Vitamin D

Vitamin D is a fat-soluble vitamin that the skin synthesizes from sunlight (UVB) and that is also obtained from foods (fatty fish, egg yolk, fortified milk and cereals) and supplements. Its central role is enabling calcium and phosphorus absorption for bone mineralization (see Calcium); it also supports muscle function and immune health. Severe deficiency causes rickets in children and osteomalacia in adults, and low status contributes to bone loss and fracture risk.

Recommended intake (Dietary Reference Intakes, Institute of Medicine 2011): the RDA is 600 IU/day (15 mcg) for ages 1–70, rising to 800 IU/day (20 mcg) after age 70 (infants: AI 400 IU). The tolerable upper intake level from age 9 is 4,000 IU/day (100 mcg). These values were set assuming minimal sun exposure, so they are cautious for most people.

How common is deficiency? Sun exposure varies (latitude, indoor lifestyles, sunscreen; darker skin pigmentation reduces synthesis) and few foods are naturally rich in vitamin D, so some groups do run low: older adults (see Senior Training (Redirect → Older Adults)), people with little sun exposure, those with darker skin, and people with malabsorption. But the 2011 DRI committee warned that deficiency is considerably overestimated when labs use high, unvalidated cut-points. On its reading of the evidence, people are at risk of deficiency for bone health below a serum 25-hydroxyvitamin D of 30 nmol/L (12 ng/mL), and practically everyone is sufficient at 50 nmol/L (20 ng/mL) or above.

Testing and supplements. The 2024 Endocrine Society guideline suggests against routine 25(OH)D testing in people without an established medical reason. It suggests going above the DRI without testing only for children and teens aged 1–18, adults over 75, pregnancy and high-risk prediabetes, and suggests against it for other healthy adults under 75. When a clinician confirms low status, supplementation (commonly D3) is the usual correction.

• Fat-soluble; made in skin from UVB sunlight; few rich food sources.
• Enables calcium/phosphorus absorption for bone; aids muscle and immunity.
• RDA 600 IU (15 mcg) ages 1–70; 800 IU (20 mcg) after 70; UL 4,000 IU (100 mcg)/day from age 9 (DRI, 2011). Set assuming minimal sun.
• Deficiency risk below 25(OH)D 30 nmol/L (12 ng/mL); practically all sufficient at ≥50 nmol/L (20 ng/mL). Deficiency is often overestimated by high cut-points.
• At-risk: older adults, low sun, darker skin, malabsorption.
• Endocrine Society 2024: no routine 25(OH)D testing without an indication; extra vitamin D suggested only for ages 1–18, over 75, pregnancy, high-risk prediabetes.