Low vitamin D levels linked to higher diabetes risk in older adults, study finds

In a recent systematic review and meta-analysis published in the journal Nutrients, researchers from Italy updated a systematic review and meta-analysis to examine whether low serum vitamin D (25-hydroxyvitamin D or 25OHD) levels can predict the onset of type 2 diabetes (T2D) in older adults. Despite adjusting for several confounders, they found that low 25OHD levels were associated with an increased risk of developing T2D in older adults.

Vitamin D and Risk of Incident 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 worldwide prevalence of diabetes in individuals aged 20–79 was 536.6 million people in 2021 and is projected to rise to 783.2 million by 2045. Diabetes prevalence is highest among older adults, particularly those aged 75–79, leading to a substantial increase in health expenditures in the near future.

Vitamin D deficiency, common in older adults, is associated with an increased risk of T2D, potentially due to its role in pancreatic insulin secretion, metabolic syndrome, inflammation, and genetic factors. While observational studies and meta-analyses indicate an inverse relationship between 25OHD levels and diabetes risk, intervention studies yield mixed results. Some meta-analyses show that vitamin D supplementation reduces diabetes risk, especially in non-obese individuals. However, these studies primarily focus on younger adults, with limited research on older populations, despite their higher risk for both conditions. Therefore, researchers in the present study updated a previous systematic review and meta-analysis to investigate whether low serum 25OHD levels (hypovitaminosis D) can predict the onset of T2D in the elderly population.

About the study

The present study searched PubMed and SCOPUS databases to include longitudinal, prospective studies with self-reported diabetes diagnoses, medical records, or diagnostic criteria per the American Diabetes Association. Cross-sectional studies, studies using non-serum 25OHD assessments, and those with only subclinical diabetes estimates were excluded. The updated review and meta-analysis included 12 studies comprising a total of 40,664 older adults from European and North American populations. The participants’ mean age was 69.1 years, and 66% were female. The median follow-up period was 7.3 years.

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

Results and discussion

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

After adjusting for potential confounders, lower baseline serum 25OHD levels still significantly increased the risk of developing diabetes by 19% (hazard ratio = 1.22). The analysis included 12 studies with a median of 11 adjustments, and no publication bias was found (Egger’s test = −0.34).

No significant heterogeneity was found among the outcomes, so no meta-regression analyses were conducted. Further, the length of the follow-up did not significantly moderate the unadjusted or adjusted data. Cumulative analysis, which removed one study at a time, confirmed that the results were robust and unchanged.

As per the study, vitamin D affects T2D risk through multiple mechanisms, including modulation of insulin secretion and action, reduction of insulin resistance, regulation of calcium and magnesium metabolism, attenuation of chronic inflammation, and potential impacts on adipose tissue metabolism. Understanding these mechanisms is crucial for elucidating the complex interplay between vitamin D status and metabolic health, particularly in the context of diabetes prevention and management.

The study uniquely examines the link between vitamin D and incident T2D in older adults, boasting a large sample size, extensive covariate adjustments, and a long follow-up period with low heterogeneity in outcomes. However, the study is limited by its observational design, lack of causal inferences, lack of focus on the very old population, insufficient gender-specific studies, and reliance on radioimmunoassay for measuring serum 25OHD, which may be less accurate than chemiluminescence.

Conclusion

In conclusion, the present meta-analysis shows that low vitamin D levels are linked to an increased risk of diabetes in older adults, even after adjusting for various potential confounders. This reaffirms and updates the findings from a 2017 study. The results highlight vitamin D’s broader impact 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 findings in very old populations.