studies on the site of fat absorption: 1. the sites of absorption of
TRANSCRIPT
Gut, 1961, 2, 23
Studies on the site of fat absorption1. The sites of absorption of increasing doses of 3'I-labelled
triolein in the rat
C. C. BOOTH, A. E. READ, AND E. JONES
From the Department of Medicine, Postgraduate Medical School ofLondon, and theDepartment of Physics, Hammersmith Hospital, Du Cane Road, London
SYNOPSIS These studies on fat absorption in rats show that a small dose was relatively rapidlyabsorbed in the jejunum. With larger doses it was also absorbed in the ileum. When larger doseswere given the jejunum responded to the increased load with an increased absorption, and fatwas also absorbed in the ileum.
Absorption of different substances does not occuruniformly in the small intestine. Glucose, forinstance, is absorbed rapidly from the jejunum butmore slowly in the distal small intestine (Cumminsand Jussila, 1955; Verzar and McDougall, 1936) andunder physiological conditions vitamin B12 is pre-dominantly absorbed from the ileum (Booth, 1958;Booth and Mollin, 1959).Although the site of fat absorption has been
studied by several workers, the relative importanceof the proximal and distal small intestine in theabsorption of fat is uncertain. Kremen, Linner, andNelson (1954), from observations in dogs subjectedto intestinal resection, have concluded that fat isprimarily absorbed in the ileum, and the upperileum is said to play a major r6le in the absorptionof '311-labelled olive oil in the rat (Benson, Chandler,Vansteenhuyse, and Gagnon, 1956). On the otherhand, Turner (1958) considers that the jejunum isthe main site of absorption of fat in the dog, andBorgstrom, Dahlqvist, Lundh, and Sj6vall (1957)have suggested that in man fat is absorbed in theproximal 100 cm. of the jejunum.
In view of these conflicting reports, we have madea further study of the site of fat absorption in therat, using '31L-labelled triolein. Animals were killedat frequent intervals during absorption of differentdoses of labelled fat and the amount of radioactivityin the various parts of the small intestine wasmeasured. The use of 1311-labelled triolein providesa simple method for this purpose. Digestion of thelabelled material does not release significant amountsof free iodide (Balint, Pendower, and Ramsey, 1960)and 1311 therefore remains bound to triolein during
absorption at least until it has reached the intestinalmucosal cell. Studies of thoracic duct lymph afteroral administration of 1311-labelled triolein suggestthat it may be partially de-iodinated during absorp-tion (Van Handel and Zilversmit 1958), but thisprobably occurs within the intestinal mucosal cellssince free iodide may be released from '311-labelledtriolein by incubation with freshly prepared extractsof intestinal mucosa (Balint et al., 1960).
This paper is concerned with the detection oflabelled triolein only during its passage along theintestinal tract and entry into the mucosal cell andthe measurement of the total radioactivity in thevarious parts of the gut is therefore a satisfactorymethod of tracing the absorption of 1311-labelledtriolein. The techniques used were similar to thosereported by Parkins, Dimitriadou, and Booth (1960).
MATERIALS AND METHODS
EXPERIMENTAL ANIMALS Laboratory-bred male or femalewhite rats weighing approximately 200 g. were usedthroughout the experiments.
1311-LABELLED TRIOLEIN Triolein was iodinated with 1311using a modification of the method described by Lubranand Pearson (1958). The 1311-labelled triolein was purifiedfirst by dialysis and then by passage through an ion-exchange resin to remove free iodide. The resultingmaterial usually contained a concentration of 30 to 40 mg.of triolein per ml., of specific activity 0.8 to 0.9 1£c. permg. It was tested for purity by ascension chromatographyin a butanollacetic acid/water medium, the distributionof radioactivity in the chromatograms being determinedby autoradiography. Chromatograms were also madeafter incubation of the 1311-labelled triolein with pan-
23
C. C. Booth, A. E. Read, and E. Jones
creatic extracts or with rat intestinal juice for 24 hours.As reported by Balint and his colleagues (1960), digestionfailed to release significant amounts of 1311 from thematerial.
Test doses of 10, 50, 100, and 500 mg. of 1311-labelledtriolein in 1-ml. volume were prepared by dilution andthe addition of suitable amounts of unlabelled triolein,the radioactivity of the test doses being adjusted toapproximately 2 fuc. of 131J.
EXPERIMENTAL PROCEDURE
ABSORPTION OF 131I-LABELLED TRIOLEIN BY INTACTANIMALS Successive doses of 10, 50, 100, and 500 mg.of 13I-labelled triolein were given to a group of six rats.The animals were starved for 12 hours beforehand and
the labelled fat was fed from a graduated syringe througha fine polythene tube passed into the stomach. Eachanimal was then placed in an individual metabolismcage and the faeces were collected for 48 hours, urinebeing collected separately. The excretion of radio-activity in the faeces of these animals was measured andabsorption was calculated by assuming that the radio-active material not recovered in the faeces had beenabsorbed.
Measurement of radioactivity The faecal specimenswere collected in waxed cartons and counted by position-ing each carton between two scintillation counters,according to the method described by Lewis and Porter(1960). This counting arrangement made it possible tocount the radioactivity without homogenization. Thesensitivity of the counter was approximately 60,000counts per minute per ,ec of 131I. Background countingrates ranged from 400 to 600 per minute.
DISTRIBUTION OF RADIOACTIVITY IN GASTROINTESTINALTRACT DURING ABSORPTION Four groups of 21 animalswere used. The animals were starved for 12 hours andeach group of rats received intragastric test doses ofeither 10, 50, 100, or 500 mg. of "3'l-labelled triolein.Three animals from each group were then killed at eachof the following times after the test doses were given:quarter, half, one, one and a half, two, three, and fourhours. The gastro-oesophageal junction, pylorus, andrectum were clamped and the entire gastrointestinal tractwas removed. The small intestine was divided into foursegments of equal length as follows: (1) duodenum andupper jejunum, (2) lower jejunum, (3) upper ileum, (4)lower ileum. The stomach and different segments of thesmall intestine and colon were then placed in separatecontainers and dissolved in 20 ml. of a solution containingequal quantities of 40% sodium hydroxide and absolutealcohol, the mixture being incubated overnight at 56°C.Measurement ofradioactivity The radioactivity of the
stomach and intestinal specimens was measured in acoronet-type counter similar to that described by Vealland Baptista (1954). With this counter, 1 jac of 1311recorded approximately 9,000 counts per minute. Back-ground counting rates on this counter ranged from 25 to45 per minute.
RESULTS
ABSORPTION OF TEST DOSES OF 1311-LABELLED TRIO-LEIN BY INTACT RATS The amounts of radioactivityrecovered from the faeces of the six animals whichreceived test doses of 10, 50, 100, and 500 mg. of1311-labelled triolein are given in Table I; the meanexcretion of radioactivity after each dose is shownin Fig. 1. The actual amounts of '311-labelled fat
TABLE IFAECAL EXCRETION OF 131I-LABELLED TRIOLEIN (% OFDOSE) AFTER INTRAGASTRIC DOSES OF 10 TO 500 MG.
RatNo.
Amount Fed (mg.)
10 50 100 500
1 11-02 11-03 1564 10-55 13-56 9-2
Mean ± S.D. = 11-8±2233
40
30
qoso 20u
L1
8-311-69-614-612-59.411-0±2.34
14-813-914-314-820-618-316-0±2.7
30.322-823-023-822-423-124-2±3:01
..
10 so 100 500
Dose given (mg.)
FiG. 1. Mean percentage ofadministered dose (±t 2 S.D.)excreted in the faeces of six rats given 10, 50, 100, or500 mg. of "31I-labelled triolein.
absorbed from the different doses, calculated byassuming that the radioactive material not recoveredin the faeces was absorbed, are given in Table II;the mean absorption at each dose level is illustratedin Fig. 2.When 10 mg. of 1311-labelled triolein was given,
the percentage of this dose excreted in the faecesranged from 9.2 to 15.6 (mean 11.8 %). Similaramounts were excreted after doses of 50 mg., butthe percentage excretion of the dose increased whendoses of 100 or 500 mg. were fed, the faecal excretionranging from 13.9 to 20.6 (mean 16.0%) when 100mg. was given, and from 22.4 to 30.3 (mean 24.2 %)after the 500 mg. dose. Although the percentage ofthe dose excreted in the faeces was greater when thelarger doses were given, there was a progressiveand linear increase in the actual amount of 1311_labelled fat which was absorbed when the concen-tration of the dose was increased (Table II, Fig. 2).
24
Studies on the site offat absorption
TABLE IICALCULATED ABSORPTION1 OF '35I-LABELLED TRIOLEIN(MG.) FROM INTRAGASTRIC DOSES OF 10 TO 500 MG.
Amount Fed (mg.)RatNo.
10 50 100 500
85286-185-785S279.481-783-9
8-9 45-88-9 44-28-4 45-28-9 42-78-6 43.79-1 45.3
Mean = 8-8 44.5
348-0386-03850381.0388-0384-03790
'The absorption of "'1I-labelled triolein has been calculated byassuming that the radioactivity not recovered in the faeces (Table I)was absorbed.
400
X 300
o 200
1100
10 50 100Dolt given (,.g.)
500
FIG. 2. Mean absorption of labelledfat by six rats givendoses of 10, 50, 100, or 500 mg. of 1311-labelled triolein.
DISTRIBUTION OF RADIOACTIVITY IN STOMACH AND Gastric emptying The percentages of gastricINTESTINES DURING ABSORPTION The percentages of emptying in the four groups of animals are detailedthe different doses found in the stomach and dif- in Tables III to VI. The mean gastric emptying inferent segments of the small intestine and the colon the three rats killed at each time after the differentof the four groups of rats killed at varying time doses is shown in Fig. 3.intervals after receiving 10 to 500 mg. of 1311-labelled At quarter of an hour after feeding, there was notriolein are given in Tables III (10 mg.), IV (50 mg.), significant difference in the percentages of theV (100 mg.), and VI (500 mg.). From these results different doses which had left the stomach, the meanthe percentage gastric emptying and percentage percentage gastric emptying ranging from 41.2 toabsorption of the test doses have been calculated 50 3% (Fig. 3). By half and one hour, the percentagesfor each animal. of gastric emptying had increased after all four
TABLE III
PERCENTAGE OF 10 MG. DOSE OF 1311-LABELLED TRIOLEIN IN STOMACH AND INTESTINES OF RATS KILLEDAT INTERVALS OF QUARTER
Rat TimeNo. Killed
(hr.)
789
101112
131415
161718
192021
222324
252627
I
39.5
45.0645
Mean =49.722-317-231-2
Mean = 23-61 36-81 29-61 295
Mean =31.914 28-514 10-414 255
Mean =21 *52 20-02 1672 17-4
Mean = 18-03 10-63 5.43 19-4
Mean= 1 1-84 17-64 15-84 9.3
Mean= 14-3
Stomach Small Intestine
1 2 3 4
14-025020-219-76-8
26-611-414-93-15.54-64.45-86-85.96-24-23.74-14-01-61-01-11-20-8091-109
33.413-812-019-79.7
21-410613-94.53.34-2403.97.44.75.32-32.57.7
4-22-2121-01-51-61-82-31-9
523-10.630
13.90210682658 1
11.7881069860887.55.94.7601.31 20608081.80o91.2
OF AN HOUR TO FOUR HOURS
Colon Total %' Gastric'Absorbed Emptying
0o00-1030-10o00.2020-10.53.4021.40o503461.81-9291-120485.74.44.97.98 6467-0
0o00203020o01.30707040.51.5080.61-01 .209
0848072.13.97.75s25s63.15.75.94.9
92.187397.992.452.766964767451 850451.751.349.935.747.944.534.737.535.736.324022231 725.931 834624 1302
7.91272.176
47.333 135.332648249648348750-164352.155.5
65362564363776077.868.374.168265475.969.8
61-555s035.550.377.782868876463270470568.171.589674.578.580.083.382682089494680688.282484290785s7
'Percentage absorption was calculated by assuming that the material not recovered had been absorbed.2Percentage gastric emptying is the amount of material not recovered from the stomach.
25
23456
lrl)l
C. C. Booth, A. E. Read, and E. Jones
60 /
v 40 .
20
!/4 J1I2 ll 2 3 4Hours
FIG. 3. Mean gastric emptying at intervals of quarterhour to four hours after feeding doses of 10 (+ +),50 (o0 o), 100 (@ ), or 500 (& A) mg. of1311-labelled triolein.
doses. Between one and three hours, however, gastricemptying was invariably less complete after the 500mg. dose than after doses of 10, 50, or 100 mg. oflabelled fat. Although more than 80% of doses of10 to 100 mg. of fat had left the stomach at two andthree hours, for instance, the mean gastric emptyingafter doses of 500 mg. was only 54-2% at two hoursand 69.6 at three hours (Fig. 3). By four hours, thedegree of gastric emptying was similar in the fourgroups of animals.
Rates of absorption The total amounts of radio-activity recovered from the intestinal tracts of thefour groups of rats are also given in Tables liI toVI. The absorption by the different rats killed ateach time interval, calculated by assuming that theradioactivity not recovered had been absorbed, isgiven in these Tables. The mean amounts absorbedby the three rats killed at each time after the fourdifferent doses are shown in Figs. 4 and 5.The rates of absorption of the different test doses
varied according to the amount of triolein whichwas given. In terms of the percentage of each doseabsorbed, absorption was most rapid after feedingthe 10 mg. dose and became progressively sloweras the amount of fat in the test dose was increased(Fig. 4). If these results are expressed as the absoluteamount of fat absorbed, on the other hand, it canbe seen that the more fat was given, the greaterwas the actual amount of fat absorbed in a giventime (Fig. 5).
Distribution of radioactivity in small intestine Thepercentages of the administered radioactivity foundin the four segments of the small intestine of eachrat are given in Tables III to VI. The mean amountsin the different segments of the three rats killed ateach time after the various doses are shown in Fig. 6.
LE IVPERCENTAGE OF 50 MG. DOSE OF M3I1-LABELLED TRIOLEIN IN STOMACH AND INTESTINES OF RATS KILLED
AT INTERVALS OF QUARTER OF AN HOUR TO FOUR HOURS
Stomach Smnall Intestine
1 2 3 4
26-87-210-214-716-06-33-08-48-75-97-17-27-1
2-34-34-67-71-42-94-02-62-82-12-74-51-42-22-7
39-611.925-225-615-29-4
32-218-95-17-32-14-84-32-916-47-913-33-412-29-61.92-01-72-94-31-42-22-5
5-30-00-01-80-919-21-77-36-412-213-410-74-5
21-822-316-28-38-212-99-81-64-41-02-30-61-41.1
10
0-00-00-00-00-20-30-00-2
150
0-005
5212-20-30-34-305
11-80-44-32-03-08-44-51515
2-52-8
Colon Total % Gastric2Absorbed Emptying
1-20-00-00-40-80-80-005
0-50-20-20-31-20-60-50-80-90-60-50-78-811-02-67-58-67-98-98-5
98-692-898-196-585-473-183-680-771-156-2
84-870-764-347-458-056-646-339-747-542-541-635-029-035-226-327-234-830-1
1-47-21-93-5
14-626-916-419-328-943-815-229-335-752-642-043-453-766-352-557-558-465-0
71-064-873-772-863-269-9
74.326-337.346-047.762-953.354-664-669-438-557.565-080-585-877-184-491*781-495-876-588-286-883-593-286-472-183-9
'Percentage absorption was calculated by assuming that the material not recovered had been absorbed.'Percentage gastric emptying is the amount of material not recovered from the stomach.
26
Rat TinmeNo. Killed
(hr.)
282930
313233
343536
373839
404142
434445
464748
X 25-74-1 . 73.7
62-7Mean 54-0
2 52-34 37-12 46-7Mean 45-4
1 35-41 30-61 61-5
Mean 42-51) 35-014 19-514 14-2
Mean 22-92 15-62 8-32 18-6
Mean 14-23 24-53 11-83 13-2
M-an 16-54 6-84 13-64 27-9
Mean- 16-1
Studies on the site oJ'fat absorption
fi60,-/ IO OOmg.
20X-/ O O g
S4 >2 i2 2 34Hours
FIG. 4. Mean percentage absorption of doses of 10(+- +), 50 (O-O), 100 ( -), or 500(&^ A) mg. of,fat at intervals of quarter ofan hour tofour hours after feeding.
0SOOmg.
DIOOmg.
50mg.K Omrg.
2Hours
FIG. 5. Mean amounts offat absorbed (mg.) at intervalsof quarter hour to four hours after,feeding doses of 10(x x), 50 (O--O), 100 (e ), or 500(A A) mg. of 1311-labelled triolein.
HOURS AFTER DOSE
1/4 '12 I1/2 2 3 4
30L230 *Duodenum & jjunum
10
12 3 4 C 2 3 4 C 2 3 4 C 2 3 4 C 1 2 3 4 C 1 2 3 4 C 1 2 3 4 C
IN TESTINAL SEGMENT NUMBER
FIG. 6. Distribution of radioactive material in the different segments of the small intestine and in the colo,i of rats killedat intervals of quarter of an hour to four hours after feeding doses of 10, 50, 100, or 150 mg. of '311-labelled triolein.The intestinal segments were numbered as follows: (1) Duodenum and upper jejunum, (2) lower jejunum, (3) upper ileum,(4) lower ileum. C indicates the colon.
An0C
0
I,
3.
el-
27
30
LO-OM9.20
10
030
500 mg.20
10
C. C. Booth, A. E. Read, and E. Jones
When 10 mg. of 1311-labelled triolein was given,the mean percentage of the dose found in each ofthe proximal two segments at quarter of an hourwas 19-7% (Fig. 6, Table HI). The radioactivity inthese segments fell gradually during the next one tothree hours as absorption took place (Fig. 4), sothat at three and four hours no more than 1-9% ofthe administered radioactivity was present.At the same time, the mean radioactivity in the
third segment rose from 3 0% at quarter of an hour to8.8% at one and one and a half hours. It thenfell to 0.8 and 1.2% at three and four hours. Thefourth segment contained only small amounts ofradioactivity until three and four hours, at whichtimes the mean radioactivity in this segment was4.9 and 7.0% respectively (Fig. 6, Table III).When the dose was increased to 50 mg., the mean
percentage of the radioactivity present in the firstsegment at quarter of an hour was less than after10 mg., being 14.7%. At this time, more radio-activity (25.6%) was found in the second segment.The radioactivity in these two segments fell moreslowly than when 10 mg. was given, 9.6% of thedose being still present in the second segment attwo hours (Fig. 6, Table IV). By three and fourhours, as absorption proceeded, less than 2.9%was present in either of segments 1 and 2.
The mean radioactivity in the third segment wasonly 1.8% at quarter of an hour but rose to a peakof 16.2% at one and a half hours. It thenfell graduallyto 2-3 % at three hours and 1 % at four hours (Fig. 6,Table IV).The fourth segment contained little or no radio-
activity at quarter or half an hour but between 4and 5% of the administered radioactivity was foundin this segment between one and three hours.
After the dose of 100 mg. the mean percentageradioactivity in the different segments was essentiallysimilar to that after a dose of 50 mg. However, agreater proportion of the administered radio-activity was present in segments 3 and 4 at two hoursthan when either 10 or 50 mg. was given (Fig. 6,Table V).When a dose of 500 mg. was given, the percentage
of the radioactivity recovered from segments 1 and 2between a quarter of an hour and two hours wassimilar to that after 50 or 100 mg. However, theradioactivity in these segments fell more slowlythan after the smaller doses, for segment 1 still con-tained as much as 4.6% and segment 2, 10.4% atthree hours, at which time the mean radioactivityin these segments was invariably less than 2 to 3%after doses of 10 to 100 mg. (Fig. 6, Table VI).The mean radioactivity in the third segment was
TABLE VPERCENTAGE OF 100 MG. DOSE OF 1'11-LABELLED TRIOLEIN IN STOMACH AND INTESTINES OF RATS KILLED
1
4
1
11
11
2
22
33
3
444
AT INTERVALS OF
Stomach Small Intestine
QUARTER OF AN HOUR TO FOUR HOURS
Colon Total %1 Gastric'Absorbed Emptying
1 2 3 4
71-056-049.3
Mean = 58.871-744.540-2
Mean = 52-159.5372570
Mean = 51-248-928-216-0
Mean =31-07.412-928-8
Mean= 16-47.3
23-516-2
Mean= 15-75 6
16-6124
Mean= 1 1.5
9.083-013-211-74.57.310-37.43-1
11-711-08-61-910810-27-61-56-9675.01.93.91-62 52-31-72-32-1
11-223-328-621-06-1
22-624-617-86-97.4877.7
3.420-417-013-68-3
19-18-8
12-13.02-31-62-31-41-11-11-2
7 61-50.030
13-79.31-08-39-19-29.99.4907.3727-818-8709.511-81-52.51-81-92-42-81-82-3
0.00.00.00*00.50.0030.31-91-00.01-09-8034.95S0
22-31-93.59.32.53-11-72-42-38-65.25.4
0-1040.00-2070-6040-61-10.0030O5040709070-61-3090911-413-217-414-013-86-811*810-8
98994-291-194.797284-376886-179-666586-977.773.467756262-459.949-158255.727-648.540348-827837-634-633.3
1-15.88-95.32-815-723-213-920-433.513-122-326-632-343-837640-150941-844.372-451559.761-272-262-465466-7
29-044.050.741-228-355.5
59847.940.562-863-068-8511
71-884-069.092-687-171-283-692-776.583-884-394.483-487.688-5
'Percentage absorption was calculated by assuming that the material not recovered had been absorbed.2Percentage gastric emptying is the amount of material not recovered from the stomach.
28
Rat TimeNo. Killed
(hr.)
495051
525354
555657
585960
616263
646566
676869
Studies on the site offat absorption
TABLE VIPERCENTAGE OF 500 MG. DOSE OF 135I-LABELLED TRIOLEIN IN STOMACH AND INTESTINES OF RATS KILLED
AT INTERVALS OF QUARTER OF AN HOUR TO FOUR HOURS
Stomach Small Intestine
1 2 3 4
1361036010-011-05s68.58.47.3385.55.55 62.7634.92.57.346481.23.19.5461.7231.61-9
6924832821.512213.0244165861671281271467 61041093.412010-58 612736
15 01043.15.09.55.9
2317.14611616611312913.621.419210.517020.133017.523.54624817615 73827.74.515 12899.4
21 619-9
8 30o00o02808040o00408460o01 83.5660.03.43.91.50.01 .89.710-515-912019712912415-0
Colon Total '' Gastric'Absorbed Emptying
0o00o00o00o00o003000-13 6690o03.50.3060-103200602090o519.40969081461-15.5
96698.099097.992691 798.594.377.782-783-382-279.390777-682-582-576-973-677.785 579.373-879.559252.555.855.8
3.42-01-02-17.48-31I55.7
22-317-316-718820-79.3
22-417 517-523 126-422-314-520-726-220 540-847.544-244.2
55.343.244.448048039.957.348464068.545.559.364.859856 760443.969359.354271.865072-069694291 790492.1
'Percentage absorption was calculated by assuming that the material nol recovered had been absorbed.'Percentage gastric emptying is the amount of material not recovered from the stomach.
always greater than when doses of 10, 50, or 100 mg.were given. It rose from 11 6% at a quarter of an
hour to a maximum of 23.5 % at one and a halfhours, and remained as high as 15-7, l5 1, and 19-9%at two, three, and four hours (Fig. 6, Table VI) whenless than 3% was recovered from this segment afterthe smaller doses.The fourth segment contained only small amounts
of radioactivity until three and four hours, at whichtimes the mean radioactivity recovered from thissegment was 12.0 and 15.0% respectively. Theseamounts were also greater than were found in thefourth segment after the smaller doses.
Radioactivity in the colon Insignificant amountsof radioactivity were recovered from the colonuntil two to three hours after the doses were given(Fig. 6, Tables III to VI). At three and four hours,the colonic radioactivity was invariably less thanthe amounts excreted in the faeces after similar doseswere given to intact rats (Table I).
DISCUSSION
AMOUNTS OF FAT ABSORBED FROM INCREASING DOSES
These results show that when increasing doses of1311-labelled fat are given to rats, there is a progres-
sive increase in the percentage of the dose excretedin the faeces (Table I, Fig. 1). In terms of the amountof fat absorbed, however, there is a progressiveincrease in absorption as increasing doses of 1311.labelled triolein are given (Table II, Fig. 2).
SITES OF FAT ABSORPTION The sites of absorption ofthe various doses varied according to the amountfed. Since absorption invariably began soon afterfeeding (Fig. 4), it seemed likely that the labelledmaterial was absorbed first from the upper intestineregardless of the amount of fat given. This was
confirmed by the studies of the distribution of radio-activity in the different segments of the smallintestine during absorption (Fig. 6). Since much ofthe radioactivity in the proximal two segments dis-appeared during the first hour after feeding withoutsubsequently appearing in similar amounts in themore distal segments, it is evident that the jejunumabsorbed considerable amounts of fat. The impor-tance ofthe ileum (segments 3 and 4) in fat absorptiondepended on the amount of fat given, for the radio-activity reaching the distal two segments was
proportional to the amount of labelled fat given,the greatest amounts being recovered from theileum after the largest doses (Fig. 6). However, the
Rat TimeNo. Killed
(hr.)
707172
737475
767778
798081
828384
858687
888990
l 44.756-855.6
Mean = 52.0i 520i 60.1i 42-7Mean= 516
1 36-01 31-51 54-5
Mean =40 711 35.211 40211 43.3
Mean = 39-62 66-12 3072 407
Mean = 45-83 28-23 35.03 28-0
Mean = 30 44 5-84 8-34 9-6
Mean= 7-9
C. C. Booth, A. E. Read, and E. Jones
recovery of radioactive material from the ileum didnot necessarily indicate that absorption occurredthere. Although as much as 10% of a dose of 10 mg.of 131L-labelled triolein reached the ileum (segments3 and 4, Table III, Fig. 6), it could not be assumedthat this material was absorbed from that area.The mean faecal excretion after a dose of 10 mg. offat by an intact rat was 11-8% (Table I, Fig. 1), andthe 10% recovered from the ileum after a dose of10 mg. was therefore likely to be merely in transitto the colon for subsequent elimination in the faeces.When larger amounts of fat were fed, however, thepercentages which reached the distal intestine weregreater than were excreted in the faeces of intactrats given similar doses (Figs. 1 and 6), indicatingthat part of these larger doses must have beenabsorbed in the ileum.An approximate estimate of the amount of each
dose which was absorbed in the ileum has beenobtained by subtracting the percentage of the doseexcreted in the faeces by intact rats given the variousdoses (Table I, Fig. 1) from the maximum percentages
_ 2 r- 'a_-_s _ _I.s * . . *..r
increasing proportion was then absorbed there (TableVII). The ileum of the rat, however, appears to be lessefficient at absorbing fat than the jejunum, for muchof the labelled material which reached the ileumpassed on into the colon and was excreted in thefaeces (Tables I and VII). Dawson and Isselbacher(1960), studying the esterification of palmitate-l-C14by rat intestinal mucosa in vitro, have shown thatcell-free homogenates prepared from the ileum area quarter to a fifth as active as those from thejejunum. The results given in this paper demonstratean interesting parallel in vivo to their observations.The observation that fat may be absorbed in
increasing amounts from both the jejunum and theileum when the dietary load is increased provides arational explanation for the unlimited capacity ofthe small intestine to absorb fat (Fig. 2). Theabsorption of the largest doses may also be facilitatedby slower gastric emptying after such doses (Fig. 3),which serves to reduce the amount of fat deliveredto the intestine at any one time.
of each dose recovered trom the ileum together with RELATIONSHIP BETWEEN RATES AND SITES OF ABSORP-the colon at any one time (Tables III to VI, Fig. 6). TION If absorption of a substance begins in theIf the amount of fat absorbed in the ileum is then upper jejunum, the site of its absorption depends onsubtracted from the amount shown to be absorbed the relationship between its absorption rate and thefrom the equivalent doses (Table IL, Fig. 2), a upper intestinal motility (Parkins et al., 1960).similar approximation of jejunal absorption may be Glucose or iodide, for instance, are absorbed rapidlyderived. The results of these estimations, set out in and their absorption occurs in the proximal half ofTable VII, indicate that when 10 mg. of 1311-labelled the small intestine of the rat (Reynell and Spray,
1956; Parkins et al., 1960). 1311-labelled albumin,TABLE VII however, is absorbed more slowly; the rapid motility
CALCULATED ABSORPTION OF 1311-LABELLED TRIOLEIN of the upper small intestine propels it more distallyFROM JEJUNUM AND ILEUM before absorption is complete and some absorption
therefore also occurs in the ileum (Parkins et al.,Amount Fed (mg.) 1960).10 50 100 500 There appears to be a similar relationship between
'Mean faecal excretion absorption rate and the site of absorption of different(% of dose)' 11-8 11.0 160 24-2 doses of fat. The 10 mg. dose of fat, for instance,Maximum mean radioactivity was absorbed relatively rapidly (Fig. 4) and absorp-in ileum plus colon (% of dose)2 13-1 21.3 22-1 40 5 tion therefore occurred in the jejunum (Table VII).Calculated % of dose 1-3 10-3 6.1 16-3 A 500 mg. dose, however, was absorbed moreabsorption mg. 01 51 61 810 slowly (Fig. 4) and a greater proportion was there-Calculated /g.ofdose 86-9 787 77.9 S96 fore propelled as far as the ileum before absorptionabsorption was complete.from jejunum mg. 87 39.4 77.9 298-0
'Data from Table I.'Data from Tables IIT, IV, V, and VI.
fat was given absorption was almost entirely in thejejunum (Table VII). Larger doses of fat were alsoabsorbed to a great extent in the jejunum, whichresponded to an increased dietary load by an in-creased absorption, but at the same time increasingamounts of fat escaped absorption in the proximalsegments and therefore passed into the ileum; an
PREVIOUS OBSERVATIONS ON SITE OF FAT ABSORPTIONThe results given in this paper are in keeping withprevious observations which have suggested that thejejunum plays a major part in fat absorption(Borgstrom et al., 1957; Favarger and Gerlach,1953; Frazer, 1943; Turner, 1958). Other workershave claimed that in the rat maximal absorption of1311-labelled olive oil occurs in the third quarter ofthe small intestine, an area said to be specificallyadapted for this purpose (Benson et al., 1956). Fig. 6
30
Studies on the site offat absorption 31
illustrates a possible reason for this conclusion.These workers killed their rats only at two hours andlater after the administration of labelled fat, andmeasured the radioactivity in the mucosa of differentsegments of the small intestine. When rats are killedat two hours and later after the test doses, most ofthe radioactive material is likely to be recoveredfrom the ileum, for most of the jejunal fat has alreadybeen absorbed in this time (Fig. 6).
SUMMARY
The faecal excretion of radioactivity was measuredin rats given doses of 10, 50, 100, or 500 mg. of1311-labelled triolein. As the dose was increased, thepercentage of the dose excreted in the faeces becamegreater but there was a progressive increase in theamount of fat which was absorbed.The sites of absorption of the 131L-labelled triolein
varied according to the dose given. A dose of 10 mg.was absorbed relatively rapidly and absorptionoccurred in the jejunum. Larger doses, however,were absorbed more slowly and the motility of theupper intestine propelled the fat more distally beforeabsorption was complete; absorption therefore alsooccurred in the ileum when the larger doses weregiven.The increase in absorption of the labelled triolein
which occurred when increasing doses were givenwas achieved first by an enhanced jejunal absorptionin response to an increased dietary load, andsecondly by an increased absorption in the ileum.
We wish to thank Miss Janet Heath for her help in carry-ing out these experiments.
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