Vitamin D boosts the body's immune system and reduces inflammation. The major natural source of Vitamin D is the skin that is dependent on exposure to ultraviolet rays from the sun. Only a few foods, such as fatty fish (salmon and tuna) contain significant amounts of vitamin D. In an analysis of 42 different studies, taking vitamin D reduced the risk of acute respiratory infections by 9% compared with placebo. Daily doses ranged from 400 to 1,000 international units (IU). There were no reported serious side effects associated with Vitamin D supplementation.Add excerpt here