The prevalence of type 2 diabetes has grown substantially in both high-income and low- to middle-income countries in recent decades, particularly in Asia and Europe.
Previous observational studies have reported a link between higher plasma vitamin D concentrations and a lower risk of diabetes, but it is uncertain if this link is causal.
At the same time, randomised trials have produced contradictory results regarding the impact of vitamin D supplementation on diabetes. However, these trials might have been too small, or used insufficient doses of vitamin D to observe any benefit.
Vitamin D for diabetes
As such, researchers at several Chinese, UK and Danish universities and hospitals conducted a meta-analysis using genetically instrumented differences in plasma vitamin D concentrations to determine the effect of vitamin D on type 2 diabetes.
First, they obtained data on four vitamin D single nucleotide polymorphisms (SNPs, the most common human genetic variation type) in 82,464 subjects, plasma vitamin D concentrations in 13,565 subjects, and 5,565 diabetic cases from the China Kadoorie Biobank (CKD).
A genetic score using two vitamin D synthesis SNPs was then used to assess the effects of vitamin D on diabetes risk. The CKB results were combined in a meta-analysis of 10 studies (58,312 subjects) for the two synthesis SNPs, and seven studies (32,796 subjects) for all four SNPs, including studies involving UK and Danish subjects.
The researchers reported: "A 25-nmol/l higher biochemically measured 25(OH)D was associated with a 9% lower risk of diabetes in CKB. In a meta-analysis of all studies, a 25-nmol/l higher genetically instrumented 25(OH)D concentration was associated with a 14% lower risk of diabetes using the two synthesis SNPs."
However, the same difference in vitamin D using a genetic score with the four SNPs was not significantly related to diabetes.
Higher dose, higher participation
The researchers also said the meta-analysis was limited by their access to study-level data, as opposed to individual-level data.
They added that trials testing the impact of higher doses of vitamin D — at least 2,000 IU per day — were required to more accurately gauge the effect of vitamin D on diabetes, and that trials with larger type 2 diabetes cases may be needed as well.
They concluded: "The results of meta-analysis of ongoing and future trials of vitamin D supplementation are required before advocating use of vitamin D supplements (or food fortification) for the prevention of diabetes.
"The concordant risks of diabetes for biochemically measured and genetically instrumented differences in 25(OH)D using synthesis SNPs provide evidence for a causal effect of higher 25(OH)D for prevention of diabetes."
Source: PLOS Medicine
"Association of vitamin D with risk of type 2 diabetes: A Mendelian randomisation study in European and Chinese adults"
Authors: Ling Lu, et al.