Conjugated Linoleic Acid Improves Insulin Sensitivity ...

Conjugated Linoleic Acid Improves Insulin Sensitivity in Young, Sedentary Humans VALERIE EYJOLFSON, LAWRENCE L. SPRIET, and DAVID J. DYCK Department of Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario, CANADA

ABSTRACT EYJOLFSON, V., L. L. SPRIET, and D. J. DYCK. Conjugated Linoleic Acid Improves Insulin Sensitivity in Young, Sedentary Humans. Med. Sci. Sports Exerc., Vol. 36, No. 5, pp. 814 – 820, 2004. Background: Preliminary evidence in obese diabetic rats suggests that conjugated linoleic acid (CLA) may have antidiabetic properties, based on reductions in fasting glucose and insulin concentrations. However, in lean rats, CLA causes hyperinsulinemia. Furthermore, experiments in humans also suggest that CLA may worsen insulin sensitivity. Objectives: The present study examined whether CLA supplementation can improve insulin sensitivity in humans. Design: Sixteen young sedentary individuals (age, 21.5 ⫾ 0.4 yr (mean ⫾ SEM); body mass, 77.6 ⫾ 3.4 kg) participated in this study. Ten subjects received 4 g·d⫺1 of mixed CLA isomers (35.5% cis-9, trans-11; 36.8% trans-10, cis-12) for 8 wk, whereas six subjects received placebo (safflower oil). Oral glucose tolerance tests were performed at baseline (0), 4 and 8 wk of supplementation. Results: After 8 wk of CLA supplementation, insulin sensitivity index (ISI) increased (14.4 ⫾ 1.0, 8 wk vs 11.3 ⫾ 1.3, 0 wk; P ⬍ 0.05), which corresponded to a decrease in fasting insulin concentrations. Six of the 10 subjects showed large increases in their ISI (range, ⫹27 to 90%), whereas two demonstrated essential no change (⫹3 to 5%), and two had a decrease in insulin sensitivity (⫺12 to ⫺13%). ISI was unchanged over 8 wk in the placebo group. Conclusions: Our results indicate that a common dosage of a commercially available CLA supplement can improve ISI in young, sedentary individuals. However, there is considerable individual variability in the response. Additional studies are required to identify underlying metabolic changes in human skeletal muscle. Key Words: CLA, HUMAN, DIABETES, OBESITY, INSULIN SENSITIVITY INDEX, GLUCOSE

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reducing agent in humans. However, relatively few human trials have been conducted to determine the effects of CLA on body mass and composition. The results of these studies have generally shown a reduction in body fat, but not body mass, with CLA supplementation, (2,23,25), although not by all (28). The cellular mechanisms underlying CLA’s metabolic effects have not been clearly established; it has been hypothesized that CLA may act as a ligand for a group of nuclear transcription factors, the peroxisome proliferator activated receptors (PPAR), in several tissues, resulting in altered expression of numerous proteins involved in hepatic fatty acid metabolism, including ␤-oxidation enzymes, fatty acid-binding protein and lipoprotein lipase (see reviews by Belury (1) and Brown and McIntosh (3)). Obesity, which has reached epidemic proportions in Western society, is a well-identified risk factor for numerous diseases, including Type 2 diabetes. Intuitively, based on changes in body composition, one might expect that CLA would also improve insulin sensitivity. By virtue of its mass, skeletal muscle is the most important tissue involved in the regulation of whole-body glucose homeostasis. Thus, it is also possible that CLA might improve insulin sensitivity in this tissue, independent of changes in body composition. This is supported by data demonstrating that the trans-10, cis-12 CLA isomer reduces fasting glucose and insulin (9) and enhances insulin-stimulated glucose uptake into skeletal muscle (8,21) of obese/diabetic rats. However, the mechanism underlying this effect is unclear. There are reports of increased muscle GLUT4 mRNA (26) in mice, and unchanged GLUT4 protein content and decreased intramuscular triacylglycerol content in obese/diabetic rats (8) after

onjugated linoleic acid (CLA) is the collective name for a group of polyunsaturated fatty acids derived from linoleic acid (18:2). Numerous geometrical and positional isomers of CLA exist, but the most naturally abundant isoform is cis-9, trans-11 (22). CLA is formed by microbial biohydrogenation in the ruminant gut, and hence the lipid fraction of meat and milk products provides a naturally occurring food source for CLA. The most studied forms of CLA are the cis-9, trans-11 and trans-10, cis-12 isomers, which are typically available as synthetic mixtures of equal proportion. CLA has been widely studied in animal models and exhibits a variety of biological effects, including the suppression of chemical induced carcinogenesis and the prevention of atherosclerosis (see review (1)). The most studied effect of CLA supplementation is its effect on body mass and composition. A reduction of body fat in rodents fed CLA has been well established (6,15–17,26,27) and appears to be independent of a reduction in food intake. Thus, there is considerable interest in the efficacy of CLA as a weight

Address for correspondence: David J. Dyck, Department of Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario, Canada N1G 2W1; E-mail: [email protected]. Submitted for publication August 2003. Accepted for publication December 2003. 0195-9131/04/3605-0814 MEDICINE & SCIENCE IN SPORTS & EXERCISE® Copyright © 2004 by the American College of Sports Medicine DOI: 10.1249/01.MSS.0000126391.42896.31

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TABLE 1. Subject characteristics. BMI (kg䡠mⴚ2)

Mass (kg)

% Body Fat

Treatment

N

Age (yr)

Height (m)

0 wk

8 wk

0 wk

8 wk

0 wk

8 wk

Placebo CLA

6 10

21.6 (0.8) 21.4 (0.5)

1.70 (0.04) 1.67 (0.02)

82.5 (10.6) 74.2 (3.0)

84.5 (10.9) 74.8 (2.8)

28.4 (3.0) 26.9 (1.5)

29.1 (3.1) 27.1 (1.5)

25.7 (3.8) 25.6 (2.8)

26.5 (4.3) 26.4 (3.0)

Values are presented as mean (SEM).

CLA treatment. However, the overall evidence for the antidiabetic properties of CLA is actually quite contentious. Studies in nonobese/nondiabetic mice have documented accumulation of hepatic lipids and the development of insulin resistance (5), i.e., lipodystrophic diabetes, which appears to be reversible by the administration of leptin (26). Unfortunately, relatively few studies have examined the antidiabetic properties of CLA in humans and much of the analyses have been confined to the determination of fasting blood glucose and/or insulin, generally with little demonstrable effect (13,14,20,23). However, a recent study utilizing a euglycemic/hyperinsulinemic clamp in abdominally obese male subjects, indicated a worsening of insulin sensitivity after supplementation with both mixed and purified trans-10, cis-12 CLA isomers (20). Thus, whether CLA alters insulin sensitivity in humans has generally not been well studied and there is virtually no evidence to indicate that insulin sensitivity is actually improved. The purpose of this study was to examine whether a commercially available CLA supplement could improve insulin sensitivity in young, sedentary, nondiabetic humans. Although there is no doubt that there would be great value in determining whether CLA supplementation could improve insulin sensitivity in diabetic individuals, it is often difficult to recruit these subjects, and the fact that these individuals are already likely to be receiving pharmacological treatment must be considered. Sedentary individuals, however, are more readily recruited, can be screened for the use of medications that might confound the outcome of an intervention study, and most importantly, represent a large portion of the population that may be overweight and becoming borderline insulin resistant or diabetic. This group is also representative of many individuals who would already be using weight loss supplements such as CLA. Thus, we felt it to be of considerable practical relevance to determine the effects of CLA on glucose management in this population. Specifically, we examined the effects of 4 g CLA per day for 4 and 8 wk on blood glucose and insulin responses during an oral glucose tolerance test.

METHODS Subjects. Sixteen sedentary individuals (12 females, 4 males) from the University community volunteered to participate in this study (age, 21.5 ⫾ 0.4 yr (mean ⫾ SEM); body mass, 77.6 ⫾ 3.4 kg). Sedentary was defined as ⬍ 3 h·wk⫺1 of nonstrenuous exercise. Nine of the females were taking oral contraceptives, and the remaining three were tested at the beginning of their follicular phase after 0, 4, and 8 wk of receiving treatment. Subject characteristics are presented in Table 1. All subjects were screened by quesCLA SUPPLEMENTATION AND INSULIN SENSITIVITY

tionnaire for health risks, as well as to determine whether any medication or oral supplements were currently being taken which might interfere with the results of the study (e.g., weight loss medications or supplementations, vitamin/ mineral mega doses, steroidal medications, etc.) Some subjects were taking a daily multivitamin/mineral pill, which was continued throughout the study. The University Human Ethics Committee approved the experimental procedures, and written consent was obtained from each subject before initiating the trials. Procedures. Ten of the subjects (nine females, one male), randomly chosen from the 16 volunteers, received 4 g of 75% CLA (35.5% cis-9, trans-11; 36.8% trans-10, cis12) per day. This dosage has commonly been used in previous studies with human subjects (e.g. 3– 6 g·d⫺1). Supplements were provided in free fatty acid form in gel capsules by PharmanutrientsTM (Lake Bluff, IL). Subjects were instructed to consume one capsule with each of their three meals, and one with a light snack in the evening. Compliance was 100%. Subjects were asked to refrain from any alcohol and caffeine containing products, as well as exercise for 48 h before each oral glucose tolerance test (OGTT). Subjects were also asked to consistently maintain their normal dietary and activity patterns during the 2 months while receiving supplementation. Subjects also completed a 48-h dietary record before each OGTT. On three occasions, separated by 4 wk, subjects reported to the Human Testing Lab at the University of Guelph after an overnight fast for an OGTT. Although not diagnosed as being diabetic, many sedentary individuals still demonstrate glucose and/or insulin responses during an OGTT that are longer sustained than observed in active individuals. Toward this end, we monitored glucose and insulin responses for a 4-h period. Upon arrival, a 20-gauge, 1-inch Teflon catheter was inserted into the antecubital vein and kept patent with a saline drip. A baseline venous blood sample was taken (0 min), followed by the ingestion of 1 g·kg⫺1 body mass of dextrose in a beverage (TRUTOL®, Source Medical, Mississauga, ON, Canada). Approximately 5 mL of venous blood was collected at 15-min intervals for the first 2 h, every 20 min for the third hour, and every 30 min for the last hour. Subjects remained quietly seated for the 4-h duration of the OGTT. In addition, body mass and composition analyzed by bioelectric impedance (Body Stat 1500, Body Stat U.S. Inc., Tampa, FL) was determined before supplementation and after 8 wk of supplementation. In addition, six subjects (three females, three males) were randomly chosen to receive a placebo treatment (4 g·d⫺1 of safflower oil in gel capsules) to note the variability of the OGTT response over an 8-wk period in sedentary individuals. To our knowledge, there are no reports assessing this Medicine & Science in Sports & Exercise姞

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TABLE 2. Fasting whole blood glucose and plasma insulin concentrations at baseline (0), 4, and 8 wk of supplementation with placebo or CLA. Insulin (pmol䡠Lⴚ1)

Glucose (mM) Treatment Placebo CLA

0 wk

4 wk

8 wk

0 wk

4 wk

8 wk

4.1 (0.1) 4.1 (0.2)

4.2 (0.2) 4.0 (0.2)

4.0 (0.2) 4.0 (0.2)

107 (21) 119 (14)

119 (21) 112 (14)

105 (21) 84 (7)a

Values are presented as mean (SEM). a Significantly different from baseline CLA.

variability. The CLA and placebo capsules were provided by PharmanutrientsTM) and were indistinguishable. After the study, all subjects completed a questionnaire asking whether they could identify the supplement (CLA or placebo) that they were receiving, as well as whether their dietary/activity patterns changed during the 8 wk. Analyses. Each blood sample was collected in a sodium-heparinized tube and immediately processed; 200 ␮L of blood was transferred into 1 mL of 0.6 N perchloric acid (PCA) and centrifuged for 2 min. The supernatant was then removed and stored at ⫺80°C until the subsequent analyses of whole blood glucose, lactate, and glycerol. These were determined in duplicate fluorometrically. The remaining blood was centrifuged for plasma collection. An aliquot of 800 ␮L of plasma was treated with 200 ␮L of NaCl, and

incubated at 56°C for 30 min to denature the enzyme lipoprotein lipase. These samples were stored at ⫺80°C for subsequent colorimetric analysis of free fatty acids (Wako, VA). The remaining plasma was stored at ⫺80°C for the analysis of insulin (Human Insulin Kit, Linco Research Inc., St. Charles, MO). Calculations and statistics. Area under the curve for insulin and glucose responses during the OGTT was calculated using Prism 3.0 software (Graph Pad Software Inc., San Diego, CA). Negative peaks were subtracted from the total AUC. Repeated measures ANOVA (within each treatment) were used to detect significant differences between 0, 4, and 8 wk for all measured parameters. A Dunnett’s posthoc test was used to compare the 4- and 8-wk trials to baseline (0 wk) differences revealed by a significant F ratio

FIGURE 1—Whole blood glucose (A and B) and plasma insulin (C and D) concentrations during an oral glucose tolerance test after 0, 4, and 8 wk of placebo and CLA supplementation.

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in the ANOVA. All values are reported as means ⫾ standard error (SEM). Whole body insulin sensitivity (unitless) was calculated with the following formula, as outlined by Matsuda and Defronzo (11): 10,000/square root of [fasting glucose ⫻ fasting insulin] ⫻ [mean glucose ⫻ mean insulin during OGTT]. In the original formula, plasma glucose and insulin concentrations were used; in the present study, whole blood glucose values have been substituted.

RESULTS Body Composition and Diet There were no significant changes in body mass, BMI, or percent body fat after 8 wk of supplementation with either placebo or CLA (Table 1). Dietary composition 48 h before each of the OGTT at 0, 4, and 8 wk were consistent in both treatment groups (placebo: CHO ⫽ 53–54%, fat ⫽ 25–30%, protein ⫽ 15–18%; and CLA: CHO ⫽ 50 –54%, fat ⫽ 33–34%, protein ⫽ 13–15%). Three of the six subjects receiving placebo correctly guessed which treatment they were receiving, whereas 3 of the 10 subjects receiving CLA guessed correctly. According to the poststudy questionnaire,

none of the subjects significantly altered their activity or dietary patterns during the study.

Oral Glucose Tolerance Tests Glucose and insulin responses. Fasting insulin (Table 2) was significantly lower after 8 wk of CLA treatment (P ⬍ 0.05). Whole blood glucose responses and AUC (Figs. 1 and 2) were not significantly different between baseline, and 4 and 8 wk of supplementation in either the placebo or CLA trials. Plasma insulin responses (Fig. 1) were not affected by either treatment; however, the calculated insulin AUC (Fig. 2) after 8 wk of CLA supplementation decreased between 10 and 38% in 8 of the 10 subjects compared with baseline. Of the remaining two subjects, insulin AUC increased 12 and 35% at 8 wk relative to baseline. When both glucose and insulin responses were taken into account in calculating the insulin sensitivity index (ISI), there was a significant (P ⬍ 0.05) improvement after 8 wk of CLA supplementation, but not at 4 wk (Fig. 3). There was no significant change in ISI over the 8 wk in the placebo group (P ⬎ 0.05). It should be noted that the individual changes in

FIGURE 2—Glucose (A and B) and insulin (C and D) area under the curve (AUC) during an oral glucose tolerance test after 0, 4, and 8 wk of placebo and CLA supplementation.

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FIGURE 3—Calculated insulin sensitivity index (ISI) during an oral glucose tolerance test after 0, 4, and 8 wk of (A) placebo or (B) CLA supplementation; * significantly different from 0 wk.

calculated ISI after 8 wk of treatment were variable. Six of the 10 subjects receiving CLA showed a large improvement in ISI after 8 wk (Table 3; range, 27 to 90%), two remained essentially unchanged (3 to 5% increase in ISI), whereas two demonstrated a worsening of ISI (12–13% decrease). Insulin sensitivity was unchanged over the 8 wk in the placebo group.

DISCUSSION This is the first study to demonstrate that CLA improves insulin sensitivity, as determined by an OGTT, in young 818


sedentary humans after 8 wk of supplementation. This effect was not evident after 4 wk. Importantly, it should be noted that the ability of CLA to improve insulin sensitivity was inconsistent. Six of the 10 subjects receiving CLA demonstrated a marked improvement in ISI after 8 wk (27 to 90%), whereas two showed essentially no change (3–5% improvement) and two showed a decrease (⫺12 to 13%). Insulin sensitivity index was unchanged in the placebo group. Conjugated linoleic acid has previously been shown to normalize glucose tolerance and decrease hyperinsulinemia in diabetic rats (9,21). The antidiabetic effects of CLA in rats appear to be specific to the trans-10, cis-12 isomer (8). Although the mechanisms by which CLA might improve insulin sensitivity have not been extensively researched, it is suggested that CLA works through a similar mechanism to that of thiazolidinedione (TZD) antidiabetic drugs, and the activation of PPAR in various tissues. In rodent skeletal muscle, CLA activates PPAR␥ (12) and increases the expression of proteins related to fatty acid oxidation, such as uncoupling protein 2 (18,21) and carnitine palmitoyl transferase I (15). Surprisingly, it did not alter the activity of phosphatidylinositol 3-kinase and Akt, important proteins in the insulin-signaling cascade (21). In humans, several studies using 3– 6 g·d⫺1 of CLA have examined the effects of supplementation on body mass and composition, and the findings have generally indicated reductions in body fat, but not total mass (2,23,25). Therefore, in this study, our lack of change in body mass is consistent with most previous studies, and our lack of reduction in body fat and BMI agrees with at least one other study failing to observe such changes (28). It may be possible that a study of longer duration would be required to detect significant changes in body composition, as each of the previously mentioned studies (2,23,25) were conducted for 12 wk. However, there has been little research directly examining the putative antidiabetic properties of CLA in humans. In nondiabetic subjects, CLA supplementation for 2 months resulted in a nonsignificant trend toward an increased fasting serum insulin (13); a CLA-induced increase in fasting insulin has been observed in other nondiabetic animals (24,26). Furthermore, a recent study utilizing a euglycemic/ hyperinsulinemic clamp in abdominally obese male subjects indicated a worsening of insulin sensitivity after supplementation with both mixed and purified trans-10, cis-12 CLA isomers (20). Thus, the effect of CLA on insulin sensitivity in humans is controversial and not adequately tested. Our findings are variable, but indicate that CLA can improve insulin sensitivity in sedentary humans. This variability should not be surprising and is a normal characteristic of human subjects. For example, variable metabolic effects of other popular supplements such as caffeine (4) and creatine (7) have also been documented, and are not effective in all individuals. Glucose and insulin AUC were not statistically different from baseline after 4 and 8 wk of supplementation. However, 8 of the 10 individuals had a smaller insulin AUC in response to the OGTT at 8 wk. The calculated ISI was significantly greater after receiving CLA for 8 wk, although as previously discussed, this did not occur in all individuals. In support of the http://www.acsm-msse.org

TABLE 3. Individual fasting insulin, glucose, and ISI responses after 8 wk of treatment with CLA. Subject ID 1 2 3 4 5 6 7 8 9 10 Mean⫾SEM

Fasting Glucose

Fasting Insulin

Mean Glucose (OGTT)

Mean Insulin (OGTT)

ISI

⫹9% ⫹15% ⫹3% 0 ⫺24% 0 ⫹18% ⫺9% ⫺10% ⫺10%

⫹35% ⫺34% ⫺6% ⫺41% ⫺25% ⫺32% ⫺39% ⫺14% ⫺34% ⫺67%

⫺3% ⫹4% ⫹22% ⫺16% ⫹9% ⫹14% ⫹13% ⫹2% ⫺24% ⫹4%

⫺8% ⫺34% ⫺17% ⫺40% ⫺18% ⫺18% ⫹16% ⫹19% ⫺27% ⫺2%

⫺12% ⫹36% ⫺13% ⫹90% ⫹41% ⫹27% ⫹5% ⫹3% ⫹74% ⫹81%

⫺1 ⫾ 4%

⫺26 ⫹ 8%

⫹3 ⫹ 4%

⫺13 ⫾ 6%

⫹33 ⫾ 12%

ISI, insulin sensitivity index; OGTT, oral glucose tolerance test.

general improvement in insulin sensitivity after 8 wk was a significant reduction in fasting plasma insulin, often used as a surrogate measure of insulin sensitivity. In the present study, percent changes in fasting insulin at 8 wk relative to baseline, that is, the degree of improvement, in individuals receiving CLA were significantly correlated to percent changes (improvement) in ISI (Fig. 4; P ⫽ 0.015); fasting glucose was not (P ⫽ 0.2403). A decrease in fasting insulin was observed in 9 of 10 subjects receiving CLA at 8 wk; no reduction in fasting insulin was observed after 4 wk of treatment, which is consistent with the lack of improvement in calculated ISI. Thus, our data indicate that fasting insulin concentration is a good reflec-

FIGURE 4 —Correlation between percent change in insulin sensitivity index (ISI) and percent change in fasting insulin after 4 and 8 wk of CLA supplementation.

tion of insulin sensitivity, and that improvements in fasting insulin concentrations may be a valuable clinical marker of improved insulin sensitivity. However, it should be appreciated that the pulsatile nature of insulin release may also introduce variability in fasting serum insulin measurements (19); therefore, fasting determinations in duplicate or even triplicate should be considered for such purposes. Clearly, more research focusing on CLA’s putative antidiabetic effects in humans is required, particularly in light of the current near epidemic proportion of obesity and diabetes in our societies. The results of our study are in contrast to those recently reported by Riserus et al. (20). Although we cannot explain this discrepancy, it should be noted that even in our own study, four individuals showed no improvement in ISI, and two of these actually showed a substantial worsening. Thus, the effects of CLA on insulin sensitivity might be highly variable. Further research needs to address this controversy, and focus on the ability of CLA to alter glucose control in both nondiabetic and diabetic humans, and the variability of such effects. In addition, it will be important to identify the underlying metabolic changes in skeletal muscle responsible for improved glucose control, including the regulation of fatty acid metabolism (i.e., balance between oxidation and storage) as well as glucose metabolism and transport and insulin signaling. In particular, alterations in stored intramuscular lipids will be important to assess, as these are strongly correlated to insulin resistance in humans (10). The authors also wish to acknowledge the generous donation of CLA and placebo supplements from Pharmanutrients™, Lake Bluff, IL, as well as the excellent technical assistance of Premila Sathasivam, Cheryl Collier, and Cyndy McClean. This study was funded by grants from the Natural Science and Engineering Research Council of Canada (D.J. Dyck and L.L. Spriet).

REFERENCES 1. BELURY, M. A. Dietary conjugated linoleic acid in health: physiological effects and mechanisms of action. Annu. Rev. Nutr. 22: 505–531, 2002. 2. BLANKSON, H., J. A. STAKKESTAD, H. FAGERTUN, E. THOM, J. WADSTEIN, and O. GUDMUNDSEN. Conjugated linoleic acid reduces body fat mass in overweight and obese humans. J. Nutr. 130:2943– 2948, 2000. CLA SUPPLEMENTATION AND INSULIN SENSITIVITY

3. BROWN, J. M., and M. K. MCINTOSH. Conjugated linoleic acid in humans: regulation of adiposity and insulin sensitivity. J. Nutr. 133:3041–3046, 2003. 4. CHESLEY, A., R. A. HOWLETT, G. J. F. HEIGENHAUSER, E. HULTMAN, and L. L. SPRIET. Regulation of muscle glycogenolytic flux during intense aerobic exercise after caffeine ingestion. Am. J. Physiol. 275:R596 – 603, 1998. Medicine & Science in Sports & Exercise姞

819

5. CLEMENT, L., H. POIRIER, I. NIOT, et al. Dietary trans-10,cis-12 conjugated linoleic acid induces hyperinsulinemia and fatty liver in the mouse. J. Lipid Res. 43:1400 –1409, 2002. 6. DELANY, J. P., F. BLOHM, A. A. TRUETT, J. A. SCIMECA, and D. B. WEST. Conjugated linoleic acid rapidly reduced body fat content in mice without affecting energy intake. Am. J. Physiol. (Regulatory Integrative Comp. Physiol.). 276:R1172–R1179, 1999. 7. GREENHAFF, P. L., K. BODIN, K. SODERLUND, and E. HULTMAN. Effect of oral creatine supplementation on skeletal muscle phosphocreatine resynthesis. Am. J. Physiol. 266:E725–730, 1994. 8. HENRIKSEN, E. J., M. K. TEACHEY, Z. C. TAYLOR, et al. Isomerspecific actions of conjugated linoleic acid on muscle glucose transport in the obese Zucker rat. Am. J. Physiol. Endocrinol. Metab. 285:E98 –E105, 2003. 9. HOUSEKNECHT, K. L., J. P. VANDEN HEUVEL, S. Y. MOYA-CAMARENA, et al. Dietary conjugated linoleic acid normalizes impaired glucose tolerance in the Zucker diabetic fatty fa/fa rat. Biochem. Biophys. Res. Commun. 244:678 – 682, 1998. 10. KELLEY, D. E., B. H. GOODPASTER, and L. STORLIEN. Muscle triglyceride and insulin resistance. Annu. Rev. Nutr. 22:325–346, 2002. 11. MATSUDA, M., and R. A. DEFRONZO. Insulin sensitivity indices obtained from oral glucose tolerance testing: comparison with the euglycemic insulin clamp. Diabetes Care 22:1462–1470, 1999. 12. MEADUS, W. J., R. MACINNIS, and M. E. DUGAN. Prolonged dietary treatment with conjugated linoleic acid stimulates porcine muscle peroxisome proliferator activated receptor gamma and glutaminefructose aminotransferase gene expression in vivo. J. Mol. Endocrinol. 28:79 – 86, 2002. 13. MEDINA, E. A., W. F. HORN, N. L. KEIM, et al. Conjugated linoleic acid supplementation in humans: effects on circulating leptin concentrations and appetite. Lipids 35:783–788, 2000. 14. NOONE, E. J., H. M. ROCHE, A. P. NUGENT, and M. J. GIBNEY. The effect of dietary supplementation using isomeric blends of conjugated linoleic acid on lipid metabolism in healthy human subjects. Br. J. Nutr. 88:243–251, 2002. 15. PARK, Y., K. J. ALBRIGHT, W. LIU, J. M. STORKSON, M. E. COOK, and M. W. PARIZA. Effect of conjugated linoleic acid on body composition in mice. Lipids 32:853– 858, 1997. 16. PARK, Y., K. J. ALBRIGHT, J. M. STORKSON, W. LIU, M. E. COOK, and M. W. PARIZA. Changes in body composition in mice during feeding and withdrawal of conjugated linoleic acid. Lipids 34: 243–248, 1999.

820


17. PARK, Y., J. M. STORKSON, K. J. ALBRIGHT, W. LIU, and M. W. PARIZA. Evidence that the trans-10,cis-12 isomer of conjugated linoleic acid induce body composition changes in mice. Lipids 34:235–241, 1999. 18. PETERS, J. M., Y. PARK, F. J. GONZALEZ, and M. W. PARIZA. Influence of conjugated linoleic acid on body composition and target gene expression in peroxisome proliferator-activated receptor alpha-null mice. Biochim. Biophys. Acta. 1533:233–242, 2001. 19. PORKSEN, N., S. MUNN, J. STEERS, S. VORE, J. VELDHUIS, and P. BUTLER. Pulsatile insulin secretion accounts for 70% of total insulin secretion during fasting. Am. J. Physiol. 269:E478 – 488, 1995. 20. RISERUS, U., P. ARNER, K. BRISMAR, and B. VESSBY. Treatment with dietary trans10cis12 conjugated linoleic acid causes isomer-specific insulin resistance in obese men with the metabolic syndrome. Diabetes Care 25:1516 –1521, 2002. 21. RYDER, J. W., C. P. PORTOCARRERO, X. M. SONG, et al. Isomerspecific antidiabetic properties of conjugated linoleic acid: improved glucose tolerance, skeletal muscle insulin action, and UCP-2 gene expression. Diabetes 50:1149 –1157, 2001. 22. SEBEDIO, J. L., S. GNAEDIG, and J. M. CHARDIGNY. Recent advances in conjugated linoleic acid research. Curr. Opin. Clin. Nutr. Metab. Care 2:499 –506, 1999. 23. SMEDMAN, A., and B. VESSBY. Conjugated linoleic acid supplementation in humans–metabolic effects. Lipids 36:773–781, 2001. 24. STANGL, G. I., H. MULLER, and M. KIRCHGESSNER. Conjugated linoleic acid effects on circulating hormones, metabolites and lipoproteins, and its proportion in fasting serum and erythrocyte membranes of swine. Eur. J. Nutr. 38:271–277, 1999. 25. THOM, E., J. WADSTEIN, and O. GUDMUNDSEN. Conjugated linoleic acid reduces body fat in healthy exercising humans. J. Int. Med. Res. 29:392–396, 2001. 26. TSUBOYAMA-KASAOKA, N., M. TAKAHASHI, K. TANEMURA, et al. Conjugated linoleic acid supplementation reduces adipose tissue by apoptosis and develops lipodystrophy in mice. Diabetes 49: 1534 –1542, 2000. 27. WEST, D. B., J. P. DELANY, P. M. CAMET, F. BLOHM, A. A. TRUETT, and J. SCIMECA. Effects of conjugated linoleic acid on body fat and energy metabolism in the mouse. Am. J. Physiol. (Regulatory Integrative Comp. Physiol.). 275:R667–R672, 1998. 28. ZAMBELL, K. L., N. L. KEIM, M. D. VAN LOAN, et al. Conjugated linoleic acid supplementation in humans: effects on body composition and energy expenditure. Lipids 35:777–782, 2000.

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