Vitamin B

 

Vitamin B would have to be one of the most complex and interesting Vitamins available. Vitamin B as a whole holds seven different compounds such as; Vitamin B1: Thiamin, B2: Riboflavin, B3: Niacin, B5: Pantothenic Acid B6: Pyridoxal or either of its 6 compounds, B7: Biotin and B12: Cobalamin. Each compound of B vitamin serves a different purpose and interacts with our cell membranes in order to intiate specific chemical responses. 


Vitamin B1


Thiamin is a water-soluble substance that occurs in free or phosphorylated forms in most plant and animal tissue. It plays an essential role in the supply of energy to the tissue, in carbohydrate metabolism and in the metabolic links between carbohydrate, protein and fat metabolism. Following ingestion, absorption of thiamin occurs mainly in the jejunum, actively at low concentrations and passively at high concentrations. It is transported in blood in both plasma and red blood cells. If intake is high, only a small amount of the thiamin is absorbed and elevated serum values result in active urinary excretion. The total body content of the vitamin is about 30 mg.


Vitamin B2


Riboflavin is a water-soluble vitamin. The bioactive forms of riboflavin are the oxidised and reduced forms of flavin adenine dinucleotide (FAD and FADH2, respectively) and flavin mononucleotide (FMN and FMNH2, respectively). They function as co-enzymes for key reactions in the catabolism of fuel molecules (eg β-oxidation of fatty acids, Krebs cycle), and in certain biosynthetic pathways (eg fatty acid synthesis). Riboflavin and its derivatives are important for the body’s handling of some other nutrients including conversion of vitamin B-6 to its bioactive form, pyridoxal phosphate; conversion of tryptophan to niacin and conversion of methylenetetrahydrofolate (MTHF) to methylTHF by the enzyme methyleneTHF reductase (MTHFR).


Vitamin B3


Niacin is a generic descriptor for the closely related compounds, nicotinic acid and its amide nicotinamide, which act similarly as nutrients. The amino acid tryptophan is converted to nicotinamide with an average conversion efficiency of 60:1 and can thus contribute to requirements although this can vary depending on a number of dietary and metabolic factors.


Vitamin B6


Vitamin B6 is made up of six compounds – pyridoxal, pyridoxine, pyridoxamine and their respective 5’ phosphates. It acts as a coenzyme in the metabolism of amino acids, glycogen and sphingoid bases. The most common form in human tissue is the 5’-phosphate form of pyridoxal (PLP) most of which is found in muscle bound to phosphorylase. The second most common is the 5’-phosphate form of pyridoxamine (PMP). Plant foods contain primarily pyridoxine (PN) and its 5’-phosphate (PNP), sometimes in the form of a glucoside.


Vitamin B5


Pantothenic acid is a component of coenzyme A (CoA) and phosphopantetheine, both of which are involved in fatty acid metabolism. It is essential to almost all forms of life and is widely distributed in foods. Chicken, beef, potatoes, oat-based cereals, tomato products, liver, kidney, egg yolks and whole grains are major sources in western diets. Little information is available about bioavailability, with estimates ranging from 40 to 61%. Neither is there much information about interactions with other nutrients, although there is some information that implies that thiamin, and to a lesser extent riboflavin, can affect pantothenate metabolism and excretion.


Vitamin B7


Biotin is a cofactor for four carboxylase enzymes found in mammals – pyruvate carboxylase, methyl-crotonyl- CoA carboxylase, proprionyl-CoA carboxylase and acetyl-CoA carboxylase. The first three of these are mitochondrial and the fourth is both mitochondrial and cytosolic. They are involved in a range of actions including catabolising acetyl CoA, carboxylation of pyruvate, degradation of leucine and carboxylation of proprionyl-CoA. Biotin is found in free and protein-bound forms in food but little is known about its bioavailability. It is found in the protein-bound form in meats and cereals, although it seems to be less bioavailable in the latter.


Vitamin B12


Vitamin B12 is the generic descriptor for those corrinoid compounds exhibiting qualitatively the biological activity of cyanocobalamin. The main cobalamins with physiological action are hydroxycobalamin, methylcobalamin and deoxyadenosylcobalamin. Vitamin B12 is required for the synthesis of fatty acids in myelin and, in conjunction with folate, for DNA synthesis. Adequate intake of vitamin B12 is essential for normal blood function and neurological function. It can be stored in the liver for many years.


References:


Allen RH, Stabler SP, Lindenbaum J. Serum betaine, N,N-dimethylglycine and N-methylglycine levels in patients with cobalamin and folate deficiency and related inborn errors of metabolism. Metabolism1993;42:1448–60.


Hamm MW, Mehansho H, Henderson LM. Transport and metabolism of pyridoxamine and pyridoxamine phosphate in the small intestine of the rat. J Nutr 1979;109:1552–9.


Horwitt MZK, Harper AE, Henderson LM. Niacin-tryptophan relationships for evaluating niacin equivalents. Am J Clin Nutr 1981;34:423–7.


McCormick DB. Niacin. In; Shils ME, Young VR, eds. Modern nutrition in health and disease. Philadelphia: Lea & Febiger,1988. Pp 370–5.


Zempleni J, Galloway JR, McCormick DB. Pharmacokinetics of orally and intravenously administered riboflavin in healthy humans. Am J Clin Nutr 1996;63:54–66.


Davis RE, Icke GC, Thom J, Reiley WJ. Intestinal absorption of thiamin in man compared with folate and pyridoxyl and its subsequent urinary excretion. J Nutr Sci Vitaminol (Tokyo) 1984;30:475–82.