Vitamin D is important for overall health. Unfortunately, only a limited number of foods contain naturally-occurring (non supplemental) Vitamin D.
Lately, various health experts have been touting the myriad of health benefits from taking Vitamin D, the "sunshine vitamin." Not only does Vitamin D help in the absorption of calcium, which is necessary to build strong teeth and bones (especially important for Asian and Caucasian women, who are more prone to osteoporosis), there is also newer evidence that Vitamin D can improve energy levels and help prevent a number of diseases including depression, some forms of cancer, heart disease, and certain autoimmune diseases.
Unfortunately, because many people work indoors and don't have as much exposure to sunlight, it's become difficult to get another Vitamin D in the form of D3, the type which is synthesized in the body when our skin has direct contact with sunlight. In the summer, as few as 10 to 20 minutes in the sun (sans sunscreen) per day can be enough exposure to sunlight to produce sufficient Vitamin D without substantially increasing your risk for skin cancer. But during the winter --- especially in Northern climates--- it's nearly impossible to get enough Vitamin D from sunlight alone.
Fortunately, Vitamin D can also be found in a limited number of foods. Some foods have naturally occurring (not added as a supplement) Vitamin D. These include:
Cod liver oil, which can be bought at most drugstores and health food stores. This oil, derived (as you might expect) from fish liver, is also available in different flavors (like lemon), or as tablets, for those who find its oily texture or slightly fishy taste too unappealing. It has the highest naturally occurring Vitamin D content of any food, and has a number of other health benefits, too!
Herring, long a staple of the Swedish diet, is also extremely high in naturally-occurring Vitamin D. While the little oily fish can be a bit pungent alone, there are a number of recipes online that suggest ways to use herring as a deliciously salty, slightly smoky accent to more traditional foods, like this potato salad recipe. Canned herring also tends to be extremely reasonably priced. Sardines are another small similarly pungent and healthy fish. They are delicious added to a classic Greek salad!
Salmon, which many people may find more palatable than cod liver oil or herring. There are so many delicious ways to prepare salmon, and it also has a number of well-documented health benefits. If you want to be environmentally conscious as well as health-conscious, try looking up which types of salmon (and other fish) are most sustainably harvested (here's a link to the Monterey Bay Aquarium's Regional Guide to Sustainable Seafood), and make an effort to purchase those. In addition to salmon, most oily cold water fish such as mackerel and catfish, are substantial Vitamin D sources.
Tuna. The old staple of various sandwiches and salads is also rich in Vitamin D (and is a great, inexpensive source of protein and healthy fats). Again, if you want to be an environmentally conscious consumer, try looking up which types of tuna are sustainably harvested.
Shrimp and oysters. Shrimp, like tuna, are also a great source of healthy protein and fats.
Beef Liver. Although it does not have as high a Vitamin D content as fish, it's a good alternative for red meat-eaters.
Eggs. For vegetarians or people who really don't like fish, egg yolks are an excellent and healthy way to add more naturally-occurring Vitamin D to your diet.
Shiitake and Button mushrooms (sun dried only). Unfortunately, there are very few foods with naturally occurring Vitamin D that can be included in a vegan diet, but these are one of them! Make sure to purchase those that are dried by sunlight.
Before drastically overhauling your diet, it may be worthwhile to have your Vitamin D levels tested by a trained physician. A simple blood test can tell you if you might benefit from adding some of these naturally Vitamin D rich foods to your diet. And as always, make sure to speak to a trained physician before you make significant changes to your diet.