Ingestion of a protein hydrolysate is accompanied by an accelerated in vivo digestion and absorption rate when compared with its
intact protein
Rene´ Koopman, Nico Crombach, Annemie P Gijsen, Ste´phane Walrand, Jacques Fauquant, Arie K Kies, Sophie Lemosquet,
Wim HM Saris, Yves Boirie, and Luc JC van Loon
Background: It has been suggested that a protein hydrolysate, as
opposed to its intact protein, is more easily digested and absorbed
from the gut, which results in greater plasma amino acid availability
and a greater muscle protein synthetic response.
Objective: We aimed to compare dietary protein digestion and
absorption kinetics and the subsequent muscle protein synthetic response
to the ingestion of a single bolus of protein hydrolysate
compared with its intact protein in vivo in humans.
Design: Ten elderly men (mean 6 SEM age: 64 6 1 y) were
randomly assigned to a crossover experiment that involved 2 treatments
in which the subjects consumed a 35-g bolus of specifically
produced L-[1-13C]phenylalanine-labeled intact casein (CAS) or hydrolyzed
casein (CASH). Blood and muscle-tissue samples were
collected to assess the appearance rate of dietary protein–derived
phenylalanine in the circulation and subsequent muscle protein fractional
synthetic rate over a 6-h postprandial period.
Results: The mean (6SEM) exogenous phenylalanine appearance
rate was 27 6 6% higher after ingestion of CASH than after ingestion
of CAS (P , 0.001). Splanchnic extraction was significantly
lower in CASH compared with CAS treatment (P , 0.01).
Plasma amino acid concentrations increased to a greater extent (25–
50%) after the ingestion of CASH than after the ingestion of CAS
(P , 0.01).
Muscle protein synthesis rates averaged 0.054 6
0.004% and 0.068 6 0.006%/h in the CAS and CASH treatments,
respectively (P = 0.10).
Conclusions: Ingestion of a protein hydrolysate, as opposed to its
intact protein, accelerates protein digestion and absorption from the
gut, augments postprandial amino acid availability, and tends to
increase the incorporation rate of dietary amino acids into skeletal
muscle protein. Am J Clin Nutr 2009;90:106–15.
INTRODUCTION
Food intake promotes net muscle protein accretion by providing
ample amino acids (AAs) as precursors for protein assembly
(1). The quantity and quality of the ingested protein, ie, its
digestibility and AA composition, represent important factors
that modulate the anabolic response of skeletal muscle to dietary
protein ingestion (2).
The ingestion of a protein hydrolysate, as opposed to its intact
protein, has been proposed to facilitate protein digestion and
absorption, increase plasma AA availability, and thereby augment
the postprandial muscle protein synthetic response (3). A
more rapid increase in circulating plasma AA concentrations has
previously been reported after the ingestion of a protein hydrolysate
compared with its intact protein (3). However, absolute
changes in plasma AA concentrations do not necessarily represent
changes in the appearance rate of exogenous (dietary) AAs
(4). Although some studies have measured gastric emptying (3),
nitrogen excretion (5), and gut endogenous nitrogen flow (6),
direct evidence that supports the proposed differences in digestion
and absorption kinetics after the ingestion of a protein
hydrolysate, compared with its intact protein in vivo in humans,
remains lacking. This is partly due to the restrictions set by the
methodology that has been used to assess the appearance rate of
AAs from the gut into the circulation. Because free AAs and
protein-derived AAs exhibit a different timing and efficiency of
intestinal absorption (7), simply adding labeled free AAs to
a protein-containing drink does not provide an accurate measure
of the digestion and absorption kinetics of the ingested dietary
protein (8). To accurately assess the appearance rate of AAs
derived from dietary protein, the labeled AAs need to be incorporated
into the dietary protein source (7, 9). Therefore, we
produced highly enriched L-[1-13C]phenylalanine–labeled milk,
purified the casein fraction, and enzymatically hydrolyzed part
of the casein. This complex approach was required to allow true
insight into the effect of different dietary protein sources on the
subsequent digestion and absorption kinetics in vivo in humans.
In this study, we hypothesize that the ingestion of a protein
hydrolysate accelerates protein digestion and the absorption rate,
which results in a greater increase in plasma AA availability and
the muscle protein synthesis rate when compared with the ingestion
of its intact protein. To test that hypothesis, elderly men
were given a single bolus of specifically produced intrinsically L-
[1-13C]phenylalanine–labeled intact casein (CAS) or casein
hydrolysate (CASH), combined with continuous intravenous
L-[ring-1-2H5]phenylalanine, L-[1-13C]leucine, and L-[ring-
2H2]tyrosine infusion.
SUBJECTS AND METHODS
Subjects
Ten healthy, elderly, male volunteers [mean 6 SEM: age: 64 6
1 y; weight: 78.8 6 3.1 kg; height: 1.78 6 0.02 m; body mass
index (in kg/m2): 24.7 6 0.7; basal glucose: 5.44 6 0.07 mmol/L;
basal insulin: 9.99 6 1.28 mU/L; homeostasis model assessment
of insulin resistance (HOMA-IR): 2.43 6 0.32] who had no
history of participating in any regular exercise program took part
in this study. Subject recruitment was initiated on 26 March
2007. All subjects were informed of the nature and possible risks
of the experimental procedures before their written informed
consent was obtained. This study was approved by the Medical
Ethics Committee of the Academic Hospital Maastricht.
Pretesting
All subjects performed an oral-glucose-tolerance test before
inclusion in the study. After an overnight fast, subjects arrived
at the laboratory at 0800 by car or public transportation. Body
weight was measured with a digital balance with an accuracy of
0.001 kg (E1200; August Sauter GmbH, Albstadt, Germany). A
catheter (Baxter BV, Utrecht, Netherlands) was inserted into an
antecubital vein, and a resting blood sample was drawn after
which 75 g glucose (dissolved in 250 mL water) was ingested.
Thereafter, blood was sampled every 30 min until t = 120 min.
Plasma glucose concentrations were measured to determine
glucose intolerance and/or the presence of type 2 diabetes according
to the 2006 American Diabetes Association guidelines
(10).
Diet and activity before testing
All subjects consumed a standardized meal (32 6 2 kJ/kg body
weight, consisting of 55% of energy from carbohydrate, 15% of
energy from protein, and 30% of energy from fat) the evening
before the experiments. All volunteers were instructed to refrain
from any heavy physical exercise and to keep their diet as constant
as possible 3 d before the initiation of the experiments.
Experiments
Each subject participated in a randomized, double-blind crossover
design. All subjects were studied on 2 occasions that were
separated by 14 d, in which drinks containing CAS or CASH
were administered. After the ingestion of the given bolus of the test
drink, plasma and muscle samples were collected during a
6-h measurement period. These experiments were designed to
simultaneously assess the exogenous and endogenous rate of
appearance of phenylalanine, splanchnic phenylalanine extraction,
and the fractional synthetic rate (FSR) of mixed muscle
protein in the vastus lateralis muscle.
Protocol
At 0800, after an overnight fast, subjects arrived at the laboratory
by car or public transportation. A polytetrafluoroethylene
catheter was inserted into an antecubital vein for stable isotope
infusion. A second polytetrafluoroethylene catheter was inserted
into a heated dorsal hand vein of the contralateral arm and placed
in a hot box (60C) for arterialized blood sampling. After basal
blood sample collection (t = 2120 min), plasma phenylalanine,
leucine, and tyrosine pools were primed with a single intravenous
dose of the AA tracers L-[ring-2H5]phenylalanine
(2 lmol/kg), L-[ring-2H2]tyrosine (0.775 lmol/kg), and L-
[1-13C]leucine (5.06 lmol/kg). Thereafter, continuous tracer
infusion was started with an infusion rate of 0.046 6 0.001 lmol
kg21 min21 for L-[ring-2H5]phenylalanine, 0.017 6 0.000
lmol kg21 min21 for L-[ring-2H2]tyrosine, and 0.110 6 0.002
lmol kg21 min21 for L-[1-13C]leucine. Thereafter, subjects
rested in a supine position for 2 h, after which an arterialized
blood sample and a muscle biopsy from the vastus lateralis
muscle were collected (t = 0 min). Subjects then received a bolus
(4.5 ml/kg) of a given test drink containing 35 g intrinsically
L-[1-13C]phenylalanine–labeled protein. Arterialized blood
samples were collected at t = 15, 30, 45, 60, 75, 90, 105, 120,
135, 150, 165, 180, 210, 240, 270, 300, 330, and 360 min with
a second muscle biopsy taken at t = 360 min from the contralateral
limb.
Blood samples were collected in EDTA-containing tubes and
centrifuged at 1000 · g and 4C for 5 min. Aliquots of plasma
were frozen in liquid nitrogen and stored at –80C. Muscle biopsies
were obtained from the middle region of the vastus lateralis
(15 cm above the patella) and ’3 cm below entry through
the fascia by using the percutaneous needle biopsy technique
(11). Muscle samples were dissected carefully and freed from
any visible nonmuscle material. The muscle sample was immediately
frozen in liquid nitrogen and stored at –80C until
analysis.
Preparation of intrinsically labeled protein and beverage
composition
Intravenous L-[1-13C]phenylalanine administration was applied
in 2 cows to produce intrinsically L-[1-13C]phenylalanine–
labeled milk proteins. Two Holstein dairy cows [mean (6SEM)
body wt (BW): 726 6 38 kg at 26 6 2 d of lactation] were
infused with a large amount of L-[1-13C]phenylalanine via the
jugular vein by using a peristaltic pump at a rate of 4.16 mL/min
(402 lmol phenylalanine/min) for 44–48 h. The cows were
milked every 12 h during infusion and for the subsequent 6 h
after cessation of infusion. Casein and whey protein were separated
from the collected milk by microfiltration and ultrafiltration
as described previously (8). Part of the casein fraction
was enzymatically hydrolyzed by specific endopeptidases and
praline-specific endoprotease (PeptoPro process) by DSM Food
Specialties (Delft, Netherlands) (12). The L-[1-13C]phenylalanine
enrichments in the CAS and CASH proteins, which were assessed
by gas chromatography-mass spectrometry after hydrolysis,
were highly enriched [29.2 and 28.9 mole percent excess (MPE),
respectively]. The proteins met chemical and bacteriologic specifications
for human consumption.
Subjects received a beverage volume of 350 ml to ensure
a given dose of 35 g CAS or CASH. The CAS and CASH were
isonitrogenous (0.070 6 0.002 compared with 0.070 6 0.002 g
N/kg BW) and provided 142 6 6 compared with 134 6 6 lmol
phenylalanine/kg BW, 141 6 6 compared with 135 6 6 lmol
tyrosine/kg BW, and 322 6 13 compared with 306 6 13
lmol leucine/kg BW, respectively. To make the taste comparable
in all treatments, beverages were uniformly flavored by adding
0.375 g sodium saccharinate, 0.9 g citric acid, and 5 ml vanilla
flavor (Quest International, Naarden, Netherlands) per liter of
beverage. Treatments were performed in a randomized order
with test drinks being provided in a double-blind fashion.
Plasma analyses
Plasma glucose (Uni kit III, 07367204; Roche, Basel, Switzerland)
concentrations were analyzed with the COBAS-FARA
semiautomatic analyzer (Roche). Insulin was analyzed by radioimmunoassay
(Insulin RIA kit; Linco Research Inc, St Charles,
MO). Plasma (100 lL) for AA analyses was deproteinized on ice
with 10 mg dry 5-sulphosalicylic acid and mixed, and the clear
supernatant was collected after centrifugation. Plasma AA concentrations
were determined by HPLC after precolumn
derivatization with o-phthaldialdehyde (13). For plasma phenylalanine,
tyrosine, and leucine enrichment measurements, plasma
phenylalanine, tyrosine, and leucine were derivatized to their
t-butyldimethylsilyl derivatives, and their 13C or 2H enrichments
were determined by electron ionization (by gas chromatographymass
spectrometry; Agilent 6890N GC/5973N MSD; Little Falls,
DE) by using selected ion monitoring of masses 336, 337, and 341
for unlabeled and labeled (1-13C and ring-2H5) phenylalanine, respectively;
of masses 466, 467, 468, and 470 for unlabeled and
labeled (1-13C, ring-2H2, and ring-2H4) tyrosine, respectively; and
of masses 302 and 303 for unlabeled and labeled leucine (14). For
plasma a-ketoisocaproate (KIC) enrichment measurements, plasma
KIC was derivatized to its N-methyl-N-(Tert-butyldimethylsilyl)
trifluoroacetamide derivative, and its 13C enrichment was assessed
by monitoring masses 301 and 302 for unlabeled and labeled KIC,
respectively (15). We applied standard regression curves in all
isotopic enrichment analysis to assess linearity of the mass spectrometer
and to control for the loss of tracer.
Muscle sample analyses
For measurement of L-[1-13C]phenylalanine and L-[1-13C]
leucine enrichment in the free AA pool and mixed muscle
protein, 55 mg wet muscle was freeze dried. Collagen, blood,
and other nonmuscle fiber material were removed from the
muscle fibers under a light microscope. The isolated muscle fiber
mass (2–3 mg) was weighed, and 8 volumes (8 times dry
weight of isolated muscle fibers · wet:dry ratio) of ice-cold 2%
perchloric acid were added. The tissue was then homogenized
and centrifuged. The supernatant was collected and processed in
the same manner as the plasma samples, such that intracellular
free L-[1-13C]phenylalanine, L-[1-13C]tyrosine, L-[ring-2H2]tyrosine,
L-[ring-2H4]tyrosine, and L-[1-13C]leucine enrichments
could be measured by using their t-butyldimethylsilyl derivatives
on a gas chromatography-mass spectrometer.
The protein pellet was washed with 3 additional 1.5-ml washes
of 2% perchloric acid, dried, and hydrolyzed in 6 mol/L HCl at
120C for 15–18 h. The hydrolyzed protein fraction was dried
under a nitrogen stream while heated to 120C and 50% acetic
acid solution was added to one vial, and the hydrolyzed protein
was passed over a Dowex exchange resin (AG 50W-X8, 100–
200 mesh hydrogen form; Biorad, Hercules, CA) by using 2
mol/L NH4OH. Thereafter, the eluate was dried, and the purified
AAs were derivatized to their N(O,S)-ethoxycarbonyl ethyl esters
for the determination of 13C/12C ratios of muscle proteinbound
phenylalanine and leucine (16). Thereafter the derivative
was measured by gas chromatography-isotope ratio mass spectrometry
(Finnigan MAT 252; Bremen, Germany) by using the
Ultra I GC-column (no. 19091A-112; Hewlett-Packard, Palo
Alto, CA) and combustion interface II and by monitoring ion
masses 44, 45, and 46. By establishing the relation between
the enrichment of a series of L-[1-13C]phenylalanine and L-[1-
13C]leucine standards of variable enrichment and the enrichment
of the N(O,S)-ethoxycarbonyl ethyl esters of these standards, the
muscle-protein-bound enrichment of phenylalanine and leucine
was determined. We applied standard regression curves to assess
the linearity of the mass spectrometer and to control for loss of
the tracer. The CV for the measurement of L-[1-13C]phenylalanine
and L-[1-13C]leucine enrichment in mixed muscle protein averaged
1.0 6 0.1% and 1.1 6 0.1%, respectively.
Calculations
Ingestion of L-[1-13C]phenylalanine-labeled protein, intravenous
infusion of L-[ring-2H5]phenylalanine, L-[ring-2H2]tyrosine, and
L-[1-13C]leucine, and arterialized blood sampling were used to
assess whole-body AA kinetics in nonsteady state conditions.
Total, exogenous, and endogenous rates of appearance (Ra) and
splanchnic extraction (ie, the fraction of dietary AA taken up by
the gut and liver during the first pass) for phenylalanine was calculated
by using modified Steele’s equations (7, 15). These variables
were calculated as follows:
Total Ra ¼ F 2pV CðtÞ dEiv=dt
Eivt
ð1Þ
Exo Ra ¼
Total Ra Epo
t
þ pV dEpo=dt
Eprot
ð2Þ
Endo Ra ¼ Total Ra 2Exo Ra 2F ð3Þ
Sp ¼ 1003
PheProt AUCExoPheRa
PheProt
ð4Þ
where F is the intravenous tracer infusion rate (lmol kg21
min21), pV (0.125) is the distribution volume for phenylalanine
(17), and C(t) is the mean plasma phenylalanine concentration
between 2 time points. dEiv/dt represents the time-dependent
variations of plasma phenylalanine enrichment (expressed in
the tracer:tracee ratio, or the TTR) derived from the intravenous
tracer, and Eiv(t) is the mean plasma phenylalanine enrichment
from the intravenous tracer between 2 consecutive time points.
Exo Ra represents the plasma entry rate of dietary phenylalanine,
Epo(t) is the mean plasma phenylalanine enrichment for the oral
tracer, dEpo/dt represents the time-dependent variations of
plasma phenylalanine enrichment derived from the oral tracer,
and Eprot is the L-[1-13C]phenylalanine enrichment in the dietary
protein. PheProt is the amount of dietary phenylalanine ingested,
AUCExoPheRa represents the area under the curve (AUC) of Exo
Phe Ra, which corresponds to the amount of dietary phenylalanine
that appeared in the blood over a 6-h period after drink
intake. To determine total leucine, Ra and Rd, calculations were
performed by using both plasma leucine MPE and KIC MPE as
precursors. Because the conclusions were identical whichever
precursor pool was used in these calculations for whole-body
fluxes, we present only the results using plasma L-[1-13C]leucine.
The total rate of disappearance of phenylalanine equals the rate
of phenylalanine hydroxylation and utilization for protein synthesis.
These variables can be calculated as follows:
Rd ¼ Total Ra 2pV 3
dC
dt
ð5Þ
Phe hydroxylation ¼ Tyr Ra 3
EpðtÞ
EtðtÞ 3
PheRd
Fp þ PheRd
ð6Þ
Protein synthesis ¼ Total Rd 2Phe hydroxylation ð7Þ
Phe net balance ¼ Protein synthesis2EndoRa ð8Þ
The FSR of mixed muscle protein synthesis was calculated by
dividing the increment in the enrichment of the product, ie, the
protein-bound L-[1-13C]phenylalanine and L-[1-13C]leucine, by
the enrichment of the precursor. Plasma L-[1-13C]phenylalanine
and L-[1-13C]KIC enrichments were used to provide an estimate
for the true FSR of mixed muscle proteins. Plasma L-
[1-13C]KIC was used as a precursor for the calculation of FSR
instead of plasma L-[1-13C]leucine enrichment because it has
been shown to be more representative of the intracellular
leucine enrichment (18). Muscle FSRs were calculated as
follows (19):
FSR ¼ DEp
Eprecursor 3t
3100 ð9Þ
where DEp is the D increment of protein-bound L-[1-13C]phenylalanine,
L-[ring-2H5]phenylalanine, and L-[1-13C]leucine during
incorporation periods. Eprecursor is the average plasma
L-[1-13C]phenylalanine and L-[1-13C]KIC enrichment during
the time period for determination of AA incorporation (20).
t indicates the time interval (h) between biopsies.
Statistics
A complete randomized design was used to assess the effect of
the ingestion of intact protein (CAS) or protein hydrolysate
(CASH) on plasma AA kinetics and whole-body and muscle
protein synthesis rates in elderly men (n = 10). All data are
expressed as means 6 SEMs. Calculation of the required sample
size was based on the effect size and variance observed in
previous studies from our laboratory (12, 19, 21). We calculated
the sample size by using the following variables: the difference
in FSR .20% and a SD of 15% with a type I error of 5% and
a type II error of 10%. Power calculations showed that 9
subjects were needed, and therefore 10 elderly men were included
in this study. The plasma insulin, glucose, phenylalanine,
tyrosine, and branched-chain AA (leucine, isoleucine, and valine)
responses were calculated as the AUC above baseline values.
A 2-factor repeated measures analysis of variance (ANOVA,
general linear model) with time (df: 19) and treatment (df: 1) as
factors was used to compare differences between treatments
over time. In the case of significant interaction between time
and treatment, a Scheffe post hoc test was applied to locate
these differences. For non-time-dependent variables, a paired
t test was performed to detect differences between treatments.
Statistical significance was set at P , 0.05. All calculations
were performed by using SPSS version 12.0 (SPSS Inc,
Chicago, IL).
RESULTS
Plasma analyses
Plasma insulin concentrations increased to a greater extent in
the CASH compared with the CAS treatment (Figure 1). Peak
plasma insulin concentrations (individual peak values) averaged
50.2 6 7.6 and 26.2 6 3.7 mU/L in the CASH and CAS
treatment, respectively (P , 0.01). The plasma insulin response,
expressed as the AUC above baseline values, was significantly
greater after the ingestion of CASH compared with CAS (Figure 1
inset; P , 0.05). Plasma glucose responses averaged 25.5 6
34.4 and 23.2 6 16.1 mmol 6 h L21 in the CAS and CASH
treatment, respectively, with no significant differences between
treatments (P = 0.46).
Plasma phenylalanine, tyrosine, leucine, valine, and isoleucine
concentrations over time are reported in Figure 2. Generally,
plasma AA concentrations increased and remained elevated
throughout the 6-h measurement period after CAS ingestion.
FIGURE 1. Mean (6SEM) plasma insulin concentrations (mU/ L) and
insulin response (expressed as area under the curve minus baseline values) in
elderly men (n = 10) after ingestion of 35 g casein (CAS) or casein
hydrolysate (CASH). The horizontal line indicates the time period over
which significant differences were observed between treatments. Data
were analyzed with a 2-factor repeated measures ANOVA (time ·
treatment): time effect: P , 0.01; treatment effect: P , 0.01; interaction
of time and treatment: P , 0.01. *Significantly different from CAS, P ,
0.05 (paired t test).
Plasma AA concentrations increased to a greater extent after ingestion
of CASH with ’25–50% higher peak AA concentrations
in the CASH compared with the CAS treatment. In contrast, 4–
6 h after ingestion of the drink, plasma leucine and isoleucine
concentrations were significantly lower in the CASH compared
with the CAS treatment (Figure 2; P , 0.05). The plasma
phenylalanine response averaged 6.7 6 0.8 compared with
5.3 6 1.5 mmol 6 h L21 in the CASH and CAS treatments,
respectively; P = 0.25). The plasma tyrosine response (AUC)
was significantly higher in the CASH compared with the CAS
treatment (18.3 6 1.1 compared with 9.7 6 0.8 mmol 6 h
L21, respectively; P , 0.01). In addition, plasma leucine, valine,
and isoleucine responses (AUC) were significantly higher
in the CASH compared with the CAS treatment (42.7 6 2.3
compared with 32.6 6 1.8, 54.9 6 2.9 compared with 36.7 6
2.5, and 22.0 6 1.2 compared with 17.7 6 0.7 mmol 6 h L21,
respectively; P , 0.01).
The time courses of the plasma L-[1-13C]phenylalanine, L-
[ring-2H5]phenylalanine, L-[1-13C]leucine, L-[1-13C]KIC, L-
[ring-2H2]tyrosine, and L-[ring-2H4]tyrosine enrichments are shown
in Figure 3. The plasma L-[1-13C]phenylalanine enrichment
(originating from the intrinsically labeled protein) quickly increased
after ingestion of the test drink with higher peak values
(individual peak values) observed after ingestion of CASH
FIGURE 2. Mean (6SEM) plasma phenylalanine (A), tyrosine (B), leucine (C), valine (D), and isoleucine (E) concentrations (lmol/L) during casein
(CAS) and casein hydrolysate (CASH) experiments in elderly men (n = 10). The horizontal lines indicate the time period over which significant differences
were observed between treatments. Data were analyzed with a 2-factor ANOVA repeated measures (treatment · time). For plasma phenylalanine, tyrosine,
leucine, valine, and isoleucine: time effect, P , 0.01; treatment effect, P , 0.01; interaction of time and treatment, P , 0.0.01. *Significantly different from
the CAS treatment, P , 0.05 (Scheffe test).
compared with CAS (0.17 6 0.01 compared with 0.12 6 0.01
TTR; P , 0.05). However, plasma L-[1-13C]phenylalanine enrichments
were lower in CASH compared with CAS during the
final 2 h of the test (Figure 3A; P , 0.05). Plasma L-[ring-2H5]
phenylalanine, L-[1-13C]leucine, L-[1-13C]KIC, and L-[ring-2H2]tyrosine
enrichments decreased during both treatments after ingestion
of the drink. Generally, lower values were observed during the first
2–3 h after protein ingestion in the CASH compared with the CAS
treatment (Figure 3, B–E; P , 0.05). In contrast, higher plasma
enrichments were observed in the CASH compared with the CAS
treatment during the final stages of the test (Figure 3, B–E; P ,
0.05). Plasma L-[ring-2H4]tyrosine enrichments decreased after
CASH intake only (Figure 3F; P , 0.05) and remained at a lower
concentration during the first 3 h when compared with the ingestion
of CAS. No differences in plasma L-[ring-2H4]tyrosine enrichments
were observed between treatments during the final 3 h of the test.
Whole-body protein metabolism
Ingestion of the intrinsically labeled protein in the CASH and
CAS treatments resulted in a rapid increase in the exogenous
phenylalanine appearance rate (Figure 4A), with significantly
higher peak phenylalanine appearance rates (individual peak
values) observed in the CASH compared with the CAS treatment
(0.35 6 0.03 compared with 0.18 6 0.01 lmol phenylalanine
kg21 min21, respectively; P , 0.001). In addition,
total exogenous phenylalanine appearance (expressed as AUC
over 6 h) was 27 6 6% (range: 8–60%) higher in the CASH
compared with the CAS treatment (P , 0.001). In addition, the
calculated percentage of ingested phenylalanine taken up by
the splanchnic area during its first pass (ie, the amount of ingested
phenylalanine not appearing in plasma) was significantly
lower in the CASH compared with the CAS treatment
FIGURE 3. Mean (6SEM) plasma L-[1-13C]phenylalanine (A), L-[ring-2H5]phenylalanine (B), L-[1-13C]leucine (C), L-[1-13C]KIC (D), L-[ring-
2H2]tyrosine (E), and L-[ring-2H4]tyrosine enrichment tracer:tracee ratios (TTR) (F) during the casein (CAS) and casein hydrolysate (CASH) experiments
in elderly men (n = 10). The horizontal lines indicate the time period over which significant differences were observed between treatments. Data were analyzed
with ANOVA repeated measures (treatment · time). For plasma L-[1-13C]phenylalanine, L-[ring-2H5]phenylalanine, L-[1-13C]leucine, L-[1-13C]KIC, L-
[ring-2H2]tyrosine, and L-[ring-2H4]tyrosine enrichment: time effect, P , 0.001; treatment effect, P , 0.001; interaction of time and treatment, P ,
0.001. *Significant differences between CAS and CASH (P , 0.05, Scheffe test).
(66.1 6 1.2% compared with 73.0 6 1.4%, respectively; P ,
0.01). Total (exogenous and endogenous) phenylalanine appearance
rates were significantly higher during the first 105
min after protein ingestion in the CASH compared with the
CAS treatment (peak rates averaged 0.92 6 0.03 compared
with 0.79 6 0.04 lmol phenylalanine kg21 min21, respectively;
P , 0.05).
Total phenylalanine appearance rates decreased to a greater
extent over time during the CASH compared with the CAS
treatment. As a result, mean total phenylalanine appearance in
plasma, measured over the entire 6-h period, did not differ between
treatments (P = 0.52). Endogenous phenylalanine appearance
rates rapidly declined after protein ingestion in both
the CASH and CAS treatments (Figure 4C). The average endogenous
phenylalanine appearance in plasma over 6 h tended
to be lower in the CASH compared with the CAS treatment
(0.39 6 0.01 compared with 0.41 6 0.01 lmol phenylalanine
kg min21, respectively; P = 0.058).
Peak plasma phenylalanine disappearance and phenylalanine
hydroxylation rates (individual peak values) were significantly
higher in the CASH compared with the CAS treatment (0.85 6
0.03 compared with 0.73 6 0.03 and 0.16 6 0.03 compared
with 0.09 6 0.01 lmol phenylalanine kg21 min21, respectively;
P , 0.05). Phenylalanine disappearance and hydroxylation
rates decreased to a greater extent over time in the
CASH compared with the CAS treatment. As a result, average
total phenylalanine disappearance in plasma over the entire 6-h
measuring period did not differ between treatments (P = 0.43).
On average, phenylalanine hydroxylation tended to be higher
during the CASH compared with the CAS treatment (0.065 6
0.008 compared with 0.053 6 0.004 lmol phenylalanine kg21
min21, respectively; P = 0.10). Average whole-body protein
synthesis did not differ between treatments and averaged 0.51 6
0.01 and 0.51 6 0.01 lmol phenylalanine kg21 min21 in the
CASH and CAS treatments, respectively (P = 0.78). Total net
protein balance (AUC synthesis minus AUC endogenous Ra)
over the 6-h period after protein ingestion tended to be higher in
the CASH compared with the CAS treatment (40.6 6 3.4
compared with 34.3 6 2.1 lmol phenylalanine 6 h kg21, respectively;
P = 0.08).
By using [1-13C]leucine as an additional intravenous tracer,
we observed similar changes in Ra and Rd over time between
the CASH and CAS treatments when compared with phenylalanine
tracer kinetics (data not shown). Peak leucine Ra and Rd
(individual peak values) were significantly higher in CASH
compared with CAS treatments (Ra: 3.26 6 0.12 compared
with 2.43 6 0.13 lmol leucine kg21 min21, respectively; Rd:
2.93 6 0.10 compared with 2.25 6 0.07 lmol leucine kg21
min21, respectively; P , 0.01). Average total leucine Ra and Rd
over the entire 6-h period was 7 6 1% and 8 6 2% higher in
the CASH compared with the CAS treatment, respectively
(P , 0.05).
FIGURE 4. Mean (6SEM) rate of exogenous (A), total (B), and endogenous (C) phenylalanine (PHE) appearance in plasma (Ra) and of total
phenylalanine disappearance (Rd) from plasma (D) in lmol kg21 min21 during the casein (CAS) and casein hydrolysate (CASH) experiments in
elderly men (n = 10). The horizontal lines indicate the time period over which significant differences were observed between treatments. Data were
analyzed with repeated-measures ANOVA (treatment · time). Exogenous Ra: time effect, P , 0.001; treatment effect, P , 0.001; interaction of time and
treatment, P , 0.001. Total Ra: time effect, P , 0.05; treatment effect, P , 0.001; interaction of time and treatment, P , 0.001. Endogenous Ra: time effect,
P = 0.06; treatment effect, P , 0.001; interaction of time and treatment, P , 0.001. Total Rd: time effect, P , 0.05; treatment effect, P , 0.001; interaction of
time and treatment, P , 0.001. *Significant differences between CAS and CASH (P , 0.05, Scheffe test).
Muscle analysis
No differences were observed in basal free L-[1-13C]phenylalanine,
L-[1-13C]leucine, L-[1-13C]tyrosine, and L-[ring-2H2]
tyrosine enrichment that was determined in the muscle biopsies
collected before the ingestion of the test drink between treatments.
Free muscle L-[1-13C]leucine, L-[1-13C]tyrosine, and L-
[ring-2H2]tyrosine enrichments increased over time. However,
no differences were observed in free AA enrichment in the biopsy
samples collected 6 h after the ingestion of the protein drink
between treatments. A significant time · treatment interaction
was observed for free muscle L-[1-13C]phenylalanine enrichment
(P , 0.01). Six hours after protein intake, muscle free L-[1-13C]
phenylalanine enrichment was significantly lower in the CASH
compared with the CAS experiment, ie, 0.0133 6 0.0011 compared
with 0.03283 6 0.0035 TTR, respectively (P , 0.001).
The increase in protein-bound L-[1-13C]phenylalanine enrichment
tended to be higher in the CASH compared with the
CAS treatment (0.00035 6 0.00011 compared with 0.00025 6
0.00002 TTR, respectively; P = 0.07). The increase in proteinbound
L-[1-13C]leucine enrichment averaged 0.00020 6
0.00002 compared with 0.00023 6 0.00002 TTR in the CAS
and the CASH treatment, respectively (P = 0.35).
Mixed muscle protein synthesis rates
Mixed muscle protein FSRs, with the mean plasma L-[1-13C]
phenylalanine enrichment as a precursor (Figure 5A), tended to
be higher (33 6 16%; P = 0.10) in the CASH compared with the
CAS treatment. By using the L-[1-13C]leucine tracer, FSR values
were similar, and no significant differences were observed between
the CASH compared with the CAS treatment (Figure 5B,
P = 0.35). A significant positive correlation was observed between
FSR values calculated by using L-[1-13C]phenylalanine
and L-[1-13C]leucine as tracers (r = 0.71, P , 0.01).
DISCUSSION
In this study, we assessed dietary protein digestion and absorption
kinetics and the subsequent muscle protein synthetic
response to the ingestion of a single bolus of protein hydrolysate
compared with ingestion of its intact protein in vivo in healthy,
elderly men. The men were studied by using specifically produced
intrinsically L-[1-13C]phenylalanine–labeled intact (CAS)
and hydrolyzed (CASH) casein. This is the first study to show
that ingestion of a casein hydrolysate, as opposed to its intact
protein, accelerates the appearance rate of dietary phenylalanine
in the circulation, lowers splanchnic phenylalanine extraction,
increases postprandial plasma amino acid availability, and tends
to augment subsequent muscle protein synthesis in vivo in humans.
The rate of dietary protein digestion and absorption and the
subsequent splanchnic amino acid extraction determine postprandial
amino acid delivery to the periphery (9). The availability
of dietary amino acids has been shown to be an important regulator
of postprandial muscle protein metabolism (22–25). To
allow the assessment of dietary protein digestion and absorption,
and the subsequent postprandial skeletal muscle protein synthetic
response in vivo in humans, we applied specifically produced
intrinsically L-[1-13C]phenylalanine-labeled casein. It has been
speculated that enzymatic predigestion of a protein source can
be applied to modulate its in vivo digestion and absorption kinetics
(3). In accordance, in this study we observed a greater
increase in plasma amino acid concentrations after ingestion of
the hydrolyzed casein (CASH) when compared with its intact
protein CAS (Figure 2). These observations are in line with
Calbet et al (3), who reported higher peak plasma AA concentrations
after intragastric administration of hydrolyzed casein
when compared with its intact protein. We extend these findings
by directly measuring the true plasma appearance rate of dietary
phenylalanine after ingestion of both the intact and hydrolyzed
intrinsically labeled L-[1-13C]phenylalanine casein (Figure 4).
The exogenous phenylalanine appearance rate increased to
a greater extent after ingestion of the hydrolysate when compared
with the intact protein (Figure 4). During the 6-h postprandial
period, ’25% more dietary phenylalanine appeared in
the circulation after ingestion of the hydrolysate when compared
with the intact protein. Consequently, this study shows that
a hydrolyzed protein is more rapidly digested and absorbed,
which results in a greater AA delivery to the periphery in vivo in
elderly men. In addition, we show that ’70% of the ingested
phenylalanine does not appear in the circulation within a 6-h
postprandial period. This finding is in line with previous work in
pigs showing that, although ’90% of the dietary phenylalanine
is absorbed, the splanchnic area extracts ’50% to sustain its
functional mass (4). Interestingly, the percentage of the AAs
extracted within the splanchnic area varies between different
FIGURE 5. Mean (6SEM) fractional synthetic rate (FSR) of mixed muscle protein after the ingestion of intact casein (CAS) or hydrolyzed casein (CASH)
in elderly men (n = 10) by using plasma L-[1-13C]phenylalanine (A) and L-[1-13C]leucine enrichment (B) as precursors. Data were analyzed with a paired t
test. No significant differences were observed between treatments.
amino acids and seems to depend on the amount, quality, and
digestibility of the dietary protein source (26) and on the coingestion
of other macronutrients (27, 28). Previously published
data from human studies suggest that when protein or AAs are
ingested in small boluses over a prolonged period of time,
’50% of dietary phenylalanine (29) and leucine (30) is extracted
by the splanchnic area in elderly men. In this study, we
show that the percentage of the ingested phenylalanine that does
not appear in plasma is significantly (’10%) lower after the
ingestion of a single bolus of casein hydrolysate when compared
with its intact protein (66 6 1% compared with 73 6 1%, respectively;
P , 0.01). Consequently, hydrolyzed casein provides
a protein source that is more rapidly digested and absorbed in
vivo in humans, which improves postprandial plasma AA
availability.
It has been reported that greater postprandial plasma AA
availability will compensate for an attenuated postprandial
muscle protein synthetic response in the elderly and augment net
muscle protein accretion (15). In this study, we observed that
whole-body protein breakdown rates tended to be further lowered
after ingestion of the protein hydrolysate compared with the
intact protein (P = 0.058), which may be due to the greater insulin
release that was observed after protein hydrolysate ingestion
(31, 32). Elevated insulin concentrations have been
shown to inhibit proteolysis (31, 33, 34), stimulate AA uptake
(35), and/or augment muscle protein synthesis (35, 36). Some
groups propose that insulin is rather permissive instead of
modulatory and that plasma insulin concentrations of ’10–15
lU/mL are already sufficient to allow a maximal muscle protein
synthetic response (37, 38). In contrast, it is also suggested that
postprandial increases in circulating insulin concentrations are
instrumental in stimulating skeletal muscle blood flow and thereby
augment AA delivery to the muscle (39, 40). Consequently, both
the increase in postprandial plasma AA availability and the
greater plasma insulin response after CASH compared with
CAS ingestion (during the initial 3-h postprandial period) might
enhance postprandial muscle protein anabolism.
We used the plasma phenylalanine rate of disappearance and
hydroxylation to calculate postprandial whole-body protein synthesis
rates. Over the entire 6-h period, whole-body protein
synthesis rates did not differ between treatments (P = 0.78).
Whole-body net protein balance (AUC synthesis minus AUC
endogenous Ra) tended to be higher in the CASH compared with
the CAS treatment (P = 0.08). This result indicates that the
intake of a protein hydrolysate, as opposed to its intact protein,
further stimulates the anabolic response to food intake mainly by
inhibiting whole-body protein breakdown. However, postprandial
whole-body protein synthesis and breakdown rates do not necessarily
reflect changes on a muscle-tissue level (19). Therefore,
we also determined the incorporation rate of L-[1-13C]phenylalanine
(from the intrinsically labeled dietary protein) into the
muscle protein pool in skeletal muscle-tissue samples, which
tended to be greater after the ingestion of casein hydrolysate
(0.00035 6 0.00011) when compared with ingestion of the intact
protein (0.00025 6 0.00002; P = 0.07). As a result, observed
FSR values tended to be ’30% higher over the 6-h
period after the ingestion of the casein hydrolysate compared
with the ingestion of the intact protein (P = 0.10). Similar differences
were observed when calculating FSR on the basis of
intravenous L-[1-13C]leucine administration. However, due to
large intersubject variability, no significant differences in the
muscle protein synthetic response to protein ingestion were
observed between treatments (Figure 5). This may be due to the
timing of the collection of muscle-tissue samples (22). On the
basis of whole-body phenylalanine flux data and circulating
plasma amino acid and insulin concentrations, it could be
speculated that net muscle protein accretion was greater during
the first 3 h after CASH ingestion when compared with CAS.
This might explain why differences in the observed FSR values
did not reach statistical significance when assessed over the
entire 6-h period. Future studies should consider differentiating
the muscle protein synthetic response to dietary protein intake
during the acute (,3 h) from that during the more prolonged
(.3 h) postprandial period. Another factor that may explain the
lack of statistical difference in FSR values after CAS and CASH
ingestion is the ingestion of a relatively large amount of dietary
protein in the present study. A bolus of 35 g dietary protein may
have been more than sufficient to maximize the postprandial
muscle protein synthetic response (38, 41, 42). More research is
warranted to assess the potential differences in the postprandial
muscle protein synthetic response to the ingestion of smaller,
meal-like amounts of hydrolyzed compared with intact protein
(’20 g). However, measuring the incorporation rate of labeled
AAs derived from even smaller amounts of intrinsically labeled
dietary protein will be methodologically challenging.
In conclusion, ingestion of a protein hydrolysate, as opposed to
its intact protein, accelerates protein digestion and absorption
from the intestine, lowers splanchnic AA, extraction, augments
postprandial plasma AA availability, and tends to increase the
incorporation of dietary AAs into mixed muscle protein in vivo in
elderly men.
We gratefully acknowledge the expert technical assistance of J Senden and
A Zorenc. We greatly appreciate the enthusiastic support of all subjects who
volunteered to participate in this study.
The authors’ responsibilities were as follows—YB, RK, and LJCvL:
designed the study; AKK, SL, and JF: assisted in the production and/or preparation
of the intrinsically labeled protein; RK and NC: organized and carried
out the clinical experiments; APG and SW: performed the stable isotope analyses;
RK and LJCvL: performed the statistical analysis of the data and wrote
the manuscript together with AKK and WHMS; and WHMS: provided medical
assistance. AKK is a researcher with DSM Food Specialties, Delft, Netherlands.
None of the authors had a conflict of interest.
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(je ne connais pas les %age d'hydrolyse par contre)
