Insulin response during the oral glucose tolerance test: The role of age, sex, body fat and the pattern of fat distribution

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Abstract
To clarify their primary roles on insulin response to oral glucose, age and sex differences in body composition should be taken into account. Oral glucose tolerance tests were performed on 472 men and 299 women of the Baltimore Longitudinal Study of Aging, ranging in age from 20 to 96 years. Subjects who were taking medications or had any diseases which could affect glucose tolerance were excluded. In addition to insulin and glucose values for the glucose tolerance test, we calculated body mass index (BMI), percentage body fat from skinfolds (% Body Fat), waist hip ratio (WHR), mean glucose level over the 2-hour test (GM), the basal insulin (I0), and the mean insulin response over the 2-hour test (IM). There was no significant sex difference in mean age, but men had significantly higher BMI (25.6 vs 24.0 kg/m2), WHR (0.93 vs 0.76), and GM (8.5 vs 7.7 mM), while % Body Fat was lower (25% vs 33%). Unadjusted I0 and IM levels were significantly higher in men than in women (51 vs 44 and 303 vs 231 pM - antilogs of log-normalized values). Insulin levels, adjusted for differences in age, % Body Fat, WHR, and GM by analysis of covariance, however, showed no sex differences (49 vs 46 and 282 vs 257 pM). Adjusted insulin levels declined significantly with age; IM fell progressively from 323 pM in 20 to 39-year olds, 267 pM in 40 to 59-year, 253 pM in 60 to 79-years, and 228 pM in 80 to 96-year olds (p<0.01). We conclude that the sex differences in insulin levels are explained by differences in body habitus and post-load glucose levels, but that insulin levels decline with age per se.