Protein turnover modifications induced by the protein feeding
pattern still persist after the end of the
diets
M. A.
Arnal1,
L.
Mosoni1,
Y.
Boirie2,
P.
Gachon2,
M.
Genest2,
G.
Bayle1,
J.
Grizard1,
M.
Arnal1,
J.
M.
Antoine3,
B.
Beaufrère2, and
P.
Patureau
Mirand1
1 Unité d'Etude du Métabolisme
Azoté, Institut National de la Recherche Agronomique et Centre de
Recherche en Nutrition Humaine, 63122 Clermont-Ferrand-Theix;
2 Laboratoire de Nutrition Humaine,
Université d'Auvergne et Centre de Recherche en Nutrition
Humaine, 63009 Clermont-Ferrand; and
3 Société Danone, 92350 Le
Plessis-Robinson, France.
 |
ABSTRACT |
This study was
undertaken to determine whether the protein feeding pattern could
induce chronic adaptation of protein turnover. After a 15-day adaptive
period, elderly (68 yr) and young (26 yr) women received, for 14 days,
a diet providing 200 KJ · kg fat-free mass
(FFM)
1 · day
1,
where the daily protein intake (1.7 g protein · kg
FFM
1 · day
1)
was either spread over 4 meals in the spread pattern or mainly (80%)
consumed at noon in the pulse pattern. One day after the end of the
dietary treatment, whole body leucine kinetics were measured by use of
a continuous [13C]leucine infusion, both in the
postabsorptive state and in the same fed state. The pulse pattern was
able to induce, in young as in elderly women, a lower postabsorptive
leucine oxidation and endogenous leucine flux than the spread pattern
and improved the responsiveness of nonoxidative leucine disposal during
4-h oral feeding. Thus the pulse pattern was able to induce chronic regulation of protein metabolism in young as in elderly women.
women; postabsorptive state; postprandial state; aging
 |
INTRODUCTION |
SEVERAL EXPERIMENTS HAVE SHOWN that protein retention
can be improved in elderly people by adequate protein feeding. This may
be useful in limiting the gradual body protein loss occurring during
aging. Thus it is generally admitted that, in elderly people, an
increase in the daily amount of protein intake improves, at least in
the short term, protein balance (9, 21, 24). We previously
demonstrated, in elderly women, that the increase of protein retention
due to an increase of daily protein intake [from 1.2 to 1.7 g
protein · kg fat-free mass
(FFM)
1 · day
1]
was modulated by the protein feeding pattern (1). Indeed, when 80% of
the daily protein intake was consumed during the midday meal (as in the
pulse pattern), the increase of nitrogen balance was higher than when
protein intake was spread over 4 meals (as in the spread pattern).
Similar protein retention was obtained in young and in elderly women
fed the pulse but not the spread pattern (2). It was suggested that
this effect was the consequence of a defect in the response of whole
body protein anabolism to the spread pattern in elderly women. We
analyze now what mechanisms are involved in the effects of the protein
pulse feeding pattern.
Indeed, protein intake levels modulate protein deposition by acting on
protein turnover regulation during the diurnal cycle of
postabsorptive/postprandial phases (22). Low protein intake can reduce
postabsorptive protein breakdown and, consequently, postabsorptive
protein losses. A high protein intake improves postprandial protein
gains by a higher stimulation of protein synthesis and a higher
inhibition of proteolysis (20). The pulse pattern, which included both
high- and low-protein meals, induced a higher protein synthesis in
elderly women than the spread pattern (1). This might correspond to a
specific recovery of protein synthesis stimulation by nutrients during
the dietary treatment. However, the pulse pattern may also induce
chronic metabolic adaptations of protein turnover, such as a better
protein-sparing effect during the postabsorptive period and/or a higher
responsiveness of protein metabolism to nutrients. In this case, these
modifications of protein turnover had to persist after the dietary
treatments had stopped.
In the present study, we tested the hypothesis that the positive effect
of the pulse pattern on protein anabolism in elderly women involves
chronic adaptations of protein turnover. Thus we measured, in young and
in elderly women, leucine kinetics on the day after the end of the
dietary pulse or spread diets both in the basal state (i.e.,
postabsorptive) and during the fed state (i.e., small repeated meals).
This allowed us to detect whether or not protein metabolism responses
to postabsorptive and fed states remained different in women previously
fed the spread or the pulse diet, according to their age.
 |
METHODS |
Subjects.
Thirty-one healthy women who had previously participated in the
nitrogen balance and flux determination studies (1, 2) were included in
the present study; they were 16 young [26 ± 1 (SE) yr]
and 15 elderly adults (68 ± 1 yr). Each subject had a normal physical
examination without any medical history of renal, cardiovascular,
endocrine, or currently evolving disease. All subjects were asked to
maintain their usual physical activity before and during the study.
Their physical characteristics were recorded. Mean body weights were
55.7 ± 2.0 and 62.4 ± 1.8 kg (in young and elderly women,
respectively, P < 0.05), FFM values (as measured with
18O water dilution) were 73.9 ± 1.6 and 61.7 ± 1.4% of
body weight (P < 0.05), and body mass index values were 20.8 ± 0.5 and 25.3 ± 0.8 kg/m2 (P < 0.05) in
young and elderly women, respectively.
The purpose and the potential risks of the study were fully explained
to the subjects, and written informed consent was obtained from each
participant. The experimental protocol was approved by the Ethical
Committee of Clermont-Ferrand.
Materials.
L-[1-13C]leucine [99 mole
percent excess (MPE)],
L-[5,5,5-2H3]leucine
(97 MPE), and [13C]bicarbonate (99 MPE) were
obtained from MassTrace (Woburn, MA). The isotopic and chemical purity
of the leucine compounds was checked by gas chromatography-mass
spectrometry (GC-MS). Solutions of tracers were tested for sterility
and pyrogenicity before use and prepared in sterile nonpyrogenic
saline. During each experiment, the tracers were filtered through
0.22-µm filters.
Experimental design.
The food intake was entirely controlled for 29 days. The experimental
design and the composition of the diets are described in detail
elsewhere (1, 2). Briefly, during the first 14 days, each subject
received a controlled diet adjusted to 1.2 g
protein · kg
FFM
1 · day
1
and 200 KJ · kg
FFM
1 · day
1.
This adaptive period was achieved to obtain a similar protein status
for all of the subjects. Between days 15 and 29, two
isonitrogenous (1.7 g protein · kg
FFM
1 · day
1)
and isoenergetic (200 KJ · kg
FFM
1 · day
1)
diets were fed to test the effects of two protein feeding patterns. The
daily protein and energy repartition of these diets is shown in Table
1. The first diet was composed of four
meals, spreading daily protein intake over 12 h (0800, 1200, 1600, and
2000): it is referred to as the spread pattern. In the second diet,
80% of daily protein intake was concentrated at midday, and the
remaining 20% was provided at breakfast (0800) and dinner (2000): it
is referred to as the pulse pattern. The protein intake
from the midday meal in the pulse pattern was significantly higher than the combined protein intake from meals fed at 1200 and 1600 in the
spread pattern. This was not the case for energy intakes (Table 1).
On the 30th day, the day after the end of the dietary treatment,
L-[1-13C]leucine infusion was
performed as described by Boirie et al. (7) to measure leucine
kinetics. Subjects were admitted to Research Center facilities at 0700 in the postabsorptive state. At 0730, a catheter was inserted
retrogradely into a dorsal vein of the hand for arterialized blood
sampling after introduction of the hand into a 60°C-heated,
ventilated box. A second catheter was inserted into a vein of the
contralateral arm for tracer infusion. At 0800, after a prime dose of
[13C]bicarbonate (6 mg), a primed (6 µmol/kg), continuous (0.10 µmol · kg
1 · min
1)
infusion of L-[1-13C]leucine was
begun and continued for 8 h. After 210 min, a semiliquid diet was given
orally for the 270 remaining minutes (from 210 to 480 min). The diet
provided 3.7 MJ/kg, 13.3% as protein (in the form of whey protein
concentrate, containing 10% leucine by weight), 52.3% as
carbohydrates (dextrin maltose hydrolyzed from potato starch of low
natural 13C abundance), and 35% as fat (in the form of
vegetable oil). The liquid meal was prepared on the day of protocol,
just before the beginning of the study, and was ingested in small
(50-ml) aliquots given every 20 min.
[2H3]leucine was added to the meals
to obtain an oral administration rate of 0.07 µmol · kg
1 · min
1.
Blood and breath samples were taken before any infusion and at 20-min
intervals during the last hour of each plateau in the postabsorptive
state (i.e., from 150 to 210 min) and in the fed state (from 420 to 480 min). The plasma supernatant was separated, an internal standard was
added, and the sample was stored at
20°C until further
analysis. Breath samples were kept in 10-ml evacuated containers
(Vacutainer, Becton-Dickinson, Grenoble, France). Total carbon dioxide
production rates were measured at isotopic plateau during the last hour
of the two postabsorptive and fed states by an open-circuit indirect
calorimeter (Deltatrac, Datex, Geneva, Switzerland).
During this infusion protocol, two elderly women (one in the pulse and
one in the spread group) did not endure the continuous feeding mode
with the semi-liquid diet. Consequently the fed state plateau could not
be achieved (not shown), and these two subjects were excluded from this study.
Analytical methods.
Plasma [13C]- and
[2H3]leucine and ketoisocaproate
(KIC) enrichments were measured by selected ion monitoring electron
impact GC-MS (model 5971A, Hewlett-Packard, Palo Alto, CA) with
t-butyldimethylsilyl derivatives, and corrections for
13C and 2H3 enrichments were
applied according to Biolo et al. (4). As shown in Fig.
1, enrichment plateaus were achieved during
both postabsorptive and postprandial states, and there was no
significant difference in [13C]- and
[2H3]leucine or KIC enrichments
between groups. 13CO2 enrichments were measured
on a GC combustion isotope ratio mass spectrometer (µGas system;
Fisons Instruments, VG Isotech, Middlewich, UK). Plasma amino acid
concentrations, after deproteinization with 0.6 M TCA, were measured on
an autoanalyzer (Biotronic LC 3000, Roucaire, Vélizy, France,
with BTC 2410 resin). Measurements of leucine content of the liquid
meal were performed by GC (model 5971A, Hewlett-Packard) after a 24-h
hydrolysis at 110°C with HCl 6N and with norleucine as internal
standard. Plasma insulin concentrations were determined by RIA (CIS,
Gif-sur-Yvette, France). Plasma glucose, total proteins, and urea
concentrations were measured using enzymatic reactions on an
autoanalyzer (Cobas Mira, Roche Diagnostic Systems, Neuilly sur Seine,
France).

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Fig. 1.
Leucine and ketoisocaproate (KIC) enrichments during
postabsorptive and fed states in young (YW) and elderly women
(EW). Values are means ± SE. MPE, mole percent excess; Pu,
pulse; Sp, spread.
|
|
Calculations.
As described previously (3), leucine kinetics were calculated as
follows
|
(1)
|
where
Leu Ra (µmol · kg
FFM
1 · min
1)
is total leucine systemic flux, F[13C]Leu is
the [13C]leucine infusion rate
(µmol · kg
FFM
1 · min
1)
corrected for isotopic purity, and [13C]Leu MPE
is the plasma [13C]leucine enrichment. This
flux includes the tracer infusions.
Leucine splanchnic extraction (%) was calculated as follows
|
(2)
|
where
Ra [13C]Leu and Ra
[2H3]Leu are total leucine fluxes
calculated according to Eq. 1 with either the intravenous
(13C) or oral (2H3) tracer.
Splanchnic extraction represents the fraction of ingested leucine taken
up by the gut and/or liver during its first pass.
Leucine oxidation (Leu Ox, µmol · kg
FFM
1 · min
1)
was calculated as follows
|
(3)
|
where
13CO2 excretion (µmol · kg
FFM
1 · min
1)
is the product of carbon dioxide production and
13CO2 atom percent excess (APE), corrected for
incomplete recovery by a factor of 0.70 in the postabsorptive state and
0.82 in the fed state, according to Hoerr et al. (16).
Nonoxidative leucine disposal (NOLD, µmol · kg
FFM
1 · min
1),
an index of whole body protein synthesis, is the difference between total leucine flux (Leu Ra) and leucine oxidation (Leu Ox).
Endogenous leucine production (µmol · kg
FFM
1 · min
1)
represents whole body protein breakdown and is the difference between
total leucine flux (minus the intravenous tracer infusion rate) and
either leucine intake (Endo Leu Ra) or leucine intake corrected for splanchnic extraction (corrected Endo Leu Ra)
calculated as follows
|
(4)
|
Finally, net leucine balance is the difference between total
leucine intake (including the tracers) and leucine oxidation.
Statistical analysis.
Results are means and residual SD (RSD) or SE. Leucine kinetics and
plasma parameters were compared by a three-way ANOVA [age (A),
protein feeding pattern (P), and nutritional state (N) as factors] with repeated measures for nutritional state. The
analyses were performed with the Superanova software (Abacus Concepts, Berkeley, CA). The error term subject was used to test the effects of
A, P, and their interactions; it was the N × subjects interaction for factors including nutritional state. Then, to analyze the effects
of age and diet in the postabsorptive state or in response to feeding
(%postabsorptive), a two-way ANOVA was performed to discriminate
between the effects of age and protein feeding pattern.
 |
RESULTS |
Plasma substrate and hormone concentrations.
In the postabsorptive state, no significant effects of age were
detected, either on plasma glucose or on insulin concentrations. Feeding induced an increase of both glucose and insulin concentrations (Table 2). In the fed state, glycemia and
insulinemia were higher in the elderly women than in the young women
(+30 and +68%, for glucose and insulin concentrations, respectively).
Plasma urea concentrations were higher in the elderly than in the young
women and were increased during feeding only in the elderly women
(Table 3). Feeding induced an increase in
plasma free amino acid concentrations (Table 3). A significant
(P < 0.05) effect of age was detected for His, Leu, Tyr, and
Val, and especially in the fed state for Ile, Leu, and Val, for which
concentrations were, respectively, 22, 17, and 15% higher in the
elderly subjects than in the young subjects. Concentrations of His,
Lys, and Phe were lower in the pulse than in the spread groups
(
13,
11, and
13%, respectively).
Whole body leucine kinetics.
A complete analysis of results regarding leucine kinetics is given in
Table 4. Feeding induced a significant
stimulation of Leu Ra and Leu Ox and an inhibition of Endo
Leu Ra. The stimulation of NOLD in the fed state was
dependent on the protein feeding pattern (N × P interaction): it
was significantly different from zero in the pulse but not in the
spread groups. Because the protein feeding pattern had no significant
effect according to age (A × P interaction not significant for
each leucine flux), the effects of age were analyzed by pooling the
data of both protein feeding patterns, and the effects of protein
feeding pattern were analyzed by pooling the data of both age groups.
Effect of age.
In the postabsorptive state, Leu Ra was not
significantly different in the young and the elderly women (Fig.
2). Leu Ox was 20% lower in the elderly
women than in the young women. Neither NOLD nor Endo Leu Ra
differed between the two age groups. However, postabsorptive leucine
balance was less negative in the elderly than in the young women
(
0.23 ± 0.01 and
0.33 ± 0.03 µmol · kg
FFM
1 · min
1,
respectively). The increase of Leu Ra and Leu Ox induced by feeding was higher in the elderly women than in the young women (Fig.
2). The significant NOLD changes were not dependent on the age of the
subjects. In the fed state, Endo Leu Ra inhibition tended
(P = 0.10) to be lower in the elderly women (65 ± 6%) than in the young women (75 ± 4%). When it was corrected for splanchnic extraction, this inhibition was significantly lower in the elderly women than in the young women (Fig. 2). Leucine balance became positive, reflecting fed state protein gains, but no significant effect
of age was detected (+1.12 ± 0.07 and +1.08 ± 0.08 µmol · kg
FFM
1 · min
1
for young and elderly women, respectively).

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Fig. 2.
Effect of age on leucine rate of appearance (Ra), leucine
oxidation (Leu Ox), nonoxidative leucine disposal (NOLD), and
endogenous leucine Ra corrected for splanchnic extraction
(corrected Endo Leu Ra). Values are means ± SE. Data are
analyzed using a 2-way ANOVA (age and protein feeding pattern) in the
postabsorptive (PA) state (A) and in responses to feeding in
steady-state conditions (B). Only comparisons between elderly
and young women are reported: n = 16 and n = 13, respectively, in young and elderly groups. * Significantly different
from young group, P < 0.05.
|
|
Effect of diet.
Postabsorptive Leu Ra, corrected Endo Leu Ra,
and Leu Ox were, respectively, 9, 8, and 26% lower in the pulse than
in the spread groups (Fig. 3). No diet
effect was detected on NOLD in the postabsorptive state. Postabsorptive
Leu balance was higher in the pulse than in the spread groups
(
0.24 ± 0.02 and
0.33 ± 0.03 µmol · kg
FFM
1 · min
1,
respectively). In the fed state, Leu Ra tended to be more
stimulated (P = 0.06) when women were previously fed the pulse
diet.

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Fig. 3.
Effect of diet on leucine Ra, Leu Ox, NOLD, and corrected
Endo Leu Ra. Values are means ± SE. Data are analyzed
using a 2-way ANOVA (age and protein feeding pattern) in PA state
(A) and in responses to feeding in steady-state conditions
(B). Only comparisons between pulse and spread groups are
reported; n = 15 and n = 14, respectively, in spread
and pulse groups. Significantly different from spread group:
* P < 0.05; P = 0.06.
|
|
The stimulation of Leu Ox in the fed state was not different in the two
groups. NOLD was increased in the fed state when women had been fed the
pulse diet but not the spread diet. No significant effect of the spread
or the pulse diets was detected on corrected Endo Leu Ra
inhibition (Fig. 3). Finally, fed-state Leu balance was higher in the
pulse group than in the spread group (+1.22 ± 0.04 and +0.99 ± 0.08 µmol · kg
FFM
1 · min
1, respectively).
 |
DISCUSSION |
The present study was undertaken to determine whether the
mechanisms involved in the improvement of protein anabolism observed in
elderly women fed a protein pulse pattern (1) are durable after the
dietary treatment (i.e., the pulse or the spread patterns) has ended.
Thus we determined protein synthesis and breakdown on the day after the
end of dietary treatments both in a basal (postabsorptive) state and in
response to the same moderate stimulus (fed state). Our results
demonstrated that the metabolic adaptation induced by the pulse pattern
continues and is detectable on postabsorptive protein losses and on fed
state protein gains.
Indeed, this specific pulse pattern was able to induce after 14 days,
in young as in elderly women, a higher postabsorptive leucine balance
than the spread pattern because of a lower protein breakdown. This may
be the consequence of the evening meal, which was a low-protein meal in
the pulse pattern (providing 14% of total daily intake vs. 28% for
the spread pattern), because low-protein diets are known to induce a
reduction of postabsorptive protein losses by decreasing protein
breakdown (20). The effect of this low-protein evening meal may also be
involved in the low postabsorptive leucine oxidation with the pulse
pattern. It could be caused by a decrease in the active form of the
liver branched-chain
-keto acid dehydrogenase, as described in
meal-fed compared with ad libitum-fed rats (5). Moreover, the protein
feeding pattern had no significant effect on NOLD during the
postabsorptive phase.
In contrast, a significant increase in whole body protein synthesis
during the fed state was observed only in women previously fed the
pulse feeding pattern. The induction of the fed state was characterized
by a moderate increase of the aminoacidemia (i.e., 53% for the sum of
the essential amino acids), which was not different between the pulse
and the spread groups. However, a higher increase (>50%) of plasma
free amino acid levels was shown to be necessary to induce a
postprandial increase in the whole body protein synthesis in young
adults (15). Such a stimulation is scarcely observed in most studies of
the fed state in steady-state conditions (3, 4, 18, 19), and it was
also the case in the spread groups. Only the pulse pattern induced an
increase in the responsiveness of NOLD to the fed state in both young
and elderly women. This suggests that, one day after the end of the pulse diet, a moderate increase in plasma amino acid concentration remained effective in activating protein synthesis. Furthermore, leucine oxidation was lower during the fed state in the pulse than in
the spread groups. This result is in agreement with the sparing effect
of a 3-discrete-meal pattern vs. a 10-small-meal pattern (11). Thus
fed-state leucine balances were higher in the pulse groups in both
young and elderly women. This is in keeping with the improvement of
nitrogen balance described in elderly women during the dietary period
(1). However, this was not in agreement with nitrogen balance data
obtained in young women, because no significant effect of the protein
feeding pattern was observed on protein retention (2). In addition,
during dietary treatments, protein turnover was higher with the pulse
pattern, whereas the previous protein feeding patterns had no effect on Leu Ra in the present study. These discrepancies could be
due to the fact that the effects observed during dietary treatments resulted from the interaction between the chronic modifications of
protein metabolism and the nutrient flux, which is specific to each
diet. In contrast, the results reported in the present paper focus only
on chronic metabolic modifications that can be detected in a steady
state. Thus the pulse pattern induced similar chronic modifications of
protein turnover in both young and elderly women. It could be
speculated that, in young women, an efficient response of protein
metabolism to the nutrient flux of the pulse pattern could rapidly
occur during the dietary treatments, leading to the lack of effect
observed with that pattern (2). It will now be of great interest to
verify this hypothesis by performing the same measurements
for each age group during dietary treatments in non-steady-state conditions.
The present study also reveals some other modifications of the
metabolic responses to feeding during aging. The higher increase of
glycemia and insulinemia observed in the same fed state in the elderly
than in young women is in keeping with the insulin resistance of
glucose metabolism that occurs during aging (12). It is associated with
a lower inhibition of corrected Endo Leu Ra. Methodological
issues must be raised concerning the accurate measurement of Endo Leu
Ra in the fed state. Calculation of splanchnic extraction
allows a more accurate estimation of protein breakdown, because dietary
leucine sequestered by splanchnic tissues during the first pass cannot
reach the metabolic pool where [13C]leucine is
infused. Corrected Endo Leu Ra takes into account this
splanchnic uptake of dietary leucine (23). Leucine splanchnic extraction was measured using free
[2H3]leucine given orally with the
liquid meal. This assumed that leucine in the whey protein was as
rapidly absorbed as free
[2H3]leucine. Although this was
probably not the case, the resulting error in the leucine flux
determination is probably minor, because the leucine of whey proteins
is readily available (6), and the 4 h of constant feeding allowed a
continuous and steady amount of leucine to enter into the body pool.
This correction for splanchnic retention provides the means to detect a
significant effect of age on proteolysis during feeding, which was less
inhibited in elderly women; this was also observed in elderly men (7).
In young adults, it was shown in leg muscles that insulin acts
predominantly on protein breakdown, resulting in decreasing amino acid
release (17). In the present study, the reduction of proteolysis
inhibition during the fed state, leading to a higher increase of plasma
free amino acids, was observed in elderly women despite the higher increase of insulinemia. Thus, as it has already been suggested in
other experiments in humans (8, 14) and in rats, at muscle level (10,
13), an insulin resistance of protein metabolism could develop during aging.
In conclusion, our data demonstrated some age-related dysregulation of
protein breakdown during the fed state. Moreover, this experiment
clearly shows that protein turnover modifications induced in elderly
women fed the protein pulse pattern for 14 days persist 1 day after
subjects have stopped this diet. These modifications provide an
explanation for the mechanisms implied in the positive effect of the
pulse pattern observed previously in elderly women (1). It improves the
responsiveness of protein synthesis to the fed state and shows its
better protein-sparing effect in the postabsorptive period.
 |
ACKNOWLEDGEMENTS |
We thank Marie-Louise Houlier, Marie-Claude Ribeyre, Liliane Morin,
and Elisabeth Verdier for technical help. We are especially grateful
for the willingness and valuable cooperation of the volunteers, who
made this study possible.
 |
FOOTNOTES |
This work was supported by a PhD studentship to MA Arnal and grants
from the Société Danone, 92350 Le Plessis-Robinson, France.
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests: P. Patureau Mirand, UEMA, INRA
Clermont-Ferrand-Theix, 63122 St Genès Champanelle, France.
Address for correspondence: M. A. Arnal, UEMA, INRA, 63122 St
Genès Champanelle, France (E-mail:
arnalma{at}clermont.inra.fr).
Received 21 October 1999; accepted in final form 9 December 1999.
 |
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