Vitamins

Vitamins

Vitamins

Vitamins are essential components of the diet that are not chemically members of any of the major categories of foodstuffs (fats, carbohydrates, proteins), which the human body is incapable of making in quantities sufficient for its normal requirements. Many populations in the world are subject to diets deficient in one or more of the vitamins. In Western societies gross vitamin deficiencies are rare, but older people make up one of the population groups in which they can occur. Deficiencies also have to be considered as possibilities in patients with severe bowel disease that prevents normal nutrition or absorption of nutrients (Brewer, 2002).

Vitamins are divided into the fat-soluble vitamins (A, D, E, and K), the water-soluble vitamins of the B complex, and vitamin C. Vitamin A precursors occur in leaves and vegetables. On a worldwide basis, vitamin A deficiency is common. It is involved in the integrity of epithelial tissues, is an important cause of blindness in children, and predisposes to cancer, particularly of the esophagus. Vitamin A deficiency is rare in Western populations but can be a cause of night blindness as it is involved in the synthesis of the visual pigments in the eye. Excess of vitamin A is poisonous and produces a characteristic syndrome of headache, flushing, hypertension, and flaking of the skin. This is sometimes seen in people who unwisely eat polar bear or shark liver, which contain vitamin A in high concentrations (Kiple, 2000).

Humans obtain vitamin D from two sources. One form is present in the diet, particularly in oily fish, but most is synthesized in the skin when ultraviolet light from the sun converts 7-dehydrocholest-erol into cholecalciferol, vitamin D3. Vitamin D formed in the skin or absorbed from the diet is transported to the liver, where it is converted into 25-hydroxyvitamin D; then a second transformation in the kidney produces 1,25-hydroxyvitamin D, which is the most active form of the vitamin. Vitamin D deficiency can arise through lack of exposure to the sun and dietary deficiency. Some drugs (such as some antiepileptics) induce enzymes in the liver that break down the 25-hydroxyvitamin D, and damage to the kidney can prevent the formation of the most active form of the vitamin. All these factors can be relevant to older people. Gross vitamin D deficiency in old age produces the syndrome of osteomalacia, which is characterized by failure of bone to calcify properly; by weakness of the muscles, particularly the proximal muscles of the legs; generalized aches and pains; and psychiatric changes, particularly depression (Heart Outcomes Prevention Evaluation Study Investigators, 2000).

Roles and Importance of Different Vitamins for Human

Vitamin E

Vitamin E is found in plant oils and green leaves. Dietary deficiency sufficient to cause identifiable disease is extremely rare. The main function of the vitamin in the body is thought to be an antioxidant protective action on cell membranes, but so far there is no convincing evidence of benefits from vitamin E in retarding aging or preventing vascular disease (Heart Outcomes Prevention Evaluation Study Investigators, ...