Endocrine Abstracts

Vitamin D is an essential hormone for skeletal metabolism across the lifecycle. Rickets and osteomalacia, uncommon manifestations of vitamin D deficiency in western countries, are still common in the Middle East and Asia (1, 2). Furthermore, low bone mass and fractures, latent manifestations of vitamin D insufficiency, are common conditions worldwide (3, 4). Serum 25-hydroxyvitamin D (25-OHD) level is the best index of vitamin D nutritional status, and whereas it is generally accepted that a level below 5–10 ng/ml (multiply by 2.5 to convert to nmol/l) represents vitamin D deficiency, what constitutes a desirable level is now emerging based on the evidence detailed below.

Vitamin D sufficiency in adults and elderly can be defined by evaluating discrete biochemical or physiological outcomes that this hormone modulates. These include intestinal calcium absorption, serum parathyroid hormone levels, bone mass, muscle function, and fractures. Intestinal calcium transport increases linearly from 15% to 35% when serum 25-OHD level rises from 10 to 32 ng/ml (5); whereas the vitamin D level at which PTH levels tends to decrease and follow a shallower curve varies from study to study, with a range of 20–40 ng/ml (6). In the NHANES III study, higher serum levels of 25-OHD were associated with higher bone mass of the hip in older (and younger) men and women; the curves being steepest for 25-OHD levels between 10 and 40 ng/ml (7). As for musculoskeletal outcomes, the elderly need a 25-OHD level of around 26 ng/ml to improve muscle function and reduce the risk of falls, and a level above 30 ng/ml to reduce the risk of hip and non-vertebral fractures (8). Therefore, based on the above body of evidence, a desirable target level for 25-OHD would be above 30 ng/ml. Using this cut-off, it is estimated that 1 billion individuals would suffer from hypovitaminosis D worldwide, regions at higher risk are the Middle East and Southern Asia (2, 3). Each 100 IU of vitamin D taken orally would raise 25-OHD level by 1 ng/ml, consequently the current recommendations for an adequate intake for vitamin D, of 400 IU in adults and 600 IU in elderly, would be sub-optimal to reach a target level above 30 ng/ml. Daily doses of 800–1600 IU have been suggested (9) and would vary depending on the nutritional status of the individual at the start of supplementation.

Hypovitaminosis D is a major public health problem across all life stages, with deleterious immediate and latent manifestations (1–4). Strategies to address this often silent disease should include public education, national health policies for screening and prevention through food fortification, and treatment of high risk patients through vitamin D supplementation. In addition to the above, further research is needed to standardize vitamin D assays, conclusively define optimal vitamin D levels, and determine the doses and regimens of vitamin D supplementation for pregnant and lactating women, infants and adolescents.