Download Layma, D.K. Increased Dietary Protein Modifies Glucose and Insulin

Human Nutrition and Metabolism
Increased Dietary Protein Modifies Glucose and Insulin Homeostasis
in Adult Women during Weight Loss1,2
Donald K. Layman,*†3 Harn Shiue,† Carl Sather,† Donna J. Erickson* and Jamie Baum†
*Department of Food Science and Human Nutrition, †Division of Nutritional Sciences, University of Illinois
at Urbana-Champaign, Urbana, IL 61801
KEY WORDS:
●
insulin
●
amino acids
●
leucine
●
obesity
Dietary requirements for amino acids remain controversial.
Most studies are focused on criteria to define a minimum
requirement to maintain short-term nitrogen balance. This
concept is particularly useful for a limiting amino acid such as
lysine, which serves as an essential amino acid for peptide
structures and has limited use as a metabolic substrate (1,2).
At the other end of the spectrum, the branched-chain amino
acids (BCAA) are essential amino acids for protein synthesis
and also participate in critical metabolic processes (3,4). These
differences in roles among amino acids suggest that a single
definition of requirements may not be adequate to encompass
the full range of human needs for each of the nine indispensable amino acids.
The three BCAA, leucine, valine and isoleucine, support
numerous metabolic processes ranging from the fundamental
role as substrates for protein synthesis to metabolic roles as
precursors for synthesis of alanine and glutamine (5,6) and as
a modulator of the insulin-signaling pathway (7–9). The potential for the BCAA to participate in each of these metabolic
processes appears to be in proportion to their availability.
●
syndrome X
Experimental evidence comparing the priority of use of the
BCAA for each of these individual processes is limited, but
suggests that the first priority is for aminoacylation of tRNA
for protein synthesis (10), whereas their contribution to the
production of alanine and glutamine or their effect on the
signaling pathway is dependent on increasing intracellular
concentrations (5,11,12). The potential effect of these amino
acids on metabolic processes under physiologic conditions
remains to be explored.
The interrelationship between BCAA and glucose metabolism was first reported to be associated with the glucosealanine cycle (5,6). These investigators found that there was a
continuous flux of BCAA from visceral tissues through the
blood to skeletal muscle where transamination of the BCAA
provides the amino nitrogen to produce alanine from pyruvate
with a corresponding movement of alanine from muscle to
liver to support hepatic gluconeogenesis. Although the importance of the glucose-alanine cycle has been debated, Ahlborg
et al. (5) reported that it accounted for ⬎40% of endogenous
glucose production during prolonged exercise.
More recently, the overall contribution of dietary amino
acids to glucose homeostasis received further support on the
basis of quantitative evaluations of hepatic glucose production.
Jungas et al. (13) provided an elegant argument that amino
acids serve as a primary fuel for the liver and the primary
carbon source for hepatic gluconeogenesis. Other investigators
(14,15) extended this thinking with the findings that endogenous glucose production in the liver is a critical factor in the
1
Presented in part at Experimental Biology 2001, April 2001, Orlando, FL
[Shiue, H., Sather, C. & Layman, D. K. (2001) Reduced carbohydrate/protein
ratio enhances metabolic changes associated with weight loss diet. FASEB J. 15:
A301 (abs.)].
2
Supported by the Cattlemen’s Beef Board, National Cattlemen’s Beef Association, Kraft Foods, USDA/Hatch, and the Illinois Council on Food and Agriculture Research.
3
To whom correspondence should be addressed.
E-mail: [email protected].
0022-3166/03 $3.00 © 2003 American Society for Nutritional Sciences.
Manuscript received 18 July 2002. Initial review completed 13 August 2002. Revision accepted 20 November 2002.
405
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ABSTRACT Amino acids interact with glucose metabolism both as carbon substrates and by recycling glucose
carbon via alanine and glutamine; however, the effect of protein intake on glucose homeostasis during weight loss
remains unknown. This study tests the hypothesis that a moderate increase in dietary protein with a corresponding
reduction of carbohydrates (CHO) stabilizes fasting and postprandial blood glucose and insulin during weight loss.
Adult women (n ⫽ 24; ⬎15% above ideal body weight) were assigned to either a Protein Group [protein: 1.6
g/(kg 䡠 d); CHO ⬍40% of energy] or CHO Group [protein: 0.8 g/(kg 䡠 d); CHO ⬎55%]. Diets were equal in energy
(7100 kJ/d) and fat (50 g/d). After 10 wk, the Protein Group lost 7.53 ⫾ 1.44 kg and the CHO Group lost 6.96 ⫾ 1.36
kg. Plasma amino acids, glucose and insulin were determined after a 12-h fast and 2 h after a 1.67 MJ test meal
containing either 39 g CHO, 33 g protein and 13 g fat (Protein Group) or 57 g CHO, 12 g protein and 14 g fat (CHO
Group). After 10 wk, subjects in the CHO Group had lower fasting (4.34 ⫾ 0.10 vs 4.89 ⫾ 0.11 mmol/L) and
postprandial blood glucose (3.77 ⫾ 0.14 vs. 4.33 ⫾ 0.15 mmol/L) and an elevated insulin response to meals (207
⫾ 21 vs. 75 ⫾ 18 pmol/L). This study demonstrates that consumption of a diet with increased protein and a reduced
CHO/protein ratio stabilizes blood glucose during nonabsorptive periods and reduces the postprandial insulin
response. J. Nutr. 133: 405– 410, 2003.
LAYMAN ET AL.
406
maintenance of blood glucose. After an overnight fast, gluconeogenesis provides ⬎ 70% of hepatic glucose release, with
amino acids serving as the principal carbon source (16). These
studies provide further evidence for a linkage between dietary
protein and glucose homeostasis.
Recent reports have highlighted the critical need to enhance regulation of blood glucose in overweight adults (17)
and during weight loss (18). This study tests the hypothesis
that a moderate increase in dietary protein with a corresponding reduction of carbohydrates (CHO) improves glucose and
insulin homeostasis during weight loss. We propose that
leucine is a critical substrate in the relationship of dietary
protein to glucose homeostasis. Currently, the minimum
leucine requirement for nitrogen balance is reported to be
⬍ 38 mg/(kg 䡠 d) (19,20), whereas positive oxidative balance
requires intakes of 89 mg/(kg 䡠 d) (20). We evaluated the effect
of changing the dietary CHO/protein ratio from 3.5 to 1.4 and
with doubling of dietary leucine on plasma BCAA, plasma
levels of gluconeogenic precursors alanine and glutamine, insulin response to meals and maintenance of fasting and postprandial blood glucose.
Women (n ⫽ 24; 45–56 y old) with body weights ⬎15% above
ideal body weight (21) were recruited from the University of Illinois
community. Subjects were screened using a medical history and a
24-h diet recall; subjects with known medical conditions, routine use
of medications or smokers were excluded from the study. Subjects
selected for the study had minimal daily physical activity, had maintained a stable body weight during the past 6 mo and consumed a diet
that contained 12–17% of energy as protein. These conditions were
selected as representative of average U.S. food intake (22) and to
standardize prestudy conditions. All protocols and consent forms were
reviewed and approved by the Institutional Review Board of the
University of Illinois Urbana-Champaign.
After the initial screening period, subjects had an additional
baseline evaluation period that included a 3-d weighed dietary record
and measurement of plasma glucose, insulin and amino acids. This
period served as an initial control period for each subject. After the
TABLE 1
Energy and macronutrient compositions of weight loss diets1
Daily intakes2
Macronutrients
Energy
CHO
Protein
Selected amino acids
Fat
Leu
Ile
MJ/d
Protein group
CHO group
6.98 ⫾ 0.19
6.94 ⫾ 0.17
Val
Phe
Thr
6.42 ⫾ 0.12
3.67 ⫾ 0.07
5.42 ⫾ 0.11
3.12 ⫾ 0.06
5.03 ⫾ 0.09
2.67 ⫾ 0.05
g/d
171 ⫾ 7
239 ⫾ 5
125 ⫾ 3
68 ⫾ 2
54 ⫾ 2
48 ⫾ 2
9.89 ⫾ 0.19
5.39 ⫾ 0.10
5.99 ⫾ 0.11
3.20 ⫾ 0.06
Test meal3
Energy
CHO
Protein
Fat
Leu
MJ
Protein group
CHO group
1.69
1.57
Ile
Val
Phe
Thr
1.71
0.36
1.92
0.41
1.69
0.44
1.36
0.28
g
39
57
33
10
13
12
2.70
0.66
1 Weight loss diets were designed to be equal in energy and fat. The protein group had a daily intake of protein of 1.5 g/(kg 䡠 d) and a ratio of
CHO/protein ⫽ 1.4 and the CHO group had a protein intake of 0.8 g/(kg 䡠 d) and CHO/protein ⫽ 3.5.
2 Values represent intakes from 3-d weighed records; means ⫾ SEM, n ⫽ 12.
3 Values represent defined intakes fed at test meal.
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SUBJECTS AND METHODS
baseline evaluation, subjects were divided into two groups (n ⫽ 12)
based on age (50.1 ⫾ 1.1 y) and body weight (85.2 ⫾ 3.6 kg).
One group of 12 women was assigned to a moderate protein diet
(Protein Group) with protein intake of 1.5 g/(kg 䡠 d) and a CHO/
protein ratio ⬍1.4. This diet provided 30% of dietary energy as
protein, 40% as carbohydrates and 30% as fats (Table 1). The second
group was designated a control group and consumed a high carbohydrate diet (CHO Group) similar to their baseline diet with protein
intake at 0.8 g/(kg 䡠 d) and a CHO/protein ratio ⬎3.5. This diet
provided 15% of the energy as protein, 55% as carbohydrates and
30% as fats. Daily intakes of individual amino acids reflected total
protein intake such that the relative amino acid composition for the
two diets remained constant. For example, leucine accounted for
7.85% of the protein consumed by the Protein Group and 7.86% for
the CHO Group. The Protein Group consumed 9.9 g/d of leucine and
22.3 g/d of BCAA and the CHO Group consumed 5.4 g/d of leucine
and 12.3 g/d of BCAA. Both diets were designed to produce a daily
energy deficit of ⬃2.09 MJ and generate weight loss of ⬃0.6 kg/wk
(23). Subjects were instructed to maintain a constant level of physical
activity throughout the study.
The overall experiment lasted for 11 wk with wk 1 used as an
Initial Control period providing baseline data for all subjects. The test
period consisted of a 10-wk diet study. During the first 4 wk of the
study, all food was prepared in the food research laboratory and all
meals were weighed by the research staff and also by the subjects to
evaluate reliability and reproducibility of the subject weighed food
records. Also during the laboratory-based diet period, subjects received daily instruction by our dietitian about the menus, food
substitutions, portion sizes and procedures for maintaining weighed
diet records. During the final 6 wk of the study, subjects continued to
use the 2-wk menu rotation while preparing meals at home. Each
week, subjects were required to report to the research laboratory for
measurement of body weight and to review their 3-d food records
with the research dietitian.
At wk 0, 2, 4 and 10 during the weight loss, subjects reported to
the research facility at 0700 h after a 12-h overnight fast. Body
weights were determined by electronic scale and blood samples were
drawn. Subjects were then given a test meal providing 1.67 MJ with
the macronutrient composition similar to the respective diet treatments. The test meals were designed to represent common breakfast
meals and to provide energy levels similar to standard oral glucose
tolerance tests (24,25). The test meals were designed to be diet
DIETARY PROTEIN MODIFIES GLUCOSE HOMEOSTASIS
FIGURE 1 Plasma glucose concentrations in fasting women assigned to a moderate protein diet (Protein Group) or a high carbohydrate diet (CHO Group) during 10 wk of weight loss. Values are means
⫾ SEM, n ⫽ 12. Means without a common letter differ, P ⬍ 0.05. The
linear decline in the CHO Group was significant (R2 ⫽ 0.982; P
⫽ 0.0086).
whereas in the CHO Group, insulin was 115% above fasting
levels. The insulin response to the test meal at 2, 4 and 10 wk
increased over time in the CHO Group (Fig. 3) and the time
⫻ diet interaction was significant.
Plasma concentrations for the indispensable amino acids
did not differ between treatment groups after an overnight fast
(Table 2). Subjects in the CHO Group had higher fasting
blood levels for glutamine and for the sum of alanine plus
glutamine. After the test meal, changes in plasma amino acids
reflected the indispensable amino acid content of the respective diets. The Protein Group received a meal containing 33 g
of protein (Table 1) and plasma concentrations of amino acids
remained significantly above fasting levels 2 h after the completion of the meal (Table 2). The magnitude of the increases
varied among the amino acids with the BCAA increasing most
(68 –108%) and threonine increasing least (22%). In the
CHO Group, the test meal provided 10 g of protein; 2 h after
the meal, most amino acids exhibited concentrations that
were not different from values after a 12-h fast. Alanine and
phenylalanine concentrations tended to be greater (P ⫽ 0.61
and 0.78, respectively), whereas glutamine concentration was
⬎ 19% lower than the fasting concentration (P ⬍ 0.05).
Meal responses for the BCAA and the two nonessential
amino acids, alanine and glutamine, reflected fundamental
differences in metabolism between the groups (Table 2). After
the higher protein meal, the sum of the BCAA increased by
76% (248 ⫾ 10 ␮mol/L, P ⬍ 0.05) for the Protein Group with
a corresponding increase in the nitrogen-transporting mole-
RESULTS
Daily intakes of macronutrients were determined from
weekly 3-d weighed food records (Table 1). The Protein
Group consumed 6.98 ⫾ 0.19 MJ/d with 125 g of protein and
171 g of carbohydrates. The CHO Group consumed 6.94
⫾ 0.16 MJ/d with 68 g of protein and 240 g of carbohydrates.
After consuming the respective diets for 10 wk, subjects in the
Protein Group lost 7.53 ⫾ 1.44 kg of body weight and subjects
in the CHO Group lost 6.96 ⫾ 1.36 kg (27).
At the beginning of the study (wk 0), fasting blood glucose
did not differ between groups (Fig. 1). After 10 wk, the CHO
Group exhibited fasting blood glucose of 4.34 ⫾ 0.10 mmol/L,
which was 11% lower than that of the Protein Group (P
⬍ 0.05) and represented a significant decline over time for the
CHO Group (Fig. 1).
For both groups, the 2-h postprandial blood glucose values
were ⬃15% lower than the values after the 12-h fast, with the
CHO Group reduced to 3.77 ⫾ 0.14 mmol/L (Fig. 2A). The
timing of these measurements at 2 h after the meal was based
on established oral glucose tolerance curves, which indicate
that a 2-h time point reflects the end of the absorptive period
and the return of glucose to baseline nonabsorptive levels
(24,25).
Fasting plasma insulin concentrations did not differ between the groups (Fig. 2B). The test meal increased insulin
levels, which was still evident 2 h after the meal. In the
Protein Group, insulin was 42% higher than fasting levels,
FIGURE 2 Plasma glucose (A) and insulin (B) concentrations after
a 12-h fast and 2 h after a 1.67 MJ test meal in women assigned to a
moderate protein diet (Protein Group) or a high carbohydrate diet (CHO
Group) during 10 wk of weight loss. Values are means ⫾ SEM, n ⫽ 12.
Means without a common letter differ, P ⬍ 0.05.
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specific to evaluate the normal metabolic response to the chronic
diet. Two hours after completion of the meal, a postprandial blood
sample was drawn. Plasma samples were analyzed for amino acids,
glucose and insulin. Plasma amino acids were determined by HPLC as
described previously (26). Plasma glucose was analyzed by a glucose
oxidase-peroxidase automated method (YSI model 2300 analyzer,
Yellow Springs Instruments, Yellow Springs, OH) and insulin was
determined by commercial RIA kit (07–26102 ICN Pharmaceuticals,
Costa Mesa, CA).
Data were evaluated using a one-way ANOVA with repeated
measures with diet treatment and time as independent variables
(SAS Institute, Cary, NC). When significant treatment or treatment
⫻ time effects were observed (P ⬍ 0.05), differences were evaluated
using Fisher’s Least Significant Difference test to determine differences between diet treatments or differences within each diet treatment over time. Values are means ⫾ SEM.
407
LAYMAN ET AL.
408
FIGURE 3 The 2-h postprandial insulin response to a 1.67 MJ test
meal in women assigned to a moderate protein diet (Protein Group) or
a high carbohydrate diet (CHO Group). The insulin response was determined as the 2-h postprandial value minus the fasting value for each
subject. Values represent means ⫾ SEM, n ⫽ 12. Means without a
common letter differ, P ⬍ 0.05.
cules, alanine plus glutamine (42%, 296 ⫾ 22 ␮mol/L). Because virtually no dietary alanine or glutamine escapes gut and
liver metabolism during meal absorption (28), these values
must reflect changes in plasma amino acid flux resulting from
either increased peripheral production or reduced visceral utilization of these nonessential amino acids. In the CHO Group,
2 h after the test meal, the sum of the BCAA and the sum of
alanine and glutamine did not differ from the fasting concentrations. Although plasma amino acid contractions do not
reflect quantitative flux measurements, plasma concentrations
do reflect changes in intracellular concentrations and rates of
BCAA catabolism (29,30).
DISCUSSION
The potential for amino acids to interact with glucose
metabolism is well established; however, the effect of prolonged modification of protein intake on glucose homeostasis
is unknown. In the 1970s, researchers reported that amino acid
availability supported glucose metabolism during prolonged
aerobic exercise (5) or during intravenous infusions (31).
Subsequently, quantitative measures of amino acid flux established the importance of liver gluconeogenesis in the maintenance of blood glucose during nonabsorptive periods
TABLE 2
Blood amino acid values in women consuming either a moderate protein or high carbohydrate (CHO) diets during weight loss1
Protein group
Fasting
Test meal
␮mol/L
Leucine
Isoleucine
Valine
Phenylalanine
Threonine
Alanine
Glutamine
⌺BCAA
⌺ala ⫹ gln
102 ⫾ 5b
50 ⫾ 3b
171 ⫾ 8b
41 ⫾ 1.5c
104 ⫾ 7b
324 ⫾ 16b
378 ⫾ 15b
323 ⫾ 5b
702 ⫾ 16c
CHO Group
Change
Fasting
␮mol/L
%
181 ⫾ 9a
104 ⫾ 6a
287 ⫾ 12a
66 ⫾ 2.6a
127 ⫾ 11a
485 ⫾ 28a
513 ⫾ 24a
571 ⫾ 9a
998 ⫾ 26a
77
108
68
61
22
50
36
77
42
Test meal
99 ⫾ 4b
50 ⫾ 2b
174 ⫾ 8b
43 ⫾ 1.8c
106 ⫾ 12b
388 ⫾ 21b
449 ⫾ 12a
322 ⫾ 5b
837 ⫾ 16b
Change
%
93 ⫾ 4b
49 ⫾ 2b
158 ⫾ 7b
51 ⫾ 2.3b
113 ⫾ 11b
452 ⫾ 24a,b
363 ⫾ 19b
300 ⫾ 4b
815 ⫾ 22b
⫺6
⫺2
⫺9
18
7
16
⫺19
⫺7
⫺3
1 Values represent means ⫾ SEM of three trials for each of the 12 women (n ⫽ 12) at wk 2, 4 and 10. Means in a row without a common letter differ,
P ⬍ 0.05.
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(13,15,16). The present study evaluated the potential of moderate changes in the dietary CHO/protein ratio to affect the
metabolic balance between glucose and insulin homeostasis
and the availability of gluconeogenic amino acids. After 10 wk
of diet modification, increasing dietary protein and reducing
the CHO/protein ratio minimized postprandial and fasting
changes in blood glucose during weight loss.
Maintenance of blood glucose within the normal range of
4.4 – 6.0 mmol/L requires a precise balance between hepatic
glucose release (16) and peripheral tissue glucose use (32). The
liver regulates glucose release by balancing the disposal of
exogenous dietary glucose with endogenous production from
gluconeogenesis and glycogenolysis. The balance achieved by
the liver among absorption, de novo synthesis and stored
glycogen is dependent on diet composition and stage of absorption (14,16,33,34). Similarly, the use of blood glucose by
peripheral tissues is a balance among both insulin dependent
and insulin independent tissues and varies widely, depending
on glucose availability, hormone status and tissue energy
needs. This balance between hepatic glucose release and peripheral clearance must be able to extend from minimum
needs of ⬃80 to 120 g/d for obligate glycolytic tissues such as
the brain, nerve tissue and blood cells to levels ⬎ 400 g/d
during conditions of high dietary carbohydrate intakes.
To test the influence of the dietary CHO/protein ratio on
glucose homeostasis, one approach would be to examine glucose absorption curves and peak insulin levels. This approach
would be essential to evaluate the meal response to a carbohydrate load including the potential to reduce hepatic endogenous production (16) or to increase insulin-driven peripheral
clearance (35). In nondiabetic subjects, the balance of these
responses maintains blood glucose within normal ranges. On
the other hand, another key regulatory challenge occurs during periods between meals when exogenous glucose is not
available. During these periods, the body must rebalance hepatic glucose production and peripheral clearance to protect
blood glucose from hypoglycemic responses. The two most
likely periods for hypoglycemia would be in the morning after
an overnight fast or at the end of a postprandial period when
insulin is elevated and blood glucose returns to nonabsorptive
levels (24,36). These two “nonabsorptive” periods were the
focus of the current study.
After 10 wk of controlled dietary intakes, subjects consuming a diet with adequate protein [0.8 g/(kg 䡠 d)] and a high
DIETARY PROTEIN MODIFIES GLUCOSE HOMEOSTASIS
minimal disposal in the gut and a relatively slow rate of
catabolism in liver (28). Changes in the BCAA, Phe and Thr
reflected these metabolic differences in tissue specificity, with
postprandial plasma Thr increased by 21%, Phe by 60% and
the sum of the BCAA by 93% in the Protein Group.
Postprandial changes in plasma levels of BCAA, alanine
and glutamine after the test meal were consistent with our
proposed relationship of dietary amino acids to glucose homeostasis. Specifically, we proposed that postprandial increases
in BCAA are associated with increased production of alanine
and glutamine and enhanced hepatic gluconeogenesis to
maintain fasting blood glucose. We found that the higher
protein meal produced anticipated increases in plasma BCAA
with corresponding increases in alanine and glutamine. For
the CHO Group, with dietary protein at levels designed to
meet minimum needs for nitrogen balance, the 2-h postprandial values for BCAA were not different from fasting levels
with no change in the sum of alanine plus glutamine.
Changes in plasma amino acid concentrations are not
equivalent to quantitative measurements of amino acid flux.
However, concentration differences for BCAA, alanine and
glutamine reflect metabolic differences. Increases in BCAA
levels in the blood relate directly to changes in intracellular
concentrations (11,29,38). Similarly, increases in tissue concentrations of BCAA increase catabolism via the aminotransferase and dehydrogenase (11,39), and increased BCAA flux
to muscle relates directly to increased production and release
of alanine and glutamine (5,29,30,39). Summing each of the
elements of the pathway, this study demonstrated that prolonged dietary modification resulting in increased postprandial
levels of BCAA, and increased levels of alanine and glutamine
can affect glucose homeostasis in free-living subjects.
The relationship of the dietary CHO/protein ratio to hepatic glucose production was also evident in subjects after
fasting overnight. After 10 wk of consuming the energyrestricted diets, subjects receiving the higher CHO diet (239
g/d) had 12% lower fasting blood glucose and the level declined throughout the study (Fig. 1). Associated with the
reduced blood glucose, subjects in the CHO Group had combined alanine plus glutamine levels that were 20% higher than
those of the Protein Group. This increase in plasma alanine
and glutamine associated with a diet containing more CHO
and less protein was unexpected. Increases in peripheral production of alanine and glutamine are unlikely with lower
dietary intake of the BCAA. A possible explanation could be
increased rates of protein breakdown associated with the high
CHO diet. A more likely explanation for the increased alanine
and glutamine levels would be a reduction in the rate of
hepatic clearance. This hypothesis is supported by reports that
the rate of clearance of plasma alanine is proportional to
plasma alanine concentration and the rate of gluconeogenesis
(40). Investigators have shown that high CHO feeding or
elevated insulin result in down-regulation of hepatic gluconeogenesis on the basis of measurements of flux (16) as well as
down-regulation of gene expression of key regulatory enzymes
(41).
This study evaluated the effect of sustained dietary changes
in the ratio of CHO to protein intake on plasma amino acid
profiles and maintenance of blood glucose during energy restriction. We found that a diet with increased protein and
reduced levels of CHO stabilizes blood glucose during nonabsorptive periods and reduces postprandial insulin response.
Additional research utilizing substrate flux measurements is
required to define the quantitative relationship between dietary intake of glucose and amino acids and hepatic vs. peripheral management of blood glucose levels. However, this
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CHO/protein ratio (⬎ 3.5) had lower fasting and postprandial
blood glucose than subjects consuming the diet with more
protein [1.5 g/(kg 䡠 d)] and a lower CHO/protein ratio (⬍ 1.4).
As outlined above, regulation of blood glucose must be
achieved through changes in either hepatic output or peripheral clearance, or both. In the CHO Group at 2 h after the test
meal, lower blood glucose was associated with blood insulin
levels higher than either the fasting concentration or those of
the Protein Group. This response to the test meal seems
reasonable because the CHO Group consumed more carbohydrates. However, although the direction of the insulin response to the test meal was expected, the magnitude of the
insulin response appeared to be disproportionate to the carbohydrates consumed. Carbohydrate intake from the test meal
was 46% higher for the CHO Group (57 g vs. 39 g), whereas
the insulin response to the meal was 115% higher in the CHO
Group (Fig. 2B) and the magnitude of the insulin response
increased over the 10-wk feeding period (Fig. 3). These data
are consistent with reduced insulin sensitivity in the CHO
Group during the study. On the other hand, subjects in the
CHO Group had lower fasting blood glucose than subjects in
the Protein Group with similar fasting insulin levels. If insulin
sensitivity is lower in the CHO Group, reduced peripheral
sensitivity should produce higher fasting blood glucose. These
data suggest that dietary changes in the CHO/protein ratio
produce changes in endogenous glucose regulation that likely
include changes in both peripheral glucose clearance and
hepatic glucose production.
Estimates of the contribution of amino acid carbon to de
novo glucose synthesis range from 0.5 to 0.7 g of glucose from
1 g of dietary protein (28,37). In addition to the direct conversion of amino acid carbon to gluconeogenic precursors,
there is also the contribution of the BCAA to the glucosealanine cycle (5,29). Because BCAA are catabolized in skeletal muscle, the amino nitrogen is transferred from the BCAA
to ␣-keto glutarate, forming glutamate. The nitrogen is then
transferred from glutamate to alanine via aminotransferase, or
glutamate is further aminated to glutamine. The net effect is a
direct stoichiometric relationship between catabolism of the
BCAA in skeletal muscle and production of alanine or glutamine. The alanine and glutamine produced in muscle are
released to the blood circulation and extracted by splanchnic
tissues, predominately the liver and gut. Production of these
nonessential amino acids affects glucose homeostasis by the
recycling of glucose carbon. Production of alanine captures
pyruvate in skeletal muscle (29,31); during catabolism of glutamine in the gut, at least 50% of the ␣-amino group of
glutamine is converted to alanine via transamination (28,29).
These metabolic pathways suggest that skeletal muscle uptake
of BCAA is directly linked to the quantity of alanine and
glutamine produced and provision of 3-carbon substrates for
hepatic gluconeogenesis.
Changes in plasma concentrations of amino acids after the
meal reflect diet composition and known metabolic responses
in the gut, liver and peripheral tissues (1,28). After a mixed
meal containing protein, there is an increased rate of disposal
of amino acids for protein synthesis and amino acid degradation. Catabolism of individual amino acids is tissue specific.
For example, after a meal, the gut is highly active in amino
acid catabolism and disposes of most glutamine and threonine
before they reach the portal circulation (28). At the other
extreme, the gut and liver have minimal capacity to initiate
amino acid degradation of the BCAA, resulting in increased
movement of the BCAA through the blood to peripheral
tissues (11,28,30). Plasma appearance of phenylalanine would
be expected to be intermediate between Thr and BCAA, with
409
LAYMAN ET AL.
410
study supports the hypothesis that the ratio of dietary protein
and carbohydrates can have a significant effect on metabolic
balance and specifically on glucose homeostasis during weight
loss.
LITERATURE CITED
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