A vitamin D pill can’t substitute for a healthy diet and sunshine, a new genetic study published in The Lancet Diabetes & Endocrinology suggests. In recent years many people have been seduced by observational studies that found low levels of vitamin D in people who developed type 2 diabetes. The new study instead suggests that the association is not causal, and that raising vitamin D by itself will not be helpful.
Researchers in the U.K. performed a Mendelian Randomization study in more than 100,000 people in which they examined the effect of four separate, single-nucleotide polymorphisms (SNPs) on genes that have a known effect on vitamin D levels. Despite the significant effect of these genetic variations on circulating levels of vitamin D (25(OH)D), the researchers found no relationship between genetically determined levels of vitamin D and the risk for developing type 2 diabetes.
Added to previous evidence, write the authors, the results “suggest that interventions to reduce the risk of type 2 diabetes by increasing concentrations of 25(OH)D are not currently justified.” Instead, they write, “our findings emphasize the need for investigation of the discrepancy between the observational evidence and the absence of causal evidence.” Two possible confounders are physical activity and adiposity, they add.
Results of several long-term randomized trials will be needed to definitively prove that vitamin D supplements are not beneficial, say Brian Buijsse in an accompanying editorial. He cautions that “Mendelian randomisation studies need careful interpretation,” but an analysis of previous trials “do not offer much hope that vitamin D supplementation can be used to prevent type 2 diabetes.” He concludes that “the sky is becoming rather clouded for vitamin D in the context of preventing type 2 diabetes.”