['The association of hypovitaminosis D with type 2 diabetes is well recognized. Although hypovitaminosis D is associated with insulin resistance, there is much less information about its impact on β-cell function in humans.', 'We enrolled 150 healthy, glucose-tolerant subjects for the assessment of β-cell function (acute insulin response) and insulin sensitivity index (ISI) using a hyperglycemic clamp. Adjusted β-cell function (ABCF) was defined as the product of acute insulin response and ISI. The relations of plasma 25-hydroxyvitamin D [25(OH)D] level with insulin sensitivity and ABCF were examined.', 'Plasma 25(OH)D levels were positively associated with ABCF (P = 0.00004) and ISI (P < 0.00001). The associations remained significant after adjustment for age, sex, body mass index, physical activity, ethnicity, and season of study.'] Are plasma 25-hydroxyvitamin D levels favorably associated with β-cell function?