Plasma and dietary vitamin E in relation to insulin secretion and sensitivity

Aim: In the Insulin Resistance and Atherosclerosis Study (IRAS), we have previously shown a protective effect of plasma alpha-tocopherol concentration against diabetes incidence among persons not taking vitamin E supplements. The biologic mechanism for such a protective effect could involve improvement in either insulin sensitivity (S-I), insulin secretion or both. Thus, we examined vitamin E in relation to insulin secretion and S-I among persons not taking vitamin E supplements. Methods: This analysis included 457 adults aged 40-69 years without a previous diabetes diagnosis or vitamin E supplement use at baseline and seen at the 5-year follow-up examination. Baseline nutrient intake was estimated from a validated 1-year food frequency questionnaire; plasma levels of alpha-tocopherol were also assessed. At follow up, a frequently sampled intravenous glucose tolerance test determined S-I, acute insulin response to glucose (AIR), and the disposition index (DI) was calculated as the sum of the log-transformed AIR and S-I to reflect pancreatic compensation for insulin resistance. Results: In multivariable regression analyses, no relationship was observed for vitamin E intake and either S-I, AIR or DI. However, plasma alpha-tocopherol concentration was positively associated with log-transformed S-I (beta = 0.27 +/- 0.09, p < 0.01) and DI (beta = 0.41 +/- 0.14, p < 0.01), but not with log-transformed AIR. Conclusions: Plasma concentration of alpha-tocopherol may improve S-I and pancreatic compensation for insulin resistance, although it does not seem to be related to acute insulin response.