Clinical evidence suggests that vitamin D may be important in maintaining proper insulin function. Epidemiological research supports this contention because there is evidence that type I diabetes is negatively associated with sun exposure. Vitamin D is hydroxylated in the liver to hydroxyvitamin D [25(OH)D] and subsequently further hydroxylated in the kidney to form 1,25 hydroxyvitamin D [1,25(OH)D]. It is these metabolites that may have important cellular effects that cause changes in insulin sensitivity. However, the relationship between vitamin D and insulin sensitivity is complicated because parathyroid hormone (PTH) and adiposity may be confounding variables. This is because low vitamin D levels are also associated with high PTH and high adiposity, both of which are associated with poor insulin sensitivity. There is therefore an interest amongst researchers to test the relationship between vitamin D and insulin sensitivity after adjustments for adiposity and parathyroid hormone.
Researchers1 have investigated the relationship between 25(OH)D and PTH concentrations with insulin sensitivity in African and European Americans women. The study involved a cross section of 25 of each ethnicity and determined the whole body insulin sensitivity index (SI) while assessing adiposity with dual-energy X-ray absorptiometry and computer tomography. Analysis of the results showed that 25(OH)D and PTH were positive and negative independent determinants of SI, respectively. This suggests that both PTH and 25(OH)D may influence insulin sensitivity through independent mechanisms. Interestingly the study reported that the mean plasma 25(OH)D concentration was just 22.3 ng/mL (55.8 nmol/L) and that 54% of the women were deficient, and 72% insufficient, in vitamin D. The researchers also suggested that the lower insulin sensitivity seen in African American women may be due to the lower 25(OH)D levels seen in this population.
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