Vitaminsare organic substances that must be provided in small quantities by the environment (usually the diet) and are generally classified in two categories: the water-soluble and the fat-soluble vitamins. These small organic molecules cannot be made in adequate amounts by the body but are required for normal metabolism.
Water Soluble Vitamins
Water-soluble vitamins consist of members of the vitamin B complex and vitamin C. They are generally found together in the same foods with the exception of B12 which is present only in meat and dairy foods. The others are found in whole grain cereals, legumes, leafy green vegetables, and fruits. The water-soluble vitamins generally function to assist the activity of important enzymes such as those involved in the production of energy from carbohydrates and fats. They are often referred to as "cofactors". Other roles may be defined with further research. The water-soluble vitamins are not stored to a great extent in the body so frequent consumption is necessary. When present in excess of the body's needs, they are excreted in the urine. Because they are readily excreted, they are generally non-toxic, although symptoms have been reported in some individuals taking megadoses of niacin, vitamin C or pyridoxine. The lack of water soluable vitamins most greatly affects tissues that are growing or metabolizing rapidly such as skin, blood, the digestive tract and nervous system. These molecules present in fruit, vegetables and grains are all unstable in the presence of heat so that processing and cooking methods can greatly affect the amount of vitamin actually available in food.
Vitamin B Complex – The vitamin B complex is traditionally made up of 10 members (listed below) that differ in their biological actions, although many participate in energy production from carbohydrates and fats. They were grouped together into a single class because they were initially isolated from the same sources, liver and yeast.
Thiamine (Vitamin B1) is important for energy metabolism and in the initiation of nerve impulses. A deficiency of thiamine causes a condition known as beriberi. In certain parts of the world where the diet consists largely of polished rice, this condition is frequently seen. In these countries, a deficiency in mothers can cause a deficiency in infants and may lead to death. In the US, thiamine deficiency is most commonly seen in alcoholics, although it can occur in the presence of several diseases. Pregnancy increases thiamine requirements slightly and when a pregnancy is associated with a prolonged period of vomiting and/or poor food intake, thiamine deficiency may result.
The major symptoms of the deficiency are related to the nervous system (i.e. sensory disturbances, muscle weakness, impaired memory) and the heart (i.e. shortness of breath, palpitations, and heart failure). Wernicke’s syndrome is a serious complication of alcoholism and thiamine deficiency that may manifest as impaired muscle coordination, impaired ability to move the eyes, and marked confusion. It may lead to Korsakoff’s psychosis, a chronic disorder in which memory and learning are impaired.
Thiamine is used to treat thiamine deficiency. There are many unproven uses of thiamine including a treatment for poor appetite, canker sores, motion sickness, poor memory, fatigue and as an insect repellant. The RDA for women over 18 years is 1.1 mg; for pregnant women, 1.4 mg; for lactating women, 1.5 mg; and for men over 14 years, 1.2 mg
Riboflavin (Vitamin B2) is important in promoting the release of energy from carbohydrates, fats and proteins. It also aids in maintaining the integrity of red blood cells. Riboflavin deficiency can occur most frequently in people with long-standing infections, liver disease, and alcoholism. A sore throat and sores at the corners of the mouth are generally the first symptoms of a deficiency. This can be followed by a swollen tongue, seborrheic dermatitis, anemia and impaired nerve function. These manifestations are commonly seen in other diseases, including many vitamin deficiencies. The RDA for women over 18 years is 1 mg; for pregnant women, 1.4 mg; for lactating women, 1.6 mg, and for men over 14 years, 1.3 mg.
A deficit of cellular energy metabolism may play a role in migraine headaches. A recent study indicated that high-dose (400 mg/day) riboflavin was effective in decreasing the frequency of migraines. Further studies are needed to confirm this effect. High dose riboflavin can cause a yellow-orange fluorescence or discoloration of the urine.
Nicotinic acid (Niacin, Vitamin B3) is important for the release of energy from carbohydrates and fats, the metabolism of proteins, making certain hormones, and assisting in the formation of red blood cells. Niacin deficiency causes pellagra, a condition that affects the skin (dermatitis), GI tract (i.e. diarrhea, nausea, vomiting and swollen tongue) and nervous system. (i.e. headache, depression, impaired memory, hallucinations and dementia). Frequent causes of a deficiency include a poor diet, isoniazid therapy (used in the treatment of tuberculosis) and carcinoid tumors. Rarely a deficiency can occur in the presence of hyperthyroidism, diabetes mellitus, cirrhosis, pregnancy or lactation.
Dietary niacin and niacin formed within the body from the amino acid tryptophan are converted to niacinamide. Niacinamide (nicotinamide) is the biologically active form of niacin and it may be preferred as a supplement because it lacks the flushing effects of niacin. The RDA for women over 14 years is 14 mg; for pregnant women, 18 mg; for lactating women, 17 mg; and for men over 14 years, 16 mg.
Niacin is used for the treatment of niacin deficiency but at large doses is also used to treat high cholesterol and triglycerides. High doses should only be taken under the supervision of a physician because there is a risk of developing serious side effects such as liver dysfunction. There are also several medical conditions that may be worsened by its use at the high, therapeutic doses. It can cause the release of histamine resulting in increased gastric acid, therefore it is generally not used in the presence of an active peptic ulcer. Large amounts can also decrease uric acid excretion, possibly precipitating a gout attack in people predisposed to this condition, and it can impair glucose tolerance, interfering with blood sugar control in diabetics. In the treatment of high cholesterol the simultaneous use of niacin with drugs that inhibit cholesterol formation, known as the HMG-CoA reductase inhibitors (i.e. Lipitor®, Baycol®, Mevacor®, Zocor® and Pravachol®) increases the occurrence of serious muscle disorders.
Due to common side effects (flushing, nausea, dizziness, itching, low blood pressure), many people do not tolerate high doses of niacin, even though some may lessen in intensity with continued usage.
Pyridoxine (Vitamin B6) is necessary for the proper function of over 60 enzymes that participate in amino acid metabolism. It is also involved in carbohydrate and fat metabolism. A deficiency in adults mainly affects the skin (seborrhea-like lesions around the eyes, nose and mouth), mucous membranes, peripheral nerves and blood forming system. Convulsive seizures may also occur. Deficiencies can manifest in people with kidney disease, cirrhosis, alcoholism, impaired gastrointestinal absorption (malabsorption), congestive heart failure and hyperthyroidism.
The RDA for pyridoxine in women from 19-50 years of age is 1.3 mg; women over 50 years, 1.5 mg; for pregnant women, 1.9 mg; for lactating women, 2 mg; for men 14 to 50 years of age, 1.3 mg; and for men over 50 years, 1.7 mg. Prolonged doses in excess of 200 mg. per day have been associated with neurotoxicity. Pyridoxine may be effective in lowering high levels of homocysteine, a risk factor for heart disease, decreasing the symptoms of premenstrual syndrome, as an adjunct to other treatments for improving behavior in autism, and for reversing some of the side effects of flurouracil in cancer patients. It is also used in treating some metabolic disorders.
Several drugs can increase the pyridoxine requirement, such as hydralazine, isoniazid and oral contraceptives. Simultaneous use of pyridoxine with amiodarone can increase the risk of drug-induced sensitivity to sunlight, and pyridoxine can decrease the effects of phenytoin and phenobarbital.
Pantothenic acid (Vitamin B5) is the precursor to coenzyme A that is vital for the metabolism of carbohydrates, the synthesis and degradation of fats, the synthesis of sterols and the resultant steroid hormones, and the synthesis of many other important compounds. A deficiency has not been seen in humans on a normal diet because it is so widely distributed in foods, however it is often included in multivitamin preparations.
There is insufficient information to establish RDAs for pantothenic acid. The Committee on Dietary Allowances provides provisional recommendations for adults of 4 to 7 mg. per day.
Folic acid (Vitamin B9) plays a major role in cellular metabolism including the synthesis of some of the components of DNA. It is necessary for normal red blood cell formation and adequate intake can reduce damage to DNA.
Folic acid deficiency is a common complication of diseases of the small intestine that interfere with the absorption of folic acid from food and the recycling of folic acid from the liver back to the intestines. Alcoholism can result in folic acid deficiency. Folic acid activity can also be reduced by several drugs including large doses of nonsteroidal anti-inflammatory drugs (NSAIDs), methotrexate, trimethoprim, cholestyramine, isoniazid, and triamterene. The simultaneous ingestion of folic acid supplements may, in theory, interfere with the effectiveness of methotrexate cancer treatments, however their combined use in the treatment of rheumatoid arthritis and psoriasis has resulted in lessened side effects from methotrexate.
Although the anemia that results from folic acid deficiency is not distinguishable than that resulting from B12 deficiency, folic acid deficiency is rarely associated with neurological abnormalities (see Vitamin B12). Excessive doses of folic acid may mask the anemia that results from B12 deficiency, preventing diagnosis of the deficiency and allowing progression of neurological damage.
Adequate folic acid intake is associated with a reduced risk of neural tube birth defects. It is recommended that all women of childbearing age consume at least 400 micrograms of folic acid each day. Folic acid supplements are also used to lower elevated homocysteine levels, a known risk factor for heart disease. Recent studies have suggested that folic acid supplements may be effective in lowering the risk of colon cancer. Topical folic acid formulations are used for gingival hyperplasia that result from phenytoin therapy and for gingivitis associated with pregnancy.
The RDA for folic acid for adults over 13 years, 400 micrograms; for pregnant women, 600 micrograms; and lactating women, 500 micrograms.
Vitamin B12 (Cyanocobalamin) is important for the proper functioning of many enzymes involved in carbohydrate, fat and protein metabolism, synthesis of the insulating sheath around nerve cells, cell reproduction, normal growth and red blood cell formation. It is essential for proper folic acid utilization. A deficiency results in anemia, gastrointestinal lesions and nerve damage. Many drugs can interfere with the absorption of vitamin B12 including drugs commonly used to treat ulcers (such as cimetidine, omeprazole), and drugs used to treat seizures (such as phenytoin and phenobarbital).
A protein called intrinsic factor is secreted by the stomach and is required for vitamin B12 absorption from the lower part of the small intestine. Signs of B12 deficiency often occur in the presence of adequate B12 intake, but result from impaired absorption. Conditions that are associated with this include some gastric surgeries, pancreatic disorders, bacterial overgrowth or intestinal parasites, and damage to the intestinal cells.
The RDA for vitamin B12 for adults is 2.4 micrograms; for pregnant women, 2.6 micrograms; and for lactating women, 2.8 micrograms. Approximately 10 to 30% of people over 50 years of age have difficulty absorbing food-bound vitamin B12, so they should eat foods fortified with the vitamin or take a supplement.
Vitamin B12 and folic acid have a close relationship. A deficiency in either one results in abnormal synthesis of DNA in any cell in which cell division is taking place. Tissues such as the blood forming system are most severely affected, therefore an early sign of deficiency of either vitamin is a type of anemia termed megaloblastic anemia.
Choline is traditionally not a vitamin, however it was identified as part of the vitamin B complex and has several important functions. Choline is a component of many biological membranes and fat transport molecules in the blood. It is able to stimulate the removal of excess fat from the liver. Choline serves as the precursor to many substances including a the transmitter of the parasympathetic nervous system, acetylcholine. Some athletes use choline to delay muscle fatigue because acetylcholine is involved in muscle contraction, but this effect has not been proven. A deficiency is uncommon except among people receiving long-term IV nutrition. It is added to infant formulas to approximate the amount found in human milk.
The Daily Reference Intake (DRI) is 550 mg for adult males and lactating females; 425 mg for adult females; and 450 mg for pregnant females. Oral choline supplements have not been proven to be effective in treating memory loss, Alzheimer’s disease, dementia and schizophrenia.
Inositol is an important part of cell membranes and is part of a signaling mechanism that transmits information from the outside to the inside of cells. Some evidence suggests it participates in the movement of fat out of the liver and intestinal cells, and that it may reverse desensitization of serotonin receptors, however this remains to be confirmed. Although it may be effective in treating panic disorders, depression and obsessive-compulsive disorders, these uses remain to be verified.
A dietary need for inositol has not been established, probably due to its production by gut bacteria, the existence of tissue stores following absorption from food, and possible synthesis in some organs. It may be added to infant formulas to approximate the content of human milk.
Biotin has an important role in carbohydrate and fat metabolism. It can be synthesized by gut bacteria and recycled. A deficiency rarely occurs in humans. If raw egg whites are consumed in large quantities, a biotin deficiency can occur. Signs of a deficiency include dermatitis, muscle pain, loss of appetite, slight anemia, an inflamed tongue, and weakness. There is no RDA for biotin.
Vitamin C (Ascorbic Acid) has many important functions in the body. It is a powerful antioxidant, protecting against oxidative damage to DNA, membrane lipids and proteins. It is involved in the synthesis of numerous substances such as collagen, certain hormones and transmitters of the nervous system, lipids and proteins. It is necessary for proper immune function, a fact that has led many to use vitamin C to prevent or treat colds, although this has not been supported by current studies. It may, however, shorten or reduce the severity of a cold.
Vitamin C deficiency causes scurvy that is characterized by capillary fragility resulting in bruising and hemorrhaging, inflammation of the gums, loosening of the teeth, anemia and general debility that can lead to death. The RDA for adults 15 years and older is 60 mg; for pregnant women, 70 mg; and for lactating women in the first six months, 95 mg decreasing to 90 mg for the second six months. There may be increased vitamin C requirements for people taking estrogens, oral contraceptives, barbiturates, tetracyclines, aspirin and for cigarette smokers. Large doses of vitamin C can interfere with many laboratory tests. Side effects from large doses include nausea, vomiting, heartburn, abdominal cramps, headache and diarrhea.
Diets containing 200mg or more of vitamin C from fruits and vegetables are associated with a lower cancer risk, particularly for cancers of the colon, lung, mouth, esophagus and stomach. The consumption of dietary supplements have not been shown to have the same effect. It may block the formation of N-nitrosamines, cancer-causing agents from certain foods. Ascorbic acid alone does not appear to prevent heart disease, however the combined use with vitamin E may reduce the risk of heart disease
Fat-Soluble Vitamins
Fat soluble vitamins are found in meats, liver, dairy, egg yolks, vegetable seed oils, and leafy green vegetables. Some foods such as milk and margarine are artificially fortified with vitamins A and D. These vitamins are metabolized along with fat in the body and require fat for absorption in the gut. The fat-soluble vitamins may be stored in large amounts, and this gives them the potential to cause toxicity if consumed in high amounts. Deficiencies are rare in adults but may be seen in children. Megadosing of fat soluable vitamins, except where indicated by a medical professional, is potentially dangerous and should be avoided. Two of the fat-soluble vitamins, A and D, have hormone like actions, causing specific cells to increase or decrease the expression of certain genes.
Vitamin A (retinol) plays a vital role in the functioning of the retina, growth and maturation of the cells lining the inner and outer surfaces of the body (the epithelial cells), growth of bone, reproduction and embryonic development. Several compounds have vitamin A activity and they are referred to as retinoids. They function with certain carotenoids to protect against the development of certain cancers and to enhance immune function. Carotenoids are substances that are consumed in the diet, some of which are converted to vitamin A. They may also have antioxidant activity.
Deficiency
Vitamin A deficiency causes night blindness, a condition in which vision is impaired in dim light. Dryness and ulceration of the eyes, skin eruptions and dryness, abnormal cells of the mucous membranes, urinary stones, and impaired taste and smell also characterize the deficiency. Many children in developing countries have irreversible blindness resulting from vitamin A deficiency. In the US, it occurs more commonly in chronic diseases that affect fat absorption such as pancreatic insufficiency and portal cirrhosis, or following removal of a portion of the stomach. Vitamin A is stored in several sites in the body, so when a deficiency occurs, supplements must be given long enough for these stores to be replenished
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