Dr. Kumar’s Take:
This is a systematic review and meta-analysis of observational studies and clinical trials in adults, not a new trial of its own. The observational signal is reasonably consistent: lower vitamin D status, and lower calcium or dairy intake, track with having and with developing type 2 diabetes and metabolic syndrome. The trial evidence is much weaker, and the authors are direct about why: the intervention studies were short, small, used different formulations of vitamin D and calcium, and some of the analyses were post-hoc. My read is that correcting an insufficiency of either nutrient is a reasonable, low-cost thing to do on its own merits, but I would not sell supplementation as diabetes prevention outside of people who already have glucose intolerance.
Key Takeaways:
✔ Low vitamin D status and low calcium or dairy intake are associated with both prevalent and incident type 2 diabetes and metabolic syndrome.
✔ Combined vitamin D and calcium supplementation may have a role in prevention only in high risk populations, meaning people with glucose intolerance.
✔ The evidence base is limited: mostly cross-sectional observational studies with incomplete confounder adjustment, and short, small intervention trials.
Actionable Tip:
If you are at risk for type 2 diabetes, particularly if you already have glucose intolerance, it is worth checking your vitamin D status and making sure your calcium intake is adequate. This review points to combined vitamin D and calcium, not either nutrient in isolation, as the combination with a possible role in prevention.
Brief Summary:
This 2007 systematic review and meta-analysis examined whether altered vitamin D and calcium homeostasis plays a role in the development of type 2 diabetes. The authors searched MEDLINE through January 2007 for observational studies and clinical trials in adults with outcomes related to glucose homeostasis, and pooled results where the data allowed. Their conclusion: vitamin D and calcium insufficiency may negatively influence glycemia, and combined supplementation with both nutrients may be beneficial in optimizing glucose metabolism.
Study Design:
The authors reviewed MEDLINE through January 2007 for observational studies and clinical trials in adults with outcomes related to glucose homeostasis. Meta-analyses were performed where data could be combined, and results are reported as summary odds ratios comparing highest versus lowest intake or 25-hydroxyvitamin D status.
Results:
- For developing type 2 diabetes, the highest versus lowest combined vitamin D and calcium intake carried an odds ratio of 0.82 (95% CI 0.72 to 0.93).
- The highest versus lowest dairy intake carried an odds ratio of 0.86 (95% CI 0.79 to 0.93) for incident type 2 diabetes.
- For prevalent type 2 diabetes, highest versus lowest 25-hydroxyvitamin D carried an odds ratio of 0.36 (95% CI 0.16 to 0.80) among non-blacks.
- For prevalent metabolic syndrome, the highest versus lowest dairy intake carried an odds ratio of 0.71 (95% CI 0.57 to 0.89).
- Trials of vitamin D and/or calcium supplementation suggest a role in preventing type 2 diabetes only in populations at high risk, meaning those with glucose intolerance.
How Vitamin D and Calcium Impact Blood Sugar
The premise the authors set out to test is that altered vitamin D and calcium homeostasis may play a role in the development of type 2 diabetes. Their conclusion is that insufficiency of these nutrients may negatively influence glycemia, and that supplying both together may be beneficial in optimizing glucose metabolism.
Related Studies and Research
Global Prevalence of Vitamin D Deficiency: A Meta-Analysis: Frames metabolic risk in the context of deficiency.
Role of Vitamin D and Calcium in Type 2 Diabetes Prevention: A Systematic Review and Meta-Analysis: Aggregates evidence on D + Ca for glycemic control.
Obesity-Induced Repression of CYP2R1 and Vitamin D Deficiency Mechanisms: Highlights how obesity disrupts D metabolism.
AJCN Estimation of Optimal Serum 25(OH)D Levels for Multiple Health Outcomes: Recommends target D concentrations for metabolic benefits.
Vitamin D Deficiency: Clinical Review by Michael F. Holick, MD, PhD: Discusses D supplementation in metabolic syndrome.
Frequently Asked Questions
Should I take vitamin D and calcium together?
This review found that the association with lower incident type 2 diabetes came from combined vitamin D and calcium intake, and that supplementation trials pointed to a possible preventive role only for the combination in high risk populations. I still prefer patients get calcium from food rather than supplements where that is practical.
Can dairy alone help reduce diabetes risk?
Highest versus lowest dairy intake was associated with lower odds of developing type 2 diabetes and with a lower prevalence of metabolic syndrome.
Who is most likely to benefit from supplementation?
The trial evidence suggests a role for combined vitamin D and calcium supplementation in prevention only among populations at high risk, meaning people who already have glucose intolerance.
How strong is this evidence?
Limited, and the authors say so. Most of the observational studies were cross-sectional and did not adjust for important confounders, and the intervention studies were short, enrolled few subjects, used a variety of vitamin D and calcium formulations, or reported post-hoc analyses.
Conclusion
This review supports an association between low vitamin D and calcium status and type 2 diabetes, both prevalent and incident, and raises the possibility that combined supplementation helps optimize glucose metabolism. The strongest caveat is in the authors’ own words: the observational data are largely cross-sectional and incompletely adjusted, and the trials were short and small. For someone with glucose intolerance, making sure both nutrients are adequate is a defensible step while better trials are awaited.
