Vitamins, Water-soluble Health Article

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.