Randomised controlled trials (RCTs) assessing the effects of vitamin D2 or D3 on the bone mineral density of adults, with no metabolic bone disease, were eligible for inclusion. Interventions could be vitamin D2 or D3, but not a vitamin D metabolite. Trials with co-interventions had to provide the same co-intervention to all groups. The primary outcome of interest was the percentage change in bone mineral density from baseline. Trials of patients with disorders that were likely to affect bone and calcium metabolism, such as chronic kidney disease, pregnancy, or glucocorticoid use, were excluded.
Most of the included trials were of vitamin D3; the remainder were of vitamin D2 or intramuscular vitamin D. The dosages varied. Most trials were placebo controlled; some compared different doses of vitamin D. Most participants were White and female, and their average age ranged from 22 to 80 years. The mean baseline serum 25-hydroxyvitamin D concentration was less than 50 nanomoles per litre, in half the trials that reported it. Bone mineral density was measured at the lumbar spine, femoral neck, total hip, trochanter, total body, or forearm. The trials were conducted in the USA, Bangladesh, Australia, or various countries in Europe, including the UK.
Two reviewers independently selected trials for inclusion.