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The eight B vitamins (B1/thiamine, B2/riboflavin, B3/niacin, B5/pantothenic acid, B6/pyridoxine, B7/biotin, B9/folate, B12/cobalamin) serve primarily as coenzymes in energy metabolism and nucleic acid synthesis. Folate and B12 are critical for one-carbon metabolism and DNA methylation; deficiency causes megaloblastic anemia and, during pregnancy, neural tube defects. Vitamin C (ascorbic acid) is a reducing agent required for collagen synthesis and non-heme iron absorption; deficiency causes scurvy. Unlike fat-soluble vitamins, B vitamins and vitamin C have limited storage and must be consumed regularly, though toxicity from dietary sources is rare.
Map each B vitamin to its coenzyme form and the metabolic pathway it supports (e.g., thiamine → TPP → pyruvate dehydrogenase). Understanding the biochemical role makes deficiency symptoms predictable rather than memorized.
From your study of enzyme cofactors and coenzymes, you know that many enzymes cannot function without a non-protein helper molecule. The B vitamins are the body's coenzyme toolkit for metabolism — each one is converted into a specific coenzyme form that enables a class of biochemical reactions. Deficiency in a B vitamin does not simply reduce one reaction; it can stall an entire metabolic pathway. That is why deficiency symptoms are often so dramatic despite the tiny quantities involved.
The energy-metabolism B vitamins work in concert. Thiamine (B1) becomes thiamine pyrophosphate (TPP), a cofactor essential at the pyruvate dehydrogenase complex — the gateway between glycolysis and the citric acid cycle. Without it, cells cannot convert glucose into usable energy via the mitochondria. Riboflavin (B2) becomes FAD and FMN, which carry electrons in the electron transport chain. Niacin (B3) becomes NAD⁺ and NADP⁺, the most abundant electron carriers in metabolism, involved in hundreds of oxidation-reduction reactions. Pantothenic acid (B5) is literally a structural component of coenzyme A. These four vitamins are not optional accessories — they are load-bearing infrastructure for cellular energy production.
Folate (B9) and cobalamin (B12) occupy a special position because they collaborate on one-carbon metabolism: the transfer of single-carbon units needed to synthesize purines and thymidine (components of DNA) and to recycle homocysteine. Folate provides the one-carbon units; B12 is needed to regenerate the active folate form (tetrahydrofolate). When either is deficient, DNA synthesis stalls — rapidly dividing cells like red blood cell precursors are hit hardest, producing large, immature cells (megaloblastic anemia). The distinction matters clinically: high folate intake can mask B12 deficiency by correcting the blood picture while neurological damage from B12 deficiency quietly progresses, because B12 has a separate and irreplaceable role in maintaining myelin sheaths.
Vitamin C is the outlier in this group — it is not a coenzyme but a reducing agent (antioxidant) that donates electrons to other reactions. Its most critical biochemical role is in collagen synthesis: the enzymes prolyl hydroxylase and lysyl hydroxylase require ascorbate to keep their iron cofactors in the reduced (active) state. Without vitamin C, newly synthesized collagen cannot be properly cross-linked, leading to structurally weak connective tissue. The resulting disease, scurvy, manifests as fragile blood vessels, bleeding gums, and wound dehiscence — all expressions of connective tissue failure. Vitamin C's role in enhancing non-heme iron absorption (reducing Fe³⁺ to Fe²⁺) is a secondary application of this same electron-donating chemistry.