More than one in three Americans is prediabetic, meaning that roughly 96 million people have higher-than-normal blood sugar levels that raise the risk of developing type 2 diabetes, heart disease, and stroke if left unmanaged. While vitamin D supplements may offer a slight benefit for blood sugar control, taking then isn’t the most effective way to significantly improve glucose levels.
Vitamin D plays a role in blood sugar regulation by supporting insulin sensitivity and secretion and by lowering inflammation, which can interfere with insulin signaling in the body. Notably, prediabetes has been linked to low levels of vitamin D.
Research examining how vitamin D supplementation may affect prediabetes has shown some promise, said Roxana Ehsani, MS, RD, CSSD, a sports dietitian. One 2025 study found that people with prediabetes who took a vitamin D supplement for three years were 30% more likely to return to normal glucose regulation when compared with those taking a placebo.
Other research suggests supplementation decreased the risk of progression to type 2 diabetes from 25% to 22% over three years, said Anne Cappola, MD, an endocrinologist and professor at the University of Pennsylvania’s Perelman School of Medicine in Philadelphia.
Still, it’s important to note that there’s no medical consensus on the optimal dose of vitamin D for this purpose. It’s also unclear if there are any adverse long-term effects of taking vitamin D supplements.
While the risks of vitamin D supplementation are generally low, it shouldn’t be the main focus of diabetes prevention, Cappola said. Research hints at only small improvements in blood sugar, and vitamin D is a “much weaker” intervention compared with lifestyle factors like a healthy diet and exercise.
“Lifestyle interventions reduce the absolute risk of progression from prediabetes to diabetes by 15 percentage points over three years,” Cappola said. “I do not want vitamin D supplementation to be a distraction.”
Before recommending vitamin D supplements, Cappola said she looks first at whether a patient can increase intake through sun exposure or dietary sources, including fatty fish, eggs, and fortified foods. The recommended daily intake of vitamin D is 600 international units (IU) per day until people hit age 70, when intake should increase to 800 IU daily. However, some providers may recommend a higher dosage for certain people based on vitamin D status.
Before taking any supplements, be sure to consult your doctor, Ehsani recommended. Because supplements are largely unregulated in the United States, it’s best to choose a reputable brand and a product that’s been tested by a third party for purity and label accuracy.