Vitamins Water Soluble Thiamin Riboflavin and B6

C. Robert Dove

University of Georgia, Athens, Georgia, U.S.A.


The water-soluble vitamins thiamin, riboflavin, and B6 are involved in a wide variety of metabolic processes. Most of these processes are involved in the digestion and metabolism of nutrients. Like most of the B vitamins, the initial deficiency symptoms of each include decreased growth rate and poor general health status. The B vitamins are found in plant and animal tissues in varying concentrations and availabilities. While dietary recommendations vary considerably from species to species, dietary supplementation of most of the B vitamins is recommended for monogastric species. Microorganisms in the rumen of ruminant animals produce enough B vitamins to meet the animal's needs under most production situations.


Thiamin was originally known as vitamin Bj, or aneurin, and was the first B vitamin identified. Early studies of the cause of beriberi led to the discovery of thiamin. It was shown that rice bran, extracts of rice bran, or whole rice could alleviate the symptoms of beriberi. Thiamin was subsequently identified as the active factor in rice bran that prevented beriberi. The word vitamin was first coined in reference to thiamin when Casimir Funk used the term vitamine to refer to thiamin as an amine that is essential for life.[1]

Thiamin absorption takes place mostly in the jejunum of the small intestine. The thiamin phosphor esters are completely hydrolyzed by intestinal phosphatases, and are present in the lumen of the intestine in free form. At low intestinal concentrations, active transport occurs. At higher concentrations, a passive diffusion system appears to be active.[2]

The biologically active form of thiamin is the coenzyme thiamin pyrophosphate (TPP). Thiamin, in the form of TPP, is essential for the metabolism of carbohydrates and proteins (branched-chain amino acids). Thiamin pyrophosphate also functions in the transketolase reaction of the pentose phosphate shunt, and it is believed that thiamin, in the form of thiamin triphosphate, plays a role in nerve conduction.1-3-1

Early symptoms of a thiamin deficiency include anorexia and an associated reduction in weight gain.[4] These nonspecific symptoms are similar to the symptoms of several other vitamin deficiencies. Other deficiency symptoms include a depression in body temperature, occasional vomiting, a flabby heart, bradycardia, hypertrophy of the heart, myocardial degeneration, and sudden death associated with heart failure. Transketolase activity has also been noted to be decreased in response to a thiamin deficiency. The most widely used and best functional test of thiamin status is the assay for eryth-rocyte transketolase.

Most cereal grains are rich in thiamin. However, thiamin is very heat sensitive and is easily destroyed during feed processing, especially in the presence of reducing sugars. Meat, milk, and egg products also contain significant amounts of thiamin.[1]


Riboflavin was originally known as vitamin B2, or ovoflavin. It was isolated from egg white and shown to be effective in promoting growth in rats. Dietary forms of riboflavin are mostly coenzyme derivatives that are released in the stomach and hydrolyzed by phosphatases in the small intestine. Riboflavin is absorbed by a specialized transport mechanism in the proximal small intestine. Intestinal absorption of riboflavin appears to be increased by the presence of food in the intestine.[5,6]

Riboflavin participates in metabolism as a component of flavin adenine dinucleotide (FAD) and flavin mono-nucleotide (FMN). As a component of FAD and FMN, riboflavin functions as a catalyst for a number of redox reactions that are critical in the metabolism of carbohydrates, proteins, and fats.[1]

Symptoms of riboflavin deficiency include a reduction in growth rate, stiffness of gait, alopecia (hair loss), seborrhea (crusty exudates), vomiting, and cataracts. Other deficiency symptoms that have been observed are increased blood neutrophil granulocytes, reduced immune response, discolored kidney and liver tissue, fatty liver, and degeneration of the myelin of the sciatic and brachial nerves. Females with severe deficiency have also been shown to have collapsed follicles and degenerating ova.[5'6]

Riboflavin in feedstuffs is primarily in the form of proteins complexed with FMN and FAD. Riboflavin is found in small amounts in most plant and animal products. Organ meats and milk products contain higher levels of riboflavin. The riboflavin present in a corn soybean meal diet was estimated to be 59% bioavailable relative to crystalline riboflavin.1-7-1 Riboflavin photo-degrades in the presence of light, and appreciable amounts of riboflavin may be lost during the processing and storage of cereal grains and feeds.[8]

ase activity has been suggested as the method of choice in assessing vitamin B6 status.[5]

Vitamin B6 occurs in feedstuffs as pyridoxine, pyridoxal, pyridoxamine, and pyridoxal phosphate. For the chick, vitamin B6 is about 40% bioavailable in corn and about 60% bioavailable in soybean meal.[11] Vitamin B6 concentrations are fairly high in most cereal grains and common feed ingredients. Vitamin B6 is normally present in adequate amounts and does not require supplementation. However, feed processing and storage can result in the destruction of 10% to 50% of the naturally occurring vitamin B6 activity.[1]



Vitamin B6 is actually a group of six related compounds. These include pyridoxal, pyridoxine, and pyridoxamine and the 5'-phosphates of each of these compounds. The primary forms of vitamin B6 found in animal tissue are pyridoxal phosphate (PLP) and pyridoxamine phosphate (PMP). In plant tissues, the primary forms of vitamin B6 are pyridoxine and pyridoxine phosphate.[5]

Vitamin B6 is absorbed from the intestinal tract by a nonsaturable, passive process. Absorption takes place mostly in the jejunum of the small intestine. The active form of vitamin B6 is primarily pyridoxal phosphate (PLP), which serves as a coenzyme in many metabolic reactions. Pyridoxal phosphate is needed in amino acid metabolism, and to a lesser extent in carbohydrate and lipid metabolism. The role of PLP in lipid metabolism is as \yet unclear, but it has been shown that carcasses of deficient animals contain less lipid than those of controls.[5,9]

As with many other vitamins, a deficiency of vitamin B6 results in a reduction in feed intake and growth rate. Other deficiency symptoms that have been observed include the development of brown exudates around the eyes, impaired vision, vomiting, ataxia, epileptiform seizures, coma, and death. Examination of blood samples taken from deficient animals has revealed microcytic hypochromic anemia; a reduction in albumin, hematocrit, hemoglobin, red blood cells, and lymphocytes; and an increase in gamma globulin. Other signs characteristic of vitamin B6 deficiency determined at necropsy include degeneration of sensory neurons and fat infiltration of the liver.[1,5,10] A reduction in antibody production as a result of vitamin B6 deficiency has also been noted. Clinical signs of a vitamin B6 deficiency in young, growing animals appear within 2 to 3 weeks following the removal of the vitamin from the diet.[5] Toxicity signs, such as ataxia, muscle weakness, neuropathy, and loss of balance, have been reported.[5,10] The assay for apotyrosine decarboxyl-

Thiamin, riboflavin, and vitamin B6 play important roles in the metabolism and efficient utilization of nutrients. These vitamins are involved in the metabolism of fats, proteins (amino acids), and carbohydrates. They are also involved in nerve conduction, vision, and heart function. Thiamin, riboflavin, and vitamin B6 are present in plant and animal products, but the availability of the naturally occurring vitamins varies considerably. Pure forms of these vitamins are normally supplemented in the diets of monogastric species. Under normal production practices, ruminant animals can produce enough of these vitamins in the rumen to meet the animal's needs.


1. Dove, C.R. Water Soluble Vitamins in Swine Nutrition. In Swine Nutrition, 2nd Ed.; Lewis, A.J., Southern, L.L., Eds.; CRC Press: New York, 2001; 315 356.

2. Rindi, G. Thiamin. In Present Knowledge in Nutrition, 7th Ed.; Ziegler, E.E., Filer, L.J., Jr., Eds.; ILSI Press: Washington, DC, 1996; 160 166.

3. Bettendorff, L.; Kolb, H.A.; Schoffeniels, E. Thiamine triphosphate activated an anion channel of large unit con ductance in neuroblastoma cells. J. Membr. Biol. 1993, 136, 281 288.

4. Hughes, E.H. The minimum requirement of thiamine for the growing pig. J. Nutr. 1940, 2, 239 241.

5. Standing Committee on the Scientific Evaluation of Dietary Reference Intakes and Its Panel on Folate, Other B Vitamins, and Choline and Subcommittee on Upper Reference Levels of Nutrients Food and Nutrition Board Institute of Medicine. Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin and Choline (1999); National Academy Press: Washington, DC, 2000.

6. McCormick, D.B. Riboflavin. In Modern Nutrition in Health and Disease, 8th Ed.; Skils, M.E., Olson, J.A.,

Shike, M., Eds.; Lea and Febiger: Philadelphia, 1994; 366 375.

7. Chung, T.K.; Baker, D.H. Riboflavin requirement of chicks fed purified amino acid and conventional corn soybean meal diets. Poultry Sci. 1990, 69, 1357.

8. Rivlin, R.S. Riboflavin. In Present Knowledge in Nutrition, 7th Ed.; Ziegler, E.E., Filer, L.J., Jr., Eds.; ILSI Press: Washington, DC, 1996; 167 173.

9. Sauberlich, H.E. Biochemical Systems and Biochemical Detection of Deficiency. In The Vitamins: Chemistry,

Physiology, Pathology, Assay, 2nd Ed.; Sebrell, W.H., Jr., Harris, R.S., Eds.; Academic Press: New York, 1968; Vol. 2, 44 80.

10. Leklem, J.E. Vitamin B6. In Present Knowledge in Nutrition, 7th Ed.; Ziegler, E.E., Filer, L.J., Jr., Eds.; ILSI Press: Washington, DC, 1996; 174 183.

11. Yen, J.T.; Jensen, A.H.; Baker, D.H. Assessment of the concentration of biologically available vitamin B6 in corn and soybean meal. J. Anim. Sci. 1976, 42, 866 870.

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