This review summarises aspects of vitamin D metabolism, the consequences of vitamin D deficiency, and the impact of vitamin D supplementation on musculoskeletal health in older age. With age, changes in vitamin D exposure, cutaneous vitamin D synthesis and behavioural factors (including physical activity, diet and sun exposure) are compounded by changes in calcium and vitamin D pathophysiology with altered calcium absorption, decreased 25-OH vitamin D [25(OH)D] hydroxylation, lower renal fractional calcium reabsorption and a rise in parathyroid hormone. Hypovitaminosis D is common and associated with a risk of osteomalacia, particularly in older adults, where rates of vitamin D deficiency range from 10–66%, depending on the threshold of circulating 25(OH)D used, population studied and season. The relationship between vitamin D status and osteoporosis is less clear. While circulating 25(OH)D has a linear relationship with bone mineral density (BMD) in some epidemiological studies, this is not consistent across all racial groups. The results of randomized controlled trials of vitamin D supplementation on BMD are also inconsistent, and some studies may be less relevant to the older population, as, for example, half of participants in the most robust meta-analysis were aged under 60 years. The impact on BMD of treating vitamin D deficiency (and osteomalacia) is also rarely considered in such intervention studies. When considering osteoporosis, fracture risk is our main concern, but vitamin D therapy has no consistent fracture-prevention effect, except in studies where calcium is coprescribed (particularly in frail populations living in care homes). As a J-shaped effect on falls and fracture risk is becoming evident with vitamin D interventions, we should target those at greatest risk who may benefit from vitamin D supplementation to decrease falls and fractures, although the optimum dose is still unclear.
Vitamin D supplementation increases true fractional calcium absorption in the absence of caloric restriction