Vitamins Organic Micronutrients
Vitamins are organic (carbon-containing) substances required in small amounts to regulate various processes within living cells (Table 8.4). Humans need 13 vitamins; of these, four are fat-soluble (A, D, E, and K), and nine are water-soluble (C and the B vitamins; thiamin, riboflavin, niacin, vitamin B-6, folate, vitamin B-12, biotin, and pantothenic acid).
Solubility affects how a vitamin is absorbed, transported, and stored in the body. The water-soluble vitamins are absorbed directly into the bloodstream, where they travel freely. Excess amounts of water-soluble vitamins are generally removed by the kidneys and excreted in urine. Fat-soluble vitamins require a more complex absorptive process. They are usually carried in the blood by special proteins and are stored in the liver and in fat tissues rather than excreted.
Functions of Vitamins Many vitamins help chemical reactions take place. They provide no energy to the body directly but help release the energy stored in carbohydrates, proteins, and fats. Other vitamins are critical in the production of red blood cells and the maintenance of the nervous, skeletal, and immune systems. Some vitamins act as antioxidants, which help preserve the health of cells. Key vitamin.
Vitamin and mineral supplements are popular, but they are not usually necessary for healthy people who eat a balanced diet.
Antioxidants include vitamin E, vitamin C, and the vitamin A precursor beta-carotene. (Antioxidants are described later in the chapter. )
Sources of Vitamins The human body does not manufacture most of the vitamins it requires and must obtain them from foods. Vitamins are abundant in fruits, vegetables, and grains. In addition, many processed foods, such as flour and breakfast cereals, contain added vitamins. A few vitamins are made in certain parts of the body: The skin makes vitamin D when it is exposed to sunlight, and intestinal bacteria make vitamin K. Nonetheless, you still need to get vitamin D and vitamin K from foods.
Vitamin Deficiencies and Excesses If your diet lacks a particular vitamin, characteristic symptoms of deficiency can develop (see Table 8.4). For example, vitamin A deficiency can cause blindness, and anemia can develop in people whose diets lack vitamin B-12. Vitamin deficiency diseases are most often seen in developing countries; they are relatively rare in the United States because vitamins are readily available from our food supply. However, many Americans consume lower-than-recommended amounts of several vitamins. Nutrient intake that is consistently below recommended levels can have adverse effects on health even if it is not low enough to cause a deficiency disease. For example, low intake of folate increases a woman’s chance of giving birth to a baby with a neural tube defect (a congenital malformation of the central nervous system). Low intake of folate and vitamins B-6 and B-12 has been linked to increased heart disease risk. A great deal of recent research has focused on vitamin D, suggesting that vitamin D supplementation can reduce the risk of cardiovascular disease and linking low vitamin D levels to an increased risk of several cancers. As important as vitamins are, however, many Americans consume less-than-recommended amounts of some vitamins.
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Extra vitamins in the diet can be harmful, especially when taken as supplements. Megadoses of fat-soluble vitamins are particularly dangerous because the excess is stored in the body rather than excreted, increasing the risk of toxicity. Even when supplements are not taken in excess, relying on them for an adequate intake of vitamins can be problematic. There are many substances in foods other than vitamins and minerals that have important health effects. Later, this chapter discusses specific recommendations for vitamin intake and when a supplement is advisable. For now, keep in mind that it’s best to get most of your vitamins from foods rather than supplements.
The vitamins and minerals in foods can be easily lost or destroyed during storage or cooking. To retain their value, eat or process vegetables immediately after buying them. If you can’t do this, store them in a cool place, covered to retain moisture either in the refrigerator (for a few days) or in the freezer (for a longer term). To reduce nutrient losses during food preparation, minimize the amount of water used and the total cooking time. Develop a taste for a crunchier texture in cooked vegetables. Baking, steaming, broiling, grilling, and microwaving are all good methods of preparing vegetables.