aspects of protein digestion absorption in health …quantity ofprotein orthe appearance in the...

POSTGRAD. MED. J. (1961), 37, 745

SOME ASPECTS OF PROTEIN DIGESTIONAND ABSORPTION IN HEALTH

AND DISEASEC. W. CRANE, B.Sc., M.B., B.S., A.R.I.C.

Senior Research Associate in Medicine and Hon. Lecturer in Medicine, Wellcome Research Wing, Department of Medicine,University of Birmingham and Queen Elizabeth Hospital, Birmingham, 15

THE study of protein digestion and absorption hasoccupied the attention and interest of physiologistsand biochemists for over I50 years since theclassical experiments of Spallanzani and the epoch-making discovery by Beaumont of the digestiveproperties of the gastric juice of his patient,Alexis St. Martin. It has been demonstrated bynitrogen balance experiments that the overallmechanism in man is an extremely efficient one.Reifenstein, Albright and Wells (I945) fed humansubjects increasing amounts of protein (from 20 to250 g. a day), but found that the daily faecalnitrogen excretion remained substantially the samelevel of between i and 2 g. Evidence regardingthe origin of the faecal nitrogen from studies onpatients undergoing a prolonged fast or receivingdiets containing negligible quantities of protein(Lusk, 1928) strongly suggest, however, that a largeproportion of the faecal nitrogen is derived fromendogenous sources, so that the efficiency ofassimilation at levels of intake of protein of 250 g.per day is thus greater than 95%.Yet Fisher, writing in 1954, pointed out that

precise information on a number of importantdetails in these events was still lacking, amongthem the rates of digestion and absorption of pro-tein and the form in which the split products areabsorbed by the intestinal mucosa. The rate ofbreakdown of proteins in vivo has long been knownto be much faster than occurs in vitro by thesuccessive actions of gastric and pancreatic en-zymes and ' erepsin ' and Fisher discusses at somelength the evidence for concluding that proteinsmay not be degraded completely to single amino-acids before absorption. There is some recentevidence for believing that dietary proteins areabsorbed from the gut unchanged. Bangham andTerry (I957) and Bangham, Ingram, Roy, Shillamand Terry (1958) have shown that plasma proteinsmay be absorbed unchanged from the gut in new-born rats and calves, while Dent and Schilling(1949) were unable to show any rise of amino-acids in the portal blood of dogs receiving homo-

logous serum protein by mouth in contrast to thehigh values found when normal dietary proteinswere given. In contrast Kekwick (1959) con-cluded that the evidence for the absorption ofantibodies by newborn babies from maternalcolostrum was not very strong. However, Wilsonand Walzer (I935) from their work claimed thatbabies could absorb whole egg white and Gruskayand Cooke (1955) reported that after bouts ofdiarrhoea in young children an increase as much asfivefold in the absorption of this protein waspossible. More recently Parish, Barrett, Coombs,Gunther and Camps (I960) have claimed that theserum of babies fed cows' milk may contain anti-bodies to the milk proteins. Thus it appearscertain that if whole protein is absorbed un-changed then this is confined to special circum-stances either during the neonatal period of lifeor in association with disease, but, in any case,the amounts must be extremely small. It is notyet certain, however, whether the conclusions applyin respect to homologous serum plasma protein. Ifproteins are not degraded completely to amino-acids prior to absorption and larger fragments areabsorbed into the mucosa, evidence for thepresence of large quantities of circulating peptidesin the blood in the post-absorptive phase aftergiving protein by mouth has not yet been demon-strated. For example, Dent and Schilling (I949)were unable to show any significant concentrationof peptide in the portal blood of their experimentaldogs after protein feeding.Methods for Investigating Protein Digestionand AbsorptionAlthough nitrogen balance experiments may give

useful information regarding the overall loss or gainof nitrogen by the body under a variety of con-ditions, such studies provide no measure of therates of digestion and absorption of protein.Clearly many of the experimental techniques usedin animal work (e.g. Wright and Wynn, 1951;;Gupta, Dakroury and Harper, 1958; Geiger,

copyright. on A

pril 13, 2020 by guest. Protected by

http://pmj.bm

j.com/

Postgrad M

ed J: first published as 10.1136/pgmj.37.434.745 on 1 D

ecember 1961. D

ownloaded from

http://pmj.bmj.com/

746 POSTGRADUATE MEDICAL JOURNAL December 1096

Human and Middleton, I958) cannot be appliedto human subjects. The occurrence of patientswith intestinal fistula such as described by Kurodaand Gimbel (I954) and suitable for absorptionstudies is extremely rare, while results of experi-ments with patients having portal collaterals on theabdominal wall and which can be sampled atintervals after giving protein by mouth were foundby Bean, Franklin, Embrick and Daum (195i)to be difficult to interpret. Techniques whichhave been used with some success in human sub-jects fall into two main groups: those whichinvolve sampling the contents of the lumen of thegut at given levels and those which measurechanges in the concentration of constituents of theperipheral blood. McGee and Emery (1940)studied the digestion and absorption of solutionsof casein and gelatin by sampling the contents ofsegments of the jejunum blocked distally by in-flated balloons, while Nasset and Davenport (1954)investigated the proteolysis of lactalbumin in thestomach by sampling the gastric contents. Themost successful of the techniques so far has beenthat devised by Blankenhorn, Hirsch and Ahrens(I955), in which a fine polythene catheter is passedalong the entire gut, enabling sampling at a numberof given levels. This method has been used mostsuccessfully by Borgstrom, Dahlqvist, Lundh andSjovall (1957) to study the digestion and absorp-tion of carbohydrate, fat and protein from anumber of human subjects and to acquire a greatdeal of useful information. A similar technique hasalso been used by Lundh (1958), who investigateda series of patients with malabsorption followingpartial gastrectomy.The other main type of study measures thanges

in amino-acid concentration in the peripheralvenous blood after the ingestion of a standardquantity of protein or the appearance in the bloodof an isotopic tracer after giving a dose of labelledprotein. Free and Leonards (I944), using un-labelled protein, compared the times to reachmaximum amino-acid concentration in normalsubjects ingesting meat, whole blood and caseinhydrolysate, while West, Wilson and Eyles (1946)followed the blood amino-nitrogen levels in normalchildren and those with deficient pancreatic func-tion after giving gelatin and casein. Christiansenand Schwachman (I949) refined this technique andmeasured the changes in concentrations in theplasma of a specific amino-acid, glycine, afterfeeding gelatin to both normal persons and thosewith pancreatic fibrosis.

Other workers have used the rates of appearanceof radioactive tracers in the peripheral blood afterfeeding isotopically-labelled protein. Lavik, Mat-thews, Spector and Friedell (1952) administered'31I-labelled casein to children suffering from pan-

creatic disorders, while Chinn, Lavik, Babb,Buckaloo, Stott and Abbott (I953) fed 311-labelledalbumin with a meal of gelatin to normal subjects.Freeark, Kozoel and Meyer (1957) and Polachek,Cope, Williard and Barnes (1959) have madeextensive studies on normal subjects and patientswith pancreatic lesions using labelled albuminalong with unlabelled gelatin. More recently35S- and 32P-labelled proteins have been used tostudy problems of digestion and absorption inpatients after gastrectomy by Nakayama, Ohtsuka,Kuvaishi, Koshibu, Arima, Fukushima, Naka-gami and Fuse (I958). These workers haveattempted to simulate normal feeding in theirexperiments by administering their test substancesalong with a meal of bread, butter and egg.Compared with radioactive tracers, the stableisotopes have, up to the present, not been usedextensively in problems of clinical interest. As faras the stable isotope of nitrogen, 15N, is concerned,besides freedom from possible hazards of radiation,its use would be expected to provide more directinfprmation regarding the digestion and absorptionof protein and subsequent metabolism of ingestednitrogen by the body, and White and Parson (I950)and Sharpe, Lassen, Shankman, Gebhardt andHazlet (1956) have used yeast protein so labelledto study some aspects of protein metabolism inhuman subjects. Crane and Neuberger (i96oa,I960b) have shown that the time to reach maxi-mum labelling of both urinary ammonia and ureain samples of urine passed at frequent intervalsafter administering the protein by mouth may beused to investigate the digestion and absorptionof protein in normal subjects and in patients suf-fering from adult coeliac disease.To some degree criticism can be levelled at all

the methods of investigation that have been men-tioned. It is difficult to be absolutely certain, forexample, that the presence of a sampling tube lyingwithin the gut does not produce abnormal peri-stalsis, while experiments involving sampling of thecontents from a segment of small bowel closeddistally by a balloon must be regarded as non-physiological. In those studies in which changesin peripheral-blood amino-nitrogen are measuredafter ingesting a quantity of test substance it isnecessary to be sure that these bear some relationwith respect to time to the changes occurring in theportal blood. In patients suffering from mal-nutrition these differences may be considerable.The rate of digestion of a test protein will bedependent upon whether considerable denaturinghas occurred during its preparation and whetherit is fed together with a quantity of a denaturedprotein like gelatin, or with an undenatured proteinlike skim milk. The fact that these rates may beappreciably different has been used by Bocci (I961)

copyright. on A


http://pmj.bm

j.com/

Postgrad M


ecember 1961. D

ownloaded from

http://pmj.bmj.com/

December 1961 CRANE: Some Aspects of Protein Digestion and Absorption in Health and Disease 747

to determine the extent of denaturing of 131I-labelled gamma globulins. Work with radioactivetracers involves usually the measurement of activityof samples of whole blood and no attempt is madeto separate and identify specifically-labelled amino-acids. Provided that one can be sure that nofree iodide is present in the sample, the method isprobably sufficiently accurate for work with 131L-labelled proteins, but when 35S and 32p tracers areused allowance must be made for the radioactivityof secondary products occurring in the plasmafrom the metabolism of these substances within thebody.The use of 15N-labelled protein offers advantages

in that the isotope appearing in the urinaryammonia and urea is almost certainly derived fromall the amino-acids entering the body and thatsmall quantities can be given alone or mixed withother proteins. The technique can also be used inill patients where intubation or large amounts ofprotein by mouth would not readily be tolerated.Furthermore, the 15N excreted in the urine andfaeces allows assessment to be made of the amountof tracer retained under different conditions.

The Digestion and Absorption of Protein inNormal Subjects

Hydrolysis of dietary protein (warmed, masti-cated and moistened with saliva) begins in thestomach by the action of pepsin producing a com-plex mixture of high-molecular weight peptidesoften referred to as ' peptones '. The extent of thisinitial proteolysis will depend upon a number offactors: the availability of hydrochloric acid andpepsin, the type of protein eaten, and the rate atwhich the stomach empties, and would thereforebe expected to vary considerably from person toperson and from meal to meal in the same person.For example, if the consumption of a meal is rapid,a greater dilution of enzymes and buffering of theavailable hydrochloric acid would be expected,while the presence of fat and a state of hungerwould both influence the rate at which the stomachempties. Lastly, a protein denatured by thoroughcooking would be attacked more readily by pro-teolytic enzymes than one only partially denatured,for example, underdone meat. Thus any generalstatement regarding the extent of gastric proteo-lysis must take into account these features. Cath-cart (192I) suggested under normal conditionsthat the degree of gastric digestion was un-important and Borgstrom, Dahlqvist, Lundh andSjovall (1957) concluded that after feeding a mealof corn oil, dextrose and skim milk powder notmore than 15% of an admixed tracer dose of3I-labelled albumin was hydrolysed in thestomach of normal subjects. The assumption bythese authors that this figure applies also to the

milk powder must be accepted with some reserve,since there may be considerable differences in therate of proteolysis of milk protein and the albumin.Using cooked pig's heart as the test protein,Elsom, Chornock and Dickey (1942), on the otherhand, found considerable proteolysis in thestomach of their subjects. Whether a proportionof these large peptides may be further hydro-lysed and absorbed by the stomach mucosa isnot at present definitely known, but it is of someimportance that Linderstrom-Lang and his col-leagues (1952) were able to find peptidases capableof hydrolysing small synthetic peptides in thefundal cells of the pig's stomach. The observationof Nasset and Davenport (I954) that after onehour the concentration of free amino-acid in thestomach contents after feeding lactalbumin hadrisen tenfold is also of some interest, but theseauthors were unable to exclude the regurgitationfrom the duodenum of powerful pancreatic en-zymes. However, it is unlikely that significantproteolysis could have occurred from the actionof these enzymes owing to the low pH of the gastriccontents. Following the emptying of the stomachcontents in small portions over about four hours,the Swedish workers found that a three- to five-fold dilution took place in the duodenum and thatextensive digestion (50 to 60%) resulted from theaction of the pancreatic enzymes. The absorptionof protein was almost complete during the passagethrough the proximal Ioo cm. of the jejunum(maximum absorption occurring within the secondhour), but that about Io% of the test proteincould be recovered unchanged from the lowestportion of the small intestine. These workerssuggested, therefore, that some dietary protein isnormally digested and absorbed in the large bowel.The pH of the contents of various sections of the

gut were also measured. Over the first three hoursthe pH of the gastric contents were found to bebetween 3 and 4, while that of the small intestinewas between 6 and 8, the latter values beingreached only in the ileum. In view of the rapiddigestion of the protein in the upper jejunum it issurprising that these values are so far removed fromthe pH optima determined experimentally for pep-sin and the pancreatic enzymes.

Recently Taylor (I959a) has demonstrated thatgastric juice from normal subjects contains pepsinwith maximum activities occurring between pH1.6 to 2.4 and also between pH 3.3 to 4. Measure-ment of proteolysis in the lower ranges of pHalone therefore underestimates the true extent ofbreakdown of which this enzyme is capable.

Estimates of the overall rates of digestion andabsorption of protein have been made by a numberof workers and, as would be expected, these showvariations according to the type and quantities of

copyright. on A


http://pmj.bm

j.com/

Postgrad M


ecember 1961. D

ownloaded from

http://pmj.bmj.com/


protein fed. Free and Leonards (I944) found thatthe maximum amino-acid concentration occurredat three and four hours respectively after the ad-ministration of a litre of blood and 400 g. ofmeat to normal subjects. In studies of normalchildren, West, Wilson and Eyles (1946) foundthat these maxima occurred at i hours afterfeeding either casein or gelatin in amountsequivalent to 1.7 g./kg. body weight. Using radio-active techniques, Lavik and others (1952) re-ported maximum radioactivity of venous bloodwithin three hours after a test meal of 0.5 g. ofgelatin/kg. body weight admixed with 131I-labelledalbumin, while Chinn and others (I953) have foundsimilar results. Nakayama and others (1958), onthe other hand, showed maximum concentrationsof radioactivity in the peripheral blood weredelayed to between six and seven hours in normalsubjects receiving test doses of 35S- and 32P_labelled proteins along with a meal consisting of86 g. of carbohydrate, 30 g. of fat and 26 g. ofprotein. Using much smaller quantities (circa 2 g.)of 15N-labelled yeast protein in fasting subjects(Fig. I).

05 -

0-3 - X

0*4

0

O'100-2

I I I I I I0 I 2 3 4 5 6

Time (hr.)FIG. i.-Comparison of the rate of change of 15N

labelling of ammonia (-o-) and urea (-*-) in urineafter giving whole yeast protein to a fasting normalsubject equivalent to o.8 mg. '5N/kg. body weight.Values are plotted at the mid-points of the collectionintervals.

Crane and Neuberger (Ig6oa) showed that diges-tion and absorption were extremely rapid, urinaryammonia being maximally labelled at about 45minutes after administration and urinary urea atabout 75 minutes. Little difference could bedetected between the rates of digestion and absorp-

07

06

0 1 2 3 4 5 6 7

0-5

Time (04r.)

FIG. 2.rThe isotopic labelling of urinary ammoniaafter giving whole yeast protein (-e-) and yeastprotein hydrolysate (-o-) to a fasting normalsubject in amounts equivalent to o.8 mg. 15N/kg.body weight. Values are plotted as for Fig. I.

0*2

0I

0 1 2 3 4 5 6 7Time (hr.)

tion of whole proteinand theabllingof urinary ammonia

equivalent amount of water-soluble enzymically

after givingwhoedlabelle yeast protein -) andit was

estimated that under the conditions of study the

hydrolysis of the proteinin(--)the intea fastines retardedsubjtheoverallct inabsorptionby alentthemost.8 mg. inutes

body weight.Simialus re plotted as for Fig. i.

Janney (I9I5), who measured theurinaryoutputofnitrogen and glucose in phloridzinized dogs after

administeringwhole protein and the abosrptionhydof an

equivalent amount of water-soluble enzymically

lysate and labelled yeast prouryein anditrper(was958),whoexaminmatedthat under the condisappearanceof studyproteinand amino-acidmixtrolysisof the protein in the intestines ofrats.ardedhen,overall absorption by at thelledmoaterial was

mixed with milk powder equivalent to o.4 g. of

protein/kgg.2). Similarbodyweightthere was considerabledbyproteinney (915), who melabellingofsuredthe urinary output of

four hours respectively, values which are in agree-ment with those already discussed. These findings,

nitrogether withthglucose iof Boridzinized dogs after

Lundh and Sjovall (I957), allow of the conclusion

thatprodministein digestionand absorptionis a verydro-

lysate and by Gupta, Dakroury and Harper (1958),

rapid processexamin the rates of disappearance of protthealimentary

and amino-acid mixtures from intestines of rats.When, however, the 15N-labelled material wasmixed with milk powder equivalent to 0.4 g. ofprotein/kg. body weight there was considerable

canal and thatthe overall digesrateion and absorption of thisprotein, maximumlaptying.Tof the urinaryam-olysis

monia and urea occurring at about 24 and three tofour hours respectively, values which are in agree-ment with those already discussed. These findings,together with those of Borgstr6m, Dahlqvist,Lundh and Sj6vall (1957), allow of the conclusionthat protein digestion and absorption is a veryrapid process in the upper part of the alimentarycanal and that the overall rate is controlled mainlyby gastric emptying. The very rapid hydrolysis

copyright. on A


http://pmj.bm

j.com/

Postgrad M


ecember 1961. D

ownloaded from

http://pmj.bmj.com/


of the protein is likely to be due to the very highconcentrations of trypsin and chymotrypsin whichare found in the small intestine. Borgstr6m andothers (1957) found values between 200 to 800tJg./ml. within a short time of giving a test meal.When yeast protein was digested in vitro withconcentrations of these enzymes equivalent toabout 400 Lg./ml., Crane and Neuberger (i96oa)estimated that more than 50% of the protein wasconverted into peptides soluble in trichloraceticacid within io minutes, and it is likely that the con-version of protein to peptides is also a very fastprocess in vivo. It has already been mentioned thatthere is considerable uncertainty regarding theform in which the products of digestion of proteinsare absorbed by the mucosa. Experiments such asthose devised by Wright and Wynn (I95I), whichdemonstrate the presence of both free amino-acids and peptides in the upper intestine afterfeeding a protein meal to dogs, do not allowconclusions to be reached regarding the amount ofprotein already absorbed either in the form ofpeptide or as amino-acids, or whether the peptidesare derived mainly from the dietary protein orfrom the breakdown of endogenous proteinsecreted within the lumen of the gut. Newey andSmyth (1959), in animal experiments, have shownthat simple peptides are hydrolysed inside themucosal cells. It is reasonable to assume thatlarger peptides may be similarly degraded by amixture of peptidases which Wright, Jennings,Florey and Lium (I940) have shown to be essenti-ally intracellular enzymes. Rapid digestion ofdietary proteins which has been observed cantherefore be regarded as mainly due to the actionof high concentrations of pancreatic enzymes, thechief products of which are peptides, which are inturn further hydrolysed to amino-acids afterabsorption into the mucosal cells. There is furtherevidence for these conclusions: it is known fromstudies such as that of Hokin (i951) that forprotein synthesis to occur all the requisite amino-acids must be present at the same moment and thatthe body is unable to store quantities of amino-acids for any length of time. Thus Geiger (1947)showed that supplementing a diet deficient inessential amino-acids by giving the missing amino-acids several hours later failed to promote growthin animals. However, addition of pure amino-acids to diets deficient in these substances whichcause cessation of growth led Rose to the classicdiscovery of the essential nature of a group ofamino-acids. More recently this type of experi-ment has been extended to human subjects.Gomez, Ramos-Galvan, Cravioto and Frenk (1958)and Scrimshaw, Brissani, Behar and Viteri (I958)have both reported increase in weight togetherwith nitrogen retention in children fed diets poor

in lysine and tryptophan, when supplemented withpure amino-acids. From these studies it is difficultto escape the conclusion that digestion and absorp-tion of dietary proteins proceed at rates comparablewith the rates of absorption of pure amino-acids.A further point of interest in this work is the

contribution made by protein from endogenoussources. Nasset and his colleagues (see Nasset1957 for review) in a series of experiments claimedthat sufficient endogenous protein is secreted intothe gut during digestion to exert a marked in-fluence on the composition of the amino-acidsabsorbed from the gut. Although some of theirconclusions have been questioned by Geiger and hiscolleagues (1958), it is certain that a consider-able quantity of endogenous protein is turned overin the intestine in the form of enzymes, mucus,shed mucosal cells and possibly plasma protein.Mention has already been made of the high con-centrations of pancreatic enzymes which aresecreted into the small intestine, but Avakian(1961) found that trypsin was rapidly inactivatedby incubation with intestinal juice at 37° C.,although chymotrypsin was more stable. Theturnover of the mucosal cells in animals has beenestimated at about 24 hours (Leblond and Stevens,1948). If the figures for human mucosa are com-parable, the addition of endogenous protein to thegut must be considerable and may well be of theorder of the 50 to Ioo g./day estimated by Nasset.Information concerning the breakdown and ab-sorption of protein in the large intestine is lacking.Borgstr6m and others (i957) estimated from theirstudies that Io% of dietary protein remained un-absorbed by the time a test meal had reached thelarge intestine and concluded that about Io% ofdietary protein is normally degraded and absorbedthere. The work of Reifenstein and his colleagues(I945) on the absorption of diets containing largeamounts of protein has already been mentioned.In those studies in which 250 g. of protein werefed daily to human subjects a total of 25 g. ofdietary protein would thus be expected to bedigested and absorbed by the colon, a quantityequal to between a quarter and one-third of thenormal daily intake. This suggests that the esti-mate by the Swedish workers is too high. On theother hand, it is possible that some of the intestinalenzymes at least, reaching the large intestine, mayescape digestion, since Avakian (1961) records that'31I-labelled protein can be recovered unchangedafter 24 hours in the human rectum.

The Effect of Gastrectomy on ProteinAbsorptionThe success of partial gastrectomy in the treat-

ment of peptic ulcer in the last 40 years, whichin general rarely produces catastrophic upsets in

copyright. on A


http://pmj.bm

j.com/

Postgrad M


ecember 1961. D

ownloaded from

http://pmj.bmj.com/


nutrition, has been responsible no doubt for thegeneral belief that protein digestion in the stomachis not really essential. The amount of proteolysisproduced by pepsin on a variety of dietary pro-teins in vitro (Fisher, 1954) is not usually marked,but Cathcart (1921), summarizing earlier views onthis point, suggested that the initial action ofpepsin on dietary proteins was nevertheless im-portant for the subsequent proteolysis by thepancreatic enzymes. The recent work of Taylor(I959a and I959b) shows that the true extent ofgastric proteolysis is underestimated if measuredonly at pH 2, and has demonstrated that inpatients with pernicious anemia, activity is presentat higher pH values.When the details of a large series of patients

with adequate follow-up after partial gastrectomywere studied, it became clear that a large majorityof the patients had lost weight and further studies(see Lundh, 1958, for review) showed that weightloss was greater in Billroth II operations than withBillroth I and that steatorrhcea and increasedfecal nitrogen losses were greater in the formeroperation. In patients with total gastrectomy thesefindings were shown to be more marked andmore persistent. A large amount of work ongastric proteolysis following gastrectomy hasshown that in general the extent of digestion isimpaired by the increased rates of emptying ofthe stomach remnant and the reduced amount ofacid and pepsin available, but studies on pan-creatic function following gastrectomy have led toconflicting results. Under normal conditionssecretin and pancreozymin which stimulate pan-creatic secretion are released from the duodenumby the passage of contents from the stomach.Nakayama and his colleagues (1958), using 32P-and 3S-labelled proteins, have shown that the rateof absorption is lower, and the faecal excretion ofisotope greater, in patients after total gastrectomythan in normal subjects. The absorption of theirtest material was more effective after oesophageal-duodenal anastomosis compared with oesophageal-jejunal anastomosis.The changes in this mechanism resulting from

gastrectomy (Annis and Hallenback, 1952) werestudied in dogs after Billroth I and II operationsand results showed that there was marked diminu-tion of pancreatic secretion during the ingestionof cooked meat although response to secretin wasnevertheless normal. Using 311-labelled proteinsa number of investigations, for example those ofBabb, Chinn, Stitt, Lavik, Levey, Kreiger andAbbott (i953), have shown that there may berapid increases in the radioactivity of the plasmaafter Billroth II operations, but a more slow riseand more prolonged activity in patients withBillroth I operations, when 131I-labelled albumin

was used. Lundh (1958), using the intubationtechniques of Blankenhorn, Hirsch and Ahrens(I955), studied a large series of patients aftergastrectomy. In patients with Billroth I type ofoperation he found gastric emptying to be morerapid than with Billroth II, but in both groupsthere was a very rapid passage of a test meal tothe distal parts of the small intestine and alsoevidence of defective digestion. Concentrationsof trypsin in the lumen of the gut were found tobe low and in a number of samples no activitywas found in patients with the Billroth II type ofgastrectomy. In Billroth I patients trypsin wasmixed with a test meal, but the poor mixing inBillroth II patients was explained by the lack ofco-ordination of pancreatic secretion followinggastric emptying. The conclusion that the poormixing of protein with pancreatic enzymes was amore important factor than rapid intestinal passageafter gastrectomy was based upon the disappear-ance of l31I-labelled albumin from the smallintestine.

It has already been pointed out that the ratesof digestion of denatured and native proteinsdiffer markedly and the assumption therefore thatthe proteins of the normal diet behave in thesame manner as albumin remains to be proved.Only one patient with a sub-total gastrectomy

has been investigated with 15N-labelled yeastprotein, so far. After feeding 15N-labelled yeastprotein to a patient who underwent an operationthree years previously for a three-quarter resectionof the stomach for suspected carcinoma and whopresented with a steatorrhtea of an average ofI gm. of fat together with 2 to 3 gm. of nitrogen

per day, it has been found that the rate of uptakeof the protein was within normal limits. Whenthe experiment was repeated and an amount ofmilk protein equivalent to 0.4 gm./kg. body weightadded to the yeast protein, maximum concentra-tions of isotope in the urinary ammonia and ureawere reached more quickly than in normal sub-jects and this was considered to result from themore rapid entry of the test materials into thesmall intestine. From the first study the amountof isotope eliminated in the urine during the firstsix hours was found to be as high as Io% of theamount ingested. In normal subjects this valuewas reasonably constant between 6 and 7%. Itis suggested that these high figures indicate con-siderable malutilization of the absorbed proteinsince not less than 80% of the nitrogen of theurine is contained in the urea fraction.

The Effect of the Loss of PancreaticSecretion on Protein Absorption

Because of their high activity the loss of thepancreatic enzymes to the body would be expected

copyright. on A


http://pmj.bm

j.com/

Postgrad M


ecember 1961. D

ownloaded from

http://pmj.bmj.com/


to produce extremely serious nutritional conse-quences. Cystic disease of the pancreas in children(Clarke and Hadfield, 1924) or surgical excisionof the pancreas may produce marked weight losswith the appearance of large quantities of fat andnitrogen in the faeces, but in disturbances of pan-creatic function following acute or chronic pan-creatitis all grades of disability can be encountered,though the finding of markedly increased faecalnitrogen and fat usually denotes quite severedamage. It is possible that apart from the twoconditions mentioned, there is never completeloss of the external secretion of the pancreas.Some estimate of pancreatic insufficiency may beobtained by duodenal intubation of patients afterstimulation of the pancreas by secretin or pan-creozymin and measuring the concentrations ofvarious enzymes. For trypsin and chymotrypsin,Lundh (I957) has recently introduced a verysensitive method.

West, Wilson and Eyles (1946) fed gelatin tochildren with deficient pancreatic function andshowed that the rise of blood amino-acid nitrogencompared with normal children was extremelydepressed, but this situation could be improvedby administering pancreatin. Test proteins taggedusually with 131I have been used extensively toinvestigate pancreatic disease, and Chinn, Lavik,Stitt and Buckaloo (1952) fed 13I-labelled albuminto patients with pancreatic disease and showedthat the faecal excretion of the isotope was greaterthan normal. Freeark, Kozoel and Meyer (1957)showed that the increase of radioactivity appearingin the blood after giving a dose of '31I-labelledalbumin was low compared with healthy subjects.Polachek and others (I959) carried out a similarinvestigation on patients with severe pancreaticdysfunction and found identical results for theplasma, but the faecal excretion in their patientswas seldom abnormal. These studies are of diag-nostic interest and confirm what would be ex-pected from the loss of pancreatic enzymes by thebody, but are, nevertheless, subject to a criticismwhich has already been discussed. The test pro-tein used is a native one and hence may behavein a different way from the cooked and denatureddietary proteins that are eaten by man.The disturbance of protein metabolism is much

more marked following total pancreatectomy.Whitfield, Gourevitch and Thomas (1952) pub-lished a detailed metabolic study on a patientwho had a partial gastrectomy and total pan-createctomy. They showed that although fatabsorption improved with pancreatin the excre-tion of nitrogen was not greatly affected andremained between 5 and 8 g. a day on a normaldiet. A preliminary communication has alreadybeen given (Crane, 1961) on the fate of 15N-yeast

protein in this patient who is alive and wellii years after his operation. After three dayswithout supplements of pancreatin it has beenshown that this patient did absorb a considerablequantity of his test protein although the absorp-tion was delayed as measured by the rates andextent of labelling of urinary ammonia and urea.These findings in a patient who has lost thebenefit of practically all his proteolytic enzymes,thought essential for the digestion of dietaryprotein, is of some interest. Nakayama and others(I958) have recorded also that in ten patientsundergoing combined gastrectomy and pan-createctomy, digestion and absorption of labelledprotein, although incomplete, was neverthelssspossible. Handleman, Golden and Pratt (I934)have recorded that dogs deprived of pancreaticsecretion can also absorb large quantities of foodand that weight loss after operation is regained byexcessive dietary intake but fxecal nitrogen lossesrise accordingly. More recently Coles (I96I)reported that after the occlusion of the pancreaticduct of young cats there was hypertrophy of thesmall intestinal mucosa and increased proteolyticactivity was found but positive evidence on theselines has not yet been obtained for man.

The Absorption of Protein in Coeliac DiseaseThere has not been much work reported on

protein absorption in cceliac disease. In one ofthe few studies Angfanger and Heavenrich (I949)investigated a series of children with the con-dition using a test dose of gelatin, and concludedthat digestion and uptake were within normallimits. Prior to the publications of Taylor, Wol-laeger, Comfort and Power (1952) and Cooke,Thomas, Mangall and Cross (I953) it was gener-ally held that in adult coeliac disease there waslittle disturbance of nitrogen assimilation, sinceestimates of faecal nitrogen were reported as littledifferent from normal values. Further interest inthe problem arose from studies by these twogroups who showed that considerable losses ofnitrogen can nevertheless occur and the observa-tions of Paulley (I954) and Shiner and Doniach(I958) from biopsy specimens that there was oftengross pathological change in the jejunal mucosawith partial or complete atrophy of the villi.Crane and Neuberger (i96ob and c) fed quantitiesof 15N-labelled yeast protein and protein hydro-lysate to four adult patients with severe coeliacdisease and found that the times for maximumlabelling of both urinary ammonia and urea(Fig. 3) when whole protein was used werereached between 60 and 90 minutes later thanwas found for normal subjects (Fig. i). When ahydrolysate was given there was again a delay inmaximum labelling (Fig. 4), but this was more

copyright. on A


http://pmj.bm

j.com/

Postgrad M


ecember 1961. D

ownloaded from

http://pmj.bmj.com/

752 POSTGRADUATE MEDICAL JOURNAL December I96I

03

(O

~0-2

io.I

I I I I0 1 2 3 4 5

Time (hr.)FIG. 3.-'5N content of urinary ammonia (-o-) and urea

(-*-) after giving labelled whole yeast protein(0.4 mg. :5N/kg. body weight) mixed with skimmilk powder equivalent to 0.4 g. protein/kg. bodyweight to a fasting subject. Values are plotted asfor Fig. i.

variable and considerably less than when wholeprotein was fed. These results were explained byassuming that while atrophy of the villi of thejejunum reduced its total absorbing surface andhence caused some delay in the uptake of the hy-drolysate, the reduced total activity of intracellularpeptidases necessitates a greater digestion of theprotein by pancreatic enzymes within the lumenof the gut. Analysis of the stool nitrogen for 15Nshowed that there was little difference betweenthe amounts of isotope excreted when hydrolysatewas substituted for whole protein and that in eachcase about two-thirds was in a form insoluble intrichloacetic acid indicating a loss of whole pro-tein. It seemed possible, therefore, that this lossof protein, which was larger than that found fornormal subjects, could contribute to the hypo-albuminaemia, muscle wasting and negative nitro-gen balance often encountered in this disease.In one patient who responded rapidly to with-drawal of gluten from the diet it was found afterthree weeks that the absorption of labelled pro-tein and the elimination of the isotope in thefaces had returned to normal. In more recentinvestigations from this laboratory, yet unpub-lished, it has been found that faecal losses ofisotope may diminish before improvement inabsorption and digestion of the labelled protein.The Absorption of Protein in Massive Resec-tion of the Small Intestine

Immediately following massive resection of thesmall bowel for such conditions as mesentericthrombosis, injury, etc., the body is faced with asudden decrease in total absorbing area of boweland mucosal cells capable of elaborating enzymes.

0'2

0 1 2 3 4' 5 6Time (hr.)

FIG. 4.-Comparison between the 15N labelling ofurinary ammonia after giving whole yeast protein(-q-) and hydrolysate (-o-) to a fasting adult patientwith cceliac disease (0.4 mg. SN/kg. body weight).Values are plotted as for Fig. I.

Experimental work in dogs (Flint, I9I2) has shownthat up to 50% of the small bowel can be resectedwith complete recovery, but in human beings thedisability appears to be more variable. This maybe due to disease in the remaining portions of thegut-for example, Crohn's disease-or formationof blind loops, but Jackson (1958) believes that inthe absence of complications the resection of lessthan two-thirds of the small bowel causes noserious metabolic disturbances. After massiveresection large losses of faecal nitrogen occur; thesemay be aa s 50% of the nitrogen intak andamino-acid tolerance tests are abnormal (Altha sen,Uyeyama and Simpson, 1949). Perhaps the moststriking feature of reported cases is that within afew months of massive resection, bowel move-ments, initially 20 or so a day, return to normaland patients often regain their pre-operative bodyweights. Flint (I9I2) showed that the remainingportion of the small bowel of his dogs underwentconsiderable hypertrophy with increased size ofthe villi, and Jackson (1958), summarizing thefindings of a number of investigations of patients,reports considerable dilatation and hypertrophy ofthe remaining portion of the small bowel seeneither at autopsy or on radiological examination,but there was no convincing demonstration ofvillous hypertrophy. However, from the clinicalcondition of some of the patients, a considerablecompensation in digestion and absorption of pro-tein must take place. Perhaps the dilatation andhypertrophy serve as a functional reservoir bywhich protein, as has been suggested in adultcoeliac disease, may be more thoroughly attacked

copyright. on A


http://pmj.bm

j.com/

Postgrad M


ecember 1961. D

ownloaded from

http://pmj.bmj.com/


by pancreatic enzymes, or a larger surface area ofthe villi enables peptides to be more readilyabsorbed and broken down, but further investiga-tion on these patients is obviously required toelucidate the problem.

It seems likely from the evidence that has beendiscussed in these few selected topics that thedigestion and absorption of dietary protein byman is a more complicated and more rapid affairthan would appear from the accounts usually given.In some conditions evidence has been put forwardsuggesting that some degree of compensation takesplace for the loss of particular enzymes or the loss

of absorbing surface of the intestine due to surgeryor disease. It is hoped that a combination and ex-tension of some of the techniques described willhelp to eliminate some of the points on whichthere is at present still some doubt.

I wish to acknowledge with gratitude the support ofProfessor W. Melville Arnott and Dr. J. M. French ofthe Department of Medicine, University of Birminghamand of Professor A. Neuberger, Department of ChemicalPathology, St Mary's Hospital, London, in whoseDepartment the earlier 15N work reported here wascarried out. I also wish to thank the United Birming-ham Hospitals Endowment Research Fund and the Med-ical Research Council for financial help.

REFERENCESALTHAUSEN, T. L., UYEYAMA, K., and SIMPSON, R. G. (1949): Digestion and Absorption after Massive Resection of

the Small Intestine, Gastroenterology, 12, 795.ANGFANGER, H., and HEAVENRICH, R. M. (I949): Aminoacid Tolerance in Children, Amer. J. dis. Child., 77, 425.ANNIS, D., and HALLENBECK, G. A. (1952): The Effects of Partial Gastrectomy on Canine External Pancreatic Secretion,

Surgery, 31, 517.AVAKIAN, S. (I96I): Current Concepts in Therapy: Chymotrypsin and Trypsin, New Engl. J. Med., 264, 764.BABB, L. I., CHINN, A. B., STITT, R. M., LAVIK, P. S., LEVEY, S., KREIGER, H., and ABBOTT, W. E. (1953): Evaluation

of Protein and Fat Metabolism in Postgastrectomy Patients, Arch. Surg., 67, 462.BANGHAM, D. R., INGRAM, P. L., ROY, J. H. B., SHILLAM, K. W. G., and TERRY, R. J. (1958): The Absorption of 131I-

labelled Serum and Colostral Proteins from the Gut of the Young Calf, Proc. roy. Soc., B 149, 184.-, and TERRY, R. J. (1957): The Absorption of 131I-labelled Homologous and Heterologous Serum Proteins Fed

Orally to Young Rats, Biochem. J., 66, 579.BEAN, W. B., FRANKLIN, M., EMBRICK, J. F., and DAUM, K. (195 ): Absorption Studies Using Portal Anastomotic

Veins, J. clin. Invest., 30, 263.BLANKENHORN, D. H., HIRSCH, J., and AHRENS, E. H. (1955): Transintestinal Intubation Technique for Measurement

of Gut Length and Physiologic Samping at Known Loci, Proc. soc. Exp. Biol. (N. Y.), 88, 356.BoccI, V. (1961): Proteolysis of 31I-labelled Gamma Globulin, Int. J. appl. Radiat., 10, 94.BORGSTROM, B., DAHLQVIST, A., LUNDH, G., and SJOVALL, J. (1957): Studies of Intestinal Digestion and Absorption

in the Human, J. din. Invest., 36, 1521.CATHCART, E. P. (1921): ' The Physiology of Protein Metabolism '. London: Longmans, Green.CHINN, A. B., LAVIK, P. S., BABB, L. I., BUCKALOO, G. W., STITT, R. M., and ABBOTT, W. E. (1953): Blood Isotope

Levels following a Test Meal of 13I-labelled Protein, J. Lab. clin. Med., 42, 377., -, STITT, R. M., and BUCKALOO, G. W. (I952): Use of 13I-labelled Protein in the Diagnosis of PancreaticInsufficiency, New Engl. J. Med., 247, 877.

CHRISTIANSEN, H. N., and SHWACHMAN, H. (1949): Determination of the Plasma Glycine after Feeding Gelatin as aDiagnostic Procedure for Pancreatic Fibrosis, J. clin. Invest., 28, 319.

CLARKE, C., and HADFIELD, G. (I924): Congenital Pancreatic Disease with Infantilism, Quart. J. Med., 17, 358.COLES, B. L. (1961): Report of Symposium on Small Intestinal Function, Lancet (1961), i, 1342.COOKE, W. T., THOMAS, G., MANGALL, D., and CROSS, H. (1953): Observations on the Faecal Excretion of Total Solids,

Nitrogen, Sodium, Potassium, Water and Fat in the Steatorrhoea Syndrome, Clin. Sci., 12, 223.CRANE, C. W. (I96I): Report on Symposium on Small Intestinal Function, Lancet, i, 1342.

, and NEUBERGER, A. ( 96oa): The Digestion and Absorption of Protein by Normal Man, Biochem. J., 74, 3 3.(,(960b): Absorption and Elimination of "'N after Administration of Isotopically Labelled Yeast Protein

and Yeast Protein Hydrolysate to Adult Patients with Cceliac Disease: I. Rate of Absorption of 5N Yeast Proteinand Yeast Protein Hydrolysate, Brit. med. J., ii, 815., (I96oc): Absorption and Elimination of 15N after Administration of Isotopically Labelled Yeast Proteinand Yeast Protein Hydrolysate to Adult Patients with Cceliac Disease: II. Elimination of the Isotope in the Urineand Faeces, Ibid., ii, 888.

DENT, C. E., and SCHILLING, J. A. (I949): Studies on the Absorption of Proteins: The Amino-acid Pattern in thePortal Blood, Biochem. J., 44, 318.

ELSOM, K. A., CHORNOCK, F. W., and DICKEY, F. G. (I942): Intubation Studies of the Human Small Intestine, J. clin.Invest., 21, 795.

FISHER, R. B. (1954): ' Protein Metabolism'. London: Methuen.FLINT, J. M. (1912): The Effect of Extensive Resection of the Small Intestine, Bull. Johns Hopk. Hosp., 23, 127.FREE, A. H., and LEONARDS, J. R. (1944): Studies on the Ingestion of Large Quantities of Protein and Aminoacids,

J. Lab. clin. Med., 29, 963.FREEARK, R. J., KOZOEL, D. D., and MEYER, K. A. (1957): The use of 13I-labelled Albumin in the Diagnosis of Pan-

creatic Disease, Surgery, 41, 268.GEIGER, E. (1947): Experiments with Delayed Supplementation of Incomplete Aminoacid Mixtures, J. Nutr., 34, 97.

, HUMAN, L. E., and MIDDLETON, M. J. (1958): Nitrogen Content of Gastrointestinal Tracts of Rats during Absorp-tive Period, Proc. Soc. exp. Biol. Med., 97, 232.

GOMEZ, F., RAMOS-GALVAN, R., CRAVIOTO, J., and FRENK, S. (1958): Prevention and Treatment of Chronic SevereInfantile Malnutrition (Kwashiorkor), Ann. N.Y. Acad. Sci., 69, 969.

copyright. on A


http://pmj.bm

j.com/

Postgrad M


ecember 1961. D

ownloaded from

http://pmj.bmj.com/


GRUSKAY, F. L., and COOKE, R. E. (1955): The Gastrointestinal Absorption of Unaltered Protein in Normal Infantsand in Infants Recovering from Diarrhcea, Peaiat., x6, 763.

GUPTA, J. D., DAKROURY, A. M., and HARPER, A. E. (1958): Observations on Protein Digestion in vivo,J. Nutr., 64, 447.·HANDLEMAN, M. B., GOLDEN, L. A., and PRATT, J. H. (1934): The Effect of Variations in the Diet on the Absorption

of Food in the Absence of Pancreatic Digestion, Ibid., 8, 479.HOKIN, L. E. (1951): Aminoacid Requirements of Amylase Synthesis by Pigeon Pancreas Slices, Biochem. J., 50, 2x6.JACKSON, W. P. U. (1958): Massive Resection of the Small Intestine, in ' Modem Trends in Gastroenterology ', second

series, ed. by Jones, F. A. London: Butterworth.JANNEY, N. W. (1915): A Note on the Rate of Metabolism of Protein and Aminoacids, J. biol. Chem., 22, I91.KEKWICK, R. A. (1959): The Serum Proteins of the Fetus and Young of Some Mammals, 'Advances in Protein

Chemistry ', vol. 14. New York Academic Press.KURODA, Y., and GIMBEL, N. S. (1954): Selective Disappearance of Aminoacids from the Human Small Intestine, J.

appl. Physiol., 7, 148.LAVIK, P. S., MATTHEWS, L. W., SPECTOR, S., and FRIEDELL, H. (1952): Utilization of 13I-labelled Casec in the Study

of Protein Digestion and Absorption, Amer. J. dis. Child., 84, 736.LEBLOND, C. P., and STEVENS, C. E. (1948): The Constant Renewal of the Intestinal Epithelium in the Albino Rat,

Anat. Rec., 100, 357.LINDERSTROM-LANG, K. U. (1952): ' Lane Medical Lectures: Proteins and Enzymes '. California: Stanford University

Press.LUNDH, G. (1957): Determination of Trypsin and Chymotrypsin in Human Intestinal Contents, Scand. . clin. Lab.

Invest., 9, 229.(1958): Intestinal Digestion and Absorption after Gastrectomy, Acta chir. scand., Suppl. 23I.

LUSK, G. (1928): ' The Elements of the Science of Nutrition ', fourth edition. London: Saunders.McGEE, L. C., and EMERY, E. S. (1940): Rate of Absorption of Aminoacids from the Small Intestine of Man, Proc.

Soc. exp. Biol. Med., 45, 475.NAKAYAMA, K., OHTSUKA, A., KUVAISHI, T., KOSHIBU, M., ARIMA, M., FUKUSHIMA, M., NAKAGAMI, T., and FUSE, S.

(1958): 'Study of Digestion and Absorption with Radioactive Isotopes following Gastrointestinal Surgery:Isotopes in Medicine'. Geneva: United Nations.

NASSET, E. S. (1957): The Role of-the Digestive Tract in the Utilization of Protein and Aminoacids, J. Amer. med. Ass.,I64, 172.

- , and DAVENPORT, A. (1954): Canine and Human Digestion of Proteins in vivo, J. appl. Physiol., 7, 447.NEWEY, H., and SMYTH, D. H. (1959): The Intestinal Absorption of Some Di-peptides, J. Physiol., 145, 48.PARISH, W. E., BARRETT, A. M., COOMBS, R. R. A., GUNTHER, M., and CAMPS, F. E. (1960): Hypersensitivity to Milk

and Sudden Death in Infancy, Lancet, ii, I I6.PAULLEY, J. W. (I954): Observations on the iEtiology of Idiopathic Steatorrhcea, Brit. med. J., ii, 1318.POLACHEK, A. A., COPE, C. B., WILLIARD, R. F., and BARNES, F. W. (1959): 13'I-labelled Protein and Fat Meals in

Patients with Chronic Pancreatitis, Gastroenterology, 37, 38.REIFENSTEIN, E. C., ALBRIGHT, F., and WELLS, S. L. (1945): The Accumulation, Interpretation and Presentation of

Data Pertaining to Metabolic Balances, Notably Those of Calcium, Phosphorus and Nitrogen, J. clin. Endocr.,5, 367.

SCRIMSHAW, N. S., BRISSANI, R., BEHAR, M., and VITERI, F. (1958): Supplementation of Cereal Proteins with Amino-acids, J. Nutr., 66, 485.

SHARPE, G. S., LASSEN, S., SHANKMAN, S., GEBHART, A. F., and HAZLET, J. W. (1956): Studies of Protein AbsorptionUsing Nitrogen 15 as a Tag, Ibid., 58, 443.SHINER, M., and DONIACH, I. (1958): Histopathologic Studies in Steatorrhcea, Proc. World Congr. of Gastroenterology,Washington, 1958, i, 586. Baltimore: Williams and Wilkins.TAYLOR, W. H. (1959a): Studies on Gastric Proteolysis, I, Biochem. J., 71, 73..- (I959b): Gastric Proteolysis in Disease, I, J. clin. Path., 12, 2Io.

TAYLOR, A. B., WOLLAEGER, E. E., COMFORT, M. W., and POWER, M. H. (1952): The Effect of Cortisone on Non-tropical Sprue, Gastroenterology, 20, 203.

WEST, C. D., WILSON, J. L., and EYLES, R. (1946): Changes in Blood Amino Nitrogen Levels following Ingestion ofProtein and of a Protein Hydrolysate in Infants with Normal and with Deficient Pancreatic Function, Amer. J.dis. Child., 72, 25 .

WHITFIELD, A. G. W., GOUREVITCH, A., and THOMAS, G. (1952): Metabolic Effects of Total Pancreatectomy in Man,Lancet, i, I8o.WHITE, A. G. C., and PARSON, W. (1950): Comparative Studies on Nitrogen Excretion, Arch. Biochem., 26, 205.WILSON, S. J., and WALZER, M. (1935): Absorption of Unaltered Egg Protein in Infants and Children, Amer. J. dis.

Child., 50, 49.WRIGHT, R. D., JENNINGS, M. A., FLOREY, H. W., and LIUM, R. (1940): The Influence of Nerves and Drugs on

Secretion by the Small Intestine and an Investigation of the Enzymes in the Intestinal Juice, Quart. J. exp. Physiol.,30, 73-,and WYNN, V. (I951): The Digestion of Protein in the Alimentary Canal, Aust. J. exp. Biol. med. Sci., 29, 281.

copyright. on A


http://pmj.bm

j.com/

Postgrad M


ecember 1961. D

ownloaded from

http://pmj.bmj.com/

aspects of protein digestion absorption in health …quantity ofprotein orthe appearance in the...

Documents