Biomedical Hypertexts Glossary

Vitamins: Introduction and Index


Vitamins are organic molecules that are necessary for normal metabolism in animals, but either are not synthesized in the body or are synthesized in inadequate quantities. Consequently, vitamins must be obtained from the diet. Most vitamins function as coenzymes or cofactors. Deficiency states are recognized for all vitamins, and in many cases, excessive intake also leads to disease.

Fat-Soluble Vitamins

Names Major Sources Comments
Vitamin A
(retinol)
Present in many animal tissues, especially fish and liver. Carotinoids in green plants serve can be converted to vitamin A following ingestion. Necessary for a broad range of bodily function, including production of vision pigments, resistance to infectious agents and maintenance of health in many epithelial cells. Disease results from both deficiency and excess.
Vitamin D
(cholecalciferol)
Synthesized in the skin when exposed to sunlight (and thus not a true vitamin). Also present at low concentration in some natural foods, and in many artificially-fortified food products. A steroid hormone. Major effect is to facilitate absorption of calcium from the intestine, and thereby assist in maintaining calcium homeostasis. Receptors are present in most cells and it likely has many additional effects.
Vitamin E
(tocopherol)
Vegetable oils, leafy green vegetables and whole grains. A family of molecules that function as antioxidants, particularly to prevent oxidation of unsaturated fatty acids and maintain the integrity of cell membranes. Deficiency can lead to reproductive function, leading to the nickname of "antisterility vitamin".
Vitamin K Majority is synthesized by bacteria in the large intestine. Dietary sources include the photosynthetic (green) parts of plants. Necessary for formation of several blood-clotting factors in the liver, and deficiency leads to bleeding disorders.

Water-Soluble Vitamins

Names Major Sources Comments
Vitamin C
(Ascorbic acid)
Present in fruits and vegetables. Rich sources include citrus fruits, strawberries, tomatoes and leafy green vegetables. Most animals can synthesize ascorbic acid; those that cannot include primates (including humans), guinea pigs and (I'm not making this up) Mongolian fruit bats. A major function is synthesis of hydroxyproline, an important component of collagen and, thereby, all connective tissues. Essential for growth of cartilage, bone and teeth, and for wound healing. Deficiency results in the disease scurvy.
Thiamine
(Vitamin B1)
Present in meats, leafy green vegetables, grains and legumes. Functions as the thiamine pyrophosphate, a coenzyme for several enzymes involved in decarboxylation reactions. Required, for example, in the oxidation of glucose due to its role in decarboxylation of pyruvate.
Riboflavin
(Vitamin B2)
Present in wide variety of foods, including milk, meats and grains. Precursor to the coenzymes flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN), which serve as hydrogen carriers in a number of important oxidation-reduction (respiration) reactions within mitochondria.
Niacin
(Nicotinamide)
Present in meats, leafy green vegatables, potatoes and peanuts. Can be synthesized in small amounts within the body from tryptophan. Precursor to the coenzymes nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADPH), which serve as hydrogen carriers in such important processes as glycolysis, Kreb's cycle and oxidative phosphorylation.
Pyridoxine
(Vitamin B6)
Present in meat from mammals, fish and poultry. Also present in a number of vegetables, including potatoes and tomatoes. The precursor to pyridoxal phosphate, a coenzyme for several important reactions involving protein metabolism, including the transamination reactions necessary for synthesis of amino acids.
Pantothenic Acid Present in a broad variety of foods, including grain, legumes, egg yolk, and meat. Also synthesized by intestinal bacteria. The precursor to coenzyme A, which is an enzyme critical to the oxidation and/or synthesis of carbohydrates and fatty acids.
Biotin Found in egg yolk, legumes, nuts and liver. Also synthesized by intestinal bacteria. Functions as a coenzyme for several enzymes that catalyze carboxylation, decarboxylation and deamination reactions. One example is pyruvate carboxylase, an essential enzyme of Kreb's cycle.
Cobalamin
(Vitamin B12)
Microbial synthesis is the sole source of this vitamin in nature. It is obtained almost exclusively from ingestion of animal products, and is essentially absent from plant products. A cobalt-containing coenzyme involved in numerous metabolic pathways. Deficiency usually results from failure to absorb the molecule due to inadequate quantities of intrinsic factor, and is typically manifest as a defect in red blood cell formation (pernicious anemia).
Folic Acid Present in many natural foods, including dark-green vegetables (spinach!), beef, eggs, whole grains. Also synthesized by intestinal bacteria. Serves as a coenzyme in the synthesis of several amino acids, as well as purines and thymine, and therefore DNA. Deficiency is typically manifest as growth failure and anemia.

Last updated on August 16, 2003
Author: R. Bowen
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