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....Vitamin A deficiency is common in developing countries but rarely seen in the United States.
....Thiamin deficiency - is generally found in countries where the dietary staple is white rice.
....Scurvy is caused by a dietary deficiency of vitamin C. Although scurvy is uncommon, it may occur in malnourished individuals.
....Indications to vitamin D reception are: hypo-and avitaminosis D (rickets), crises of bones, an osteoporosis, senile and against reception corticosteroids,

In November, researchers from the Physicians' Health Study-II reported that neither vitamin E nor vitamin C reduced the odds of major cardiovascular problems. A few days later, researchers said that more data from the study showed those vitamins didn't help stave off cancer, either. And another recent study found that supplemental B vitamins, including folic acid, didn't lower the risk of breast or other cancers. Because much of our food is fortified with nutrients, once common deficiency diseases such as scurvy and rickets (caused by a lack of vitamin C and D, respectively) have nearly disappeared. Researchers generally believe that with a few exceptions, like pregnant women or the elderly, most people don't need supplements. Vitamin C (for example) is Vitamin C, whether it was present in a fruit, eaten in red meat, or produced via two-step galactose fermentation in a lab. There is no difference.

Vitamins are the low-molecular organic compounds necessary for vital activity which synthesis in the organism is absent or is circumscribed. In 1880 it was found out, that natural milk in contrast to artificial, any ingredients necessary for maintenance of normal vital activity of the organism. Later, in 1912 the Polish scientist K.Funko had separeted out from rice bran substance which cured disease beriberi (arises at shortage of vitamin B1). This substance was named vitamin (from vita - life + amin because the substance contained an amides). Though many vitamins discovered later have no amides, the name "vitamin" for this class of substances was preserved till now. Vitamins come in a human body with nutriment or are synthesised by the microflora, then they are soaked up in blood and join in the compound of enzymes. ...

It is an excellent question! It is so, that people cannot synthesise vitamins, or they are developed in insufficient quantities for functioning of an organisms. They can be received as a result of special diets or complex sources, hence, vitamins are natural nutrients. The human organisms cannot produce many vitamins which are acutely necessary for the healthy life. If they are absent because of a diet, or are not digested by an organism, it threatens with development of illnesses, the person will feel tired, irritable, and our thinking will be not exact enough. ...