Because exposure to sunlight is decreased markedly during the winter, young and middle-aged adults 19-50 years of age who live in northern latitudes (above 40° N) can become vitamin D deficient. Therefore, to cover the needs of nearly all adults 19-50 years of age, NAS recommends an AI of vitamin D of 5 mcg (200 units) daily.
Adults 51-70 years of age also depend on sunlight for most of their vitamin D requirement, but this age group is more prone to developing vitamin D deficiency because of a variety of factors that reduce cutaneous production of the vitamin. Dietary intakes of vitamin D exceeding 2.5 mcg (200 units) daily appear necessary to prevent higher rates of bone loss during periods of low sun exposure in adults 51-70 years of age. At vitamin D intakes exceeding 5.5 mcg (220 units) daily in this age group, there was no seasonal variation in serum parathyroid hormone (PTH) concentrations, an indicator of vitamin D status. Therefore, to cover the needs of nearly all adults 51-70 years of age, NAS recommends an AI of vitamin D of 10 mcg (400 units) daily.
Current evidence is strong that geriatric adults are at high risk for vitamin D deficiency, resulting in secondary hyperparathyroidism, osteomalacia, exacerbation of osteoporosis, and increased skeletal fractures, and that supplementation with dosages of 10-20 mcg (400-800 units) daily can be beneficial. Based on available evidence, NAS concluded that a vitamin D intake of 7.5 mcg (300 units) daily would be prudent for adults older than 70 years of age with limited sun exposure and body stores of the vitamin. Therefore, to cover the needs of nearly all adults in this age group, NAS recommends an AI of vitamin D of 15 mcg (600 units) daily for adults older than 70 years of age, regardless of extent of exposure to sunlight or body stores of the vitamin.
In adults with severe malabsorption syndromes, at least a 5- to 10-fold increase in the physiologic dose of ergocalciferol is usually needed to prevent osteomalacia, and some clinicians have recommended oral ergocalciferol dosages of 250 mcg to 2.5 mg daily. In some patients, IM administration of the drug may be preferred.