"Few foods naturally contain vitamin D, but salmon, tuna, sardines, mackerel, shrimp and fish liver oils are the best sources," Phil Lempert, a food industry analyst, trend watcher and creator of supermarketguru.com. "Vitamin D can also be found in small amounts in beef liver, cheese, egg yolks and some mushrooms."
Other foods -- such as milk, some brands of orange juice, margarine and yogurt, along with some supplements -- have been fortified with vitamin D. Breakfast cereals often contain around 10 percent of the daily recommended intake of vitamin D, Lempert said.
The U.S. Food and Drug Administration recommends at least 400 international units of vitamin D daily, but as much as 2,000 IU is considered safe, Lempert added.
"It seems like every day researchers are finding that vitamin D plays a role in almost every aspect of human metabolism and research suggests the vitamin has a protective effect on cardiovascular events, hypertension, cancer, asthma, the insulin response, bone health and several autoimmune diseases by modulating neuromuscular and immune function and helping to reduce inflammation," Lempert said. "Vitamin D also helps control the cell life cycle keeping good cells and getting rid of cells that are no longer necessary."
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