Vitamin D Supplements Don’t Lower Type 2 Diabetes Risk: Study

Despite promising findings from past observational studies, a major new study suggests that taking a vitamin D supplement doesn’t reduce the risk for type 2 diabetes. Researchers studied how higher-than-typical levels of vitamin D affected type 2 diabetes onset in adults with sufficient levels of vitamin D who were at risk for the disease.

The vitamin D and type 2 diabetes study, dubbed D2d, was published in June 2019 in The New England Journal of Medicine (NEJM), and authors presented their findings at the American Diabetes Association’s (ADA) 79th Scientific Sessions at the Moscone Convention Center in San Francisco.

“You can trust that, for an American population, taking vitamin D is not going to reduce the risk by 25 percent or more," says Myrlene A. Staten, MD, the D2d project scientist at the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), which funded the study.

The study, the largest ever clinical trial designed to explore the effects of vitamin D supplementation on diabetes onset, was aimed at the large numbers of people who are at risk for type 2 diabetes. More than 84 million Americans — about 1 in 3 people over age 20 — are at high risk of developing diabetes, according to the study authors. In recent years, experts have explored various approaches to curtailing or delaying onset of the disease.

Researchers undertook D2d because of prior observational studies, including one published in December 2014 in the journal Diabetes Care, which suggested a low vitamin D level is associated with a range of health problems, including diabetes. Per the authors, the Recommended Daily Allowance (RDA) for vitamin D is 600 international units (IU) per day for adults up through age 70, and 800 IU per day for older adults. Yet the nutrient is generated in the body upon exposure to sunlight, and many people do not have enough sun exposure to avoid becoming deficient.

"Observational studies are important for generating hypotheses and ideas of what might be,” Dr. Staten says. “But there are so many factors that can affect observational studies. There is no way to know if changing the levels — giving them a pill of vitamin D — is going to make a difference. You have to confirm observational studies with controlled trials.”

Why D2d Was a Different Kind of Study on Vitamin D and Type 2 Diabetes

Researchers designed D2d to take a hard look at the usefulness of vitamin D supplementation via the gold standard research format: the randomized clinical trial. Launching the study in 2013, researchers enrolled 2,423 people at risk for diabetes, a definition they based on meeting at least two of three glycemic criteria. They randomly assigned participants to take a 4,000 IU vitamin D pill daily or a placebo pill. Then they followed subjects for the development of diabetes for two to five years and underwent blood tests every six months. Researchers told participants to avoid taking any other diabetes-specific or weight loss medication.

Researchers designed the study to detect a reduction in the risk of developing diabetes of 25 percent or more with vitamin D, but they observed no such reduction. Fewer people taking the vitamin D supplement developed diabetes compared with the placebo group — 24.2 compared with 26.7 percent — but that difference was not statistically significant.

"The D2d trial was a well-conducted randomized, controlled trial that addresses an important hypothesis in diabetes prevention," says Deborah J. Wexler, MD, the author of a commentary on the study, also published in June 2019 in The New England Journal of Medicine. Dr. Wexler, who was not involved in the study, is an associate professor of medicine at Harvard Medical School and co-clinical director of the Massachusetts General Hospital Diabetes Center in Boston. "Any benefit of vitamin D for diabetes prevention, if present, is modest and clearly does not pertain to a vitamin D-sufficient population."

The ability of vitamin D supplementation to prevent type 2 diabetes onset in this study could be due to the high number of people in the trial who already had sufficient vitamin D levels, the authors report.

“If you’re vitamin D deficient, you should take vitamin D to get up to sufficient levels," Staten says.

"Our trial hints that it may prevent diabetes if you're deficient. But we can't be firm in that result. That finding is a very soft finding."

The data also hinted that vitamin D supplementation might confer a lower level of risk, such as a 10 percent reduced risk, Staten says. “The trial was designed to be able to detect a 25 percent reduction," says Staten, adding that there's a chance vitamin D reduces diabetes risk by a smaller percentage, though the team didn't test for that.

The D2d results were not entirely surprising. A review published in October 2014 in the Journal of Clinical Endocrinology & Metabolism looked at studies that compared the effect of vitamin D supplementation with placebo or a non–vitamin D supplement in adults with normal glucose levels, prediabetes, or type 2 diabetes, and found no effect from vitamin D supplementation on glucose levels or diabetes prevention.

What to Know if You’re Looking to Help Prevent Type 2 Diabetes

For now, people at risk for diabetes can adopt proven lifestyle modification practices, such as following a healthy diet and exercising regularly, to lower their risk, Wexler noted in the NEJM commentary. A healthy diet is one that is high in fresh, whole foods that are rich in nutrients, and low in packaged, processed foods that are high in saturated or trans fat, added sugar, and sodium, notes the National Institute of Diabetes and Digestive and Kidney Diseases. As for exercise, experts recommend a mix of strength training and cardiovascular exercise, equal to at least two hours and 30 minutes per week, according to the Department of Health and Human Services.

The Centers for Disease Control and Prevention (CDC) notes that the Diabetes Prevention Program is a highly effective public-private partnership aimed at helping people who are at risk for type 2 diabetes avoid developing the disorder through modifiable risk factors, such as diet and exercise.

“In the Diabetes Prevention Program, the risk of progression to type 2 diabetes over a period of 2.8 years was lower by 58 percent with a lifestyle intervention than with placebo, and lower by 31 percent with metformin than with placebo, with similar-sized treatment groups as in the D2d trial,” Wexler writes.

The research on vitamin D will go on, though. The D2d researchers are looking at the role of vitamin D on how the body uses and creates insulin, as well as its impact on other conditions, such as heart disease and cancer.

“There are other disciplines that are looking at those types of outcomes,” Staten says. “Maybe we didn’t have a major effect, but who knows for them?”