Vitamin D deficiency is common worldwide and the links between the vitamin and many chronic diseases are increasingly being recognized and studied, but correcting a deficiency is not always as simple as popping a daily supplement or spending time in the sun, according to experts.

According to Susan Williams, MD, a physician at Cleveland Clinic’s Endocrinology and Metabolism Institute, although the detrimental effect of insufficient sun exposure in childhood was established centuries ago, the beneficial effects of vitamin D sufficiency have only recently been established. “Increasingly, studies have associated vitamin D deficiency with various chronic diseases, while others established a positive link between vitamin D levels and optimal functioning of the immune and nervous system.”

Causes of Vitamin D Deficiency

Dr. Williams adds that vitamin D-rich foods can provide only a small percentage of the recommended daily intake, with the rest coming from the sun or supplements. Limited sun exposure – whether as a result of inclement weather or in the interest of skin cancer prevention – is a common cause of vitamin D deficiency, but there are other risk factors.

These include having dark skin, being elderly, having obesity or past gastric bypass surgery, having certain digestive diseases that result in malabsorption, and for infants, being exclusively breastfed without vitamin D supplementation.

Vitamin D Supplementation

Suspected vitamin D deficiency can be confirmed by a simple blood test, and there are two factors to consider if supplementation is deemed necessary, says Dr. Williams. “First, there are two types of vitamin D supplements, namely vitamin D2 and vitamin D3. While both types have been found to increase vitamin D levels, recent studies have indicated that vitamin D3 has a more potent effect over time.

“Second, we need to consider how the supplement is taken. Recent studies in this regard have indicated that supplements are more effective at raising vitamin D levels when taken with a meal that includes at least 15 g of fat. More simply stated, taking the supplement with one’s largest meal appears to promote improved absorption,” she says.

Dr. Williams stresses that patients need to stick to the supplement amounts prescribed by their physician or else they run the risk of vitamin D toxicity.

Some patients do not respond to vitamin D supplements because they have problems absorbing them, Dr. Williams adds. These include people with Crohn’s disease, a history of malabsorptive-type bariatric surgery, celiac disease, cystic fibrosis, steatorrhea, short bowel disease, inflammatory bowel disease, and severe cholestasis.

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Vitamin D Tests

A problem with absorption can be confirmed through blood tests by comparing the levels of vitamin D in blood taken immediately before an oral dose of vitamin D is given, and 12 to 24 hours afterward. If malabsorption is confirmed, phototherapy may be used, which involves careful skin-typing and carefully metered exposure to UV-B light to increase D levels.

Dr. Williams warns patients against trying to increase their vitamin D by using tanning beds or spending hours sunbathing as this can be ineffective and carries the risks of skin damage and skin cancer.

“Sunshine is composed of approximately 95% UV-A and 5% UV-B, but only UV-B is required for vitamin D synthesis. UV-A is the predominant or sole light source used in tanning beds, and the dose of UV-A in tanning beds can be up to 12 times that provided by the sun,” she says. “UV-A and UV-B are both implicated in skin cancers.

“UV-A is thought to damage skin and increase the risk of melanoma by causing oxidative stress-induced DNA damage. UV-B damage is more direct, with photo products that are implicated in skin carcinogenesis.”

Dr. Williams notes that skin type and age are factors in the response to UV exposure, but in general, exposing 5% of the body surface twice weekly for 20 minutes in warm months may be equivalent to 430 international units (IU) of vitamin D per day, but a plateau is reached after 20 minutes, so time in the sun should be limited given the associated risks.