Vitamin D supplements in vitamin D-sufficient individuals with a high risk of developing type 2 diabetes does not likely reduce their likelihood of developing the  disease, according to research presented at the American Diabetes Association’s (ADA) 79th Scientific Sessions and simultaneously published in The New England Journal of Medicine. 1
 
Vitamin D insufficiency has been cited as a potential key contributor to the development of type 2 diabetes, but long term studies on the matter have been lacking. 

 A  team of researchers led by Anastassios G. Pittas, MD, of the Division of Endocrinology, Diabetes, and Metabolism at Tufts Medical Center, conducted a randomized, placebo-controlled trial assessing the time-to-event association between daily 4000 IU vitamin D3 and new-onset diabetes.  The researchers collected data from 2,423 adults with a high risk for type 2 diabetes across 22 US cities. Every participant was above 30 years of age and met at least 2 of the 3 glycemic criteria for prediabetes. Participants were randomly split into 2 groups: 1 group that would take a pill containing 4000 IU of vitamin D3 daily while the other took a placebo. Blood tests were conducted every 6 months for an average of 2.5 years to test for new-onset diabetes.  
 
Eighty percent of the study participants had adequate levels of vitamin D when the study started, which investigators acknowledged could have reduced their ability to determine the overall benefit of vitamin D in the study population.  
 
By the end of the study, 24.2% of individuals taking the vitamin D supplement developed diabetes. In the placebo group, 26.7% of participants had developed diabetes. The 12% difference between the 2 groups was found not to be statistically significant by the reserarchers and was well off the 25% target level.    
 
“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” Anastassios G. Pittas, MD, MS, endocrinologist and co-director of the Diabetes and Lipid Center at Tufts Medical Center and the study’s principal investigator said in a press release about the study findings.2 “The results of our research underscore the need for clinical trials to confirm hypothesis raised in observational studies, and this is an important step in developing additional public and clinical recommendations.” 
 
The researchers noted in the study that they plan to continue analyzing the data from the study, and they hope to determine the effects vitamin D supplementation has on how the body creates insulin as well as the impact it has on other conditions in individuals with a risk of developing type 2 diabetes, such as cardiovascular disease and kidney health.  
 
Reference
1. Pittas AG, Dawson-Hughes BD, Sheehan P, et al. Vitamin D Supplementation and Prevention of Type 2 Diabetes. N Engl J Med. 2019; doi: https://www.nejm.org/doi/full/10.1056/NEJMoa1900906?query=main_nav_lg 
2. Vitamin D Supplements Do Not Significantly Reduce Risk of Developing Type 2 Diabetes, According to Research Presented Today at the ADA’s Scientific Sessions American Diabetes Association Newsroom [press release] Published June 7, 2019. http://www.diabetes.org/newsroom/press-releases/2019/vitamin-d-supplements-do-not-reduce.html