B-vitamin lab tests (B1, B2, B3, B6, B12, Folate) — useful for clinical evaluations of fatigue, neuropathy, mood disorders, anemia, and metabolic dysfunction.

B1 (Thiamine- transketolase activity Energy metabolism (especially carbs), nerve functionFatigue, beriberi, Wernicke’s encephalopathy (confusion, ataxia, nystagmus), peripheral neuropathy RareAlcoholism, malabsorption, high carb intake, diuretics

B2 (Riboflavin-Erythrocyte glutathione reductase activity (EGRAC)FAD coenzyme for energy, redox balanceCracked lips, glossitis, sore throat, redness of eyesNone reportedPoor diet, malabsorption, alcohol

B3 (Niacin)Indirect (measured via N-methylnicotinamide or urinary metabolites)NAD/NADP production for energy, DNA repairPellagra: "4 D's" – dermatitis, diarrhea, dementia, death Flushing, liver damage with high dosesIsoniazid use, alcoholism, corn-heavy diet

B6 (Pyridoxine)Plasma PLP (pyridoxal 5'-phosphate)Amino acid metabolism, neurotransmitter synthesis, homocysteine regulationFatigue, neuropathy, glossitis, irritability, seizures Sensory neuropathy (esp. >100 mg/day long-term)Isoniazid, alcohol, oral contraceptives, malabsorption

B12 (Cobalamin)Serum B12, MMA, homocysteineRBC production, DNA synthesis, myelin maintenanceMacrocytic anemia, fatigue, memory issues, neuropathy Rare (may cause acne-like rash)Vegan diet, pernicious anemia, metformin, PPIs

Folate (B9)Serum folate, RBC folate, homocysteineDNA/RNA synthesis, methylation, fetal developmentMacrocytic anemia, fatigue, neural tube defectsMay mask B12 deficiency symptomsAlcohol, pregnancy, poor diet, malabsorption

✅ Best Functional Markers:

• B6 → Plasma
• B12 → Methylmalonic Acid (MMA)
• Folate → RBC
• B1 → Transketolase activity or whole blood thiamine
• Homocysteine → Elevated in B6, B12, or folate deficiency