Digestão acelerada de proteínas após cirurgia bariátrica

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Accelerated protein digestion and amino acid absorption after
Roux-en-Y gastric bypass1,2
Kirstine N Bojsen-Møller,3,5,9* Siv H Jacobsen,3,5,9 Carsten Dirksen,3,5 Nils B Jørgensen,3,5 Søren Reitelseder,7
Jens-Erik B Jensen,3 Viggo B Kristiansen,4 Jens J Holst,5,6 Gerrit van Hall,6,8 and Sten Madsbad3,5
3Department of Endocrinology and 4Department of Surgical Gastroenterology, Hvidovre Hospital, Copenhagen, Denmark; 5Novo Nordisk Foundation Centre
for Basic Metabolic Research and 6Department of Biomedical Sciences, University of Copenhagen, Copenhagen, Denmark; 7Institute of Sports Medicine
Copenhagen, Bispebjerg Hospital, Copenhagen, Denmark; 8Department of Clinical Metabolomics, Rigshospitalet, Copenhagen, Denmark
ABSTRACT
Background: Roux-en-Y gastric bypass (RYGB) involves exclu-
sion of major parts of the stomach and changes in admixture of
gastro-pancreatic enzymes, which could have a major impact on
protein digestion and amino acid absorption.
Objective: We investigated the effect of RYGB on amino acid
appearance in the systemic circulation from orally ingested protein
and from endogenous release.
Design: Nine obese glucose-tolerant subjects, with a mean body
mass index (in kg/m2) of 39.2 (95% CI: 35.2, 43.3) and mean glycated
hemoglobin of 5.3% (95% CI: 4.9%, 5.6%), were studied before and
3 mo after RYGB. Leucine and phenylalanine kinetics were determined
under basal conditions and during 4 postprandial hours by intrave-
nous infusions of [3,3,3-2H3]-leucine and [ring-
2D5]-phenylalanine
combined with ingestion of [1-13C]-leucine intrinsically labeled ca-
seinate as the sole protein source of the meal. Changes in body
composition were assessed by dual-energy X-ray absorptiometry.
Results: After RYGB, basal plasma leucine concentration did not
change, but marked changes were seen postprandially with 1.7-fold in-
creased peak concentrations (before—mean: 217 mmol/L; 95% CI: 191,
243 mmol/L; 3 mo—mean: 377 mmol/L; 95% CI: 252, 502 mmol/L; P =
0.012) and 2-fold increased incremental AUC (before—mean: 4.1 mmol $
min/L; 95% CI: 2.7, 5.5 mmol $ min/L; 3 mo—mean: 9.5 mmol $ min/L;
95% CI: 4.9, 14.2 mmol $ min/L; P = 0.032). However, the postprandial
hyperleucinemia was transient, and concentrations were below basal
concentrations in the fourth postprandial hour. These concentration dif-
ferences were mainly caused by changes in leucine appearance rate from
orally ingested caseinate: peak rate increased nearly 3-fold [before—
mean: 0.5 mmol/(kg fat-free mass $ min); 95% CI: 0.4, 0.5 mmol/(kg
fat-free mass $ min); 3 mo—mean 1.4 mmol/(kg fat-free mass $ min);
95% CI: 0.8, 1.9 mmol/(kg fat-free mass $ min); P = 0.002], and time to
peak was much shorter (before—mean: 173 min; 95% CI: 137, 209 min;
3 mo—mean: 65 min; 95% CI: 46, 84 min; P , 0.001). Only minor
changes were seen in endogenous leucine release after RYGB.
Conclusions: RYGB accelerates caseinate digestion and amino
acid absorption, resulting in faster and higher but more transient
postprandial elevation of plasma amino acids. Changes are likely
mediated by accelerated intestinal nutrient entry and clearly
demonstrate that protein digestion is not impaired after RYGB. This
trial was registered at clinicaltrials.gov as NCT01559792. Am J
Clin Nutr 2015;102:600–7.
Keywords: protein digestion, amino acid absorption, bariatric sur-
gery, intrinsically labeled protein, caseinate, milk protein, gastric
emptying, glucagon-like peptide-1, glucagon
INTRODUCTION
Roux-en-Y gastric bypass (RYGB)10 induces weight loss of
20–30% that is maintained for at least 20 y (1, 2). The surgery
involves extensive rearrangements of the upper gastrointestinal
tract, resulting in formation of a gastric pouch (25–50 mL) that
drains directly into the lower part of the jejunum, thus excluding
the remaining stomach, duodenum, and upper jejunum from
food exposure. Secretions from the bypassed stomach, exocrine
pancreas, and biliary system are mixed with food through an
entero-entero anastomosis located approximately 100–150 cm
distal to the pouch. RYGB was initially believed to inhibit food
intake through restriction, but recent studies have demonstrated
accelerated pouch emptying (3) and rapid absorption of the
liquid phase marker, paracetamol (acetaminophen) (4), as well
as glucose postoperatively (5–8). The absorption kinetics of
lipids and proteins are largely unknown. Exclusion of major
parts of the stomach and changed admixture of food with gastro-
pancreatic enzymes have been suggested to impair or delay
1 Supported by the Danish Council for Independent Research for Medical
Sciences, the Novo Nordisk Foundation, and the Strategic Counsel for the
Capital Area of Copenhagen (Denmark). This work was carried out as a part
of the program of the UNIK: Food, Fitness & Pharma for Health and Disease
(see www.foodfitnesspharma.ku.dk). The UNIK project is supported by the
Danish Ministry of Science, Technology and Innovation. None of the fund-
ing sources had any influence on study design, implementation, analysis, or
interpretation of the data.
2 SupplementalMaterial and Supplemental Figures 1–4 are available from
the “Supplemental data” link in the online posting of the article and from the
same link in the online table of contents at http://ajcn.nutrition.org.
9 These authors contributed equally to this work.
*To whom correspondence should be addressed. E-mail: kirstine.
bojsen-moeller@regionh.dk.
ReceivedFebruary 13, 2015. Accepted for publication July 2, 2015.
First published online August 5, 2015; doi: 10.3945/ajcn.115.109298.
10Abbreviations used: FFM, fat-free mass; GLP-1, glucagon-like peptide-1;
Ra, rate of appearance; Rd, rate of disappearance; RYGB, Roux-en-Y gastric
bypass.
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protein digestion (9). Therefore, we investigated the effect of
RYGB on the appearance of amino acids in the systemic cir-
culation from orally ingested protein as well as from endoge-
nous sources during a mixed meal. Leucine and phenylalanine
kinetics were determined under basal conditions and 4 h post-
prandially by intravenous infusions of [3,3,3-2H3]-leucine and
[ring-2D5]-phenylalanine combined with ingestion of [1-
13C]-
leucine intrinsically labeled caseinate as the sole protein source
of the meal. Importantly, leucine has been suggested as a regu-
lator of anabolic and metabolic signaling in human skeletal
muscles (10–12). Compared with leucine, phenylalanine has less
special properties and is less abundant in casein, which poten-
tially could influence systemic kinetics (13).
METHODS
Subjects
The study group included 10 obese subjects undergoing lap-
aroscopic RYGB at Hvidovre Hospital (Copenhagen, Denmark).
Before enrollment in the study, all participants fulfilled the in-
clusion criteria for bariatric surgery in Denmark and had com-
pleted a preoperative diet-induced total body weight loss of 8%
required by health authorities. All subjects presented preopera-
tively with fasting plasma glucose #6.1 mmol/L, 2-h plasma
glucose of ,7.8 mmol/L after a 75-g oral glucose load, and
a preoperative glycated hemoglobin ,6.0% (,42 mmol/mol)
(Table 1). One participant who was examined both before and
after surgery experienced serious postoperative complications
requiring prolonged hospitalization and was excluded from data
analysis. Data on glucose absorption and metabolism in the
same study population have been reported previously (5).
Ethics
Written, informed consent was obtained from all subjects, and
the study was approved by the Municipal Ethical Committee of
Copenhagen in accordance with the Helsinki declarationand by
the Danish Data Protection Agency and was registered at www.
clinicaltrials.gov as NCT01559792.
Methods
Subjects were studied within 2 wk before RYGB and 3 mo
after the operation. Each study session included a meal test and
a dual-energy X-ray absorptiometry scan. On each study day,
subjects arrived after an overnight fast (10–12 h) and were
weighed and placed in a reclined position in a hospital bed
allowing no physical activity. Cannulas were inserted in veins
of each arm, one for tracer infusions and one for blood
sampling. Primed continuous infusions of [3,3,3-2H3]-leucine
[3 mmol/kg fat-free mass (FFM), 0.07 mmol/kg FFM per
min) and [ring-2D5]-phenylalanine (1.5 mmol/kg FFM, 0.03
mmol/(kg FFM $ min)) were provided with use of a volu-
metric infusion pump (mVP5000; Arcomed ag). The exact,
measured infusion concentration was used for later calcula-
tions. All stable isotopes (Cambridge Isotope Laboratories)
were prepared under sterile conditions by the Capital Region
Pharmacy (Herlev, Denmark). After 2 h of tracer infusions,
the test meal was provided and infusions continued for 4 h
postprandially.
Test meal was a semiliquid mixed meal (200 mL, 394 kcal,
energy percentage: carbohydrate 50%, protein 15%, fat 35%)
consisting of glucose (50 g), rapeseed oil (14.1 g), and [1-13C]-
leucine intrinsically labeled calcium caseinate (15.2 g) with
a [1-13C]-leucine to leucine ratio of 10.08%. Details on pro-
duction of intrinsically labeled caseinate by the infusion of
[1-13C]-leucine in cows have been described previously (14).
Aroma was added to the mixed meal to disguise the distinct
flavor of caseinate. The meal was consumed slowly over 30 min,
and meal ingestion was supervised to secure that intake was
evenly distributed over this period. Blood was sampled be-
fore initiation of tracer infusions to determine natural isotope
abundance. Thereafter, blood samples were obtained at 230,
215, 0, 15, 30, 45, 60, 90, 120, 180, and 240 min relative to
meal intake.
Dual-energy X-ray absorptiometry scans were performed by
using a Hologic Discovery A (S/N 83487) dual-energy X-ray
absorptiometry scanner (Hologic Inc.) with software package
Apex 2.3 to determine fat percentage and FFM.
Surgical procedure
Operations were performed at the Department of Gastroen-
terology at Hvidovre Hospital with use of a standard laparoscopic
RYGB technique. A gastric pouch was created (w25 mL) that
drained directly into an alimentary jejunal limb of w100 cm.
The remaining stomach, duodenum, and first part of the jejunum
TABLE 1
Body composition and glycemic control before and 3 mo after Roux-en-Y gastric bypass in subjects with normal glucose
tolerance (n = 9)1
Before 3 mo Change P value
Male/female patients, n 3/6 3/6
Days from surgery 24 (27, 21)2 92 (83, 102)
BMI, kg/m2 39.2 (35.2, 43.3) 32.9 (29.0, 36.8) 26.3 (26.9, 25.7) ,0.001
Weight, kg 116 (105, 126) 97 (86, 108) 218.5 (219.9, 217.1) ,0.001
Fat-free mass, kg 64 (57, 71) 57 (52, 63) 26.6 (28.8, 24.4) ,0.001
Fat mass, kg 49 (42, 56) 39 (31, 46) 210.5 (213.0, 28.0) ,0.001
Fasting plasma glucose, mmol/L 5.2 (4.9, 5.5) 4.9 (4.6, 5.1) 20.3 (20.6, 20.1) 0.025
HbA1c,3 % 5.3 (4.9, 5.6) 5.2 (5.0, 5.5) 20.1 (20.4, 0.2) 0.451
1Results have previously been reported (5). Paired t tests were used for comparisons.
2Mean; 95% CI in parentheses (all such values).
3HbA1c, glycated hemoglobin.
PROTEIN UPTAKE AFTER ROUX-EN-Y GASTRIC BYPASS 601
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(intestinal length ofw75 cm) were excluded from food passage,
but gastro-pancreatic secretions and bile (i.e., biliopancreatic
limb) were mixed with food through an entero-entero anasto-
mosis w100 cm distal to the pouch.
Analytic procedures
Blood was collected into prechilled EDTA-coated tubes and
was immediately centrifuged at 2000 3 g for 10 min at 48C.
Plasma samples were frozen and stored at 2808C until analysis.
A 200-mL aliquot of plasma was mixed with 100 mL internal
standard (50 mmol/L L-[U-13C9]-phenylalanine + 150 mmol/L
[U-13C6]-leucine) and 500 mL 50% acetic acid before being
passed through a strong cation exchange column (Dowex AG
50W-X8; Bio-Rad). The purified amino acids were eluted with
3 mL 2 M NH4OH, dried under a stream of N2, and converted
to their t-butyldimethylsilyl derivatives by adding 200 mL
N-methyl-N-(tert-butyldimethylsilyl)-trifluoroacetamide and ace-
tonitrile (1:3) and heating for 1 h at 708C. The enrichment of the
sample and the internal standards was determined with use of gas
chromatography–tandem mass spectrometry (Tracer GC Ultra-
TSQ Quantum; Thermo Scientific) with electron impact ioniza-
tion and selective ion monitoring.
Calculations and statistical analysis
Data are expressed as means (95% CIs) of absolute values and
means (95% CIs) of absolute postoperative change unless oth-
erwise stated. Mean basal concentrations were calculated as
a mean of the samples taken within the last 30 min before meal
ingestion, and for the 4-h postprandial period, the total AUC was
calculated by using the trapezoidal method with subtraction
of basal concentrations to yield incremental AUC. Total rate
of appearance (Ra) and disappearance (Rd) of leucine and
phenylalanine were estimated with standard non–steady-state
equations (15) by using enrichments of [3,3,3-2H3]-leucine and
[ring-2D5]-phenylalanine, respectively. Oral leucine appearance
rate was calculated from [1-13C]-leucine enrichments multiplied
by total leucine appearance rate and corrected for time-dependent
variations in plasma [1-13C]-leucine enrichment, and endog-
enous leucine appearance rate was calculated by subtracting the
oral leucine Ra from the total leucine Ra. Total systemic ap-
pearance of leucine in the 4 postprandial hours (total AUC of
oral leucine Ra) was expressed as a percentage of the total
leucine content in the caseinate. Equations and enrichments are
provided in the Supplemental Material and Supplemental Fig-
ure 1, respectively.
Single parameters (basal/peak concentrations and AUCs) were
compared with paired t tests. Postprandial time courses of leu-
cine and phenylalanine kinetics were analyzed by ANOVA in
a linear mixed-effects model by using postprandial time (time)
and study session (session) and the time 3 session interaction
as fixed categorical effects and individual subjects as a ran-
dom effect. Logarithmic transformation of data was used if
required for optimal model fit. Time 3 session interaction
was reported as the primary readout, and P , 0.05 was in-
terpreted as a change in the postprandial time course after
RYGB. Statistical analyses were performed in R version 3.0.2
(www.R-project.org).
RESULTS
Weight loss and changes in body composition after RYGB
have been reported previously along with changes in oral and
endogenous glucose kinetics and secretion of insulin, glucagon,
glucagon-like peptide-1 (GLP-1), and glucose-dependent insu-
linotropic polypeptide (5). Three months after RYGB, patients
had a mean reduced body weight of 18.5 kg, which included
a mean of 6.6 kg of FFM (Table 1). Furthermore, patients ex-
perienced reductions in hepatic insulin resistance (mean relative
change from before: 239%; 95% CI: 262%,217%; P = 0.005)
and fasting insulin concentration (mean relative change: 241%;
95% CI: 261%, 221%; P = 0.003), as well as accelerated
postprandial glucose absorption (mean relative change in
peak Ra of oral glucose: +165%; 95% CI: +96%, +236%; P ,
0.001) and increased postprandial secretion (incremental AUC)
of glucoregulatory hormones: insulin (mean relative change:
+83%; 95% CI: +50%, +116%; P , 0.001), GLP-1 (mean
relative change: +485%; 95% CI: +150%, +819%; P , 0.001),
and glucagon (incremental AUC reversed from negative to
positive, P = 0.026) (5).
Leucine concentration and kineticsBasal plasma leucine concentration was unchanged 3 mo after
RYGB (before—mean: 173 mmol/L; 95% CI: 148, 197 mmol/L;
3 mo—mean: 161 mmol/L; 95% CI: 132, 191 mmol/L; change—
mean: 211 mmol/L; 95% CI: 238, 15 mmol/L; P = 0.345).
Marked changes were seen in the postprandial leucine concen-
trations (Figure 1A and Supplemental Figure 2 for individual
concentration curves) with a 1.7-fold increased peak concen-
tration (before—mean: 217 mmol/L; 95% CI: 191, 243 mmol/L;
3 mo—mean: 377 mmol/L; 95% CI: 252, 502 mmol/L; change—
mean: +160 mmol/L; 95% CI: +46, +274 mmol/L; P =
0.012) and 2-fold increased incremental AUC (before—mean:
4.1 mmol $ min/L; 95% CI: 2.7, 5.5 mmol $ min/L; 3 mo—
mean: 9.5 mmol $ min/L; 95% CI: 4.9, 14.2 mmol $ min/L;
change—mean: +5.4 mmol $ min/L; 95% CI: +0.6, +10.2 mmol $
min/L; P = 0.032). However, the postprandial hyperleucinemia
was transient after RYGB, and in the fourth postprandial hour
(at 180 and 240 min), leucine concentrations were statistically
significantly below basal concentrations only after RYGB, re-
sulting in unchanged total AUC after surgery (P = 0.531).
Basal leucine Ra and Rd did not change post-RYGB when
corrected for changes in FFM [mmol/(kg FFM $min); Figure 1B,
C] but was lower after surgery when expressed in absolute rates
(mmol/min; Figure 1D, E). The postprandial changes were
largely independent of the corrections for FFM (Figure 1B–E).
Postprandial total Ra and Rd of leucine changed after RYGB
(Figure 1B, C) with 1.5-fold increased peak rates and w20%
lower nadir rates after surgery (P = 0.004). The mean (95% CI)
total Ra peak before and 3 mo after surgery was 2.0 (1.8, 2.1)
mmol/(kg FFM $ min) and 3.0 (2.3, 3.8) mmol/(kg FFM $ min),
respectively, representing a change of +1.1 (+0.4, +1.8) mmol/
(kg FFM $ min) (P = 0.006). The mean (95% CI) total Rd peak
before and 3 mo after surgery was 1.9 (1.8, 2.1) mmol/(kg
FFM $ min) and 2.9 (2.3, 3.5) mmol/(kg FFM $ min), respec-
tively, representing a change of +1.0 (+0.4, +1.5) mmol/(kg
FFM $ min) (P = 0.003). Total Ra and Rd were below basal at
180 min and 240 min both before and after surgery but were
significantly lower after than before.
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Endogenous Ra of leucine was unchanged in the basal state, but
the postprandial time course changed after RYGB (Figure 1F) with
w20% lower nadir values: the mean (95% CI) values before and
3 mo after surgery were 1.1 (1.0, 1.2) mmol/(kg FFM $ min)
and 0.9 (0.8, 1.0) mmol/(kg FFM $ min), respectively, repre-
senting a change of 20.2 (20.3, 20.1) mmol/(kg FFM $ min)
(P = 0.009). Maximal postprandial suppression from basal to
nadir tended to increase (P = 0.079), but changes in the duration
of postprandial suppression could not be determined from the
present study because endogenous Ra of leucine at the end of
the experiment (t = 240 min) remained suppressed at a similar
rate (P = 0.910) before and after surgery.
Systemic appearance of leucine from orally ingested caseinate
was dramatically changed after RYGB (Figure 1G and Sup-
plemental Figure 3 for individual curves). Peak Ra of oral
leucine increased nearly 3-fold, with mean (95% CI) values be-
fore and 3 mo after surgery of 0.5 (0.4, 0.5) mmol/(kg FFM $ min)
and 1.4 (0.8, 1.9) mmol/(kg FFM $ min), respectively, repre-
senting a change of +0.9 (+0.4, +1.4) mmol/(kg FFM $ min) (P =
0.002). The time to peak also was much shorter, with mean
(95% CI) values before and 3 mo after surgery of 173 (137, 209)
min and 65 (46, 84) min, respectively, representing a change of
2108 (2158, 259) min (P , 0.001) and reflecting accelerated
systemic uptake of leucine after RYGB. Expressing rate per
kilogram of FFM or as whole-body Ra did not influence con-
clusions (data not shown). Total systemic appearance of leucine
in the 4 postprandial hours was expressed as a percentage of the
total leucine content in caseinate and increased after RYGB
from a mean (95% CI) of 49% (40%, 58%) to 70% (61%, 79%),
representing an absolute change of +21% (+10%, +31%) (P =
0.002), equivalent to the systemic appearance of leucine from
7.4 g and 10.6 g of the ingested protein, respectively. However,
protein uptake was not completed within the 4 h of postprandial
blood sampling neither before nor after surgery (Figure 1G).
FIGURE 1 Leucine concentration (A), total Ra of leucine in mmol/min per kilogram of FFM (B), Rd of leucine in mmol/min per kilogram of FFM (C),
total Ra of leucine in mmol/min (D), Rd of leucine in mmol/min (E), Ra of endogenous leucine in mmol/min per kilogram of FFM (F), and Ra of oral leucine in
mmol/min per kilogram of FFM (G) before (open squares) and 3 mo after (solid squares) Roux-en-Y gastric bypass in subjects with normal glucose tolerance
(n = 9). Values are means 6 SEMs. Changes in the basal state were evaluated by paired t tests: P = 0.345 (A), P = 0.133 (B), P = 0.163 (C), P , 0.001 (D),
P , 0.001 (E), P = 0.133 (F), and P = 1.0 (G). Postprandial time courses were evaluated by ANOVA in a linear mixed-effects model using the time 3 session
interaction: P, 0.0001 (A), P, 0.0001 (B), P, 0.0001 (C), P, 0.0001 (D), P, 0.0001 (E), P, 0.004 (F), and P, 0.0001 (G). Endo., endogenous; FFM,
fat-free mass; Ra, rate of appearance; Rd, rate of disappearance.
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Phenylalanine concentration and kinetics
Basal phenylalanine concentration was significantly lower
3 mo after RYGB (before—mean: 61 mmol/L; 95% CI: 56,
67 mmol/L; 3 mo—mean: 52 mmol/L; 95% CI: 46, 57 mmol/L;
change—mean: 29 mmol/L; 95% CI: 215, 24 mmol/L; P =
0.003), and changes in the entire postprandial period were
prominent (Figure 2A). Preoperatively, phenylalanine concen-
trations increased slowly after meal intake, reaching a mean
(95% CI) maximal increment (Dpeak-basal) of 12 (10, 13)
mmol/L without returning to basal values at 240 min (Dbasal-
240 significantly below zero, P = 0.032). After RYGB, the mean
(95% CI) maximal increment was 37 (22, 51) mmol/L (P =
0.004), and in the third and fourth postprandial hours, phenyl-
alanine concentrations had reached basal values (Dbasal-180
and Dbasal-240 were not statistically significantly different from
zero).
Systemic total Ra and Rd of phenylalanine declined in the
basal state after RYGB, whether expressed as mmol/(kg FFM $
min) or mmol/min (Figure 2B–E). Postprandial kinetics of
phenylalanine changed dramatically after RYGB (Figure 2B,C).
Before surgery, the meal induced a minor increase in total Ra
and Rd of phenylalanine in the first 4 postprandial hours, but
after surgery peaks were clearly increased. For Ra, mean (95%
CI) values before and 3 mo after surgery were 0.71 (0.64, 0.78)
mmol/(kg FFM $ min) and 0.99 (0.79, 1.20) mmol/(kg FFM $ min),
respectively, representing a change of +0.29 (+0.09, +0.48)
mmol/(kg FFM $ min) (P = 0.010). For Rd, mean (95% CI)
values before and 3 mo after surgery were 0.70 (0.63, 0.77)
mmol/(kg FFM $ min) and 0.99 (0.76, 1.22) mmol/(kg FFM $ min),
respectively, representing a change of +0.29 (+0.08, +0.50) mmol/
(kg FFM $ min) (P = 0.012). Endogenous Ra and oral Ra of
phenylalanine were not calculated because caseinate was only in-
trinsically labeled with leucine and not phenylalanine.
DISCUSSION
In this study, we demonstrate that RYGB results in marked
changes of the systemic leucine and phenylalanine kinetics after
ingestion of a mixed meal containing caseinate as the sole protein
source. A faster and higher but more transient postprandial el-
evation of the amino acids was observed in the systemic circu-
lation. The accelerated appearance of the meal protein–derived
amino acids in plasmais most likely caused by accelerated entry
of nutrients into the small intestine combined with accelerated
protein digestion and amino acid absorption. A lower splanchnic
extraction of the amino acids may contribute but is likely secondary
FIGURE 2 Phenylalanine concentration (A), total Ra of phenylalanine in mmol/min per kilogram of FFM (B), Rd of phenylalanine in mmol/min per
kilogram of FFM (C), total Ra of phenylalanine in mmol/min (D), and Rd of phenylalanine in mmol/min (E) before (open squares) and 3 mo after (solid
squares) Roux-en-Y gastric bypass in subjects with normal glucose tolerance (n = 9). Values are means6 SEMs. Changes in the basal state were evaluated by
paired t tests: P = 0.003 (A), P = 0.051 (B), P = 0.052 (C), P = 0.006 (D), and P = 0.007 (E). Postprandial time courses were evaluated by ANOVA in a linear
mixed-effects model using the time 3 session interaction: P , 0.0001 (A), P , 0.0001 (B), P , 0.0001 (C), P , 0.0001 (D), and P , 0.0001 (E). FFM, fat-
free mass; Ra, rate of appearance; Rd, rate of disappearance.
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to the accelerated amino acid absorption after RYGB (13, 16–
18). Furthermore, our results confirm that the postprandial
increases in plasma amino acids observed in previous RYGB
studies (19, 20) result from accelerated appearance of amino
acids from orally ingested protein and not from increased en-
dogenous release.
Accelerated glucose absorption has previously been demon-
strated after RYGB (5–8) and is a consequence of the surgical
changes in gastrointestinal anatomy, resulting in faster passage
of food through the pouch and the upper anastomosis and not
weight loss in general (8). Accelerated amino acid absorption
after protein intake is, however, somewhat surprising because
protein in contrast to glucose needs to be digested. A major
novel finding in this study is that protein digestion is not im-
paired after RYGB despite exclusion of major parts of the
stomach and distal admixture of gastro-pancreatic secretions.
This is likely a reflection of an almost instantaneous delivery of
nutrients not only through the upper anastomosis but also all the
way to the common limb. Scintigraphic studies after RYGB
have shown detectable amounts of solid food in the small in-
testine 15 min after meal start but could not distinguish between
the alimentary and common limb (3). Transit of liquids may be
even faster as suggested by reports of cecal arriving time of
26 min after ingestion of a glucose drink (21) and maximal
concentrations of paracetamol almost immediately after intake
(4). Consistent findings of exaggerated secretion of L-cell
products after RYGB (22) also support a rapid passage of nu-
trients to distal segments of the small intestine where L cells are
located with the highest density (23). Studies in rodents have
demonstrated significant hypertrophia of gut mucosa of the al-
imentary limb, which contributes to enhanced postprandial
glucose uptake (24), but it has not been investigated whether
these gut mucosal adaptations affect protein uptake. Intriguingly,
the rates of appearances of oral leucine and oral glucose seem
to be superimposable only after RYGB [Figure 1G and (5),
Supplemental Figure 4 for superimposed curves]. However,
protein uptake in the alimentary limb may be questionable be-
cause it would require brush-border enzymes to digest protein in
the absence of gastro-pancreatic secretions. Gastric secretions of
acid and pepsin may not be essential for protein digestion,
because complete gastrectomy does not cause severe protein
malabsorption (25). Exclusion of major parts of the stomach
post-RYGB may in fact limit the acid-mediated clotting of ca-
sein, which is believed to delay gastric emptying (26, 27). In
contrast, absence of secretions from the exocrine pancreas will
cause severe protein malabsorption (28), although protein di-
gestion can occur even with small outputs (28). Therefore, se-
cretions from the exocrine pancreas must still be delivered in
adequate amounts to the common limb for protein digestion not
to be impaired after RYGB.
Accelerated protein digestion and amino acid absorption may
have metabolic as well as nutritional consequences. Exaggerated
postprandial releases of GLP-1 and insulin have been reported
consistently after RYGB (22), including in the present study (5),
and are important for improved glucose tolerance in patients with
type 2 diabetes after surgery (29). Amino acids, especially
leucine and phenylalanine, are potent stimulators of insulin se-
cretion (15, 16, 30, 31) and are able to stimulate GLP-1 secretion
as well (32). Accelerated digestion of protein may act in concert
with glucose to enhance postprandial insulin and GLP-1 release
in response to mixed meals after RYGB, although glucose seems
to mediate substantial effects by itself (33). Moreover, high
concentrations of amino acids could contribute importantly to the
paradoxical increase in postprandial glucagon release (30, 34,
35), which is a consistent finding after RYGB (22), also in the
present population (5). Indeed, the postprandial glucagon release
is especially pronounced in response to mixed meals (33), al-
though also observed after glucose alone (36). Recently, it has
been demonstrated that post-RYGB hyperglucagonemia repre-
sents fully processed (and therefore biologically active) gluca-
gon, but the source (L cells or a cells) and metabolic importance
are still unknown (37).
The nutritional anabolic effect of ingested protein depends on
the rate of digestion and the postprandial profile of plasma amino
acids, as clearly demonstrated when assessing protein turnover
after ingestion of slowly digested micellar casein compared with
fast whey (14, 16, 17, 26, 27, 38) or casein hydrolysate (15, 16).
Slow or fast digestion rates are, however, reflections of the gastric
emptying rate and not the protein itself, as clearly demonstrated
in this study, in which RYGB changed the digestion rate of
caseinate. Protein turnover may be altered post-RYGB as a
consequence of accelerated digestion but is also influenced by
weight loss itself. Marked weight loss will inevitably decrease
FFM along with fat mass, and in the present study, patients lost
w7 kg of FFM, equivalent tow10% of initial FFM orw36% of
the total weight lost (5). Similar decreases in FFM have been
found in other studies 3–6 mo after RYGB (36, 39), as well as
after equivalent weight loss induced by gastric banding (8) or
gastric sleeve (40). The degree of caloric restriction determines
the proportion of FFM lost during major weight loss (40, 41) as
well as the initial FFM (42). Severe protein malnutrition (hy-
poalbuminemia) has been observed after RYGB but is rare (43,
44) and occurs primarily in patients with low protein intake (45),
after severe postoperative complications, or after long limb
procedures (43). Based on our results, decreased uptake of
protein is not a likely contributor to protein loss after RYGB;
however, the transient postprandial hyperaminoacidemia may
potentially limit the anabolic phases over 24 h. Therefore, it
seems reasonable to recommend small but frequent meals post-
RYGB. As for the source of protein, this study demonstrates that
the concept of slow and fast digested proteins may not apply to
RYGB patients.
In the present study, protein uptake was not completed within
the 4 h of postprandial blood sampling, which limits the eval-
uation of total protein uptake and splanchnic extraction, espe-
cially presurgery. To determine the nutritional consequences of
RYGB, we need further studies investigating 24-h systemic
protein turnover and in particular postprandial turnover rates of
individual protein stores, particularly skeletal muscle. For the
evaluation of proteinturnover, patients should be studied more
than 1 y post-RYGB to avoid the influence of calorie restriction.
The strength of the study is the use of intrinsic labeled protein,
which allows the investigation of the combined effect of di-
gestion, absorption, and metabolism of protein. However, the use
of intrinsic labeled protein excluded the use of a more everyday
protein source such as meat, which may be digested more slowly
due to the solid texture.
In conclusion, RYGB accelerates caseinate digestion and
amino acid absorption, resulting in a faster and higher but also
more transient postprandial elevation of plasma amino acids.
PROTEIN UPTAKE AFTER ROUX-EN-Y GASTRIC BYPASS 605
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These findings demonstrate that protein digestion is not impaired
after RYGB but in contrast accelerated as a result of the extensive
changes in upper gastrointestinal anatomy. Furthermore, our
results are consistent with gastric emptying rate as a rate-limiting
step in protein digestion and supports previous findings of ac-
celerated nutrient passage to distal intestinal segments after
RYGB, potentially stimulating the secretion of L-cell products.
This work would not have been possible without the professional help of
Dorthe Baunbjerg Nielsen, Sussi Polmann, and Alis Sloth Andersen (Hvidovre
Hospital, Denmark) and Nina Pluszek and Lene Foged (Clinical Metabolomics
Core Facility, Rigshospitalet, Copenhagen, Denmark). We thank Steen
Ladelund (chief statistician at Hvidovre Hospital, Denmark) for statistical
support.
The authors’ responsibilities were as follows—KNB-M and SHJ: conducted
the research; KNB-M: performed the data analysis, wrote the manuscript, and
has primary responsibility for the final content of the manuscript; CD, NBJ,
SR, J-EBJ, and VBK: contributed to the data analysis and discussion and
reviewed/edited the manuscript; JJH, GvH, and SM: contributed to the de-
sign of the study, data generation and analysis, and discussion and reviewed/
edited the manuscript; and all authors: approved the final version of the
manuscript. KNB-M, SHJ, CD, NBJ, JJH, and SM are affiliated with the
Novo Nordisk Foundation Centre for Basic Metabolic Research, which is
funded by the Novo Nordisk Foundation. SHJ and NBJ have one-third of
their PhD scholarship funded by Novo Nordisk. SR has received funding
from Arla Foods Ingredients Group P/S and the Danish Dairy Research
Foundation. J-EBJ, VBK, and GvH declared no conflicts of interest related
to this study.
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