Vitamin D and Type 2 Diabetes: What the Research Says

Written by Kelsey Schaffstall Young MS, RDN, LDN

What are the functions of vitamin D?

Vitamin D is a fat-soluble vitamin with a variety of functions throughout the human body. It is highly involved in calcium homeostasis by promoting calcium absorption in the gut, regulating blood calcium levels, and promoting bone mineralization.2 It also reduces inflammation and supports cell growth, neuromuscular function, and immune function.2 It also supports glucose metabolism by stimulating the pancreatic beta cells to secrete insulin and acting in adipose tissue, muscle cells, and liver cells to increase insulin’s action.3

Emerging research

Observational studies have suggested that low serum Vitamin D levels (25-hydroxy vitamin D) are inversely associated with the incidence of type 2 diabetes.3 However, a number of small trials have been conducted over time, and systematic reviews and meta-analyses of those studies have shown no significant effect of vitamin D supplementation on the risk of developing type 2 diabetes in those with prediabetes.3

Recently, in 2020, a large systematic review and meta-analysis which investigated this topic was published. Only randomized controlled trials with > 6-month duration were included, and the results were impressive. Vitamin D supplementation reduced the risk of developing T2DM from prediabetes by 11% and Vitamin D supplementation increased the reversion of prediabetes to normal by 48%.3

Vitamin D deficiency

Traditionally, a vitamin D level of < 30 nmol/L has been considered to be deficient, however research has shown that a level < 50 nmol/L is insufficient for bone and overall health and a level of at least 75 nmol/L could be required to maximize the effect of vitamin D on calcium, bone, and muscle.2

Symptoms of vitamin D deficiency include osteomalacia or the softening of bones, teeth deformities, muscle pain and weakness, fatigue, and muscle spasms.1,2

Those at risk of vitamin D deficiency include older adults, individuals with dark skin, people with limited sun exposure, people with a history of gastric bypass surgery, and those with conditions of fat malabsorption such as liver disease, cystic fibrosis, celiac disease, Crohn’s disease, and ulcerative colitis.1,2

How to ensure adequate vitamin D intake

The best source of vitamin D is exposure to UVB rays from the sun, which stimulates vitamin D production.2 However, it is important to know that these rays cannot penetrate glass and a number of factors can impact the body’s ability to produce vitamin D.2 These include a person’s age, the melanin content of their skin, the UV index (affected by geographic location and the time of year), cloud cover, and sunscreen.1,2

In general, a person should aim to get at least 5-30 min of sun exposure to bare face, arms, hands, and legs between 10am-4pm at least 2x weekly and up to daily to maximize their body’s vitamin D production.2

Vitamin D is relatively difficult to obtain from the diet, however good sources include fatty fish (salmon, trout, tuna), fish liver oils, beef liver, egg yolks, and cheese.2 Fortified foods provide most of the vitamin D in the typical American diet.2 These include milk, plant-based milk alternatives, breakfast cereal, orange juice, and yogurt.2

Depending on your level of sun exposure and dietary intake, you may require vitamin D supplementation to maintain adequate vitamin D levels. Vitamin D2 and Vitamin D3 are both well absorbed in the gut and have been shown to be effective at raising serum vitamin D levels.2

If you are interested in optimizing your vitamin D status and reducing your risk of diabetes through diet and lifestyle changes, consider reaching out to a Registered Dietitian who can support you in developing a personalized plan.


  1. Dawson-Hughes, Bess. “Vitamin D deficiency in adults: Definition, clinical manifestations, and treatment.” UpToDate, May 23, 2023.
  2. Vitamin, D. “Fact sheet for health professionals.” National Institutes of Health. Office of Dietary Supplements. Available online: https://ods. od. nih. gov/factsheets/VitaminC-HealthProfessional (2017).
  3. Zhang, Yu, et al. “Effects of vitamin D supplementation on prevention of type 2 diabetes in patients with prediabetes: a systematic review and meta-analysis.” Diabetes Care 43.7 (2020): 1650-1658.

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