Vitamin D is a nutrient found in some foods in small amounts, including fatty fish such as herring, mackerel, sardines and tuna. However, most vitamin D – 80% to 90% of what the body gets – is obtained through exposure to sunlight. Vitamin D is needed in cells throughout the body. For many people it’s hard to get enough vitamin D during long, dark, northern winters. The elderly have more difficulty synthesizing vitamin D from sunlight, and as well some medications interfere with its absorption. Therefore vitamin D is often recommended to be added to the diet in the form of supplements.