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A genetic analysis of a large U.S. clinical trial suggests that vitamin D supplementation may reduce the risk of progression from prediabetes to type 2 diabetes, but only for those people who harbor specific variants of the vitamin D receptor gene. The study, led by researchers at Tufts University and published in JAMA Network Open, found that daily high-dose vitamin D lowered diabetes risk by 19% in participants with certain genotypes, opening the possibility of using vitamin D as a diabetes prevention strategy.

The new findings build on data from the Vitamin D and Type 2 Diabetes (D2d) clinical trial, a multi-site randomized study that enrolled more than 2,000 U.S. adults with prediabetes. Study participants were assigned to receive either 4,000 IU of vitamin D3 daily or a placebo. The subjects were then followed for a median of 2.5 years to assess progression to diabetes. The original trial did not show a statistically significant reduction in diabetes risk across all participants.

“But the D2d results raised an important question: Could vitamin D still benefit some people?” said lead author Bess Dawson-Hughes, MD, a senior scientist at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University. “Diabetes has so many serious complications that develop slowly over years. If we can delay the time period that an individual will spend living with diabetes, we can stop some of those harmful side effects or lessen their severity.”

In their follow-on research, the Tufts noted that subsequent analysis of the D2d trial data showed that outcomes varied based on achieved blood levels of vitamin D in participants. The new study also found a genetic link to those who had improved outcomes.

To explore the role genetics might play, the investigators conducted a post hoc analysis of 2,098 D2d participants who consented to genetic testing. They focused on three common polymorphisms in the vitamin D receptor (VDR) gene: ApaI, BsmI, and FokI. The researchers first examined how vitamin D levels correlated with diabetes risk across genotypes, then evaluated how genetic variants influenced response to supplementation.

The data showed that the ApaI polymorphism is a key determinant of response. Participants with the AA genotype, which was about 30% of the cohort, did not experience a reduction in diabetes risk with vitamin D supplementation. By comparison, those with the AC or CC genotypes, the remaining 70% of participants, showed a 19% lower risk of developing diabetes when treated with vitamin D compared with placebo.

The biological basis for this effect is linked to the role the VDR gene plays in pancreatic β cells, where it influences insulin secretion and glucose regulation. Variations in the receptor may alter how effectively vitamin D exerts these effects, explaining why some individuals benefit from supplementation while others do not.

Earlier research has suggested there is a connection between vitamin D and diabetes risk. In earlier analyses of the D2d trial, participants who maintained higher blood levels of vitamin D experienced substantial reductions in diabetes incidence. These findings were supported by meta-analyses and observational studies, including research from the UK Biobank, which found that genetic variation in VDR could modify its activity.

“We hypothesized that VDR gene variants modify the association between achieved intratrial 25-hydroxyvitamin D (25(OH)D) level and diabetes risk and may modify the effect of vitamin D3 supplementation on the risk of developing diabetes,” the researchers wrote. 25(OH)D is the main form of vitamin D circulating in the blood.

The current study broadens knowledge on the role vitamin D can play in diabetes prevention by identifying the specific polymorphisms at play. The overlap between ApaI and BsmI variants provides further evidence of the role of VDR genetics, although the researchers noted that ApaI alone may be sufficient to identify likely responders.

“This genetic association analysis of the D2d study suggests that genetic variation in the VDR, specifically the ApaI polymorphism, is associated with diabetes risk at higher intratrial 25(OH)D levels and is associated with response to 4000 IU/d of vitamin D3 supplementation among adults with prediabetes,” the researchers wrote.

The implications for clinical care include the potential use of genetic testing to guide preventive treatment. A single test for the ApaI polymorphism could help identify patients with prediabetes who are most likely to benefit from higher-dose vitamin D supplementation.

While the results have established a link between variations in the VDR gene and diabetes development, the research noted that the study was not designed to assess the mechanisms underlying the genetic effects. Further, its sample size limited subgroup analyses by race and ethnicity.

“Our findings suggest we may eventually be able to identify which patients with prediabetes are most likely to benefit from additional vitamin D supplementation,” Dawson-Hughes said. “In principle, this could involve a single, relatively inexpensive genetic test.”

Next steps in this line of research include replicating the findings in independent cohorts and conducting prospective trials designed to test genotype-guided supplementation strategies.

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