Long-Term Vitamin D3 Supplementation Does Not Reduce Type 2 Diabetes Risk in Older Adults

Topline Findings:

A recent study reveals that long-term daily supplementation with moderate (1600 international units [IU]) or high (3200 IU) doses of vitamin D3 has no significant impact on reducing the risk of developing type 2 diabetes (T2D) in generally healthy older adults who already have sufficient serum vitamin D levels for bone health.

Methodology:

• Previous observational studies suggested a link between low vitamin D levels and an increased risk of T2D, and short-term trials showed some protective effects of vitamin D supplementation in those with impaired glucose metabolism. However, these benefits were not observed in populations without increased risk.
• The Finnish Vitamin D Trial, conducted between 2012 and 2018, examined the effects of five years of vitamin D3 supplementation on the incidence of major chronic diseases in healthy men aged 60 years or older and women aged 65 years or older with no history of cardiovascular disease or cancer.
• This particular analysis focused on T2D incidence among 2,271 participants, with an average age of 68.2 years and a mean BMI of 27.5 kg/m², none of whom were using diabetes medications at the study’s start.
• Participants were randomly assigned to one of three groups: placebo (n=760), 1600 IU/d of vitamin D3 (n=744), or 3200 IU/d of vitamin D3 (n=767), and followed for a mean duration of 4.2 years. T2D cases were identified using diagnostic codes extracted from health registries.
• A subcohort of 505 participants provided detailed data, including blood samples taken at months 0, 6, 12, and 24 for 25-hydroxyvitamin D3 (25(OH)D3), plasma glucose, and insulin levels.

Key Takeaways:

• No significant difference in T2D incidence was found among the three groups: placebo (5.0%; 38 people), 1600 IU/d (4.2%; 31 people), and 3200 IU/d (4.7%; 36 people), with a p-value of .731 indicating no statistical trend.
• The study noted a potential lower incidence of T2D with vitamin D supplementation, specifically in those with lower body mass indexes (BMI).
• During the 24-month follow-up of the subcohort, no significant differences were observed in changes in plasma glucose, insulin concentrations, BMI, or waist circumference among the three treatment groups (p ≥ .19).
• An analysis excluding T2D cases from the first two years suggested a potentially higher risk of T2D with increasing vitamin D dose, though with wide confidence intervals that indicate uncertainty.

Implications for Practice:

The authors concluded that long-term moderate- or high-dose vitamin D3 supplementation does not offer benefits for T2D incidence, glucose metabolism, or body size among generally healthy older adults who are vitamin D-sufficient and not at high risk for T2D. These findings challenge earlier suggestions that vitamin D supplementation could prevent diabetes.

Study Limitations:

The study had several limitations. It relied on national health registries to identify T2D cases, potentially missing some diagnoses. Additionally, 25(OH)D3 levels were only measured for a subcohort, limiting the ability to determine if deficient participants might have benefited from supplementation. The study was not designed or powered specifically for diabetes prevention, and detailed baseline diabetes history was unavailable. Given the wide confidence intervals, there remains uncertainty around some findings. Participants were primarily White and older, restricting the generalizability of the results to other demographics and vitamin D statuses.

Conclusion:

While vitamin D is essential for bone health and may contribute to other health aspects, this comprehensive study indicates that long-term supplementation at moderate to high doses does not reduce the risk of developing type 2 diabetes in healthy, vitamin D-sufficient older individuals. Healthcare providers and consumers should interpret results cautiously and consider individual health needs before relying on vitamin D supplements for diabetes prevention.

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