OBJECTIVE--The purpose of this study was to determine whether improvements in insulin sensitivity with weight loss are mediated by changes in inflammation in obese, postmenopausal women.
RESEARCH DESIGN AND METHODS--We studied 58 sedentary, overweight, and obese (BMI 33 [+ or -] 1 kg/[m.sup.2], means [+ or -] SEM) postmenopausal (58 [+ or -] 1 year) women at baseline and 37 women who completed 6 months of weight loss induced by diet and exercise. The women underwent 3-h hyperinsulinemic-euglycemic clamps (40 mU x [m.sup.-2] x [min.sup.-1]) to determine glucose utilization (M). Insulin sensitivity was determined as bill, the amount of glucose metabolized per unit of plasma insulin (I). Visceral adipose tissue (VAT) and plasma concentrations of C-reactive protein (CRP), cytokines interleukin (IL)-6, and tumor necrosis factor (TNF)-[alpha] as well as their soluble receptors, were measured.
RESULTS--At baseline, CRP concentration was a predictor of both glucose utilization and insulin sensitivity, independent of adiposity, race, and aerobic fitness (M: partial r = -0.30, P = 003, and M/I: partial r = -0.32, P = 0.02). Weight loss resulted in significant reductions in body weight, fat mass, VAT, and lasting glucose and insulin levels (P < 0.05). Both glucose utilization and insulin sensitivity increased by 16% (P < 0.05). CRP, IL-6, and soluble TNF receptor (sTNFR)-1 concentrations decreased (P < 0.05), but concentrations of TNF-[alpha] sT-NFR 2, and soluble 11 -6 receptor (IL-6sR) did not change. In stepwise regression models to predict changes in glucose homeostasis, changes in VAT and sTNF-R1 independently predicted changes in glucose utilization (r = -0.49 and cumulative r = -0.64, P < 0.01), while changes in VAT and IL-6 were both independent predictors of changes in insulin sensitivity (r = -0.57 and cumulative r = 0.68. P < 0.01).
CONCLUSIONS--Improvements in glucose metabolism with weight loss programs are independently associated with decreases in cytokine concentrations, suggesting that a reduction to inflammation is a potential mechanism that mediates improvements in insulin sensitivity.
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Elevated circulating concentrations of C-reactive protein (CRP) and interleukin (IL)-6 predict the development of type 2 diabetes in middle-aged women (1) and in elderly adults (2,3). These inflammatory markers are also increased in obesity (4,5) and associated with features of the insulin resistance syndrome (4,6). Another proinflammatory cytokine, tumor necrosis factor (TNF)-[alpha], is also increased in obesity and type 2 diabetes (7). These data suggest that the inflammatory process may play a critical role in the pathogenesis of insulin resistance.
In the few studies that examined the associations between plasma cytokine levels and insulin sensitivity assessed via the glucose clamp, negative correlations are reported between soluble TNF receptor (sTNFR)-2 and glucose utilization in obese young women (8) and between CRP and glucose utilization in obese postmenopausal women (9). In addition, glucose utilization is negatively associated with plasma concentrations of IL-6 in the Pima Indians (10), but this relationship disappears after adjustment for obesity. Thus, inflammation may be involved in the causal pathway between obesity and insulin resistance.
Emerging evidence shows that behavioral treatments, such as weight loss and exercise, that improve glucose metabolism (11-13) also decrease inflammatory markers. Dietary-induced weight loss results in decreases in CRP, IL-6, and TNF-[alpha] concentrations in obese women (14-17). It is also postulated that physical activity plays a role in inflammation, such that more active individuals have lower concentrations of IL-6 and CRP (18), and exercise training decreases several markers of inflammation (19,20). In other studies, the combined effects of weight loss and increased physical activity result in decreases in CRP and/or IL-6 levels in obese premenopausal women (21,22).
These previous studies suggest that improvements in insulin sensitivity after weight loss may be mediated by changes in inflammation. Therefore, the purpose of this study was to test our hypotheses that improvements in insulin sensitivity with diet- and exercise-induced weight loss will be associated with reductions in inflammation in obese postmenopausal women and that this association will be independent of decreases in total and regional body fat.
RESEARCH DESIGN AND METHODS--The women in this study were postmenopausal (had not menstruated for at least 1 year and had plasma follicle-stimulating hormone levels >30 mIU/ml) and were 50-70 years old, healthy, overweight, and obese (BMI >25 kg/[m.sup.2], range 25-41). Only women who were weight stable (<2.0-kg weight change in previous year) and sedentary (<20 min of aerobic exercise twice a week) were recruited to participate. Women were screened by medical history questionnaire, physical examination, fasting blood profile, and a graded exercise treadmill test in an attempt to exclude those with cardiovascular disease. The women underwent a 2-h 75-g oral glucose tolerance test during screening to exclude women with diabetes (23). All women were nonsmokers and showed no evidence of cancer, liver, renal, or hematological disease, or other medical disorders. The Institutional Review Board of the University of Maryland approved all methods and procedures. Each participant provided written informed consent to participate in the study.
Sixty-two women met the study inclusion/exclusion criteria and were enrolled. Fifty-eight of these women (46 Caucasians and 12 African Americans) had complete baseline data, including completion of a 3-h glucose clamp. Nineteen of these women were on hormone replacement therapy for at least 3 years before enrollment, which did not change for the duration of the study. Thirty-seven of the women studied at baseline completed 6 months of dietary- or exercise-induced weight loss (30 Caucasians and 7 African Americans, 12 of which were on hormone replacement therapy).
Weight loss treatments
For 6 months, all women attended weekly weight loss classes led by a registered dietitian for instruction in the principles of a hypocaloric diet that followed the American Heart Association Step 1 (24) guidelines with restriction of caloric intake by 250-350 kcal/day, as previously described (13). Compliance to the weight-loss classes was ~80%. Twenty-five women also exercised three times a week on nonconsecutive days using treadmills and cycle ergometers at >60% V[O.sub.2max] for 45 min (n = 16) or using eight exercises on pneumatic variable-resistance machines (Keiser K-300; Keiser Sports Health, Fresno, CA) and dumbbells (n = 9) (13,25).
V[O.sub.2max] and body composition
V[O.sub.2max] was measured using a continuous treadmill test protocol that has been previously described (25). Height and weight were measured to calculate BMI as weight (kg)/height ([m.sup.2]). Fat mass, lean tissue mass, and bone mineral content (BMC) were determined by dual-energy X-ray absorptiometry (Model DPX-L; Lunar Radiation, Madison, WI) using the 1.3z DPX-L extended analysis program. Fat-free mass (FFM) is reported as lean tissue plus BMC. A single 5-mm computed tomography scan was taken at the [L.sub.4]-[L.sub.5] region using a General Electric Hi-Light Scanner to determine relative proportions of visceral adipose tissue (VAT) area and subcutaneous adipose tissue (SAT) area (12).
Hyperinsulinemic-euglycemic clamps
All subjects were weight stabilized (<1 kg) for at least 2 weeks before metabolic testing before and after the interventions and were provided with a eucaloric diet for 2 days before the clamp by a registered dietitian to control nutrient intake, as previously described (26). All testing was performed in the morning after a 12-h overnight fast. Glucose clamps were performed 36-48 h after any exercise. Peripheral tissue sensitivity to exogenous insulin was measured using the hyperinsulinemic euglycemic clamp technique (27). Arterialized blood was obtained from a dorsal heated hand vein (28). Basal glucose, insulin, and cytokine levels were measured in the fasted state. A 10 min, priming and continuous infusion of insulin (240 pmol x [m.sup.-2] x [min.sup.-1] Humulin; Eli Lilly, Indianapolis, IN) and a 20% glucose solution infusion was performed for 180 min. During the clamp, blood samples were obtained every 5 and 10 min for the determination of plasma glucose and insulin levels.
The plasma glucose and insulin levels during all clamps averaged 5.19 [+ or -] 0.05 mmol/l and 478 [+ or -] 7 pmol/l (n = 96, i.e., 58 pre- and 37 postintervention). This was 97.6 [+ or -] 0.1% of the desired goal with a coefficient of variation (CV) of 4.9 [+ or -] 0.2% in all clamps (n = 96).
