Serum 25-Hydroxyvitamin D Levels in Subjects with Reduced Glucose Tolerance and Type 2 Diabetes - The Tromsø OGTT-Study

The relationships between vitamin D concentrations, hyperglycemia, and insulin resistance remain uncertain. During 2008 - 2010, an oral glucose tolerance test was performed in 3520 subjects from Tromsø, Norway. Serum 25-hydroxyvitamin D [25(OH)D] was measured in 1193 subjects with normal glucose tolerance, in 304 with isolated impaired fasting glucose, in 254 with isolated impaired glucose tolerance, in 139 with a combination of the two, and in 194 subjects with type 2 diabetes. Serum 25(OH)D did not differ between subjects with isolated impaired fasting glucose or impaired glucose tolerance, but was lower in all groups of deranged glucose metabolism as compared with normal subjects. These differences could not be explained by differences in intakes of vitamin D from cod liver oil or other supplements and remained statistically significant after adjustment for gender, age, body mass index, physical activity score, and month of examination. When the cohort was divided according to serum 25(OH)D quartiles, there was an improvement in all measures of glucose metabolism (fasting and 2-hour plasma glucose, serum insulin, HbA_1c) and estimates of insulin resistance (QUICKI , HOMA-IR, ISI_0.120) with increasing serum 25(OH)D quartile. However, interventional studies are needed to prove a causal relationship between vitamin D and glucose metabolism.