Vitamin D deficiency is common in winter, with earlier sunsets and weaker rays, and few foods are sources of vitamin D, a U.S. food expert says. Phil Lempert, a food industry analyst, trend watcher and creator of the Web site supermarketguru.com, said vitamin D refers to a group of fat-soluble vitamins that can be obtained several ways -- from exposure to sunlight (ultraviolet B), by consuming certain foods and by taking supplements. In the summer months, most people meet their vitamin D needs through planned sun exposure like sunbathing, or unintentionally from exercising outdoors in a T shirt. Direct sun exposure for 15 minutes three times a week is thought to keep the body\'s vitamin D stores at healthy levels, Lempert said. \"Few foods naturally contain vitamin D. Salmon, tuna, sardines, mackerel, shrimp and fish liver oils are the best sources. Vitamin D can also be found in small amounts in beef liver, cheese, egg yolks and some mushrooms,\" Lempert said in a statement. \"Other foods have been fortified with vitamin D, thus do not naturally contain the vitamin, and include milk -- cow, soy and rice -- some brands of orange juice, margarine, and yogurt. Breakfast cereals often contain around 10 percent of the daily recommended intake of vitamin D as well.\" The Food and Drug Administration recommends at least 400 International Units of vitamin D daily and about 2,000 IU is considered safe.