Press release

Vitamin D Supplements Do Not Significantly Reduce Risk of Developing Type 2 Diabetes, According to Research Presented Today at the ADA’s Scientific Sessions

June 7, 2019 | San Francisco, California

New research shows that daily oral vitamin D supplementation does not effectively reduce the risk of type 2 diabetes among vitamin D-sufficient adults at high risk for developing the disease, according to the “The Vitamin D and Type 2 Diabetes (D2d) Study – A Multicenter Randomized Controlled Trial for Diabetes Prevention” study presented today at the American Diabetes Association’s® (ADA’s) 79th Scientific Sessions® at the Moscone Convention Center in San Francisco.

More than 84 million Americans have prediabetes, which is a precursor of symptoms indicating higher risk of progression to type 2 diabetes. The prevalence of type 2 diabetes (T2D) and related costs are expected to continue to increase in the next quarter century, resulting in a need for effective prevention interventions for populations at high risk. Vitamin D insufficiency has emerged as a potential key contributor to the development of T2D. Observational studies have consistently reported an inverse association between blood 25-hydroxyvitamin D (25[OH]D) concentration and incident diabetes; however, data from long-term trials are lacking. D2d, a multicenter randomized controlled trial focused on diabetes prevention, was conducted in 22 U.S. cities to determine whether vitamin D supplementation is safe and effective in delaying the onset of TD2 in people at risk for the disease and to gain a better understanding of how vitamin D affects glucose metabolism.

The study included a total of 2,423 adults at high risk for diabetes, and this study utilized the ADA’s 2010 guidelines for high risk as meeting at least two of three glycemic criteria for prediabetes (fasting glucose 100–125 mg/dL (5.6–6.9 mmol/L); 2-h post load glucose after 75-g glucose load 140–199 mg/dL (7.8–11.0 mmol/L); and hemoglobin A1c 5.7–6.4% (39–47 mmol/mol)). Trial participants were ages 30 years and older, with body mass indexes of ≥25 (≥23 for Asian Americans) to 42 kg/m2, and approximately 40% were non-Caucasian. The participants were randomized to take a pill containing 4000 IU of vitamin D3 (cholecalciferol) per day or a matching placebo.

During the study, participants were followed for new-onset diabetes with blood tests every six months for a median of 2.5 years. The study was designed as an event-driven trial with a target total number of diabetes events of 508. The study was designed to detect a reduction in the risk of developing diabetes of 25% or more with vitamin D. To maximize the study’s ability to observe a treatment effect, participants were asked to refrain from using diabetes-specific and/or weight loss medications during the study and to limit the use of outside-of-study vitamin D to 1000 IU per day from all supplements including multivitamins.

Researchers specifically designed D2d to include participants at high risk for T2D, regardless of their vitamin D level. When the study began, about 80% of participants had vitamin D levels considered sufficient by U.S. nutritional standards (equal to or greater than 20 ng/mL). The study was unable to show that vitamin D decreases the risk of diabetes by the target level of 25% or more in the total study population. At the end of the study, fewer participants in the vitamin D group (293 out of 1211 participants in the vitamin D group; 24.2 percent) developed diabetes compared to the placebo group (323 out of 1212 participants in the placebo group; 26.7 percent), a reduction of 12%, which was not found to be statistically significant. The high proportion of participants with adequate vitamin D levels may have reduced the study’s ability to detect an overall benefit of vitamin D in the total study population.

“Even though many previous studies had observed that people with low vitamin D levels have an increased risk of type 2 diabetes, it was not known whether taking steps to increase people’s vitamin D levels would actually reduce their risk of diabetes. Our study results did not show a statistically significant benefit for vitamin D in decreasing progression to type 2 diabetes,” says Anastassios G. Pittas, MD, MS, endocrinologist and co-director of the Diabetes and Lipid Center at Tufts Medical Center and principal investigator of the D2d study. “The results of our research underscore the need for clinical trials to confirm hypotheses raised in observational studies, and this is an important step in developing additional public and clinical recommendations.”

Researchers plan to continue to analyze the information collected during D2d to determine if vitamin D supplementation has an effect on how the body creates or responds to insulin and to examine the impact on other conditions including heart disease, cancer and kidney health among people at risk for developing T2D. They will also conduct more analyses to study the overall safety of taking 4,000 units per day of vitamin D for longer periods of time.

The full manuscript is publishing simultaneously in The New England Journal of Medicine.

To speak with Dr. Pittas, please contact the ADA Press Office on-site at the Moscone Convention Center on June 7-11, by phone at 415-978-3606 or by email at

The American Diabetes Association’s 79th Scientific Sessions, the world’s largest scientific meeting focused on diabetes research, prevention and care, will be held June 7-11, 2019, at the Moscone Center in San Francisco, California. Nearly 15,000 leading physicians, scientists, health care professionals and industry representatives from around the world are expected to convene at the Scientific Sessions to unveil cutting-edge research, treatment recommendations and advances toward a cure for diabetes. During the five-day meeting, attendees will receive exclusive access to more than 850 presentations and 2,000 original research presentations, participate in provocative and engaging exchanges with leading diabetes experts, and can earn Continuing Medical Education (CME) or Continuing Education (CE) credits for educational sessions. The program is grouped into eight thematic areas: Acute and Chronic Complications; Behavioral Medicine, Clinical Nutrition, Education and Exercise; Clinical Diabetes/Therapeutics; Epidemiology/Genetics; Immunology/Transplantation; Insulin Action/Molecular Metabolism; Integrated Physiology/Obesity; and Islet Biology/Insulin Secretion. Gretchen Youssef, MS, RDN, CDE, President of Health Care and Education, will deliver her address, “It’s All About Access!,” on Saturday, June 8, and Louis H. Philipson, MD, PhD, FACP, President of Medicine and Science, will deliver his lecture, “Precision Medicine—Addressing the Many Faces of Diabetes,” on Sunday, June 9. Join the Scientific Sessions conversation on social media using #ADA2019.

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, About the American Diabetes Association

Every day more than 4,000 people are newly diagnosed with diabetes in America. Nearly 115 million Americans have diabetes or prediabetes and are striving to manage their lives while living with the disease. The American Diabetes Association (ADA) is the nation’s leading voluntary health organization fighting to bend the curve on the diabetes epidemic and help people living with diabetes thrive. For nearly 80 years the ADA has been driving discovery and research to treat, manage and prevent diabetes, while working relentlessly for a cure. We help people with diabetes thrive by fighting for their rights and developing programs, advocacy and education designed to improve their quality of life. Diabetes has brought us together. What we do next will make us Connected for Life. To learn more or to get involved, visit us at or call 1-800-DIABETES (1-800-342-2383). Join the fight with us on Facebook (American Diabetes Association), Twitter (@AmDiabetesAssn) and Instagram (@AmDiabetesAssn).