Vitamins, Water-Soluble

Vitamins, Water-Soluble

Vitamins are essential organic substances that are needed in small amounts in the diet for the normal function, growth, and maintenance of body tissues. Water-soluble vitamins consist of the B vitamins and vitamin C. With exception of vitamin B₆ and B₁₂, they are readily excreted in urine without appreciable storage, so frequent consumption becomes necessary. They are generally nontoxic when present in excess of needs, although symptoms may be reported in people taking megadoses of niacin, vitamin C, or pyridoxine (vitamin B₆). All the B vitamins function as coenzymes or cofactors, assisting in the activity of important enzymes and allowing energy-producing reactions to proceed normally. As a result, any lack of water-soluble vitamins mostly affects growing or rapidly metabolizing tissues such as skin, blood, the digestive tract, and the nervous system. Water-soluble vitamins are easily lost with overcooking.

Thiamin (Vitamin B1)

Thiamin functions as the coenzyme thiamin pyrophosphate (TPP) in the metabolism of carbohydrate and in conduction of nerve impulses. Thiamin deficiency causes beri-beri, which is frequently seen in parts of the world where polished (white) rice or unenriched white flour are predominantly eaten. There are three basic expressions of beriberi: childhood, wet, and dry. Childhood beriberi stunts growth in infants and children. Wet beriberi is the classic form, with swelling due to fluid retention (edema) in the lower limbs that spreads to the upper body, affecting the heart and leading to heart failure. Dry beriberi affects peripheral nerves, initially causing tingling or burning sensations in the lower limbs and progressing to nerve degeneration, muscle wasting, and weight loss. Thiamine-deficiency disease in North America commonly occurs in people with heavy alcohol consumption and is called Wernicke-Korsakoff syndrome. It is caused by poor food intake and by decreased absorption and increased excretion caused by alcohol consumption.

Riboflavin (Vitamin B2)

Riboflavin is stable when heated in ordinary cooking, unless the food is exposed to ultraviolet radiation (sunlight). To prevent riboflavin breakdown, riboflavin-rich foods such as milk, milk products, and cereals are packaged in opaque containers. Riboflavin is a component of two coenzymes—flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD)—that act as hydrogen carriers when carbohydrates and fats are used to produce energy. It is helpful in maintaining good vision and healthy hair, skin and nails, and it is necessary for normal cell growth.

Riboflavin deficiency causes a condition known as ariboflavinosis, which is marked by cheilosis (cracks at the corners of the mouth), oily scaling of the skin, and a red, sore tongue. In addition, cataracts may occur more frequently with riboflavin deficiency. A deficiency of this nutrient is usually a part of multinutrient deficiency and does not occur in isolation. In North America, it is mostly observed in alcoholics, elderly persons with low income or depression, and people with poor eating habits, particularly those who consume highly refined and fast foods and those who do not consume milk and milk products.

Niacin (Vitamin B3)

Niacin exists in two forms, nicotinic acid and nicotinamide. Both forms are readily absorbed from the stomach and the small intestine. Niacin is stored in small amounts in the liver and transported to tissues, where it is converted to coenzyme forms. Any excess is excreted in urine. Niacin is one of the most stable of the B vitamins. It is resistant to heat and light, and to both acid and alkali environments. The human body is capable of converting the amino acid tryptophan to niacin when needed. However, when both tryptophan and niacin are deficient, tryptophan is used for protein synthesis.

There are two coenzyme forms of niacin: nicotinamide adenine dinucleotide (NAD⁺) and nicotinamide adenine dinucleotide phophate (NADP⁺). They both help break down and utilize proteins, fats, and carbohydrates for energy. Niacin is essential for growth and is involved in hormone synthesis.

Pellagra results from a combined deficiency of niacin and tryptophan. Long-term deficiency leads to central nervous system dysfunction manifested as confusion, apathy, disorientation, and eventually coma and death.

Pellagra is rarely seen in industrialized countries, where it may be observed in people with rare disorder of tryptophan metabolism (Hartnup's disease), alcoholics, and those with diseases that affect food intake.

Recommended intake is expressed as milligrams of niacin equivalents (NE) to account for niacin synthesized from tryptophan. High doses taken orally as nicotinic acid at 1.5 to 2 grams per day can decrease cholesterol and triglyceride levels, and along with diet and exercise can slow or reverse the progression of heart disease. The nicotinamide form of niacin in multivitamin and B-complex tablets do not work for this purpose. Supplementation should be under a physician's guidance.

Pantothenic Acid (Vitamin B5)

Pantothenic acid is stable in moist heat. It is destroyed by vinegar (acid), baking soda (alkali), and dry heat. Significant losses occur during the processing and refining of foods. Pantothenic acid is released from coenzyme A in food in the small intestine. After absorption, it is transported to tissues, where coenzyme A is resynthesized. Coenzyme A is essential for the formation of energy as adenosine triphosphate (ATP) from carbohydrate, protein, alcohol, and fat. Coenzyme A is also important in the synthesis of fatty acids, cholesterol, steroids, and the neurotransmitter acetylcholine, which is essential for transmission of nerve impulses to muscles.

Dietary deficiency occurs in conjunction with other B-vitamin deficiencies. In studies, experimentally induced deficiency in humans has resulted in headache, fatigue, impaired muscle coordination, abdominal cramps, and vomiting.

Biotin (Vitamin B8)

Biotin is the most stable of B vitamins. It is commonly found in two forms: the free vitamin and the protein-bound coenzyme form called biocytin. Biotin is absorbed in the small intestine, and it requires digestion by enzyme biotinidase, which is present in the small intestine. Biotin is synthesized by bacteria in the large intestine, but its absorption is questionable. Biotincontaining coenzymes participate in key reactions that produce energy from carbohydrate and synthesize fatty acids and protein.

Avidin is a protein in raw egg white, which can bind to the biotin in the stomach and decrease its absorption. Therefore, consumption of raw whites is of concern due to the risk of becoming biotin deficient. Cooking the egg white, however, destroys avidin. Deficiency may develop in infants born with a genetic defect that results in reduced levels of biotinidase. In the past, biotin deficiency was observed in infants fed biotin-deficient formula, so it is now added to infant formulas and other baby foods.

Vitamin B6

Vitamin B₆ is present in three forms: pyridoxal, pyridoxine, and pyridoxamine. All forms can be converted to the active vitamin-B₆ coenzyme in the body. Pyridoxal phosphate (PLP) is the predominant biologically active form. Vitamin B₆ is not stable in heat or in alkaline conditions, so cooking and food processing reduce its content in food. Both coenzyme and free forms are absorbed in the small intestine and transported to the liver, where they are phosphorylated and released into circulation, bound to albumin for transport to tissues. Vitamin B₆ is stored in the muscle and only excreted in urine when intake is excessive.

PLP participates in amino acid synthesis and the interconversion of some amino acids. It catalyzes a step in the synthesis of hemoglobin, which is needed to transport oxygen in blood. PLP helps maintain blood glucose levels by facilitating the release of glucose from liver and muscle glycogen. It also plays a role in the synthesis of many neurotransmitters important for brain function. This has led some physicians to prescribe megadoses of B₆ to patients with psychological problems such as depression and mood swings, and to some women for premenstrual syndrome (PMS). It is unclear, however, whether this therapy is effective. PLP participates in the conversion of the amino acid tryptophan to niacin and helps avoid niacin deficiency. Pyridoxine affects immune function, as it is essential for the formation of a type of white blood cell.

Populations at risk of vitamin-B₆ deficiency include alcoholics and elderly persons who consume an inadequate diet. Individuals taking medication to treat Parkinson's disease or tuberculosis may take extra vitamin B₆ with physician supervision. Carpal tunnel syndrome, a nerve disorder of the wrist, has also been treated with large daily doses of B₆. However, data on its effectiveness are conflicting.

Folic Acid, Folate, Folacin (Vitamin B9)

Folacin or folate, as it is usually called, is the form of vitamin B₉ naturally present in foods, whereas folic acid is the synthetic form added to fortified foods and supplements. Both forms are absorbed in the small intestine and stored in the liver. The folic acid form, however, is more efficiently absorbed and available to the body. When consumed in excess of needs, both forms are excreted in urine and easily destroyed by heat, oxidation, and light.

All forms of this vitamin are readily converted to the coenzyme form called tetrahydrofolate (THFA), which plays a key role in transferring single-carbon methyl units during the synthesis of DNA and RNA, and in interconversions of amino acids. Folate also plays an important role in the synthesis of neurotransmitters. Meeting folate needs can improve mood and mental functions.

Folate deficiency is one of the most common vitamin deficiencies. Early symptoms are nonspecific and include tiredness, irritability, and loss of appetite. Severe folate deficiency leads to macrocytic anemia, a condition in which cells in the bone marrow cannot divide normally and red blood cells remain in a large immature form called macrocytes. Large immature cells also appear along the length of the gastrointestinal tract, resulting in abdominal pain and diarrhea.

Pregnancy is a time of rapid cell multiplication and DNA synthesis, which increases the need for folate. Folate deficiency may lead to neural tube defects such as spina bifida (failure of the spine to close properly during the first month of pregnancy) and anencephaly (closure of the neural tube during fetal development, resulting in part of the cranium not being formed). Seventy percent of these defects could be avoided by adequate folate status before conception, and it is recommended that all women of childbearing age consume at least 400 micrograms (μg) of folic acid each day from fortified foods and supplements. Other groups at risk of deficiency include elderly persons and persons suffering from alcohol abuse or taking certain prescription drugs.

Vitamin B12

Vitamin B₁₂ is found in its free-vitamin form, called cyanocobalamin, and in two active coenzyme forms. Absorption of vitamin B₁₂ requires the presence of intrinsic factor, a protein synthesized by acid-producing cells of the stomach. The vitamin is absorbed in the terminal portion of the small intestine called the ileum. Most of body's supply of vitamin B₁₂ is stored in the liver.

Vitamin B₁₂ is efficiently conserved in the body, since most of it is secreted into bile and reabsorbed. This explains the slow development (about two years) of deficiency in people with reduced intake or absorption. Vitamin B₁₂ is stable when heated and slowly loses its activity when exposed to light, oxygen, and acid or alkaline environments.

Vitamin B₁₂ coenzymes help recycle folate coenzymes involved in the synthesis of DNA and RNA, and in the normal formation of red blood cells. Vitamin B₁₂ prevents degeneration of the myelin sheaths that cover nerves and help maintain normal electrical conductivity through the nerves.

Vitamin-B₁₂ deficiency results in pernicious anemia, which is caused by a genetic problem in the production of intrinsic factor. When this occurs, folate function is impaired, leading to macrocytic anemia due to interference in normal DNA synthesis. Unlike folate deficiency, the anemia caused by vitamin-B₁₂ deficiency is accompanied by symptoms of nerve degeneration, which if left untreated can result in paralysis and death.

Since vitamin B₁₂ is well conserved in the body, it is difficult to become deficient from dietary factors alone, unless a person is a strict vegan and consumes a diet devoid of eggs and dairy for several years. Deficiency is usually observed when B₁₂ absorption is hampered by disease or surgery to the stomach or ileum, damage to gastric mucosa by alcoholism, or prolonged use of anti-ulcer medications that affect secretion of intrinsic factor. Agerelated decrease in stomach-acid production also reduces absorption of B₁₂ in elderly persons. These groups are advised to consume fortified foods or take a supplemental form of vitamin B₁₂.

Choline

For many years, choline was not considered a vitamin because the body makes enough of it to meet its needs in most age groups. However, research now shows that choline production in the body is not enough to cover requirements. Choline is not considered a B vitamin because it does not have a coenzyme function and the amount in the body is much greater than other B vitamins. Choline not only helps maintain the structural integrity of membranes surrounding every cell in the body, but also can play a role in nerve signaling, cholesterol transport, and energy metabolism. An "adequate intake" is 550 milligrams per day for men and 425 milligrams per day for women. Choline is widely found in foods, so it is unlikely that a dietary deficiency will occur.

Vitamin C (Ascorbic Acid)

In 1746, James Lind, a British physician, conducted the first nutrition experiment on human beings in an effort to find a cure for scurvy. However, it was not until nearly 200 years later that ascorbic acid, or vitamin C, was discovered. Vitamin C participates in many reactions by donating electrons as hydrogen atoms. In a reducing reaction, the electron in the hydrogen atom donated by vitamin C combines with other participating molecules, making vitamin C a reducing agent, essential to the activity of many enzymes. By neutralizing free radicals, vitamin C may reduce the risk of heart disease, certain forms of cancer, and cataracts.

Vitamin C is needed to form and maintain collagen, a fibrous protein that gives strength to connective tissues in skin, cartilage, bones, teeth, and joints. Collagen is also needed for the healing of wounds. When added to meals, vitamin C increases intestinal absorption of iron from plant-based foods. High concentration of vitamin C in white blood cells enables the immune system to function properly by providing protection against oxidative damage from free radicals generated during their action against bacterial, viral, or fungal infections. Vitamin C also recycles oxidized vitamin E for reuse in cells, and it helps folic acid convert to its active form, (THF). Vitamin C helps synthesize carnitine, adrenaline, epinephrine, the neurotransmitter serotonin, the thyroid hormone thyroxine, bile acids, and steroid hormones.

A deficiency of vitamin C causes widespread connective tissue changes throughout the body. Deficiencies may occur in people who eat few fruits and vegetables, follow restrictive diets, or abuse alcohol and drugs. Smokers also have lower vitamin-C status. Supplementation may be prescribed by physicians to speed the healing of bedsores, skin ulcers, fractures, burns, and after surgery. Research has shown that doses up to 1 gram per day may have small effects on duration and severity of the common cold, but not on the prevention of its occurrence.

SEE ALSO VITAMINS, FAT-SOLUBLE.

Kiran B. Misra

Bibliography

Insel, Paul; Turner, Elaine R.; and Ross, Don (2002). Nutrition. Sudbury, MA: Jones and Bartlett.

Wardlaw, Gordon M.; Hampl, Jeffrey S.; and Disilvestro, Robert A. (2004). Perspectives in Nutrition, 6th edition. New York: McGraw-Hill.

Whitney, Eleanor Noss, and Rolfes, Sharon Rady (2002). Understanding Nutrition, 9th edition. Belmont, CA: Wadsworth/Thomson Learning.