Vitamin D is commonly referred to as the "sunshine vitamin" because it's the only vitamin that can be made by the human body from sun exposure. It plays a very important role in health. The active form of vitamin D is known as vitamin D3 or cholecalciferol. The recommended adequate intake (AI) level for vitamin D3 is 5mcg daily for adults 31 to 50 years old. After age 50, the AI increases to 10mcg daily. Very good food sources of vitamin D include include seafood, fortified milk, cereals and soy products.
How it's Made
Vitamin D is made by the body when the skin is exposed to the sun's ultraviolet rays. When this happens, a cholesterol-like compound is converted first to a precursor to vitamin D and after to vitamin D3, (or cholecalciferol). Vitamin D3 is activated by enzymes from your liver and kidney. After activation, vitamin D functions as a hormone.
As little as 15 minutes under the sun (without sunscreen), three times a week enables your body to manufacture enough vitamin D, which can be stored in the body for several months. Although the AI is expressed in mcg, most vitamin D supplements are expressed in International Units (IU). One IU = 0.025 mcg cholecalciferol.
As with all nutrients, vitamin D is essential for optimum health. It's a fat-soluble vitamin (along with vitamins A, E, and K). Vitamin D is important for cardiovascular, bone and mental health. There is a significant connection between vitamin D status and mental function, mood and age-related cognitive decline. In addition, vitamin D plays a role in cancer prevention, particularly cancer of the breast.
Fish and Milk
Fatty fish are considered the best food sources of vitamin D--especially herring, catfish, salmon, trout and halibut. One serving of any of the above offers 650 IU to 2200 IU of vitamin D, or more than 100 percent of the Recommended Daily Value (DV). For example, A 3 oz. portion of Atlantic herring offers 1384 IU of vitamin D, or 346 percent of the DV. A 4 oz. portion of each wild catfish and chinook salmon offer over 400 IU of vitamin D (approximately 100 percent of the DV). Fortified milk is also a great food source of vitamin D. A 1-cup serving of vitamin A and D fortified cow's milk provides 97.6 IU of vitamin D, or nearly 25 percent of the DV.
Fortified Soy Products and Cereals
Most tofu is fortified with additional nutrients. One serving (79g, or about 1/4 of the package) of Nasoya light firm tofu offers 124 IU of vitamin D (31 percent of the DV); 1 cup Silk light plain soymilk provides about 338 IU. Soymilk (all flavors, nonfat, with added calcium) and vitamins A and D provide 297 IU to 313 IU, depending upon the brand. Soy yogurt provides 161 IU of vitamin D.
Many cereals are fortified with vitamins and minerals, including vitamin D. A 1/2-cup serving of Kellogg's All-Bran with extra fiber provides 219 IU of vitamin D. A 1/2-cup serving of (original) Kellogg's All-Bran provides 131 IU. A 3/4-cup serving of Kellogg's Fruit Harvest offers 111 IU of vitamin D; the same amount of General Mills Berry Burst Cheerios provides 109 IU vitamin D.
Fruits and Vegetables
One cup of orange juice (fortified with calcium and vitamin D) offers 259 IU of vitamin D. The only vegetable that is naturally an excellent source of vitamin D is the mushroom (canned, raw or cooked). One cup of canned mushrooms (drained, solids only) provides 168 IU of vitamin D; 1 cup of sliced white mushrooms offers 164 IU vitamin D.