv es cause for co ncern . F ibe r m u st n ow be co nsidered w ith o th er d ieta ry co nst itu en ts in allnu tritiona l stu d ie s. m J C lin Nu t r 31: S 2l-S 29 , 19 78 .

In add itio n to the inc rea sing ly w e ll-d ocu-m en ted e ffects f ibe r has on co lon ic fun ctio n ,it is now c lea r th at f ibe r is a nu trien t o fim portance in its ow n rig h t. T he am oun t an dty pe of fib er in clu ded in th e d ie t m ay h avesign if ican t n u tr itiona l im p licatio ns. T ab le Ilis ts the a rea s in w hich pub lished ev idencesugg ests th at fib er a lte rs the d igestio n ab so rp -tio n o r su bsequ en t m e tabo lism of va riousnu tr ien ts . A t the p resen t tim e, o ne w ou ld beu nw ise to stud y the n u tritio na l con seq uenceso f any g ro up o f sub stances in the d iet w ithou tin clu d ing som e refe rence to the ir in te rac tionw ith d ie tary f ibe r . W ith th e w e ll-p ub lic izedsug gestion tha t fib er de ficiency is a m a jo rp ub lic h ea lth p rob lem , expans ion o f resea rchin th is fie ld is in ev itab le. It is the re fo re im -p ortan t to e stab lish c lear p rio r ities . A lthou ghit is rela tive ly easy to show tha t f ibe r h as ane ffec t o n a pa rticu la r a sp ec t o f nu tr ition andnu tr ien t m etabo lism , it is m ore d iff icu lt andm ore im portan t to m easu re the sig n if icanceo f these ch an ges, p a rticu la rly in the lo ngt e rm .

T rad ition al co ncep ts o f n u tr ition a re o f noh elp in consid era tions o n the ro le o f d ie ta ryfibe r . F ibe r can no t b e te rm ed an essen tia lnu tr ien t, an d it is no t ea sy to asce rta in m ansda ily requ irem en t fo r so va ried a sub stan ceas fib er . S tu d ie s w ith e lem en tal d ie ts d em -on stra te tha t m an can h iv e satisfacto rily fo r 6m on th s on a fibe r-free d iet 1 , the o n ly no -tab le ch an ge be ing a d ecrease in sto o l b u lka nd freq ue nc y.

W he the r a long -te rm requ irem en t ex ists fo rf ibe r in m an rem a ins to b e answ ered .

FatF ib er a lte rs lip id abso rp tion . W hen w ho le

w hea t p ro duc ts such as b ran a re fed , feca l fa tex c re tion rise s . F ig u re 1 sh ow s fecal fatty ac idexc retio n m easu red b y gas liqu id ch rom a to g-rap hy in six m ed ical s tud en ts d u ring the 3 rdw eek o f a m e tab o lic s tudy in w hich th ey a teeith er a co n tro lled d ie t o r a d ie t to w hichw ho le w heat p rodu cts w ere added 2 . F ib erin tak e on the con tro lled d ie t w as 17 g /d ayand w as inc rea sed to 4 5 g /day b y ex chang in gw hite fo r w ho lem ea l b read and A ll B ran fo rco rn flakes, and by ad d ing a b ran b iscu it and3 0 g of b ran . A sm a ll chang e in fatty ac idin tak e from 86 to 9 6 g /day occurred , bu tfeca l fa tty ac id ou tpu t in creased sign if ican tlyfrom 1 .7 to 2 .7 g /day . F at exc retio n m easu redby the V an d e K am er techn ique 3 ro se from3 to 4 .5 g /day w ith the add itio na l fibe r .

T h is inc rea se in fecah fa t is no t o ne th atw ou ld be consid ered sign ifican t in te rm s ofo ve ra ll d ig es tib ility and abso rp tion of fat, n o rd oes it rep re sen t a p ath o log ica l ris e in fecahfat ex cre tion in g as troen teroho g icah te rm s. Itis , how eve r, a w e ll-d escr ibed phenom enon .

F rom th e M RC D unn N utrition L abora to ry , M iltonR oad , C am br idg e CB 4 1X J , Eng lan d .

M em be r o f sc ien tific sta ff, D unn N utri tio n L abora -to ry an d hono rary co nsu lta n t p hy sic ian , A dd enb roo kesH os pita l, C am bridg e.


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Su gg est ed n ut ri ti on alimplications

Relevantb ib l iography

Shor t-term requi rements for (1)f iber

L ong- term def i ciency states W idely suggested butas yet unproven

Ef fect on grow th U nknow n for manA bsorpt ion and metabol i sm

Carbohydrate (17-27)Protein (2, 4-6, 9, 12)L ipid (2, 4-8)

E f f ect on appeti te, sat iety , See other papers in thi sand obesi ty sy mposi um

B i le acid and cholesterol me- See other papers i n thi stabol i sm sy mposi um

V i tam i ns (39-44)M i neral absorpt i on

Calcium (45-62)Iron (73-77)Z i nc (47, 63-72)

parti cularl y about the ef f ect of f i ber f romcomm only eaten f rui ts and v egetables.

A t the m oment w e can tentati v el y conchudethat the ov eral l balance of f at absorpti on isnot ser iousl y al tered by f iber but that sm al li ntesti nal ev ents leading to l i pid absorpti onare af fected. I n addi ti on, some f atty acidsm ay be associated w i th f i ber in such a w ay asto lead to thei r m alabsorpti on.

A n increase in ni trogen as w el l as f at ex-creti on occurs w hen w hole w heat productsare fed (Fig. 2) (2, 4-6). T his increase in fecalni trogen is readi l y detectable but not nutri -ti onal l y signi f i cant in W estern industr i al i zedcomm uni ti es w here high protein intakes areusual . I t may of course be more important insocieti es such as those in rural A f ri ca w heredietary f i ber intakes are considerably higher.

T he source of this fecal ni trogen remainsto be establ i shed. T w o suggestions are that i trepresents ei ther an increase in endogenousni trogen excreti on (5, 6) or i s due to thepresence of unav ai l able protein complex es (9)that pass through the gut undigested. T hesecomplexes are f orm ed during cook ing orother pretreatment of food bef ore eating andresul t f rom the degradation, condensation,

DIET

3

2

ME AN 1 7 2 7FIG . I . Fecal f atty aci d excretion in g/day (m ea-

sured by gas l i qui d chrom atography ) i n six m edi calstudents during the 3rd w eek of m etabol i c diets of stan-dard W estern ty pe food or the sam e diet w i th w holem eal bread, A l l -B ran, and bran biscui ts (2).

C O NT R O L 8 6 F IB R E 9 6

M cCance and G laser (4) and M cCance andW alsham (5) noted fat absorpti on to be be-tw een only 54 and 62% complete f rom variousvarieti es of w hole w heat bread and onlysl i ghtl y greater f rom oatm eal . T hese f iguresare w el l below norm al fat absorpti on coef f i -cients observed w hen tradi ti onal W esternsty le food is eaten.

D oes thi s change represent a true al terati onin the digesti ve and absorpti v e processes forfat i n the gut? O r i s a proporti on of the fatassociated w i th f i ber in such a w ay as to beunav ai l able for digestion? A l ternati v el y , doesthi s represent an increase in endogenousl i pid excreti on as suggested by W alker (6)?D etai l ed analyses of the f ecal f atty acids inour studies have suggested that at l east partof the f ecal fat consti tutes fatty acids associ -ated w i th f i ber in such a w ay that these acidspass through the gut undigested (H . S. W ig-gins, personal communication). K ay andT rusw el l (7) have show n that 15 g/day ofpectin can double f ecal fat ex creti on, andJenk ins et ah. (8) have show n guar gum alsoincreases fecal fat al though both the pectinand guar as f ed are l i pid f ree. T his suggeststhat the actual process of l i pid absorpti on canbe al tered in some w ay by f iber al though themechanism remains unclear. Few studieshave been done that throw much l ight on thi sf i nding, especial l y ones in w hich f iber otherthan that f rom w hole w heat or cereal prod-ucts i s used. M ore information i s needed

S22 CU M M IN GS

T A B L E 1N utri t i onal i mpl i cations of f i ber i nm an w ith selected bibl iography

Nitrogen

F A E C A LFAT T YAC I DS

9 d a y


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1-25 201

N U T RI T IO N A L IM PL ICA T I ON S O F D IET A RY FIB ER S23

D IE TA R Y N C O N T R O L 1 3 6 F IB R E 1 5 l

2F A E C A LN I T R OGE N

9 d a y

M E AN

1

FI G. 2. Fecal ni trogen excretion i n g/day f rom samesubj ects as in Figure 1.

and polym eri zati on of carbohy drates andam ino acids (10). V an Soest (11) has sug-gested that up to a thi rd of the total proteinin some break f ast cereals may be bound upin these complexes. T hey are know n as M ai l -hard complexes and interf ere w i th hignin m ea-surement.

Fiber may also interf ere w i th the normaldigesti ve and absorpti ve pathw ays of protein.I t has recentl y been reported that in v i troacti v i ty of trypsin and chy motrypsin can beinhibi ted by v ar ious plant f i bers (12).

A f urther possible explanation f or in-creased fecal ni trogen excreti on i s that i t rep-resents an increase in fecah m icrobial f l ora.A l though experiments using caref ul quanti -tati on of f ecal m icrobial f l ora w i th high-f i berdiets have so f ar fai l ed to show signi f i cantchanges in the f lora (13-16), the preci sion ofthe techniques used in these studies does notrule out the possibi l i ty of i ncreased numbersof m icrof l ora in the feces of f i ber-supple-m ented diets.Carbohydrate

I t m ust have come as a surpri se to m anypeople in 1972 to f i nd T row el suggesting thatdiabetes w as a disorder related to our f i ber-def i cient diet (17); how ever, the relati v el y lowfasti ng blood sugar levels in rural A f r i canpeoples (18) and the rari ty of thi s condi ti onin com muni ti es consum ing high-f i ber dietshav e led sev eral people to take thi s suggestionseriousl y . I t i s now clear that dietary f i ber can

al ter the process of carbohydrate absorpti onin man. Jenk ins et ah. (19) have show n thatadding 14.5 g of guar gum to a l i quid testm eal containing glucose or 10 g of pectin toa sol i d test meal containing bread, butter,m armalade, m i l k , and tea, or a com binationof these substances, leads to a signi f i cantf l attening of the blood glucose response tothese test m eals and that thi s i s accompaniedby a signi f i cant low ering of serum insul i nlev els measured sim ul taneously . A sim i lar butless str i k ing change in blood glucose af ter atest m eal containing bran w as show n by Jef -f reys (20), w hi l e bagasse and w ood cel l ulosetended i f any thing to have the opposi te ef f ect.T his w ork has been ex tended by Jenk ins eta . (21) to diabeti c subjects. T he response inblood glucose to a sol i d test meal containingboth 10 g of pectin and 16 g of guar gum w asconsiderably reduced in both insul i n-depend-ent and noninsuhin-dependent diabeti cs; sim -i l ar changes in the serum insul i n responsesw ere observ ed in the hatter group.

T hese exper im ental changes are upheldw hen diabeti cs are f ed diets containing in-creased am ounts of dietary f i ber (22, 23).A l though in these dietary studies the intakesof f at, protein, and carbohydrate have al lbeen al tered in addi ti on to the change in f i berintake, i t i s clear that insul i n requi rementscan be reduced and f luctuations in bloodglucose m inim ized by these regimens. T heproblem of the etiology of diabetes remains,but thi s w ork prov ides a rati onal basis for anal ternati ve form of treatment.

T he w ay in w hich these changes in glucoseabsorpti on are brought about remains to beestabl i shed. Jenk ins et a . (19) have suggestedseveral w ay s thi s could occur. Fi rst, glucoseabsorpti on rates could be al tered through thegel - form ing properti es of the w ater-solublef racti ons of f i ber al ter i ng the rate of di f f usionof glucose in the upper smal l i ntesti ne. G lu-cose absorpti on w ould, therefore, be show erand w ould be spread out along a greaterlength of intesti ne, thus enabl i ng ti ssue up-tak e to keep pace w i th absorpti on af ter ini ti alstim ulati on of insul i n release. T hese authors(19) dem onstrated no increase in the ev olu-ti on of breath hydrogen der i ved f rom thecolon so that ov eral l glucose mahabsorpti ondoes not occur. Second, f i ber may ef f ect therelease of gastrointesti nal horm ones andmodi f y pancreati c secretory and digesti v e


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S24 CU M M I N G S

processes. A l ternati vel y , changes may occurin gastri c empty ing or in sm al l i ntesti ne tran-si t time. M outh-to-cecum transi t time, asjudged by the evoluti on of breath hy drogen,may be delayed by these f orms of f i ber (24).T he l i kel y m echanism probably relates tolocal i nteracti ons of these v ar ious foodstuf f sin the sm al l i ntesti ne rather than to long-termchanges in the body s m etabol i c response toa glucose load al though the ev idence is some-w hat conf l i cting on thi s point (25-27). W hat-ev er the mechanism , how ev er, the observ a-ti on that carbohydrate absorpti on i s al teredby f iber remains a val i d one.

F ib er d igest ionI n a considerati on of the nutr i ti onal impl i -

cati ons of dietary f i ber, i t i s important toremember not only that f i ber af fects the ab-sorpti on of other nutr i ents but al so that i t i si tsel f digested in the human gut. Problem sw i th m ethodology hav e considerablyhindered research in thi s f i eld, but digestionof f i ber has been show n f rom sev eral sourcesincluding bran (28), m ixed diets (29), assortedvegetables (30), cel l ulose (31), and possibl ypectin (32). Southgate (33) pointed out thathum an capaci ty to digest f i ber v ar ies w idely ,but in general at l east hal f the f i ber i s digestedin the gut, most probably by the colonicm icrof l ora. T he breakdow n products of f i berinclude short-chain f atty acids, hydrogen,methane, and carbon diox ide.

T he impl i cati ons of thi s are clearl y im por-tant and as yet incompletel y understood. O nekey aspect that has receiv ed some attention i sthe role of f i ber in fecal bulk ing. W i th ev i -dence now accumulati ng that short-chainfatty acids are absorbed f rom the hum anlarge bow el (34, 35) as they are f rom theanim al colon, i t seems l i kel y that the abi l i tyof a particular f i ber to resi st digesti on w ouldbe closely related to i ts f ecal bulk ing capaci ty .O ther im portant al terati ons in colonic phy si -ology are also l i kel y to occur.

T he f actors control l i ng the break dow n off i ber in the hum an large bow el are uninves-ti gated, but, i f any paral l el s can be draw nw i th the nonrum inant animal , the composi -ti on of the f iber, i ts degree of higni f i cati on,mean transi t time through the gut, and theintesti nal m icrof l ora are al l l i k el y to be im-portant (36-38).

V i t am i n sFew investigators hav e turned thei r atten-

ti on to the possible ef f ect of f i ber on v i tam inmetabol i sm and absorpti on in the gut. W i ththe know n capaci ty of f i ber to adsorb organiccom pounds and the general associati on ofmalnutr i ti on and v i tam in def i ciency w i th sta-ple diets containing large amounts of f i ber,thi s seems a reasonable area f or research. I nv i tro studies have show n that v egetabhe ma-terial i s able to adsorb fol i c acid and i tsmonoglutamates (39), but no impai rment offol i c absorpti on f rom f iber-r i ch cereal prod-ucts occurs w hen f ed in test m eals to man(40). I ndeed i t w ould appear that high-f i berdiets stimulate intesti nal m icrobial synthesisof som e B v i tam ins (41, 42). R ats f ed veryhigh supplements of pectin or cel l ulose hav ebeen show n to have im pai red v i tam in B 12absorpti on, but i t i s doubtful i f these sort ofstudies w i l l hav e many paral l el s for m an (43).Investigati ons of the interacti ons of f i ber w i thv i tam ins i s partl y compl i cated by the pres-ence of signi f i cant quanti ti es of B v i tam insbound w i thin w hole w heat products (44).M in er al a bso r pt io nCalcium

I t has long been know n that subjects f ed ahigh proporti on of w hole w heat products gointo negati ve calcium balance. Figure 3 show sthe calcium balance data for f our m edicalstudents doing a metabol i c study in w hich w ew ere reex am ining thi s question. I ni ti al l y adiet containing 88 g of f at, 13 g of protein,396 g of carbohydrate, 960 mg of calcium ,and 22 g of dietary f i ber w as f ed. Fiber intak ew as then increased by substi tuting 200 g ofw holem eal bread for w hi te bread, 25 g of A lB ran for cornf l akes, and 50 g of bran biscui tsfor N ice biscui ts, and adding 30 g of A hhinsonsB ran Plus. T hese addi ti ons increased dietaryf i ber intak e to 53 g/day , and, as a resul t ofcal cium hav ing been added to the biscui ts,cal cium intake increased to 1302 mg/day .D espi te the increase in calcium intake, thesubjects al l w ent into negati ve calcium bal -ance dur ing the 3rd w eek of the diet per iod.

T his observation w as prev iously noted andstudied in some depth 30 years ago byM cCance, W iddow son, and others (5, 4 5, 46)and more recentl y by Rheinhold et al . (47,


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N U T RIT I ON A L IM PL IC A T IO N S OF D IET A RY FI B ER S25

FIG. 3. Cal cium balances in four medical students duri ng the 3rd w eek of m etabol i c diets. e ft c olum n f or eachsubject show s fecal and urinary calcium excretion in m g/day duri ng control W estern type diet. ig ht c o lum n f or eachsubject show s calcium ex cretion during the same diet i n w hi ch w hole w heat f oods had been substi tuted for equivalentref ined foods. Calci um ex cretion above the balance l ine represents negativ e calcium balance.

48). T hei r w ork perhaps more than any otherev idence has cast doubt on the value ofw holem eal bread in our diet and has beenci ted as one of the f ew possible hazards toincreasing dietary f i ber intake f rom w holegrain sources. I t has even been suggested thatw hole w heat products may be responsible f orthe dev elopment of osteom alacia (49), andrecentl y these foods have been impl i cated inthe developm ent of ri ckets in chuppati -eatingim migrants in B ri tain 50 , 5 ).

T he associati on of calcium malabsorpti onw i th w holegrain products i s tradi ti onal l y as-cribed to the phy ti c acid content of thesefoods (52, 53). W hi le i t i s undoubtedly truethat phy ti c acid im pai rs calcium absorpti on(46, 47), i t now seem s disti nctl y possible thatf i ber i tsel f may also hav e a role. Some of thequesti ons w e need to answ er in relati on tof i ber and m ineral balance are these:

1) I s o ver al l balance af fected by f iber, or i schange one in route of ex creti on?

2 ) Can changes be induced by commonlyeaten f iber sources and i f so at w hat lev els ofintake?

3) W hich component of f i ber i s responsiblef or the changes in absorpti on (e.g., cel l ulose,l i gnin, noncel l ulosi c poly sacchar ides, or othercel l w al l consti tuents), and w hat are them echani sm s of thi s change?

4) D o long-term adapti ve ef f ects occur w i thhigh-f i ber diets?

5) W hat interacti ons occur betw een f iberand other m ineral s and dietary consti tuents?

6) Is the change in m ineral balance signi f i -cant in biological terms?

I t i s possible that f i ber i tsel f , i ndependentof phy tate, i nterf eres w i th m ineral absorpti onand metabol i sm . Rev iew of the botanical l i t-erature quick l y heads one to the bel i ef thatnot only do cel l w al l substances themselvesbind calcium and other cations but al so thatthese cations play an essential role in thedevelopm ent of the plant cel l w al l and inplant grow th. D etai l ed calcium -binding stud-ies hav e been reported w i th poly sacchar idesf rom cel l w al l s 54) , and in v i tro w ork sug-gests that f i ber may indeed bind calcium (55),possibl y in relati on to the uronic acid com -ponent of the cel l w al l structures (56). Ca -cium binding di rectl y to f i ber may be part ofthe explanation f or negati v e m ineral balancesin subjects fed high-f i ber diets (48). A t thepresent time l i ttl e is know n about the ef f ectof f eeding f iber f rom sources other than cer-eals on m ineral balance in man; w ork in thi sarea is clearl y needed.

Equal l y important i s the suggestion thatthese changes in calcium balance are in factonly short term and that over a course ofw eeks and m onths man is able to adapt to theal terati ons in intesti nal physiology and returnto normal calcium balance. W alk er et a . (57)w ere among the f i rst to suggest that, i f sub-jects w ere studied f or long per iods (over 4w eeks), cal cium balance w ould return to nor-mal . A num ber of f actors in W alker s studymakes interpretati on di f f i cul t, parti cularl y


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S26 CU M M IN G S

the large f al l i n calcium intak e w hen subjectsw ere transferred f rom thei r normal diet to thestudy diets ( i .e., a change in calcium intakef rom 1000 to 500 m g/day). I nteresti ngl y , thesubject w ho norm al l y took the low est calciumintak e show ed no change in calcium balancew i th w hole m eal bread. Campbel l et a . (58)w ere unable to show adaptati on in calciumbalance to w hole w heat diets in tw o subjectsw hom they studied over a per iod of 5 to 6w eeks. T hei r tw o subjects remained in nega-ti ve calcium balance throughout the ex peri -m ent, and both show ed a f al l i n serum cal -cium levels.

Serum calcium lev els have not f al l en inother studies in w hich the ef f ect of supple-m enting a W estern-type diet w i th bran orw hole w heat products has been observ ed ov erper iods of 3 to 19 w eeks (59-6 ) . A signi f i cantexception to thi s pattern, how ever, w as seenin a study of 27 elder l y patients w hose dietsw ere supplemented w i th 10 or 20 g of branfor 6 w eek s (62). T hese f indings clearl y needfurther study .

Studies in w hich magnesium absorpti onhave been measured at the same time ascalcium absorpti on tend to show the tw ocations behav ing sim i larl y w hen w hole w heatdiets are fed.Zinc

T he possibi l i ty that m an might becom ezinc def i cient has been taken ser iously onlyin recent years; that f i ber m ay play a part i nthe developm ent of thi s def i ciency i s a newconcept. I ni ti al impetus to thi s w ork cameoriginal l y f rom Prasad et al . (63-66) w hodescr ibed a zinc-def i ciency syndrom e in I ra-nian chi l dren characteri zed by i ron def i ciencyanaem ia, hepatosphenomegaly , hypogonad-i sm , dw arf i sm , and geophagia. T his w as latershow n to be associated w i th impai red zincabsorpti on, and i t i s now clear that grow thcan be improv ed in young people by addingzinc to thei r diet (67). Equal l y startl i ng hasbeen the demonstrati on by H am bidge andW al rav ens (68) that zinc def i ciency occursev en am ong the chi l dren of m iddle- and up-per-i ncome f am i l i es in the U ni ted States. T herole of diet i n promoting or impai ri ng zincabsorpti on therefore becomes crucial .

Rhinehold et al . (47, 48) show ed that dietshigh in w hole w heat products lead to negati vezinc balance and ini ti al l y suggested that thi s

w as due to the phy ti c acid content of thesef oods analogous to the calcium mahabsorp-ti on-phy ti c acid story . M ore recentl y he hasshow n that zinc binds qui te strongly , perhapsmore strongly than calcium , to dephy tini zedf iber in v i tro 5 5 69). I n fact the removal ofphy tate f rom bran or w hole m eal bread orthe addi ti on of calcium to the preparati onseem s to increase zinc binding in v i tro. Z inchas also been show n to bind in v i tro to starchand pure cel l ulose.

I n contrast to thei r f i ndings for calciumabsorpti on, Campbel l et al . (58) suggest thatadaptati on to impai red zinc absorpti on doesin fact occur af ter a period of w eeks. H ow -ev er, f ew long-term studies have been done.Prel im inary ev idence on the properti es ofother f i ber preparati ons has y ielded conf l i ct-i ng resul ts (70, 71).

Further interest in thi s aspect of m ineralabsorpti on has been stim ulated by the sug-gestion that the rati o of zinc to copper ab-sorbed could be important in the dev elop-ment of atherosclerosi s through an ef f ect oncholesterol m etabol i sm (72). M any questionsrelati ng to zinc absorpti on and f iber remainunanswered.I r o n

I ron absorpti on i s know n to vary consid-erably depending on the type and composi -ti on of the diet being eaten. I ts av ai l abi l i tyf rom w hole w heat products i s parti cularl ypoor (73). B alance studies show that i ronabsorpti on i s impai red f rom diets containingw holemea bread (74) and f rom test mealscontaining r i ce and vegetables (75). Otherinvestigators have show n decreased serumi ron levels af ter 3 or 5 w eek s of diets contain-ing bran and w hole w heat products (62, 76).O nce again phy ti c acid has been incrim inatedas the main factor dim ini shing i ron absorp-ti on in these ci rcumstances and in parti cularthe f orm in w hich the i ron is bound to phy-tate. I n the rat the monof err i c phy tate presentin bran w as readi l y avai l able f or absorpti onw hen isolated f rom the bran w hi le other bran-i ron-phy tate com phex es w ere not (77). I t i sal so possible that i ron binds di rectl y to plantcel l w al l material other than phy ti c acid (55),so w e need to study i ron absorpti on speci f i -cal l y f rom the point of v iew of dietary f i ber.

T he interacti on in the gut of cel l w al l ma-ter ial s w i th m ineral absorption represents a


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possible disadvantage of high-f i ber diets andperhaps parti cularl y so w hen W estern-sty lediets are supplem ented w i th high-f i ber prep-arati ons. One area w here caution w ould seemto be needed is in supplementing the diet ofold people w i th w hole w heat products toimprove bow el habi ts w hen thei r dietary in-takes of i ron and calcium m ay be barelysuf f i cient f or thei r normal requi rements. I t i sdi f f i cul t to di sentangle the relati ve roles ofphy ti c acid and dietary f i ber in al ter i ng m in-eral absorpti on, but current ev idence suggeststhat f i ber m ay hav e an independent role.Further inf ormation i s needed parti cular l y onthe m ineral -binding properti es of dietary f i -ber as present in commonly eaten f rui ts andvegetables. Equal l y im portant to bear in m indw i l l be the possibi l i ty that i n the long termthe hum an system is able in som e w ay toadapt to the dim ini shed avai l abi l i ty of m in-eral s as a resul t of these dietary changes andthat w e may ul tim atel y hav e seemed ov ercau-ti ous about thi s aspect of dietary f i ber m etab-olism.Con c lu s i o n s

Suf f i cient prel im inary ev idence is avai l ableto suggest that f i ber has important nutr i ti onalim pl i cati ons in addi ti on to i ts al ready w el l -k now n gastroenterological ef fects. Fiber ap-pears to af f ect the rate and route of absorp-ti on and metabol i sm of dietary fat, carbohy-drate, and protein as w el l as al teri ng sterolmetabol i sm and m ineral balance.

T he important question of the signi f i canceof these changes remains to be answ ered. W eneed f urther inf orm ation bef ore the role ofdietary f i ber can be assessed. For example w eneed to know more about the amount of f i bereaten by various populati ons, i ts type, and i tsphy siological ef fects. G athering thi s infor-mation i s unl i kel y to prov e easy as the essenceof the f iber story i s one of long-term changesin diet l eading to al tered patterns in humandisease. Furtherm ore w e need to recognizethat the nutr i ti onal signi f i cance of a f i ber-r i chor f i ber-depleted diet w i l l depend on otherconsti tuents of the diet, w hich v ary greatl y indi f f erent parts of the w orld. Fiber cannot beseen in i solati on f rom the rest of the diet.

I t i s w isest to assume at the m oment thatthe absorpti on and metabol i sm of m any i fnot al l nutr i ents are af f ected by f iber. Some

of these changes appear to be nutr i ti onal l ybenef i cial al though the role of f i ber in mm -eral absorpti on giv es grounds f or concern. I fthe ef f ect of f i ber on colonic f unction prov esto be important in rel i ev ing constipati on andprev enting diverti cuhar di sease and largebow el cancer, the need to increase f iber in-takes w i l l be clear. T his alone means that thenutr i ti onal impl i cati ons need to be evaluated.H ow ever, i t i s now possible to conceiv e thatsom e of these nutr i ti onal changes may be ofequal im portance in thei r ow n right.

T he author is indebted to D r. Phi l ip James for hel pfuldiscussions during the preparat ion of thi s paper and toW i l l B ranch, H elen H ouston, D r . D av id Jenk i ns, andD r. H ugh W iggins for col l aborat ion in studi es on dietaryf iber .R e f e r e n c e s

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3. J O V E R A ., AN D R. S. G O R DO N . P r oced u r e for q u a n -ti tati v e analysis of f eces w i th special reference tofecal f atty acids. J. L ab. Cl in. M ed. 59: 878, 1962.

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5. M C C A N C E R. A ., AN D C. M . W A L S H A M . T he di ges-tib i l i ty and absorption of the calori es, protein, pu-rines, f at and calcium in w holemeal w heaten bread.B ri t. J. N utr. 2: 26, 1948.

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35. M C N E I L L N . I ., J. H . C UM M IN G S AN D W . P. T .J A M E S . S h or t ch a in fa t t y a cid a b sor p t ion in t h e h u -man large bow l . Gut 18: A 425, 1977.

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76. J E N K I N S D . J. A ., M . J. H IL L AN D J. H . C U M M I N G S .Ef fect of w heat f i bre on blood l i pids, f ecal steroidexcreti on and serum i ron. A m . J. Chin. N utr. 28:1408, 1975.

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Am J Clin Nutr 1978 Cummings S21 9

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