Vitamin D isn't just for strong bones. You also need it to help cells grow, reduce inflammation, support neuromuscular functions and power your immune system. Vitamin D-3 is a type of vitamin D you'll see regularly in dietary supplements. You'll even get it from several foods, and your body makes it every time you expose your skin to sunlight. So unless your doctor recommends it, you may not need a D-3 supplement.
Vitamin D-3 doesn't have its own separate recommendation. It fits in with the overall vitamin D daily requirement of 600 international units, or IUs, the Food and Nutrition Board of the Institute of Medicine reports. Sometimes though, vitamin D is listed in micrograms. Fifteen micrograms of vitamin D is the same as the 600 IU recommendation.
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D-2 versus D-3
Vitamin D-2 isn't usually as prevalent as D-3, although you might find some supplements with D-2 and you can get it from mushrooms. Both forms of the vitamin are beneficial for biological processes and keeping your bones strong. But vitamin D-3 tends to be better for preventing fractures. Research on the exact fracture benefits of D-3 is mixed, but the Linus Pauling Institute notes that after comparing multiple studies, ingesting at least 700 international units of D-3 daily offers the greatest prevention against fractures.
Not many foods contain vitamin D, but ones that do or foods that are fortified with the vitamin typically provide vitamin D-3. Cod liver oil is one of the richest sources, giving you 1,360 international units in just 1 tablespoon. You'll get over 565 international units from 3 ounces of broiled swordfish, or nearly 450 IUs from the same amount of cooked sockeye salmon. If you have a whole egg, you'll ingest more than 40 international units of vitamin D -- it's found in the yolk. Fortified milk has between 115 and 125 IUs per cup, fortified yogurt has around 80 international units in 6 ounces, while fortified breakfast cereals with 10 percent of the daily value have 40 IUs of vitamin D per serving.
Getting Too Much
No matter which type of vitamin D you take from supplements or get from food, you need to stay below the tolerable upper intake level, known as the UL. This amount, which is 4,000 international units or 100 micrograms, is the most vitamin D you can have before it starts becoming problematic. Too much vitamin D can cause a drop in weight, excessive urination and an abnormal heart rhythm. In more severe cases, going over the UL raises your calcium levels, explains the Office of Dietary Supplements. This can damage blood vessels, tissues throughout your body and even your kidneys. If you decide to take a D-3 supplement, add up the vitamin D from all sources you eat to make sure you're not going over the UL.