While calcium is a critical dietary component, vitamin D (theoretically) is not. That's because with enough exposure to sunlight, your body can make all the vitamin D it needs.
Unfortunately, blood levels of this vitamin have been declining due to lack of sun exposure and the rising obesity epidemic. Since vitamin D is a fat-soluble vitamin, it's easily trapped in fat cells and thus less available in the blood — where it's needed to regulate blood calcium and maintain bone integrity.
Plus, vitamin D also functions as a hormone once activated by sunlight. In this way, it affects how cells grow, multiply, and specialize. Not only is vitamin D deficiency a risk factor for osteoporosis, but epidemiological studies and randomized, controlled trials suggest a correlation with diseases including cardiovascular disease, some cancers, respiratory infections, and multiple sclerosis.
How To Get More: Dietary sources of vitamin D include cod liver oil, sardines, salmon, mackerel, tuna, and fortified foods like milk, orange juice, and cereals. If these foods don't appeal to you, then just five minutes of direct sunlight without sunscreen, or 10 to 30 minutes with sunscreen, for light-skinned people will promote the synthesis of several days' worth of vitamin D.
Since the pigments of darker skin protect against UV irradiation, dark-skinned people require up to three hours of direct sunlight to produce a comparable amount of vitamin D. But there are also other factors that could interfere with this conversion, like advanced age, pollution, mobility, season, and the time of day.
To ensure you get enough vitamin D, have your levels checked at your annual physical. If your blood levels are under 40 ng/ml then you may need to take a supplement of 1,000 to 5,000 IUs per day. However, since this vitamin is the most potentially toxic among vitamins, it should only be taken on an as-needed basis.