A new study from Norway concludes that supplementation with vitamin D does not reduce the risk of developing diabetes. Experts criticized the fact that study participants already had a sufficient vitamin D status before supplementation, so no effect could be expected.
In the randomized controlled trial, an oral glucose tolerance test was used to identify type 2 diabetes cases among 511 participants with a pre-form of diabetes (impaired glucose tolerance) who had received either 20,000 IU of vitamin D3 or a placebo once a week for one year (1). The study results showed that, com- pared to the placebo group, participants supplemented with vitamin D did not have improved glucose me- tabolism, insulin secretion or sensitivity. The mean serum 25(OH)D concentration was 59.9 nmol/L and
61.1 nmol/L in the vitamin D and placebo groups, respectively, at the beginning of the study.
Experts commented that the results were no surprise, as positive effects of additional vitamin intakes cannot be expected in participants who already have a sufficient nutritional status. The Institute of Medicine defined blood vitamin D concentrations of 50 nmol/L or above as sufficient. In future clinical studies, researchers should be mandated to define nutrition status criteria and use them as inclusion/exclusion criteria prior to supplementation. Conducting nutrition interventions in individuals with adequate nutrition status is a waste of money. In observational studies, low serum vitamin D3 levels have been associated with an increased risk of diabetes (2). It was reported that each 10 nmol/L increase in serum vitamin D3 concentration was associated with a 25% reduced risk in progression to type 2 diabetes from pre-diabetes (3).