STEPTRACKERBIOCHEMISTRY + NEUROLOGY | NUTRITION | HIGH-YIELD REVIEW

B Vitamins I — B1, B2, B3, B5, B6

Water-soluble cofactors that run energy metabolism — deficiency shows first in nerves, skin, mouth and heart.

FOCUS ON
B1 thiamine (TPP) is a cofactor for 4 enzymes: PDH, α-KG dehydrogenase, transketolase, branched-chain α-ketoacid dehydrogenase. Deficiency → Wernicke-Korsakoff (alcoholics; give thiamine before glucose), dry/wet beriberi. B3 niacin → pellagra: diarrhea, dermatitis, dementia (Hartnup, carcinoid, isoniazid). B6 → isoniazid neuropathy, sideroblastic anemia, seizures.
VITAMIN
COENZYME · FUNCTION
DEFICIENCY
CAUSE · CLUE
B1 — thiamine
Thiamine pyrophosphate (TPP) for PDH, α-KG DH, transketolase, BCKDH
Wernicke: confusion, ophthalmoplegia, ataxia · Korsakoff: amnesia, confabulation · dry beriberi: polyneuropathy · wet beriberi: high-output heart failure
Alcohol, malnutrition, bariatric surgery, hyperemesis · ↑ RBC transketolase activity after thiamine
Wernicke lesion
Low ATP in high-demand brain regions
Mammillary bodies, periaqueductal gray, medial thalamus damage
Glucose before thiamine worsens it
B2 — riboflavin
FAD and FMN (electron carriers: succinate DH, complex I/II)
Cheilosis, angular stomatitis, glossitis (magenta tongue), corneal vascularization, seborrheic dermatitis
"2 Cs": Cheilosis, Corneal vascularization
B3 — niacin
NAD⁺/NADP⁺; made from tryptophan (needs B2, B6)
Pellagra: diarrhea, dermatitis (Casal necklace, sun-exposed), dementia, (death)
Hartnup (↓ Trp absorption), carcinoid (Trp → serotonin), isoniazid (↓ B6), corn-based diet
Niacin excess
lipid drug
↓ VLDL secretion, ↑ HDL
Facial flushing (prostaglandins — prevent with aspirin), hyperglycemia, hyperuricemia (gout)
Hepatotoxicity at high doses
B5 — pantothenic acid
Part of coenzyme A and fatty acid synthase
Dermatitis, enteritis, alopecia, adrenal insufficiency, burning feet
Very rare — present in almost all foods (Greek pantos = everywhere)
B6 — pyridoxine
Pyridoxal phosphate: transamination (ALT, AST), decarboxylation (GABA, serotonin, dopamine, histamine), ALA synthase, glycogen phosphorylase, cystathionine synthase, niacin synthesis
Convulsions, irritability, peripheral neuropathy, cheilosis, sideroblastic anemia (↓ heme)
Isoniazid, OCPs · excess → sensory neuropathy
Water-soluble vitamins (general)
B complex and C; washed out in urine
Deficiency develops in weeks to months (except B12, years; folate, months)
Toxicity rare (except B3, B6)

Mechanism — why thiamine runs out the brain first

Thiamine ↓
→
PDH + α-KG DH stall
→
↓ ATP, ↑ lactate
→
Glucose-dependent neurons failMammillary bodies, thalamus

A glucose load pushes more pyruvate into a blocked PDH — always give thiamine first in malnourished or alcoholic patients.

Cofactor cheat sheet

VITAMIN
COFACTOR
KEY ENZYMES
B1
TPP
PDH, α-KG DH, transketolase, BCKDH
B2
FAD, FMN
Succinate DH, ETC
B3
NAD⁺, NADP⁺
Dehydrogenases
B5
CoA
Acyl transfers, FA synthase
B6
PLP
Transaminases, decarboxylases, ALA synthase

How it's tested

COMMON CONFUSIONWet beriberi = high-output heart failure with edema; dry beriberi = symmetric neuropathy with muscle wasting.
THE TRAPA TB patient on isoniazid with numb feet needs B6 — isoniazid depletes pyridoxine, which can also cause pellagra-like symptoms.
YOUR APPROACHOrgan pattern (brain, heart, mouth, skin) + setting (alcohol, TB drugs, carcinoid, Hartnup) → vitamin → its cofactor role.

If you see… think…

Alcoholic with confusion, nystagmus, ataxia → Wernicke (B1)
Confabulation after Wernicke → Korsakoff (mammillary bodies)
Edema + high-output heart failure in malnutrition → wet beriberi (B1)
Angular cheilitis + corneal vessels → B2
Diarrhea, sun-exposed dermatitis, dementia → pellagra (B3)
Pellagra + neutral aminoaciduria → Hartnup disease
Flushing after a lipid drug, relieved by aspirin → niacin
Peripheral neuropathy on isoniazid → B6 deficiency
Sideroblastic anemia in a TB patient → B6 (ALA synthase)
Cofactor of transketolase measured in RBCs → thiamine