Study finds low vitamin D levels increase diabetes risk in older adults

A recent systematic review and meta-analysis published in the journal Nutrients, Italian researchers updated a systematic review and meta-analysis to investigate whether low serum vitamin D (25-hydroxyvitamin D or 25OHD) levels can predict the development of type 2 diabetes (T2D) in older adults. They found that low 25OHD levels were associated with an increased risk of developing T2D in older adults, despite adjusting for several confounding factors.

Vitamin D and risk of developing type 2 diabetes in older adults: an updated systematic review and meta-analysis. Image credit: Jane Vershinin/Shutterstock

background

According to the International Diabetes Federation (IDF) Diabetes Atlas, the global prevalence of diabetes among people aged 20-79 years is expected to be 536.6 million in 2021 and increase to 783.2 million by 2045. Diabetes prevalence is highest among older adults, especially those aged 75-79 years, leading to a significant increase in healthcare costs in the near future.

Vitamin D deficiency, common in older adults, is associated with an increased risk of type 2 diabetes due to its role in pancreatic insulin secretion, metabolic syndrome, inflammation, and genetic factors. Observational studies and meta-analyses have shown an inverse correlation between 25OHD levels and diabetes risk, while intervention studies have produced mixed results. Some meta-analyses have shown that vitamin D supplementation reduces diabetes risk, especially in non-obese individuals. However, these studies have focused mainly on young adults, and there are limited studies on elderly populations, despite the high risk of both diseases. Therefore, the researchers in this study updated their previous systematic review and meta-analysis to investigate whether low serum 25OHD levels (vitamin D deficiency) can predict the development of type 2 diabetes in elderly populations.

About the Research

In this study, we searched the PubMed and SCOPUS databases and included longitudinal prospective studies based on self-reported diabetes diagnosis, medical records, or American Diabetes Association diagnostic criteria. We excluded cross-sectional studies, studies that used non-serum 25OHD assessments, and studies that used only estimates of subclinical diabetes. The updated review and meta-analysis included 12 studies with a total of 40,664 older adults from European and North American populations. The mean age of participants was 69.1 years, and 66% were women. The median follow-up period was 7.3 years.

Data on study characteristics, demographics, sample size, follow-up period, serum 25OHD levels, diabetes diagnostic criteria, and covariates in the analysis were extracted. Study quality was assessed using the Newcastle-Ottawa Scale. Pooled relative risks (RRs) were calculated by random-effects meta-analysis, adjusted for covariates in secondary analyses. Further statistical analysis was performed using the chi-square test, I-square test, Egger bias test, and Duval and Tweedie method.

Results and discussion

Study quality was found to be moderate. In unadjusted analyses, lower baseline 25OHD levels are associated with a higher risk of developing diabetes in older adults. The meta-analysis included 15,924 participants (RR = 1.20) and showed a significant association. No publication bias was detected (Egger’s test = 0.26), and trim-and-fill analysis did not change these results.

Even after adjusting for potential confounders, low baseline serum 25OHD levels were associated with a significant 19% increased risk of developing diabetes (hazard ratio = 1.22).12 studies were included in the analysis, with a median adjustment of 11, and no publication bias was found (Egger’s test = -0.34).

No significant heterogeneity was found among the results, so meta-regression analyses were not performed. Furthermore, length of follow-up did not significantly adjust the unadjusted or adjusted data. Cumulative analyses excluding one study at a time confirmed that the results were robust and unchanged.

Research shows that vitamin D influences type 2 diabetes risk through multiple mechanisms, including regulating insulin secretion and action, reducing insulin resistance, regulating calcium and magnesium metabolism, attenuating chronic inflammation, and potentially affecting adipose tissue metabolism. Understanding these mechanisms is essential to unravel the complex interplay between vitamin D status and metabolic health, especially in the context of diabetes prevention and management.

This study uniquely examines the association between vitamin D and incident type 2 diabetes in older adults and boasts a large sample size, extensive covariate adjustment, and a long follow-up period with low heterogeneity in results. However, the study has limitations including its observational design, lack of causal inference, lack of focus on the very elderly population, lack of sex-specific studies, and reliance on a radioimmunoassay method for serum 25OHD measurement that may be less accurate than chemiluminescence.

Conclusion

In conclusion, this meta-analysis shows that low vitamin D levels increase the risk of diabetes in older adults, even after adjusting for a range of potential confounding factors. It reaffirms and updates the findings of a 2017 study. The results highlight the broad impact of vitamin D beyond bone health. Given the prevalence of vitamin D deficiency in older adults and the focus of existing clinical trials on younger populations, further well-designed studies are needed to confirm these results in the very elderly population.