Breastfed infants are at increased risk of vitamin D deficiency, because human milk alone does not usually provide enough vitamin D to meet adequate intake levels. In addition, the American Academy of Pediatrics (AAP) advises parents to keep infants under 6 months old out of direct sunlight, so vitamin D through UV exposure does not usually occur in infants.
Older adults are also at increased risk of deficiency, partly because vitamin D synthesis in the skin reduces with age, and partly due to the fact that older adults are more likely to spend more time indoors.
Other populations at increased risk of deficiency include individuals with limited sun exposure (e.g. wear head coverings for religious purposes or have occupations that limit sun exposure) and individuals with darker skin (increased skin melanin reduces vitamin D synthesis).
Conditions that limit fat absorption (e.g. celiac disease, Crohn’s disease) reduce the absorption of vitamin D in the gut, as it is a fat-soluble vitamin. Obesity is also associated with an increased risk of deficiency, as the increased subcutaneous fat in obese individuals sequesters more of the vitamin D synthesized from sun exposure.
Diets low in vitamin D are more common in individuals whom have milk allergy or lactose intolerance and those following a vegan diet.
Genetic variation also influences the risk of vitamin D deficiency. Whether vitamin D is made in the skin or acquired from food, vitamin D needs to be activated before the body can use it. The enzyme cytochrome P450 2R1 (CYP2R1) activates vitamin D. Activated vitamin D is then distributed around the body via the binding protein encoded by the GC gene. Variants in the CYP2R1 and GC genes affect the activation and distribution of vitamin D, increasing the risk of vitamin D deficiency.