A new meta-analysis of prospective studies published in the journal Diabetes Care confirms that low vitamin D is associated with an increased risk of developing type II diabetes (T2D).
To date, there have been many cross-sectional studies examining the relationship between vitamin D and T2D. These studies have shown an association between low vitamin D and impaired b-cell function, insulin resistance, and impaired glucose intolerance. However, these types of cross-sectional studies do not give us great insight on whether low vitamin D is associated with an increased risk of developing T2D months or years later.
Recently, there have been some prospective studies that have looked at this relationship, where they measure vitamin D at baseline, and then follow participants many months and years after to see who develops T2D and who doesn’t.
Some of these prospective studies show an association between high vitamin D levels and lower incidence of T2D, and some do not. In this new meta-analysis, Professor Yiqing Song and colleagues pooled all of these prospective studies together to answer the question, what do all the prospective studies combined say?
The researchers searched MEDLINE and Embase databases for all published prospective studies on 25(OH)D levels and risk of T2D. This means that only studies met their criteria if they were prospective by design, measured 25(OH)D levels at baseline, and then looked to see which participants got T2D later in life.
They found 21 independent prospective studies that met inclusion criteria; 11 cohort studies, 8 nested-case control studies and 2 case-control studies. These studies in total involved 76,220 participants and 4,996 subsequent incidents of T2D. Diabetes was determined in these studies by a variety of criteria, including diabetes-specific pharmacotherapy, diabetes-related hospitalization, self-report, and glycemic status information.
When all the studies were pooled together in the meta-analysis, here’s what they found:
- The summary relative risk (RR) for developing T2D was 0.62 (95% CI 0.54–0.70) for people that had the highest vitamin D level at baseline compared to the lowest vitamin D level at baseline. In simpler terms, those with higher vitamin D levels at baseline had a 38% reduced risk of developing T2D compared to lower levels.
- For every 4 ng/ml increase in vitamin D levels ranging from 8 ng/ml to 64 ng/ml, there was a relative risk of 0.96 in developing T2D. In simpler terms, this means for every 4 ng/ml increase in vitamin D levels, there was 4% reduced risk in developing T2D upon follow-up.
- However, because most studies examined people with vitamin levels below 40 ng/ml, current evidence for the relationship between levels higher than 40 ng/ml and T2D is weak.
The authors conclude,
“In conclusion, our meta-analysis of 21 prospective studies demonstrates a significant inverse association between circulating 25(OH)D levels and risk of incident type 2 diabetes in a dose response manner across a wide spectrum of 25(OH)D levels.”
They note that while this meta-analysis can’t prove that low vitamin D causes T2D or increases your risk of developing T2D, this study should complement future randomized controlled trials if they prove that vitamin D supplementation can lower risk of developing T2D. While prospective studies, specifically prospective cohort studies, aren’t top-tier evidence, they are the next best thing. So in the meantime, it appears that having a good vitamin D level isn’t a bad idea if you’re at risk of developing T2D.
Please see our page “How do I get the vitamin D my body needs?” for our recommendations on maintaining a good vitamin D level.