Nowadays, various health care products have been developed in the market. More and more people have made use of these products to supplement various nutrients. One of the common nutrients supplemented by people is vitamins. However, many people are not clear about the variety and the functions of vitamins. In fact, blind supplementation of vitamins and excessive vitamins can seriously endanger the human health. 

Vitamin is a kind of essential nutrient needed by the human body to conduct life activities. It has three obvious features: vitamins are only contained in natural foods; vitamins can not be synthesized inside the human body; and vitamins can not provide energy for the life activities.

Theoretically, people can get vitamins they need just by balanced diet. However, because of food processing, cooking styles and dietary habits, people can get more vitamins or fewer vitamins than they really need. Therefore, some special groups should supplement vitamins by vitamin tablets or some other ways. For example, people who are choosy about the foods or have dinner irregularly should supplement some vitamins in everyday life. But, people should supplement vitamins under the guidance of the doctors to know clearly about the amount of vitamins they need.

Vitamins can be divided into -soluble ones and fat-soluble ones. Water-soluble vitamins can be discharged from the human body by urination. While fat-soluble vitamins like and can not discharged from the human body by urination and excessive fat-soluble vitamins can be accumulated inside the human body to cause poisoning. Vitamin A can maintain the normal eyesight reactions and the normal functions of the bones and epithelial tissues. It is mainly contained in milk, eggs, animal livers, cod-liver oil, dark-green vegetables and fruits. Excessive vitamin A can decalcify the bones, reduce the appetite and cause some other poisoning symptoms. Vitamin D can accelerate the absorption of calcium and further maintain the normal bones. However, excessive vitamin D can cause anorexia, nausea, emesis and myalgia.

Although most of the vitamins are nonprescription drugs, it is better for people to eat vitamin tablets under the guidance of the doctors. Generally speaking, people should supplement several kinds of vitamins and minerals rather than single kind of vitamin. Vitamins should put in some shady and dry places and be taken during the period of validity. If people have to take some other medicines when they take vitamins, they should ask the doctors for advice. Some kinds of vitamins can not be eaten with some other medicines. For example, vitamin E can not be eaten with blood thinner.