Vitamins; Introduction, absorption and functions

 

Vitamins: Essential Dietary Components

Vitamins are essential, organic substances needed in small amounts in the diet. They are not a source of energy. Instead, they aid in energy metabolism as well as in the growth, development, and maintenance of body tissues. 

During the first half of the 20th century, scientists identified each of the 13 vitamins now recognized as essential. For the most part, as the vitamins were discovered, they were named alphabetically: A, B, C, D and E. Later, some substances originally classified as B-vitamins were dropped from the list because they were shown to be non-essential substances. The B-vitamins originally were thought to have a single chemical form but turned out to exist in many forms. Thus, the label "vitamin B" now comprises 8 B-vitamins.

Vitamins A, D, E and K dissolve in organic solvents, such as ether and benzene, and are referred as fat-soluble vitamins. The B-vitamins and vitamin C, in contrast, dissolve in water and referred as water-soluble vitamins.

Vitamins are indispensable in human diets because they either cannot be synthesized in the body at all or are synthesized in insufficient quantities. However, a substance does not qualify as a vitamin merely because the body can't make it. When vitamin intake is insufficient to meet needs, a deficiency develops, accompanied by a measurable decline in health. If the deficiency is not in advanced stages, the deficiency and related symptoms can be alleviated by increased intakes of the vitamin.

In addition to preventing deficiency diseases, a few vitamins have been useful as pharmacological agents in treating several non-deficiency conditions. These treatments often require the administration of megadoses, amounts much higher than typical human needs for the vitamin. For example, megadoses of a form of niacin can be used as part of blood-cholesterol-lowering treatment for certain individuals.

Foods of both plant and animal origin supply vitamins in the diet. Dietary supplements also can provide needed vitamins. Whether vitamins in supplements are isolated from food or synthesized in a laboratory, these vitamins are usually similar chemical compounds and generally work equally well in the body. Contrary to claims in the health-food literature, "natural" vitamin supplements isolated from foods are, for most part, no more healthful than those synthesized in a laboratory. However, vitamins consumed in foods as part of a varied diet may be more beneficial than vitamins taken separately as dietary supplements. Because some vitamins exist in several related forms that differ in chemical or physical properties, it is important to consume enough vitamins in the forms the body can use and in amounts that are in balance with other nutrients in the diet.

Absorption of Vitamins:

Fat-soluble vitamins are absorbed along with dietary fat. Thus, adequate absorption of fat soluble vitamins depends on efficient use of bile and pancreatic lipase in the small intestine to digest dietary fat and adequate intestinal absorption. Under optimal conditions, about 40 to 90% of the fat-soluble vitamins are absorbed when they are consumed in recommended amounts. In contrast, absorption of  B vitamins and vitamin C typically ranges from 90 to 100% and occurs in the small intestine independent of dietary fat.

Malabsorption of Vitamins:

Vitamins consumed in food must be absorbed efficiently from the small intestine to meet body needs. If the absorption of a vitamin is decreased, a person must consume larger amounts of it to avoid deficiency symptoms. For example, fat malabsorption may cause poor absorption of fat-soluble vitamins. Alcohol abuse and certain intestinal diseases also can lead to malabsorption of some B-vitamins. Individuals with these diseases usually require vitamin supplements to prevent deficiencies.

Digestion of Vitamins:

1. All Vitamins:

Digestive processes in the stomach begin the release of vitamins from food. Digestive enzymes produced by the pancreas aid in the release of vitamins from food.

2. Fat-Soluble Vitamins only:

Bile produced in the liver and stored in the gallbladder aids in fat-soluble vitamin absorption. Fat-soluble vitamins are absorbed in the small intestine, along with dietary fat, and carried by chylomicrons into the lymphatic circulation.

3. Water-Soluble Vitamins only:

Water-soluble vitamins are absorbed in the small intestine and released directly into the blood.

4. Vitamin K only:

Small amounts of vitamin K are made by bacteria in the ileum of the small intestine and in the large intestine.

Transport of Vitamins:

Once absorbed, fat-soluble vitamins are packaged for transport through the lymphatic system and delivered by the bloodstream to target cells throughout the body in a manner similar to that for dietary fats __ namely, by way of chylomicrons and other body lipoproteins. Recall that, as a chylomicron circulates, much of its triglyceride content is removed by body cells. The remaining, the remnant __ is taken up by the liver. This remnant contains the fat-soluble vitamins absorbed from the diet. The liver then "repackages" fat-soluble vitamins with new proteins for transport in the blood, or it stores them in adipose tissue or the liver for future use. In contrast to the fat-soluble vitamins, water-soluble vitamins are delivered directly to the bloodstream and distributed throughout the body.

Storage of Vitamins in the Body:

With the exception of vitamin K, fat-soluble vitamins are not readily excreted from the body. Instead, they are often stored in the liver and adipose tissue. In contrast, most water-soluble vitamins are excreted from the body quite readily, resulting in limited stores. Two exceptions are vitamin B-12 and vitamin B-6, which are stored to a greater extent than the other water-soluble vitamins. Because of the limited storage of many vitamins, they should be consumed daily. However, the signs and symptoms of a deficiency usually do not occur until the vitamin is lacking in the diet for at least several weeks and body stores are essential depleted. Thus, an occasional lapse in dietary intake of most vitamins is not a serious health concern in otherwise healthy individuals.

Vitamin Toxicity:

Although the toxic effects of an excessive intake of any vitamin is theoretically possible, toxicity from the fat-soluble vitamins A and D is the most likely to occur. However, these vitamins are unlikely to cause toxicities unless taken in amounts at least 5 to 10 times greater than the DRI guidelines. Because the daily use of balanced multivitamin and mineral supplements usually supplies less than twice the Daily Value of the components, this practice is unlikely to cause toxic effects in adults.