Plasma and dietary vitamin E in relation to incidence of type 2 diabetes - The Insulin Resistance and Atherosclerosis Study (IRAS)

OBJECTIVE - To evaluate the association of vitamin E with incidence of type 2 diabetes and to do so separately among individuals who did and those who did not report regular use of vitamin supplementation. RESEARCH DESIGN AND METHODS - The Insulin Resistance Atherosclerosis Study (IRAS) included 895 nondiabetic adults at baseline (including 303 with impaired glucose tolerance [IGT]), 148 of whom developed type 2 diabetes according to World Health Organization (WHO) criteria during the 5-year follow-up. At baseline, dietary vitamin E was estimated by a validated food frequency interview, usual supplement use was confirmed by supplement label, and plasma alpha-tocopherol was measured. Analyses were conducted separately for individuals who did (n = 318) and did not (n = 577) use vitamin E supplements. RESULTS - Among supplement nonusers, reported mean intake of vitamin E (mg alpha-tocopherol equivalents [alpha-TE]) did not differ between those who remained nondiabetic (n - 490) and those who developed diabetes (n = 87) (10.5 +/- 5.5 vs. 9.5 +/- 4.8 [means +/- SD], respectively, NS). After adjustment for demographic variables, obesity, physical activity, and other nutrients, the association remained nonsignificant (odds ratio [OR] 0.80, 95% Cl 0.13-5.06) for the highest level of intake (greater than or equal to20 mg alpha-TE) compared with the lowest level (1-4 alpha-TE). However, results or plasma concentration of alpha-tocopherol showed a significant protective effect both before and after adjustment for potential confounders (adjusted OR 0.12, 95% CI 0.02-0.68, for the highest quintile vs. the lowest quintile; overall test for trend, P < 0.01). Among individuals who reported habitual use of vitamin E supplements (at least once per month in the year before baseline; 259 remained nondiabetic and 59 developed diabetes), no protective effect was observed for either reported intake of vitamin E or plasma concentration of α-tocopherol CONCLUSIONS - A protective effect of vitamin E may exist within the range of intake available from food. This effect may go undetected within studies of high-dose supplement use, which appears to hold no additional protective benefit.