More on Vitamin D:

Vitamin D is a vitamin that plays a major role in the maintenance of serum calcium and phosphate concentrations in the body, and is important in bone health. Vitamin D has also been associated with fracture prevention, improved muscoloskeletal function and reduced rates of falls, cancer prevention,and protection against some autoimmune diseases.
A major source of vitamin D for most people is exposure of the skin to sunlight. Synthesis of vitamin D in the skin depends on several factors. The amount of UVB radiation available in sunlight is related to wavelength, amount of ozone in the atmosphere, and angle of the sunlight, which in turn depends on latitude, season, and time of day. For every 10 degrees change in latitude away from the equator, the probability of hip fracture increases by 0.6%. During winter in places above 35° North latitude, sunlight will not provide adequate UVB radiation for vitamin D3 production. Sunscreen, when applied appropriately, will decrease the body’s ability to manufacture vitamin D. UVB is filtered out by windows. [8-10] Vitamin D production is reduced in darker skinned persons and in older individuals.
NEJM studies have shown a positive association between vitamin D deficiency, BMD (bone mineral density) and fracture risk.
Vitamin D supplementation can improve BMD and reduce fracture risk.