Vitamin D against diabetes? The “sunshine” nutrient may hold potential for prediabetics, study says

08 Feb 2023 --- Adults with prediabetes who take vitamin D supplements are 15% less likely to develop diabetes, according to a recent review of three clinical trials published in the Annals of Internal Medicine.

NutritionInsight speaks with lead author Anastassios Pittas, chief of endocrinology, diabetes and metabolism at Tufts Medical Center in Boston, about the results. 

“We found that vitamin D has a beneficial effect in people with prediabetes,” Pittas details. According to the study, around 374 million people have prediabetes worldwide. 

“The most important point this paper makes – that is often missed when discussing the role of vitamin D in health and disease – is that the benefit-to-risk ratio for vitamin D depends on the target population and medical condition.” 

Where’s the harm in trying?
The extensive study included 4,190 participants. However, Pittas adds that the “results do not apply to populations that are not at risk for type 2 diabetes.” 

Moreover, he states that more research is needed to determine optimal levels and formulation of vitamin D intake.  

The study found few adverse effects of high-level vitamin D intake used in the three clinical trials. 

One of the trials used 20,000 IU of cholecalciferol (vitamin D3) weekly, another used 4,000 IU of cholecalciferol daily, and the third clinical trial used 0.75 mg of eldecalcitol, which is a synthetic analog of vitamin D. 

Lowered risk, increased intake? 
Of the clinical trial participants with prediabetes taking vitamin D supplements, 22.7% got diabetes, while 25% of the participants taking a placebo got diabetes, according to data on individual participants from three trials. Participants in the clinical trials were investigated for three years. 

Adults with prediabetes taking vitamin D supplements were 15% less likely to develop diabetes.“These results should not be extrapolated to the general population at low or average risk for diabetes as the benefit-risk ratio of high dose for diabetes prevention may not be favorable,” warns Pittas. 

Dr. Katarina Kos, senior lecturer in diabetes and obesity at the University of Exeter, adds that the trials did not assess weight loss during the testing period. 

“The National Diabetes Prevention Trial showed that people with prediabetes who lose 5% of their weight and increase physical activity reverse the risk of developing diabetes by 58%, says Kos.”

“People shouldn’t be distracted by vitamin D supplementation and vitamin D blood levels, which naturally increase after weight loss, and keep in mind the primary importance of lifestyle change,” she adds.

The paper adds that the reduction in risk for diabetes with vitamin D (15%) is slight when compared to other prevention practices such as intensive lifestyle modification (58%) or treatment with the medication metformin (31%). 

Moderation in vitamin D intake
The results of the reviewed clinical trials suggest that the vitamin D level in the blood needed to reduce the risk for diabetes is around 125 to 150 nmol/L (50-60 ng/ml).

Authors from University College Dublin and the Food Safety Authority of Ireland reviewed the paper and warned that high doses of vitamin D might also cause harm. They add that previous data demonstrated significant adverse effects of high vitamin D intake. 

Pittas adds that guidelines for vitamin D apply to the “general, healthy population to promote bone health.” In contrast, for adults with high-risk prediabetes, lowering the risk for type 2 diabetes may be favorable to the risk of high vitamin D intake. 

The paper found no statistically significant differences between the vitamin D and placebo groups when combining the analysis of the three clinical trials. 

Clinically adverse events reported were rare. Moreover, according to the paper, clinical trials are not designed to evaluate safety as they exclude people who may be at risk for adverse events. 

Doctors and researchers have also warned against overconsumption of vitamin D, as consumption of the supplement increased in recent years. 

Adults with prediabetes taking vitamin D supplements were 15% less likely to develop diabetes. Vitamin D doses used in the trials were around or above tolerable upper intake levels. 

Unanswered questions 
Pittas notes that there are still questions to be answered, and that the optimal vitamin D dose or formulation is still unknown. Furthermore, it must be established whether a focus on a specific vitamin D blood level is needed to maximize benefit in this population, with little or no risk of side effects. 

The review covers different vitamin D formulations and doses, as clinical trials were conducted in countries with other clinical practices. 

Professor Naveed Sattar, a professor of metabolic medicine from the University of Glasgow, would like a more extensive and prolonged trial to confirm the paper’s results. 

“The study is interesting, but given the results show borderline significance for lower diabetes risk is only evident with data from three trials combined, this could easily be a chance finding.”

Vitamin D is linked to other health benefits. Studies have linked vitamin D intake to a lower risk of cancer, while a deficiency has been linked to a higher risk of cardiovascular diseases. 

By Jolanda van Hal