**2. Vitamin D deficiency**

Vitamin D deficiency is found in 30–50% of the general population, and prevalence estimates suggest that more than 1 billion people worldwide are vitamin D insufficient or deficient [4, 11].

Vitamin D deficiency is indicated by serum levels of 25(OH)D < 20 ng/mL [4]. Serum levels >30 ng/mL are likely optimal for bone health, but some studies have shown benefits with lesser values. Parathyroid hormone (PTH) suppression appears to plateau at levels between 30 and 40 ng/mL [12]. There has been no agreement on optimum 25(OH)D levels required for purported health benefits beyond skeletal health. One study suggested that 25(OH)D levels below 11–14 ng/mL signify increased CVD risk [13]. Levels in the range of 21–29 ng/mL are considered by some as insufficient, a definition that would label the majority of the U.S. population vitamin D insufficient [4].

The Endocrine Society does not recommend screening for vitamin D deficiency in individuals who are not at risk for it [12]. Many of the risk factors for vitamin D deficiency have been identified. Some of these include inadequate cutaneous synthesis stemming from insufficient sun exposure or dark skin pigmentation and inadequate dietary intake. Other noted risk factors include aging, obesity, renal disease, liver disease, disorders that affect fat absorption (e.g. celiac disease, inflammatory bowel diseases, types of bariatric surgery), increased catabolism due to medications (e.g. steroids, anticonvulsants etc.), and other hereditable (e.g. rickets) and acquired disorders (e.g. hyperthyroidism) [4].
