Vitamin D3 (also known as cholecalciferol) is the most common and bioavailable form of vitamin D. Some foods help provide D3, but the most common route is from sunlight on your skin. In the modern world, especially with the use of high SPF sunscreens, many people do not receive enough exposure to the sun and can therefore benefit from taking a D3 supplement. Some research indicates that having healthy levels of vitamin D, such as one would have by taking a vitamin D3 supplement, can help promote our immune response and may protect against respiratory illnesses in general. Sufficient vitamin D is associated with increased T-cell function (a good thing!), as well as a decreased susceptibility to infection. Within the context of vitamin D deficiency, repercussions on the immune system are exposed and there appears to be increased susceptibility to infection. Further, those low in vitamin D may find that taking supplemental vitamin D3 will help them lose weight. Roughly 40 percent of North American adults are vitamin D-deficient.