Vitamins included in this classification are all water-soluble, but their levels of dissolution in water vary. .Below is a table that presents the active forms and accepted nomenclatures for vitamins belonging to each vitamin group.Vitamin C (ascorbic acid) and the B vitamins, which include thiamin (vitamin B1), riboflavin (vitamin B2), vitamin B6, niacin (nicotinic acid), vitamin B12, folic acid, pantothenic acid, and biotin, are water-soluble vitamins.Carbon, hydrogen, and oxygen are the main elements in these molecules, but some also contain nitrogen, sulfur, or cobalt.
.To allow enzyme-catalyzed reactions to occur, an active coenzyme must combine with an appropriate protein component (an apoenzyme).
.This coenzyme (coenzyme A) is an important part of the tricarboxylic acid cycle (also known as the Krebs, or citric acid, cycle), which links carbohydrate, fat, and protein metabolism. It is involved in the response between fats, proteins, and carbohydrates and is an important controlling factor in the conversion of these substances into energy.When carbohydrates and proteins are converted into metabolic energy during the citric acid cycle, the coenzymes Thiamin and Vitamin B6 are involved.As a coenzyme, riboflavin and niacin aid in the transfer of hydrogen ions or electrons (negatively charged particles) during the reactions of the tricarboxylic acid cycle.Besides their roles in synthesis of structural compounds, all of these coenzymes also function in transfer reactions. These reactions do not participate in the tricarboxylic acid cycle.
.Perhaps it performs this function because it is a strong reducing agent (meaning it readily gives electrons to other molecules).
These water-soluble vitamins are absorbed in the intestines of animals, absorbed into the blood, and then transported to the tissues in which they are needed.To absorb vitamin B12, it requires a substance known as intrinsic factor.
Some forms of the B vitamins cannot be used by animals. .This binding can also be prevented using digestive-tract enzymes, so the biotin cannot be utilized. The protein avidin can also bind biotin, which is found in raw egg whites.Animal products (such as meat) have biotin, vitamin B6, and folic acid bound together to form complexes or conjugates; although none of these vitamins is active in their complex form, they are released from the binding forms by enzymes in the intestinal tract and tissues (in the case of biotin and vitamin B6), and can then be absorbed and utilized.Most metabolizing tissues of plants and animals contain B vitamins.
A large portion of water-soluble vitamins in humans is excreted in the urine. .Thiamin, niacin, and vitamin B6 are also metabolized to produce urine products (also called metabolites).Biotin, riboflavin, and pantothenic acid also form urinary metabolites.When intake is sufficient for proper body function, the body excretes very little of these nutrients (or their metabolites).Vitamins are stored in tissues if intake exceeds minimum requirements.Due to limited tissue capacity, the rate of excretion increases sharply as tissues become saturated.Unlike the other water-soluble vitamins, however, vitamin B12 is only excreted in the feces.Folic acid and biotin are also normally excreted in this manner.Even though animals excrete water-soluble vitamins (other than vitamin B12, folic acid, and biotin), the vitamins' source probably derives from the intestinal bacteria that synthesize them, rather than the vitamins they eat and utilize.
If taken in excess, water-soluble vitamins are generally not considered toxic. .Providing animals with 100 times the amount of Thiamin needed can lead to death from respiratory failure.The therapeutic doses (100–500 mg) of thiamin in humans are not known to cause toxic effects (other than rare instances of anaphylactic shock in individuals with sensitivity).Neither other B vitamin has any known toxicity.