Subclinical vitamin D deficiency

The optimal vitamin D status, as defined by serum 25-hydroxyvitamin D [25(OH)D], is still controversial. Some individuals are at risk for subclinical vitamin D deficiency, as defined by serum 25(OH)D levels between 25 and 75 nmol/L, and up to 80-100% of the entire population can display inadequate serum 25(OH)D values depending on latitude and seasonality. The clinical manifestation of extreme vitamin D deficiency, i.e. rickets and osteomalacia, are rare. Levels of 25(OH)D ≥ 50 nmol/L are required for optimal musculoskeletal health. However, levels of 25(OH)D above 75 nmol/L may be necessary to maximize musculoskeletal benefits and take advantage of the extraskeletal actions of vitamin D. This review will summarize the actual positions on the boundaries of subclinical vitamin D deficiency, the main available evidence on the effects of inadequate vitamin D status on skeletal and extraskeletal targets and supplementation strategies.