1 Center for Human Nutrition and Department of Internal Medicine, Washington University School of Medicine, St. Louis, Missouri 63110; 2 Department of Animal Science and Faculty of Nutrition, Texas A&M University, College Station, Texas 77843; 3 Department of Nutritional Sciences, Rutgers University, New Brunswick, New Jersey 08901; and 4 St. Bartholomew's and The Royal London School of Medicine, London E1 1BB, UK
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ABSTRACT |
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The effect of obesity on regional skeletal
muscle and adipose tissue amino acid metabolism is not known. We
evaluated systemic and regional (forearm and abdominal subcutaneous
adipose tissue) amino acid metabolism, by use of a combination of
stable isotope tracer and arteriovenous balance methods, in five lean
women [body mass index (BMI) <25 kg/m2] and five women
with abdominal obesity (BMI 35.0-39.9 kg/m2; waist
circumference >100 cm) who were matched on fat-free mass (FFM). All
subjects were studied at 22 h of fasting to ensure that the
subjects were in net protein breakdown during this early phase of
starvation. Leucine rate of appearance in plasma (an index of whole
body proteolysis), expressed per unit of FFM, was not significantly
different between lean and obese groups (2.05 ± 0.18 and
2.34 ± 0.04 µmol · kg
FFM1 · min
1, respectively).
However, the rate of leucine release from forearm and adipose tissues
in obese women (24.0 ± 4.8 and 16.6 ± 6.5 nmol · 100 g
1 · min
1,
respectively) was lower than in lean women (66.8 ± 10.6 and 38.6 ± 7.0 nmol · 100 g
1 · min
1, respectively;
P < 0.05). Approximately 5-10% of total whole body leucine release into plasma was derived from adipose tissue in
lean and obese women. The results of this study demonstrate that the
rate of release of amino acids per unit of forearm and adipose tissue
at 22 h of fasting is lower in women with abdominal obesity than
in lean women, which may help obese women decrease body protein losses
during fasting. In addition, adipose tissue is a quantitatively
important site for proteolysis in both lean and obese subjects.
obesity; stable isotope tracers; protein metabolism
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INTRODUCTION |
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OBESITY is associated with altered hormone production, lipolytic activity, and glucose metabolism in adipose tissue (17, 20). However, little is known about the effect of obesity on adipose tissue amino acid metabolism. The relationship between adiposity and amino acid metabolism may have important physiological and clinical implications. For example, differences in protein metabolism between lean and obese subjects are probably responsible for conserving muscle protein and improving survival during fasting in obese persons (6).
The effect of obesity on whole body amino acid kinetics is not clear because of conflicting data from different studies, which have reported that postabsorptive whole body protein breakdown is either increased (15, 16, 24, 36) or the same (22, 25) in obese compared with lean subjects. These studies measured leucine rate of appearance (Ra) into the systemic circulation, as an index of whole body protein breakdown, and did not investigate individual organ- or tissue-specific amino acid metabolism. By use of arteriovenous balance techniques, studies conducted in rats (18, 19) and in humans (3, 8) have shown that adipose tissue makes an important contribution to systemic amino acid release during postabsorptive conditions (8, 19). However, the relative importance of muscle and adipose tissues to whole body amino acid kinetics and the effect of obesity on regional amino acid metabolism are not known.
The present study was performed to examine whole body and regional (forearm and adipose tissue) amino acid metabolism in lean and obese human subjects. Subjects were studied at 22 h of fasting to ensure the presence of net protein breakdown during this early phase of starvation. We hypothesized that obesity is associated with a decreased rate of amino acid release (protein breakdown) from skeletal muscle and adipose tissue, which could help explain the greater conservation of body protein observed during fasting in obese than in lean persons (7). A combination of stable isotope tracer infusion and arteriovenous balance techniques, which involved forearm and adipose tissue blood flow measurements and radial artery, deep forearm vein, and abdominal vein blood samples, was used to quantify whole body and regional amino acid kinetics.
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METHODS |
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Subjects.
Five lean women [body mass index (BMI) <25 kg/m2] and
five women with class II abdominal obesity (BMI 35.0-39.9
kg/m2; waist circumference >100 cm) participated in this
study. These subjects also participated in studies evaluating the
effect of fasting on lipid and glucose metabolism that were reported
previously (12, 13). Lean and obese women were matched on
fat-free mass (FFM). Although there was a trend [nonsignificant (NS)]
for the obese subjects to be older than the lean subjects (37 ± 4 and 29 ± 3 yr, respectively), all subjects were premenopausal and were studied within the first 2 wk of the follicular phase of their
menstrual cycle. After completing a comprehensive medical evaluation
including an oral glucose tolerance test, all subjects were considered
to be in good health except for the presence of obesity. All subjects
were weight stable for 2 mo before the study, and none had been
involved in any regular exercise program for
6 mo before the study.
No subjects were taking any medications. This study was approved by the
Human Studies Committee and the General Clinical Research Center at
Washington University School of Medicine, and all subjects gave
informed consent before their participation.
Body composition analysis. Body fat and FFM values were assessed by dual-energy X-ray absorptiometry (DEXA; Hologic QDR 1,000/W, Waltham, MA) in all subjects as outpatients within 3 days of the isotope infusion study. Windows were set on the forearms to estimate fat and lean tissue forearm content.
Experimental protocol.
Subjects were admitted to the General Clinical Research Center at
Washington University School of Medicine in the evening before the
study. At 1800, they ingested a meal consisting of 12 kcal/kg
body wt for lean subjects and 12 kcal/kg adjusted body weight for obese
subjects [adjusted body weight = ideal body weight + (actual
body weight ideal body weight) · 0.25]. Carbohydrate, fat, and protein represented 55, 30, and 15%, respectively, of total
energy intake. At exactly 2000, all subjects ingested a snack
containing 40 g carbohydrate, 6.1 g fat, and 8.8 g
protein (Ensure; Abbott Laboratories, Columbus, OH). The subjects then remained fasted until completion of the study on the following day.
Blood flow measurements. Abdominal adipose tissue blood flow was measured by the xenon washout technique (21). Approximately 120 µCi of 133Xe, dissolved in normal saline, was injected into subcutaneous adipose tissue, ~3 cm lateral to the umbilicus (21). A cesium iodide detector (Oakfield instruments, Eynsham, UK) was placed directly over the site of injection (28) to measure radioactive decay in adipose tissue over a 15-min period.
Forearm blood flow was measured by venous occlusion plethysmography (37) while subjects remained supine with their arms extended at heart level. Changes in forearm circumference in response to occlusion of forearm venous drainage were measured as a change in voltage with a mercury-in-Silastic strain gauge (Hokanson, Bellevue, WA). To eliminate the influence of venous return from the hand, blood flow from the hand was occluded for 2 min before measurement by increasing the pressure in a wrist cuff to 200 mmHg. Five blood flow measurements were made every 5 min between 2145 and 22 h of fasting. The strain gauge was electrically calibrated (11) before and after each set of five blood flow measurements. The average of the 15 blood flow measurements over a 15-min period was used to represent forearm blood flow.Analyses. Plasma amino acid concentrations were determined by using a fluorometric HPLC method involving derivatization with o-phthaldialdehyde, as previously described (39). Amino acids in samples were quantified on the basis of known amounts of authentic standards with a Waters model 810 baseline work station (Waters, Milford, MA).
The tracer-to-tracee ratio (TTR) of plasma leucine was measured by electron impact ionization gas chromatography-mass spectrometry of the tert-butyldimethylsilyl derivative after isolation of leucine from deproteinized plasma (27). The measured instrument response (m+3/m+0 isotopomer area ratio) was calibrated against the measured ratio for [2H3]leucine standards of known isotopic TTR.Calculations. Local net amino acid arteriovenous differences were calculated as the amino acid concentration in arterial plasma minus the concentration in venous plasma. Local net fluxes were calculated as the arteriovenous difference multiplied by local plasma flow.
Whole body plasma leucine Ra was determined from the steady-state isotope dilution equation Ra = I/TTRA (29), where I is the tracer infusion rate (in µmol · kg
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Statistics. The significance of differences between lean and obese groups was evaluated by a two-tailed Student's t-test for independent samples. The statistical significance of arteriovenous amino acid concentration differences was determined by Wilcoxon's test, because the data were not normally distributed. A probability value of P < 0.05 was considered to be statistically significant. Results are presented as means ± SE.
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RESULTS |
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Subject characteristics.
Body composition characteristics of the study subjects are listed in
Table 1. Total fat mass was more than
threefold greater in obese than in lean subjects, but FFM was similar
in both groups. At 22 h of fasting, arterial plasma insulin
concentrations were higher in obese than in lean subjects (11.2 ± 0.6 and 4.5 ± 0.7 µU/ml, respectively; P 0.001),
but arterial plasma glucagon concentrations were the same in both
groups (78.4 ±7.9 and 78.3 ±8.3 ng/ml). Plasma flow rate was slower
in obese than in lean subjects, both in forearm (0.7 ±0.1 and 1.7 ±0.2 ml · 100 ml
1 · min
1,
respectively) and in adipose (1.4 ± 0.2 and 3.1 ± 0.6 ml · 100 g
1 · min
1,
respectively) tissues (both P < 0.05).
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Leucine kinetics.
Arterial plasma leucine concentrations were 135 ± 8 µM in lean
and 124 ± 4 µM in obese subjects (NS). Systemic leucine
Ra, expressed per kilogram FFM, tended to be greater in
obese than in lean subjects, but the difference was not statistically
significant (Table 2). However, leucine
Ra, expressed per kilogram body weight, was >30% lower in
obese than in lean subjects (P < 0.05; Table 2).
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Regional amino acid net balance.
Arterial plasma amino acid concentrations and net arteriovenous
differences and fluxes of amino acids across subcutaneous abdominal
adipose tissues are shown in Table 3.
Data obtained from lean and obese subjects were similar, so values from
both groups were pooled. In general, net arteriovenous differences across adipose tissue were small except for alanine release, glutamate uptake, and glutamine release.
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DISCUSSION |
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Although it is known that the regulation of lipid and carbohydrate metabolism is altered in persons with abdominal obesity, the effect of obesity on the regulation of protein metabolism is less clear. To our knowledge, this is the first report to detail regional (i.e., deep forearm muscle and abdominal subcutaneous adipose tissue) amino acid metabolism in obese subjects. A major finding of this study was that leucine release from both forearm and subcutaneous adipose tissue was reduced (per unit of tissue) in our obese compared with our lean subjects. Furthermore, our findings indicate that adipose tissue is a quantitatively important site for proteolysis in both lean and obese subjects; ~5-10% of total whole body leucine Ra was derived from adipose tissue (Table 2). Muscle tissue tended to make a greater contribution to systemic leucine Ra in lean than in obese women, but the contribution of subcutaneous adipose tissue was similar in the two groups.
The arteriovenous difference data for the amino acids listed in Table 3 showed only small differences across subcutaneous adipose tissue. One previous study of human adipose tissue (8), which measured net arteriovenous differences for some amino acids, showed that adipose tissue took up glutamate and released glutamine and alanine. A detailed in vivo study of rat adipose tissue showed release of several amino acids (19), but the arteriovenous differences tended to be small. Our measurements of net arteriovenous differences generally confirm those findings and demonstrate net fluxes for several amino acids for the first time in humans. However, the rate of adipose tissue amino acid uptake and release can be underestimated or missed by evaluating only arteriovenous concentration balance and blood flow, particularly when arteriovenous concentration differences are small, because active uptake and release can cancel each other out. Therefore, the true rate of release of amino acids from adipose tissue (local rate of appearance into plasma) requires the infusion of isotopically labeled amino acid tracers, which can detect unlabeled amino acids released from tissue into the local circulation by the isotope dilution principle (23, 35). The arteriovenous balance technique cannot determine which cell types within a tissue are responsible for a net metabolic action. For example, we cannot determine whether amino acid flux across adipose tissue occurred within adipocytes, stromal cells, vascular endothelia, or interstitial white blood cells.
Leucine Ra is often used as an index of whole body proteolysis (4, 15, 16, 22-25, 36). Leucine Ra values seen in this study are similar to those reported previously (31). Although our values are slightly lower than those of some reports, this is perhaps due to the sex of our volunteers (all female) and the fact that we measured leucine kinetics in plasma rather than in whole blood (33). It is less likely that leucine Ra was reduced because the subjects were being studied after fasting for 22 h. Leucine Ra, expressed per kilogram FFM, tended to be greater in our obese than in our lean subjects, which is consistent with several (15, 16, 24, 36) but not all (22, 25) previous studies. However, in the present study, the differences between lean and obese groups did not achieve statistical significance, which may represent a type II statistical error because of small sample size.
Our data on deep forearm leucine release rate and body composition suggest that whole body muscle mass accounts for ~35% of whole body leucine Ra in lean subjects. This value is in close agreement with previous attempts to extrapolate regional data to whole body muscle mass, even though others measured arteriovenous differences across the leg (30). Nonetheless, conclusions made by extrapolating data from a single muscle bed to whole body muscle mass should be treated with caution because of the possibility of regional heterogeneity between different muscles.
Extrapolating from the depots studied, we estimated that leucine release from adipose tissue accounted for 6% of whole body leucine Ra in our lean women. A higher value (12%) has been reported during postabsorptive conditions (a 12-h fast) in lean men on the basis of infusion of a phenylalanine tracer (3). The discrepancy between studies might reflect sex differences or duration of fasting (12 vs. 22 h). It is also possible that leucine release underestimated protein breakdown in adipose tissue. Leucine can be transaminated and oxidized in adipose tissue (9), thereby preventing its release into the abdominal venous blood, whereas phenylalanine is not metabolized in extrahepatic tissues (10).
The mechanism responsible for the lower rates of leucine release in forearm muscle and abdominal adipose tissues in obese than in lean subjects is not known. Although adipose tissue and forearm blood flows were lower in our obese than in our lean subjects, as has been shown previously (14, 17, 26), it is unlikely that differences in blood flow are responsible for differences in regional amino acid catabolism, because amino acid release from either forearm or adipose tissues should not be limited by blood flow. However, it is possible that the higher plasma insulin concentration observed in our obese than in our lean subjects may have contributed to the differences in proteolysis between groups, because insulin inhibits protein breakdown in lean (15, 22, 31-33) and obese (15, 22) persons.
The reduced leucine release per kilogram of muscle is presumably responsible for the reduced contribution from whole body muscle to systemic leucine Ra, because our lean and obese subjects were matched on FFM. Although absolute leucine release rate per unit of adipose tissue was lower in our obese than in our lean subjects, the contribution of total body fat mass to systemic leucine Ra was greater in the obese group, because adipose tissue mass was much greater in obese than in lean subjects.
Leucine rate of appearance in plasma that was not derived from muscle or subcutaneous adipose tissue tended to be greater in our obese than in our lean subjects. This observation suggests that protein turnover at other sites (e.g., splanchnic bed and kidneys) (30) may be higher in obese than in lean persons. However, it is possible that protein metabolism is heterogeneous within tissues at different locations and that the individual muscle and fat depots that we evaluated in the present study were not representative of whole body muscle and fat masses. It is also possible that the depots studied represented whole body tissue in lean but not in obese subjects. Although deep forearm tissue may be a valid model for whole body muscle mass in lean subjects, its validity in obese subjects is more questionable because of possible "contamination" from an uncertain amount of forearm fat. We attempted to "correct" for this problem by assuming that the relative decrease in deep forearm muscle content in our obese subjects was the same as the relative decrease in total forearm lean tissue, as measured by DEXA.
In conclusion, the present study demonstrates that the rate of release of amino acids per unit of forearm and adipose tissue at 22 h of fasting is lower in abdominally obese than in lean women. Moreover, our data suggest that obesity is associated with a lower fractional contribution from skeletal muscle to systemic leucine Ra. These findings help explain why obese persons are more effective than lean persons in preserving body protein during fasting.
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ACKNOWLEDGEMENTS |
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We thank the nursing staff of the General Clinical Research Center for help in performing the studies and the study subjects for participation.
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FOOTNOTES |
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This study was supported by National Institutes of Health Grants DK-37948, RR-00036 (General Clinical Research Center), RR-00954 (Biomedical Mass Spectrometry Resource), and DK-56341 (Clinical Nutrition Research Unit) and The Wellcome Trust.
Address for reprint requests and other correspondence: S. Klein, Washington Univ. School of Medicine, 660 S. Euclid Ave., Box 8031, St. Louis, MO 63110-1093.
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. Section 1734 solely to indicate this fact.
10.1152/ajpendo.00359.2001
Received 9 August 2001; accepted in final form 4 December 2001.
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REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Andres, R,
Cader G,
and
Zierler KL.
The quantitatively minor role of carbohydrate in oxidative metabolism by skeletal muscle in intact man in the basal state. Measurements of oxygen and glucose uptake and carbon dioxide and lactate production in the forearm.
J Clin Invest
35:
671-682,
1956[ISI].
2.
Clasey, JL,
Kanaley JA,
Wideman L,
Heymsfield SB,
Teates CD,
Gutgesell ME,
Thorner MO,
Hartman ML,
and
Weltman A.
Validity of methods of body composition assessment in young and older men and women.
J Appl Physiol
86:
1728-1738,
1999
3.
Coppack, SW,
Persson M,
and
Miles JM.
Phenylalanine kinetics in human adipose tissue.
J Clin Invest
98:
692-697,
1996
4.
Darmaun, D,
Welch S,
Rini A,
Sager BK,
Altomare A,
and
Haymond MW.
Phenylbutyrate-induced glutamine depletion in humans: effect on leucine metabolism.
Am J Physiol Endocrinol Metab
274:
E801-E807,
1998
5.
Elia, M,
Folmer P,
Schlatmann A,
Goren A,
and
Austin S.
Carbohydrate, fat, and protein metabolism in muscle and in the whole body after mixed meal ingestion.
Metabolism
37:
542-551,
1988[ISI][Medline].
6.
Elia, M,
Stubbs RJ,
and
Henry CJK
Differences in fat, carbohydrate, and protein metabolism between lean and obese subjects undergoing total starvation.
Obes Res
7:
597-604,
1999[Abstract].
7.
Frayn, KN,
Coppack SW,
Humphreys SM,
and
Whyte PL.
Metabolic characteristics of human adipose tissue in vivo.
Clin Sci (Colch)
76:
509-516,
1989[ISI][Medline].
8.
Frayn, KN,
Khan K,
Coppack S,
and
Elia M.
Amino acid metabolism in human subcutaneous adipose tissue in vivo.
Clin Sci (Colch)
80:
471-474,
1991[ISI][Medline].
9.
Frick, G,
Blinder L,
and
Goodman HM.
Transamination and oxidation of leucine and valine in rat adipose tissue.
J Biol Chem
263:
3245-3249,
1988
10.
Gelfand, RA,
and
Barrett EJ.
Effect of physiologic hyperinsulinemia on skeletal muscle protein synthesis and breakdown in man.
J Clin Invest
80:
1-6,
1987[ISI][Medline].
11.
Hallbook T, Mansson B, and Nilsen R. A strain gauge plethysmograph
with electrical calibration. Scand J Clin Lab Invest
413-418, 1970.
12.
Horowitz, JF,
Coppack SW,
and
Klein S.
Whole body and adipose tissue glucose metabolism in response to short-term fasting in lean and obese women.
Am J Clin Nutr
73:
517-522,
2001
13.
Horowitz, JF,
Coppack SW,
Paramore D,
Cryer PE,
Zhao G,
and
Klein S.
Effect of short-term fasting on lipid kinetics in lean and obese women.
Am J Physiol Endocrinol Metab
276:
E278-E284,
1999
14.
Jansson, PA,
Larsson A,
Smith U,
and
Lonnroth P.
Glycerol production in subcutaneous adipose tissue in lean and obese humans.
J Clin Invest
89:
1610-1617,
1992[ISI][Medline].
15.
Jensen, MD,
and
Haymond MW.
Protein metabolism in obesity: effects of body fat distribution and hyperinsulinemia on leucine turnover.
Am J Clin Nutr
53:
172-176,
1991[Abstract].
16.
Kanaley, JA,
Haymond MW,
and
Jensen MD.
Effects of exercise and weight loss on leucine turnover in different types of obesity.
Am J Physiol Endocrinol Metab
264:
E687-E692,
1993
17.
Klein, S,
Coppack SW,
Mohamed-Ali V,
and
Landt M.
Adipose tissue leptin production and plasma leptin kinetics in humans.
Diabetes
45:
984-987,
1996[Abstract].
18.
Kowalski, TJ,
and
Watford M.
Production of glutamine and utilization of glutamate by rat subcutaneous adipose tissue in vivo.
Am J Physiol Endocrinol Metab
266:
E151-E154,
1994
19.
Kowalski, TJ,
Wu G,
and
Watford M.
Rat adipose tissue amino acid metabolism in vivo as assessed by microdialysis and arteriovenous techniques.
Am J Physiol Endocrinol Metab
273:
E613-E622,
1997
20.
Krotkiewski, M,
Björntorp P,
Sjöström L,
and
Smith U.
Impact of obesity on metabolism in men and women.
J Clin Invest
72:
1150-1162,
1983[ISI][Medline].
21.
Larsen, AO,
Lassen NA,
and
Quaade F.
Blood flow through human adipose tissue determined by radioactive xenon.
Acta Physiol Scand
66:
337-345,
1966[ISI][Medline].
22.
Luzi, L,
Castellino P,
and
DeFronzo RA.
Insulin and hyperaminoacidemia regulate by a different mechanism leucine turnover and oxidation in obesity.
Am J Physiol Endocrinol Metab
270:
E273-E281,
1996
23.
Matthews, DE,
Motil KJ,
Rohrbaugh DK,
Burke JF,
Young VR,
and
Bier DM.
Measurement of leucine metabolism in man from a primed, continuous infusion of L-[1-13C]leucine.
Am J Physiol Endocrinol Metab
238:
E473-E479,
1980
24.
Nair, KS,
Garrow JS,
Ford C,
Mahler RF,
and
Halliday D.
Effect of poor diabetic control and obesity on whole body protein metabolism in man.
Diabetologia
25:
400-403,
1983[ISI][Medline].
25.
Nair, KS,
Halliday D,
Matthews DE,
and
Welle SL.
Hyperglucagonemia during insulin deficiency accelerates protein catabolism.
Am J Physiol Endocrinol Metab
253:
E208-E213,
1987
26.
Negrão, CE,
Trombetta IC,
Batalha LT,
Ribeiro MM,
Rondon MUPB,
Tinucci T,
Forjaz CLM,
Barretto ACP,
Halpern A,
and
Villares SMF
Muscle metaboreflex control is diminished in normotensive obese women.
Am J Physiol Heart Circ Physiol
281:
H469-H475,
2001
27.
Patterson, BW,
Carraro F,
and
Wolfe RR.
Measurement of 15N enrichment in multiple amino acids and urea in a single analysis by gas chromatography/mass spectrometry.
Biol Mass Spectrom
22:
518-523,
1993[ISI][Medline].
28.
Samara, JS,
Frayn KN,
Giddings JA,
Clark ML,
and
Macdonald IA.
Modification and validation of a commercially available portable detector for measurement of adipose tissue blood flow.
Clin Physiol Lond
15:
241-248,
1995.
29.
Steele, R.
Influences of glucose loading and of injected insulin on hepatic glucose output.
Ann NY Acad Sci
82:
420-430,
1959[ISI].
30.
Tessari, P,
Garibotto G,
Inchiostro S,
Robaudo C,
Saffioti S,
Vettore M,
Zanetti M,
Russo R,
and
Deferrari G.
Kidney, splanchnic, and leg protein turnover in humans. Insight from leucine and phenylalanine kinetics.
J Clin Invest
98:
1481-1492,
1996
31.
Tessari, P,
Pehling G,
Nissen SL,
Gerich JE,
Service FJ,
Rizza RA,
and
Haymond MW.
Regulation of whole-body leucine metabolism with insulin during mixed-meal absorption in normal and diabetic humans.
Diabetes
37:
512-519,
1988[Abstract].
32.
Tessari, P,
Trevisan R,
Inchiostro S,
Biolo G,
Nosadini R,
Vigili de Kreutzenberg S,
Duner E,
Tiengo A,
and
Crepaldi G.
Dose-response curves of effects of insulin on leucine kinetics in humans.
Am J Physiol Endocrinol Metab
251:
E334-E342,
1986
33.
Tessari, P,
Zanetti M,
Barazzoni R,
Vettore M,
and
Michielan F.
Mechanisms of postprandial protein accretion in human skeletal muscle. Insight from leucine and phenylalanine forearm kinetics.
J Clin Invest
98:
1361-1372,
1996
34.
Wahren, J.
Quantitative aspects of blood flow and oxygen uptake in the human forearm during rhythmic exercise.
Acta Physiol Scand
67, Suppl269:
5-93,
1966.
35.
Waterlow, JC.
Lysine turnover in man measured by intravenous infusion of L-[U-14C]lysine.
Clin Sci (Colch)
33:
507-515,
1967[ISI][Medline].
36.
Welle, S,
Barnard RR,
Statt M,
and
Amatruda JM.
Increased protein turnover in obese women.
Metabolism
41:
1028-1034,
1992[ISI][Medline].
37.
Witney, RJ.
The measurement of volume changes in human limbs.
J Physiol Lond
121:
1-27,
1953[ISI][Medline].
38.
Wolfe, RR.
Radioactive and Stable Isotope Tracers in Biomedicine: Principles and Practice of Kinetic Analysis. New York: Wiley-Liss, 1992.
39.
Wu, G,
and
Knabe DA.
Free and protein-bound amino acids in sow's colostrum and milk.
J Nutr
124:
415-424,
1994[ISI][Medline].
40.
Yeh, S,
and
Peterson RE.
Solubility of krypton and xenon in blood, protein solutions, and tissue homogenates.
J Appl Physiol
20:
1041-1047,
1965[ISI][Medline].