Vitamin D is a key vitamin that the body absorbs from food and produces when exposed to sunlight.  It has also  been advocated as a supplement by many people in helping to prevent several conditions, most notably osteoporosis in adults.  However, a new study published in The Lancet medical journal found no significant improvement in the bone density of healthy individuals.  Looking at 23 different studies involving more than 4,000 individuals, researchers in New Zealand found no differences in bone density in the spine, hip, arm bone, or skeleton.  Increased bone density in the neck of the femur bone (a common area of the hip that is fractured) was noted but was considered not clinically relevant.
With such findings, Vitamin D supplementation to prevent osteoporosis in healthy adults would not be warranted.  The US Preventative Task Force also notes that currently, there is insufficient evidence to recommend the use of vitamin D and calcium supplementation in preventing fractures in men or pre-menopausal women.  It also recommends against Vitamin D and calcium supplementation at daily recommended doses for primary prevention of fractures in post-menopausal women.