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.

References

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.