Yes, your body does make this vitamin when exposed to sunlight. But there are several factors that affect how much.
Age: Vitamin D is a hormone, as well as a nutrient. Our bodies make vitamin D when skin is exposed to UVB sunlight. However, a person’s ability to manufacture vitamin D decreases with age, so that by the time a person enters his/her senior years, the body is making as little as 40% of the vitamin D made in childhood. As a result, dietary/supplemental intake becomes increasingly more important with each passing decade.
Location: People living north of the Latitude running generally through Los Angeles and Atlanta typically are sun-deprived and, consequently, low in vitamin D, especially during Fall and Winter. Even in the summer, the use of sunscreen blocks UV light and reduces vitamin D synthesis. A study from the University of Georgia, found that 75% of young girls had blood levels for vitamin D below 80nmol/L (100nmol/L is better, while early humans had levels above 130nmol/L). Researchers suspect that everyone living in the North is not getting enough UVB light to synthesize adequate vitamin D. In short, the farther north a person lives, the more critical dietary/supplemental sources of the vitamin become.
Skin Color: The darker your skin, the more it filters out those UVB rays, thus increasing your chances of a vitamin D deficiency.
Diet: Few foods contain this vitamin. Natural sources of vitamin D are limited to fish and cod liver oils, while the FDA allows only a few kinds of food to be fortified with vitamin D, including milk, fortified OJ and soymilk, or fortified cereals. Even then, most fortified cereals provide very little vitamin D. The wealth of new research shows that the only realistic and dependable source of the vitamin – to reach levels found effective for disease control – is through supplementation. Currently, most multis only include 400IU of the vitamin, far below the new recommended upper limit. Your best bet is to include fortified foods in your daily diet AND take a supplement of at least 400IU a day.