Age or waist as determinant of insulin action?

Several studies have shown that insulin action deteriorates with age, possibly mediated through accumulation of abdominal fat. We determined peripheral insulin action in elderly and younger men who had participated in a large population study (the Tromsø Study). To 15 elderly participants aged 71 to 77 years, we individually matched 15 younger participants aged 31 to 33 years (Y1) by body mass index (BMI). A second young group (Y2) comprised 15 participants also aged 31 to 33 years, but with BMI representative of this age group in the population study. All underwent hyperinsulinemic euglycemic clamps (0.4 mU/kg/min), oral glucose tolerance tests, and determinations of Vo2max. Insulin sensitivity index (ISI=glucose disposal per kg fat-free mass [FFM] divided by steady-state insulin concentration) did not differ between the elderly and Y1, but was higher in Y2 (0.10+/-0.01, 0.12+/-0.01, and 0.17+/-0.02, P=.0011 by analysis of variance [ANOVA]). Adjustment by waist circumferences (analysis of covariance [ANCOVA]) abolished this difference. In univariate analysis of pooled data, ISI correlated negatively to body fat indices, serum triglycerides, and free fatty acids (FFA), and positively to Vo2max. In multiple regression analysis, waist circumference and triglycerides were the only independent predictors of insulin sensitivity, whereas age had no impact. The results confirm that the decline in insulin action seen in elderly people is related to increased abdominal fat rather than aging per se.