Vitamin D has well-established hormonal effects, and like other hormones it can act as a "molecular switch" by influencing the expression of multiple genes. One of its major hormonal effects is to regulate calcium's homeostasis activities in the body. This is of vital importance, since calcium is involved in the functioning of all cells but most cells require only very little of it. Vitamin D helps the body manage calcium homeostasis (keep calcium within healthy ranges in all the body's cells, tissues and organs). Vitamin D research has rapidly expanded into exploring its diverse roles in human health. Vitamin D's clinically proven bone benefits are directly related to its management of calcium homeostasis and specifically to calcium balance in bone. Calcium balance is essential for bone growth and formation. Vitamin D is essential to ensure healthy control over calcium absorption and the availability of calcium to bone. Multiple studies have shown its support for healthy bone structure from childhood through old age. The body's activated vitamin D (25-hydroxy vitamin D3) not only influences bone health but helps support immune system responses, the heart and circulation, the lungs, the musculoskeletal system and virtually all the body's other functional systems. It is increasingly becoming clear that optimal vitamin D status may have beneficial impact on longevity. In a recently published prospective study of thousands of elderly men in Sweden, Hong Kong, and the USA, a significant association was found between healthy blood 25(OH)D levels and a healthy lifespan. Recent research also suggests the prevalence of subclinical vitamin D deficiency is increasing worldwide, and this is paralleled by an increasing number of overweight individuals. It has been proposed that being overweight may contribute to lower blood 25(OH)D levels because the storage of vitamin D in adipose tissue decreases its bioavailability.