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Vitamin D in Diabetes

Yushi Li, PharmD,

Narinder Duggal, BSc(Pharm), CDE, MD, FRCPC

Vitamin D is a fat-soluble vitamin that is derived from cholesterol in the liver, and activated by ultraviolet B rays from the sun. Activated vitamin D (also known as 1,25-hydroxycholecalciferol, calcitriol, or vitamin D3) acts as a hormone that distributes throughout the body. Vitamin D receptors are found in almost all of the cells in the body; as a result, vitamin D may have important regulatory roles in various tissues in the body.      

Vitamin D has long been recognized to play an important role in building strong bones. Increasingly, new research is discovering more beneficial roles of vitamin D, including prevention of cardiovascular disease, breast and prostate cancers, chronic pain, and diabetes.

Diabetes type I, type II, and metabolic syndrome have all been associated with deficiency in vitamin D. Several mechanisms of how vitamin D affects diabetes have been proposed:

·      Vitamin D is present in insulin-producing beta cells. Animal studies have shown that vitamin D deficiency impairs insulin secretion, which is reversed when vitamin D deficiency is corrected.

·      Vitamin D may help activate insulin receptors, which are required in order for cells to take in glucose and lower blood sugar.

·      Vitamin D has been shown to reduce inflammatory markers called cytokines. Consequently, vitamin D may prevent long-term complications like atherosclerosis (hardening of the arteries).

Given the many benefits of vitamin D, what amount of vitamin D intake is effective and safe? Studies that looked at the amount of vitamin D needed to prevent fractures and cancers, or to improve immune functions and insulin response, have seen benefits with blood levels of vitamin D between 30 to 80 ng/mL (the Institute of Medicine defines sufficient vitamin D intake as a blood level greater than 30 ng/mL). This corresponds to a daily dose of vitamin D between 5000 to 10,000 IU. Side effects (most common is the increase in blood calcium) from vitamin D supplementation are rare, and are associated with intake of vitamin D greater than 20,000 to 50,000 IU daily, or vitamin D blood level greater than 200 ng/mL. Based on newer research in the past decade, the American Journal of Clinical Nutrition recommends vitamin D dose of 10,000 IU daily as a safe upper limit.

Contrary to popular belief, modest sun exposure is not likely to achieve the high level of vitamin D needed for disease prevention. Given the seasonal and latitudinal variation in sun exposure, the amount of vitamin D production obtained from sun can be variable. In addition, the present of sunscreen can reduce vitamin D production by over 90%. Lastly, while brief exposure to sun activates vitamin D, prolonged exposure in fact destroys vitamin D.

Likewise, dietary vitamin D does not provide adequate amount of vitamin D. For example, vitamin D from fatty fish such as salmon, trout, and sardines contain 46 IU and 794 IU of vitamin D per 3-ounce serving. An egg yolk contains 25 IU. Fortified milk contains 100 IU in an 8-ounce serving. Consuming large amount of such foods to achieve the needed amount of vitamin D is unfeasible considering the high cholesterol and caloric content of such foods.

In summary, new evidence suggests vitamin D supplementation greater than 5,000 IU daily may provide a number of health benefits. Vitamin D obtained from sun exposure and diet alone are not sufficient to maintain adequate blood level of vitamin D. Individuals at higher risk for vitamin D insufficiency include:

·      Persons with minimal exposure to sunlight

·      Persons with diabetes

·      Persons who are overweight or obese

·      Breast-fed infants

·      Elderly persons

These individuals are encouraged to discuss with their health care providers to measure their vitamin D levels and obtain appropriate vitamin D supplementation.

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References:

1.    Heaney, RP. Vitamin D: criteria for safety and efficacy. Nutr Rev. 2008;66(10S2):S178—S81.

2.    Hathcock JN, Shao A, Vieth R, Heaney R. Risk assessment for vitamin D. Am J Clin Nutr. 2007;85(1):6-18.