Water-soluble vitamins include the B-complex group and vitamin C. Unlike fat-soluble vitamins, they are not stored in significant amounts and must be obtained regularly from the diet. Excess water-soluble vitamins are excreted in urine, making toxicity uncommon.

Vitamin B1

Thiamine consists of a pyrimidine ring and a thiazole ring linked by a methylene bridge. Its active form, thiamine pyrophosphate, is a cofactor for enzymes catalyzing decarboxylation of alpha-keto acids and transketolation. Pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, and transketolase require TPP. Thiamine is found in whole grains, legumes, pork, and nuts. Beriberi occurs in two forms: wet beriberi with cardiovascular symptoms including edema and heart failure, and dry beriberi with peripheral neuropathy and muscle wasting. Wernicke-Korsakoff syndrome, seen in alcoholism, causes confusion, ataxia, and memory impairment.

Vitamin B2

Riboflavin forms the coenzymes FMN and FAD, which function as electron carriers in oxidation-reduction reactions. Riboflavin is found in dairy products, eggs, organ meats, and green vegetables. Deficiency causes angular stomatitis, cheilosis, glossitis, and photophobia. Riboflavin deficiency rarely occurs in isolation, typically accompanying other B-vitamin deficiencies.

Vitamin B3

Niacin is converted to NAD+ and NADP+, the central electron carriers in cellular metabolism including oxidative phosphorylation. Nicotinamide, the amide form, is the predominant dietary source. Nicotinic acid, the acid form, is also used as a lipid-lowering drug. Niacin is found in meat, fish, poultry, and fortified grains. Pellagra presents with the three Ds: dermatitis, diarrhea, and dementia. Niacin in high doses effectively lowers LDL and triglycerides and raises HDL, but causes flushing as a side effect.

Vitamin B5

Pantothenic acid is a component of coenzyme A and acyl carrier protein, making it essential for fatty acid metabolism and the citric acid cycle. It is widely distributed in foods, and deficiency is extremely rare. Symptoms of deficiency include fatigue, insomnia, and numbness.

Vitamin B6

Pyridoxine, pyridoxal, and pyridoxamine are converted to pyridoxal phosphate, the active coenzyme for over 100 enzymes involved in amino acid metabolism. PLP is essential for transamination, decarboxylation, and deamination reactions. Good sources include poultry, fish, potatoes, and non-citrus fruits. Deficiency causes dermatitis, microcytic anemia, and neurological symptoms. Isoniazid treatment for tuberculosis can cause B6 deficiency.

Vitamin B7

Biotin is a cofactor for carboxylation reactions, covalently attached to carboxylases through a lysine residue. Biotin-dependent enzymes include acetyl-CoA carboxylase, pyruvate carboxylase, and propionyl-CoA carboxylase. Biotin is found in egg yolk, liver, and nuts. Raw egg white contains avidin, which binds biotin and prevents absorption, causing deficiency. Symptoms include dermatitis, hair loss, and neurological abnormalities.

Vitamin B9

Folate in its reduced form, tetrahydrofolate, carries one-carbon units for nucleotide synthesis and amino acid metabolism. Folate is essential for purine synthesis, thymidylate synthesis, and methionine regeneration. Good sources include leafy green vegetables, legumes, and fortified grains. Folate deficiency causes megaloblastic anemia and neural tube defects in pregnancy. Adequate periconceptional folate supplementation prevents spina bifida and anencephaly.

Vitamin B12

Cobalamin is unique among vitamins in containing cobalt and requiring intrinsic factor for absorption. Methylcobalamin and adenosylcobalamin are the active coenzyme forms. Methylcobalamin is a cofactor for methionine synthase, converting homocysteine to methionine. Adenosylcobalamin is a cofactor for methylmalonyl-CoA mutase. B12 is found only in animal products. Deficiency causes megaloblastic anemia and neurological symptoms including peripheral neuropathy and subacute combined degeneration of the spinal cord. Pernicious anemia results from autoimmune destruction of gastric parietal cells that produce intrinsic factor.

Vitamin C

Ascorbic acid is a water-soluble antioxidant and a cofactor for enzymes involved in collagen synthesis, neurotransmitter production, and iron absorption. As a cofactor for prolyl and lysyl hydroxylases, vitamin C is essential for collagen maturation. It also facilitates iron absorption by reducing ferric to ferrous iron. Scurvy causes impaired wound healing, gingival bleeding, petechiae, and fatigue. Severe deficiency is fatal without treatment.