XIV. Water-Soluble Vitamins (B and C)

What Are B Vitamins?

The B vitamins (also called B complex vitamins) are originally grouped together because of their similar metabolic functions. The nine chemically unrelated organic molecules function as metabolic catalysts (coenzymes) for energy metabolism pathways, for cellular maintenance, or for blood cell formation in the animal body. A list of water-soluble vitamins, coenzymes, and functions are shown in table 14.1. The B vitamins discussed in this section include thiamine, riboflavin, niacin, pyridoxine, pantothenic acid, biotin, folic acid, cobalamin, and choline . In ruminant animals and in herbivores, microbial synthesis meets the requirements, while in monogastric animals, such as pigs and poultry, daily supplementation is essential. B complex vitamins are also prone to loss during feed processing.

Water-Soluble Vitamins

Thiamine

Thiamine consists of one molecule of pyrimidine joined with one of thiazole. Thiamin is also referred to as vitamin B1, as it is the first vitamin identified. Thiamine is a component of the enzyme thiamin pyrophosphate (TPP), which is involved in several key reactions in energy-producing pathways.

Dietary thiamine is converted into TPP inside cells to participate in the energy-producing pathway. Oxidation decarboxylation reactions, such as pyruvate dehydrogenase and α-ketoglutarate dehydrogenase, require TPP. It also is used in transketolase reactions for nicotinamide adenine dinucleotide phosphate (NADPH) production in the pentose cycle. Due to its role in carbohydrate metabolism, an animals’ thiamine requirement is influenced by the level of carbohydrates in their diet. Cereal grains are good sources of thiamine. However, since thiamine is heat labile, feed processing can destroy thiamine content. Thiamin requirement is linked to the energy content of the diet (0.5 mg/1,000 kcal diet)

Deficiency: Thiamin also plays a specific role in neurophysiology because a typical thiamin deficiency is beriberi, a dysfunction in the nervous system. Polyneuritis is another typical symptom of thiamin deficiency in chicks. Several compounds resemble thiamine in chemical structure and can function as antagonists, causing a thiamine deficiency. Raw fish and bracken ferns (a perennial) contain an enzyme, thiaminase, which destroys thiamin, causing a deficiency that causes a neurological disorder called Chastek paralysis, named after a farmer who observed similar condition in silver foxes. Heat treatment denatures thiaminase and prevents the problem. Amprolium (coccidiostat) blocks activation of TPP and can cause a thiamine deficiency.

Riboflavin

Riboflavin is named for its yellow color (flavin) and sugar (ribose). Riboflavin (vitamin B2) is relatively heat stable but easily destroyed by light. Riboflavin functions in the body as a component of two different coenzymes: flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). Both of these enzymes are involved in dehydrogenation/oxidation reactions that function in the release of energy from carbohydrates, fats, and proteins (the tricarboxylic acid [TCA] cycle, oxidation, electron transport chain).

Deficiency: As with most B vitamins, deficiency leads to a reduction in growth in young animals. Diets low in riboflavin can cause lesions in the corners of the mouth and anorexia and can cause loss of hair and diarrhea in young animals.

Niacin

Niacin is the accepted generic description for pyridine 3-carboxylic acid and its derivatives showing the nutritional activity of nicotinic acid. Niacin functions as a constituent of two important coenzymes nicotinamide adenine dinucleotide (NAD) and NADPH. These coenzymes serve as hydrogen carriers in several important metabolic processes involving carbohydrate metabolism (glycolysis) and other energy deriving pathways involving carbohydrates, fats, and proteins, such as the TCA cycle and oxidative phosphorylation.

In animal diets, niacin present in cereal grains is in a bound form and is not biologically available to the animal. For example, one of the bound forms of niacin in wheat is called niacytin and is not biologically available. Corn contains niacinogen, which binds niacin tightly and makes it unavailable for absorption. Niacin from animal sources is highly available.

In addition to dietary sources, most animals (except cats) are capable of synthesizing niacin from the essential amino acid tryptophan. As a result, levels of tryptophan can affect niacin requirements. However, feeds low in niacin are usually also usually low in tryptophan. Cats should receive the entire supply of niacin from their diet.

Deficiency: This causes a condition called black tongue disease in dogs, and in chickens, it causes poor feathering around the eyes, also called spectacled eyes. The requirement is a 10–90 mg/kg diet. Pellagra (thick skin, dermatitis) is a typical deficiency symptom of niacin in humans associated with poor diet (high grain, no meat) and poverty.

Pyridoxine

Pyridoxine comprises three different forms: pyridoxine (plant), pyridoxal (animal), and pyridoxamine (animal). Pyridoxal, which is a component of the coenzyme pyridoxal 5-phosphate is the biologically active form. Pyridoxal 5-phosphate participates in a wide variety of biochemical reactions, most of them involving amino acid metabolism, such as transamination, deamination reactions, and decarboxylation reactions. Pyridoxal phosphate is also required for the synthesis of hemoglobin and the conversion of tryptophan to niacin. Vitamin B6 deficiency can precipitate niacin deficiency. Therefore, deficiency symptoms are similar for these two vitamins.

Deficiency: Symptoms include convulsions and reduced immune response. The requirement is a 1–3 mg/kg diet and is linked to the protein level in the diet.

Pantothenic Acid

Pantothenic acid occurs in all tissues of the body. The vitamin name is derived from the Greek term pan meaning “all,” or “everywhere.” Pantothenic acid was identified as a constituent of coenzyme A, the coenzyme required for acetylation of numerous compounds in energy metabolism. CoA is required in the formation of two-C fragments from fats, amino acids, and carbohydrates for entry into the citric acid cycle and for the synthesis of steroids. Deficiency of this vitamin is extremely rare, and in extreme cases, in addition to the reduced growth rate, in pigs, it leads to a condition called goose-stepping, an abnormal gait, due to nerve degeneration. Other signs of deficiency include a rough coat, anorexia, and impaired productivity. The Ca salt is the most common form in which the vitamin is added to diets.

Biotin

Feeding raw egg whites to rats causes skin lesions and loss of hair and were cured by a protective factor found in the liver. The original name given to this compound was vitamin H because it protected haut, a German word for “skin.” Biotin was isolated from egg yolk in 1936, a growth factor for yeast.

Biotin is a prosthetic group that binds to the lysine of the enzyme via a peptide bond to form biocytin, which serves as a cofactor in carboxylase reactions such as acetyl CoA carboxylase carboxylase  (the first step in lipogenesis) and pyruvate carboxylase (the first step in gluconeogenesis). These important metabolic pathways make biotin very important in lipid and carbohydrate metabolism. Biotin acts as a carbon dioxide carrier (carbon fixation) in reactions in which carbon chains are lengthened.

Deficiency: Biotin deficiency is rare. It causes dermatitis and hair loss. This is usually caused not by lack of biotin in the diet but instead, the antivitamin avidin binds biotin and makes it unavailable for digestion and absorption. Eggs are a rich source of biotin. But egg whites contain avidin. However, cooking denatures avidin, making the biotin available for absorption. Deficiency symptoms may be found in swine kept in pens with slotted floors with limited or no access to fecal matter as hindgut bacteria produce biotin. Lack of biotin has been shown to cause a condition called footpad dermatitis in chickens fed wheat-based diets. Requirements of biotin are a 0.1–0.3 mg/kg diet (dry basis). Animals subjected to antibiotic therapy that causes a decrease in bacterial population may need an extra supply of biotin.

Folic Acid

Folacin is a generic term used to describe folic acid and related compounds. The active form of folacin in the body is called tetrahydrofolic acid. Dietary sources of folacin are converted mainly in the liver to tetrahydrofolic acid. Vitamin B12 enhances the conversion of folacin to tetrahydrofolic acid. The function of tetrahydrofolic acid is as a transport vehicle for single carbon units. Tetrahydrofolic acid is required for purine, pyrimidine, glycine, serine, and creatine synthesis. Both purine and pyrimidine synthesis is required for DNA synthesis and thus cell replication.

Deficiency: Lack of folic acid leads to less DNA and cell multiplication and affects all mitotically active cells. These include hematopoietic cells and all epithelial cells. Since rapidly dividing cells are most affected, it causes a condition called megaloblastic anemia. Folic acid deficiency is the most prominent human vitamin deficiency. Up to one-third of all pregnant women in the world may experience a folic acid deficiency during pregnancy. Folic acid and vitamin B12 have a close relationship—vitamin B12 deficiency will precipitate folic acid deficiency. The inclusion of antimicrobials increases the possibility of folate deficiency. Requirements of folic acid are a 0.25 mg/kg diet.

Cobalamin (Vitamin B12), the last B vitamin, was discovered in 1948. Cyanocobalamin is the vitamin and deoxyadenosyl cobalamin is the coenzyme form. Vitamin B12 is unique in that it has a trace element mineral (cobalt) as its active site. It is also the only vitamin that is synthesized only by microorganisms. Similar to folic acid, cobalamin is involved in the transfer of single carbon units during various biochemical reactions. Folic acid serves as a coenzyme for several enzyme systems involving methyl transfer in fat and carbohydrate metabolism and for myelin synthesis. Cobalamin is required for the oxidation of propionic acid in ruminant animals.

The stomach plays an important role in the absorption of vitamin B12. The stomach provides the acidity and pepsin to release the tightly bound vitamin B12 from the dietary source. The stomach also secretes an intrinsic factor, a specific binding glycoprotein. The vitamin B12-intrinsic factor complex travels to the ileum and is absorbed into a portal vein. Calcium is required for B12 absorption in the ileum. The absence of glycoprotein can lead to vitamin B12 deficiency. Deficiency symptoms are very similar to folic acid deficiency. The requirement is extremely low: 5–50 µg/kg diet for nonruminants. Cobalt is required only for ruminants; the rumen microbes will synthesize cobalamin.

Deficiency: This is very similar to folic acid deficiency, causing anemia and neural disorders. Lack of vitamin B12 or folacin interferes with the absorption of nutrients. Changes in the epithelial cells of the intestine, along with shortened villi, are observed. In livestock species, loss of appetite and reduced growth are observed. In ruminants, rumen microbes can synthesize all B vitamins; therefore, there is no requirement. However, they must have cobalt to synthesize vitamin B12. Requirements of cobalamin are 5–50 µg/kg diet (nonruminants). In ruminants, only cobalt is needed.

Vitamin C

It was discovered in 1747 that scurvy can be prevented by the ingestion of lemon juice. Ascorbic acid (Vitamin C) was recognized as a vitamin in 1933. Ascorbic acid has a structure closely related to monosaccharide sugars. It is synthesized from glucose by plants and most animal species. No coenzyme form is identified. Ascorbic acid is required for hydroxylation reactions of the amino acids proline and lysine in the formation of collagen, elastin synthesis, and neurotransmitter (norepinephrine, epinephrine) synthesis. Collagen is important for normal bone formation. It also functions as an antioxidant, reducing oxidative stress. Ascorbic acid can be synthesized from glucose by all mammals except primates and guinea pigs. Therefore, there is no requirement for livestock species. Proline and OH-proline are required for collagen synthesis.

Deficiency: This results in scurvy, a disease affecting humans with impaired wound healing, capillary bleeding, faulty bone formation, and anemia; it was first reported in sailors at sea. Normally, no deficiency symptom can be detected in all mammals except primates and guinea pigs. No daily requirement is established for livestock animals.