Vitamins, Water-Soluble Health Article

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