Vitamin D is one of the only vitamins produced naturally by the body. However, in order for the body to produce vitamin D, it has to be exposed to an adequate amount of sunlight. Generally, an hour per week is more than enough, but surprisingly, many people are still not exposed to enough sun to produce beneficial amounts of vitamin D. Also, studies have shown that as we age, we tend to produce less vitamin D even with adequate sun exposure. These are the primary reasons why many adults have to monitor their diets to make sure they are taking in enough vitamin D.
Milk is often fortified with vitamin D, and you can also find good amounts of vitamin D in certain kinds of fish that are rich in omega-3 fatty acids. Cod liver oil is an herbal supplement that is rich in vitamin D. Vitamin D supplements are also widely available.
While it is important to have enough vitamin D in your diet, it is possible to take in too much vitamin D. This often happens when a person receives a good amount of sun exposure and consumes many foods that contain moderate amounts of vitamin D, but also continues to take a vitamin D supplement. For this reason, it is important to discuss with your doctor whether or not you require a vitamin D supplement as part of your diet.