Proponents of vitamin D supplementation have claimed that taking vitamin D has been linked to lower risk of cancer, depression, type 2 diabetes, heart disease, anti-inflammatory responses and of course, bone fractures (National Institutes of Health [NIH], 2018). So, it is no wonder that vitamin D is one of the most popular supplements. However, determining need for supplementation, based on screening and recommendations has been a challenge for healthcare professionals.

Vitamin D, often called the sunshine vitamin because it is absorbed through sunlight, must undergo two hydroxylations, first in the liver to convert to 25-hydroxyvitamin D [25(OH)D], then the kidney, to become active. It is this serum concentration of 25(OH)D that is used as the indicator of vitamin D status in the individual. Although 25(OH)D serves as a biomarker of exposure of vitamin D, it does not show how much vitamin D is stored in the body, nor does it show a clear picture of the health effects of such levels (NIH, 2018). For example, African Americans generally have lower 25(OH)D levels because of lower amounts of vitamin D absorbed through the skin, but paradoxically, have reduced rates of fracture and osteoporosis compared with Caucasians. Populations most at risk for vitamin D deficiency are breastfed infants, older adults, and those with minimal sun exposure, dark skin, inflammatory bowel disease, and other conditions that cause fat malabsorption, obesity, and who have had gastric bypass surgery (NIH).

There are different recommendations from various professional organizations on what constitutes a deficiency. The Institute of Medicine (IOM, 2011) at the National Academy of Sciences uses serum levels of 25(OH)D concentrations below 30 nmol/L (<12 ng/mL) as a deficiency but found that levels of 50 nmol/L (20 ng/mL) are sufficient for the general population. Thus, supplementation has been recommended at this arbitrary cutoff point by many clinicians. The IOM established adequate daily intake for various age-groups and populations, including pregnancy and lactation (600 IU/day). Evaluating daily "intake" of vitamin D is problematic. Daily intake for all other vitamins is done by a nutritional review. But although there is some vitamin D found in food such as egg yolks, fish oils, and fortified dairy and cereals, most of the body's needs are obtained by sunlight, specifically ultraviolet B (UVB) rays, absorbed through the skin. Sunscreens as low as sun protection factor (SPF) 8 will impede the absorption and UVBs cannot be absorbed through glass. Although it takes very little exposure to sunlight to meet these needs (as little as 5-15 minutes three times a week), the amount absorbed cannot be quantified. Recommendations for daily intake assume minimal skin exposure.

Recent evidence suggests we may need to rethink vitamin D supplementation and screening. A meta-analysis of 81 trials found no effect of vitamin D on total fractures, hip fractures, or falls (Bolland, Grey, & Avenell, 2018) and a randomized, placebo-controlled VITAL Clinical Trial with 25,871 participants found vitamin D supplementation did not result in a lower incidence of invasive cancer or cardiovascular events than placebo (Manson et al., 2019). The U.S. Preventive Services Task Force (LeFevre & USPSTF, 2015) does not recommend screening for vitamin D deficiency in asymptomatic adults and the American College of Obstetricians and Gynecologists (2011) does not recommend it for healthy pregnant women.

Nurses can advise women that new evidence does not show that taking vitamin D improves health outcomes previously claimed.

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