Clinical and laboratory aspects of vitamin D in relation to type 2 diabetes and osteoporosis. Results from the Tromsø Study and two randomized controlled trials.

Abstract

There has been a large interest in possible beneficial health effects of increasing the vitamin D (as measured by 25-hydroxyvitamin D (25(OH)D)) levels in the population. Data from the Tromsø Study were used to assess the relation between serum 25(OH)D levels and the risk of subsequent type 2 diabetes in 6119 participants. This risk was approximately doubled in the lowest serum 25(OH)D quartile as compared to the highest quartile. This could to a large extent be explained by higher body mass index in the lowest quartile. When thereafter comparing 52 persons with high and 108 persons with low serum 25(OH)D levels, we found more beneficial levels of glucose and lipid measures in those with high serum 25(OH)D levels. However, there was no improvement in the same measures in the low serum 25(OH)D group after six months supplementation with high dose vitamin D (approximately 150 mikrogram or 6000 international units/day) as compared to placebo. In another study, we compared one year treatment with standard dose of vitamin D (800 international units) with the same high dose vitamin D among 297 women with reduced bone mineral density. Both doses improved or maintained bone mineral density with no differences between the groups. These results do not support the use of vitamin D in such high doses. Further studies are needed to assess the ideal level of serum 25(OH)D. We have further demonstrated that smoking might interfere with 25(OH)D laboratory analyses. This is important to consider in developing and validating such methods.