Vitamins are organic compounds (compounds made up of carbon atoms bonded to hydrogen atoms) that are necessary in small amounts for good health. The body can break vitamins down, but it cannot produce them. Vitamins have to be supplied in the diet. Unlike carbohydrates, fats, and proteins, vitamins yield no energy. Instead, some serve as catalysts that enable energy nutrients to be digested, absorbed, and metabolized. Some vitamins also interact with minerals. For example, vitamin C facilitates iron absorption, vitamin D improves calcium absorption, and thiamine requires the mineral magnesium to function efficiently.Diets deficient in the RDA of vitamins may impair the physiological processes of the body and lead to deficiency diseases. The dietary allowances have been defined as the amounts of vitamins needed to prevent outright deficiency diseases, such as scurvy. This definition may soon change. The food and nutrition board of the national research council is considering revising the RDA by increasing the consumption of certain nutrients to reduce the risk of chronic disease. One major advantage of a varied diet is that it helps ensure consumption of the right amount and balance of those vitamins related to many of the major chronic diseases.
Vitamins are grouped into two categories water soluble and fat soluble. Water soluble vitamins include vitamin B complex and vitamin C. They are present in the watery components of food, distributed in the fluid components of the body, excreted in the urine, needed in frequent small doses, and are unlikely to be toxic, except when taken in megadoses (very large quantities).
Fat soluble vitamins include vitamins A, D, E, and K and are found in the fat and oily parts of food. Because they cannot be dissolved and absorbed in the bloodstream, these vitamins must be absorbed into the lymph with fat and transported in lipoproteins. When consumed in excess of the body’s need, fat soluble vitamins are stored in the liver and fat cells. Their storage makes it possible to survive for months or years without consuming them. At least three of the fat soluble vitamins (A, D, and K) may even accumulate to toxic levels. Megadoses of these vitamins should be avoided.
Antioxidant Vitamins
Three vitamins are classified as antioxidants vitamins C, E, and betacarotene . Antioxidants are protective substances because they help neutralize the activity of free radicals. Free radicals are naturally produced chemicals that arise from normal cell activity. Whenever the body uses oxygen or is exposed to a toxin, such as cigarette smoke, it forms free radicals. These unstable chemicals can damage cells throughout the body. They may damage a cell’s DNA in ways that lead to cancer, may interact with cholesterol in the bloodstream and form oxidized LDL may cause cataracts and rheumatoid arthritis, and may be a factor in the physiological changes associated with the aging process. Anything that interferes with the destructive effects of free radicals offers a health advantage. Vitamins C, E, and beta carotene are believed to prevent the oxidation of cholesterol and other molecules linked to the diseases mentioned previously, most notably heart disease and cancer. A common cooking practice illustrates this antioxidant effect: some foods like bananas, peaches, apples, and potatoes quickly turn brown when exposed to air. However, when such foods are dipped in lemon or orange juice, the vitamin C in the juice acts as an antioxidant and prevents sues of your body. It is required in the breakdown and absorption of some amino acids, other minerals (such as iron), and the formation of some hormones. It may also help the immune system prevent infections. As an antioxidant it may playa role in prevention of atherosclerosis and some forms of cancer.
Contrary to popular opinion, vitamin C does not prevent the common cold. Scientifically controlled studies reveal no difference in the incidence of colds when comparing vitamin C users with nonusers. On a positive note, however, some studies suggest the symptoms of a cold may be shortened by a day or two.
The RDA of vitamin C is 60 mg, 70 mg during pregnancy, and 90 mg during lactation. For smokers the RDA is 100 mg because smoking appears to destroy some vitamin C. Studies designed to test for antioxidant benefits typically set the dosage level at 500 mg. Megadoses (2000 mg) of vitamin C offer little benefit to the body and may be harmful. As a water soluble vitamin, doses in excess of the body’s requirement are excreted through the kidneys. In other words the body can only absorb so much. Side effects of large doses are diarrhea and abdominal discomfort. For those individuals who absorb excess iron, supplements of vitamin C could be dangerous. Large intakes may also produce errors in the results of some diagnostic tests (for example, hemoccult test, glucose in urine test).
Good sources of vitamin C are citrus fruits and green vegetables, including lemons, grapefruit, limes, oranges, tomatoes, broccoli, and green peppers .
Beta-carotene (provitamin A)
Vitamin A comes in several forms, such as preformed retinoids found in animal products that are ready for use by the body and carotenoids found in plants. Carotenoids are called provitamin A because they must first be converted to the retinoid form if they are to be used for vitamin A functions. More than 600 carotenoids are found in nature; 50 of them have potential vitamin A activity. The most potent form is beta carotene. The retinoids and carotenoids combine to form what is generally referred to as vitamin A.
Before the body can use vitamin A, it must first be separated from beta carotene. Beta carotene is a precursor of vitamin A. The process of freeing vitamin A from beta carotene prevents oxygen from releasing free radicals. This is called an antioxidant effect and is associated with the health benefits mentioned previously.
Vitamin A is needed for normal vision, skin maintenance, bone and teeth development, growth, and reproduction. It also plays a role in the DNA and RNA functions of cells.
The RDA of vitamin A is 1000 retinol equivalents (RE) for men and 800 RE for women. Dairy products, fish, and butterfat are good sources of preformed vitamin A. Beta carotene is plentiful in pigmented vegetables and fruit, such as carrots, sweet potatoes, oranges, and leafy green vegetables . Preformed vitamin A (the retinoid component) is fat soluble, can be stored in the body’s fat cells, and can be toxic in megadose quantities. This has important implications for women who are of childbearing age and/or are considering pregnancy. Vitamin A, when taken during pregnancy, is helpful in cell differentiation and is an essential nutrient in a baby’s development. Excessive amounts of vitamin A during early pregnancy, however, may impose risks of birth defects. A study conducted at boston university school of medicine involving 22,748 pregnant women found that the babies of women who consumed more than 10,000 international units (IU) (approximately 3000 retinol equivalents) of vitamin A each day, or nearly four times the recommended amount, were more likely to be born with malformations of the head, heart, brain, and spinal cord. The amounts that place babies at risk are found in a single pill in some vitamin supplements and can be readily obtained if more than one vitamin supplement is taken each day. Researchers concluded the study by urging women to avoid taking supplements that exceed 4000 to 8000 IU (1200 to 2400 RE) daily, the amount commonly found in prenatal multivitamins.
Beta carotene, on the other hand, is not associated with an increased risk of birth defects. For most people it would be nearly impossible to get too much beta carotene because it is water soluble. The major side effect of excessive consumption of beta-carotene is orange discoloration of the skin.
Vitamin E
Vitamin E is a fat-soluble vitamin found in vegetable oil, margarine, meats, nuts, and whole grain cereals and breads . The RDA is 10 mg (15 IU) for men and 8 mg (12 IU) for women. antioxidant studies typically administer 200 to 400 IU of vitamin E supplements. Vitamin E plays a role in the formation of red blood cells and maintenance of nervous tissues and aids in absorption of vitamin A. Claims that vitamin E improves the skin, heals scars, prevents stretch marks, slows the aging process, and increases fertility are more folklore than fact.
The strongest evidence of the antioxidant benefits of vitamins exists for vitamin E,17 A review of 21 epidemiological studies measuring vitamin E in either diet or blood demonstrated cancer fighting benefits. Also, two large scale studies involving 87,000 women and 40,000 men respectively found that vitamin E supplements (100 to 250 IU) for at least 2 years reduced heart disease risk by 40%.18 Its role as an antioxidant in combating heart disease and cancer has increased its popularity, and it is approaching vitamin C as the nation’s favorite supplement. Still the scientific community has not reached a consensus opinion on the use of vitamin E supplements. Some studies report contradictory findings. Also, some experts raise some of the same questions and offer the same criticisms as those for the betacarotene-lung cancer study mentioned earlier. Consequently, experts are divided in their opinion about vitamin E supplements. Some recommend supplements on the basis that there is little to lose and much to be gained; others warn of the dangers of tampering with the body’s biochemistry. It is likely to take several years before a strong consensus emerges.
Folate, Vitamin B6, Vitamin B12
Folate, previously referred to as folacin and folic acid, is a part of the vitamin B complex and combines with vitamins B6 and B12 to form parts of DNA and RNA and to make heme the iron containing protein in red blood cells. They also assist in the metabolism of amino acids. Vitamins BG and B 12 are plentiful in foods, and few people need to worry about deficiencies with the possible exception of strict vegetarians. Good sources of these vitamins are meat products, dairy products, eggs, spinach, whole wheat bread, and breakfast cereals.
Folate, as its name implies, is found in foliage leafy vegetables, such as lettuce and spinach. It is also found in citrus fruits, whole grain bread, and cereals. Of the three B vitamins mentioned here, folate is the one in which Americans are most likely to fall short.
The RDA for B6 is 2 mg for men and 1.6 mg for women. For B12 the RDA is 2 micrograms (mcg) for men and women. Folate RDA is 200 mcg for men and 180 mcg for women. Because folate has been associated with a reduction in the chances of neurological birth defects, such as spina bifida, a woman planning a pregnancy may be advised by her physician to eat foods rich in folate and possibly to take a 400 mg supplement.
Current interest in folate supplements was sparked by recent studies that demonstrated that people whose blood levels were low in folate had high homocysteine levels. Homocysteine is an amino acid that plays a role in the formation of two other amino acids, cysteine and methionine. To work properly, these amino acids require three B vitamins folate, B6′ and B12 . If these vitamins are in short supply, homocysteine levels might rise. High homocysteine levels are thought to increase the risk of heart disease. The compound has also been implicated in several other diseases, including cancer, diabetes mellitus, and neurological disorders. In a harvard study, epidemiologists checked homocysteine levels in frozen blood samples drawn several years ago from men participating in an ongoing study. Compared with healthy subjects, the 271 men who had gone on to have heart attacks had much higher homocysteine readings. A follow-up study at the university of alabama, birmingham compared 100 men who had been diagnosed with heart disease with 100 men who had not. The heart patients, like those in the harvard study and more than a dozen other studies, had substantially higher levels of homocysteine. In all of these studies the high levels of homocysteine were associated with a shortage of folate.
The precise mechanism for the association between homocysteine levels and heart disease isn’t clear. Animal studies show that injections of homocysteine can scar artery walls, triggering plaque deposits and blocked arteries. It is also speculated that homocysteine combines with low density lipoproteins to exacerbate arterial damage. Whatever the reason, one thing appears clear high homocysteine levels are associated with low folate levels. In the absence of folate, along with vitamins B6 and Bw homocysteine concentrations circulate unabated. There is speculation from the scientific community that during the next several years homocysteine levels might replace cholesterol as a major risk factor of heart disease.
The best way to lower homocysteine concentrations is to consume enough B vitamins. Emphasis should be on folate, which is more effective than B6 and B 12 in reducing elevated homocysteine levels. Getting the RDA of folate is easily achieved by following the food guide pyramid. Megadoses of folate should be avoided to prevent the possibility of a false negative for anemia (too few blood cells) caused by a vitamin B 12 deficiency. If folate supplements are taken, tell your physician so that the appropriate tests can be ordered.
Tags:amino acids, beta carotene, folate, Health Food Plan, heart disease, homocysteine levels, minerals, nutrient of food vitamins