No foods contain particularly high levels of vitamin D3, but egg yolks, fatty fish, cheese and beef liver all contain small amounts naturally. The primary health effect of vitamin D3 is in the absorption of calcium and the maintenance of bones, but there are many other possible health effects.
Severe vitamin D deficiency leads to rickets, a bone disease. Besides bone health, the health effects of vitamin D are controversial, particularly in what levels of intake are most beneficial. Newer research suggests that sufficient vitamin D levels protect against a range of conditions, including cancer, heart disease, diabetes and several infectious diseases. Higher vitamin D levels also appear to increase muscle strength in older people, helping to prevent falls, but very large annual doses actually appear to increase the risk of fractures.
Vitamin D is an unusual vitamin because it does not primarily come from dietary sources. Instead, it is generated in the skin upon sun exposure. Unfortunately, people in modern society spend a great deal of time indoors, and in latitudes far away from the equator, it can be impossible to get enough sunlight during winter. People with darker skin also have a hard time getting enough. Considering these limitations, supplementation is often required to reach the levels many researchers now suggest.