What You Will Learn in This Article
- Why niacin is uniquely synthesised in the human body among all B-vitamins
- The complete tryptophan → niacin biosynthetic pathway (kynurenine pathway)
- The cofactors and conditions that regulate niacin synthesis
- The two active coenzyme forms of niacin — NAD⁺ and NADP⁺ — and their biochemical roles
- Pellagra: clinical presentation, causes, biochemistry, and the “4 Ds”
- Pharmacological uses of niacin at high doses and side effects
- Hartnup disease and carcinoid syndrome as causes of secondary niacin deficiency
- High-yield exam facts, mnemonics, and common traps
📖 Introduction: Why This Topic Matters in Exams
In the early 20th century, a mysterious epidemic was devastating populations across the American South, parts of Europe, and Africa. Patients developed a horrifying constellation of symptoms — a rough, sun-sensitive rash, severe diarrhoea, progressive mental deterioration, and ultimately death. The disease was called pellagra, from the Italian pelle agra (“rough skin”). For years, physicians debated whether it was infectious. It wasn’t.
The culprit was a dietary deficiency of a single molecule: niacin. The victims were primarily maize-eating populations — corn is rich in bound niacin (niacytin) that is biologically unavailable without alkali treatment. The tragedy was preventable with a dietary change or supplementation.
Today, this story matters for your exam because niacin sits at the centre of an exceptionally rich web of testable concepts:
- It is the only B-vitamin synthesised in the human body — from tryptophan, making it unique among vitamins
- Pellagra has a classic, distinctive presentation — the “4 Ds” are one of the most tested clinical mnemonics in biochemistry
- Secondary niacin deficiency from Hartnup disease, carcinoid syndrome, and isoniazid use is repeatedly tested
- Pharmacological niacin — used at gram doses as a lipid-lowering agent — is a pharmacology favourite
🔬 Section 1 — Niacin: Foundational Biochemistry
1A. What Is Niacin?
Niacin is the collective term for two related compounds:
- Nicotinic acid — the acid form; also the pharmacological form used for dyslipidaemia
- Nicotinamide (Niacinamide) — the amide form; the form found in most foods and used in the body for coenzyme synthesis
Both can serve as precursors for the active coenzyme forms, but only nicotinic acid has the pharmacological lipid-lowering effect (nicotinamide does not cause flushing or lower lipids).
Chemical nature: Water-soluble, heat-stable B-vitamin (stable to cooking unlike thiamine and folate)
Absorption: Absorbed in the small intestine by both passive diffusion (at high concentrations) and carrier-mediated transport (at physiological concentrations)
Storage: Minimal body stores — making regular dietary intake necessary despite endogenous synthesis
1B. Dietary Sources of Niacin
Niacin is found in two forms in food:
- Free niacin — bioavailable; found in meat, fish, poultry, dairy, eggs
- Bound niacin (Niacytin) — covalently bound to polysaccharides/peptides in plant foods (especially maize/corn); NOT bioavailable unless treated with alkali
The corn paradox: Populations subsisting on corn (maize) develop pellagra despite corn containing niacin, because it is in bound, unabsorbable form. Traditional Mexican tortilla-making uses nixtamalisation — soaking corn in lime water (calcium hydroxide, an alkali) — which hydrolyses the bound niacin, releasing it in bioavailable form. This cultural practice protected Mexican populations from pellagra while other maize-eating populations suffered epidemics.
Rich dietary sources (free, available niacin):
- Meat (especially liver, chicken, tuna, salmon)
- Groundnuts (peanuts) — one of the richest plant sources
- Whole grains (wheat, rice with bran)
- Legumes
- Mushrooms
- Fortified cereals
1C. The Unique Feature: Endogenous Synthesis from Tryptophan
Niacin is the only B-vitamin that can be synthesised in the human body. The pathway uses the essential amino acid tryptophan as the starting material.
The conversion ratio:
60 mg of dietary tryptophan → 1 mg of niacin
This ratio (60:1) is one of the most tested facts in this entire topic. It means:
- Tryptophan is a relatively inefficient niacin precursor
- A diet adequate in tryptophan can partially substitute for dietary niacin
- However, high tryptophan intake cannot fully compensate for severe niacin deficiency
The conversion means that niacin requirements can be expressed in “Niacin Equivalents (NE)”:
- 1 NE = 1 mg niacin = 60 mg tryptophan
- Recommended daily intake: ~14–16 mg NE/day for adults
🏥 Section 2 — The Tryptophan → Niacin Pathway (Kynurenine Pathway)
2A. Overview of the Kynurenine Pathway
The synthesis of niacin from tryptophan occurs primarily in the liver via the kynurenine pathway. This is a multi-step, enzyme-rich pathway that also produces several biologically important intermediates (kynurenic acid, quinolinic acid, serotonin is a branch-point product).
The key steps are:
Tryptophan
↓ Tryptophan 2,3-dioxygenase (TDO) [liver]
↓ or Indoleamine 2,3-dioxygenase (IDO) [extrahepatic]
N-Formylkynurenine
↓ Formamidase
Kynurenine
↓ Kynurenine hydroxylase (requires NADPH + O₂)
3-Hydroxykynurenine
↓ Kynureninase (requires Pyridoxal Phosphate / Vitamin B6)
3-Hydroxyanthranilic acid
↓ 3-Hydroxyanthranilic acid oxidase
Quinolinic acid (QUIN)
↓ Quinolinate phosphoribosyltransferase (QPRT)
Nicotinic acid mononucleotide (NaMN)
↓ (further steps)
NAD⁺ (Nicotinamide Adenine Dinucleotide)2B. Critical Regulatory Points — High-Yield for Exams
1. Tryptophan 2,3-dioxygenase (TDO):
- Rate-limiting enzyme of the kynurenine pathway
- Induced by: tryptophan itself, glucocorticoids (cortisol)
- Contains heme as a cofactor (connects to iron metabolism)
- Inhibited by: serotonin (explaining the competition between serotonin and niacin synthesis from tryptophan)
2. Kynureninase — Requires Vitamin B6 (Pyridoxal Phosphate):
- This is the exam’s favourite regulatory point
- Vitamin B6 deficiency impairs the conversion of tryptophan to niacin — even if tryptophan intake is adequate
- This explains why isoniazid (INH) — a Vitamin B6 antagonist used in tuberculosis treatment — can cause secondary niacin deficiency and pellagra-like symptoms
- Oral contraceptives also impair B6, similarly reducing niacin synthesis from tryptophan
3. Competition with serotonin:
- Tryptophan is also the precursor for serotonin (5-HT)
- In carcinoid tumours, massive serotonin synthesis diverts tryptophan away from the kynurenine pathway → reduced niacin synthesis → secondary pellagra
4. Ratio of synthesis:
- Only ~2% of dietary tryptophan is converted to niacin under normal conditions; the rest goes to protein synthesis, serotonin, and complete catabolism
- The efficiency drops further in deficiency states (B6 deficiency, carcinoid)
🧪 Section 3 — Active Coenzyme Forms: NAD⁺ and NADP⁺
3A. NAD⁺ (Nicotinamide Adenine Dinucleotide) and NADP⁺
Once niacin reaches the liver, it is converted to its active coenzyme forms:
| Coenzyme | Full Name | Primary Role | Key Reactions |
|---|---|---|---|
| NAD⁺ | Nicotinamide Adenine Dinucleotide (oxidised) | Catabolism — accepts electrons (oxidising agent) | Glycolysis, TCA cycle, β-oxidation, electron transport chain |
| NADH | Reduced form of NAD⁺ | Electron donor to Complex I of ETC | Delivers electrons for oxidative phosphorylation |
| NADP⁺ | Nicotinamide Adenine Dinucleotide Phosphate | Anabolism — provides electrons (reducing agent) | Fatty acid synthesis, cholesterol synthesis, glutathione reduction, pentose phosphate pathway |
| NADPH | Reduced form of NADP⁺ | Reductive biosynthesis; antioxidant defence | HMP shunt (primary source of NADPH), cytochrome P450 reactions |
Memory rule: NAD⁺ = Catabolic (breakdown); NADP⁺ = Anabolic (synthesis)
NAD⁺ predominates in the mitochondria/cytoplasm for energy metabolism
NADP⁺ predominates in the cytoplasm for biosynthesis and antioxidant defence
3B. The Mechanism of Action
Both NAD⁺ and NADP⁺ function as hydride (H⁻) acceptors/donors — they pick up or donate a hydride ion (H⁻ = proton + 2 electrons) at the nicotinamide ring:
NAD⁺ + 2H → NADH + H⁺This redox cycling makes niacin cofactors essential to over 400 enzymatic reactions in human metabolism — more than any other single vitamin cofactor. This is why niacin deficiency has such widespread systemic effects.
💊 Section 4 — Pellagra: The Classic Niacin Deficiency Disease
4A. The “4 Ds” of Pellagra
Pellagra presents with a classic tetrad remembered as the 4 Ds:
| D | Feature | Explanation |
|---|---|---|
| Dermatitis | Photosensitive, symmetric rash on sun-exposed areas | NAD-dependent DNA repair impaired; oxidative stress in skin |
| Diarrhoea | Watery diarrhoea, glossitis, stomatitis | Atrophy of intestinal mucosa; impaired rapid cell turnover |
| Dementia | Confusion, psychosis, memory loss | NAD⁺/NADPH deficiency impairs neuronal energy metabolism |
| Death | Fatal if untreated | Progressive multi-organ failure |
Exam tip: Some sources list 3 Ds (Dermatitis, Diarrhoea, Dementia) as the classic triad, with Death added as the 4th in severe/untreated cases. Exams may ask for either “3 Ds” or “4 Ds” — know both.
4B. The Dermatitis of Pellagra — Distinguishing Features
The skin manifestation of pellagra is pathognomonic and heavily tested:
- Casal’s necklace — a band of hyperpigmented, scaly dermatitis around the neck (at the collar line — a sun-exposed area)
- Rash is bilateral and symmetric (distinguishes it from contact dermatitis)
- Rash is photosensitive — worsens with sun exposure (photodistribution: face, neck, backs of hands, forearms)
- The skin may initially look like sunburn → progresses to hyperpigmentation → scaling → thickening → fissuring
4C. Causes of Pellagra — Primary and Secondary
Primary Pellagra (dietary deficiency):
- Diet based predominantly on corn/maize (bound niacin, unavailable)
- Diet based predominantly on jowar (sorghum) — contains excess leucine, which inhibits the kynurenine pathway, blocking tryptophan → niacin conversion
- Severe malnutrition / generalised protein-calorie malnutrition
- Chronic alcoholism (poor diet + impaired absorption)
Secondary Pellagra (metabolic causes — high-yield!):
| Cause | Mechanism |
|---|---|
| Hartnup Disease | Autosomal recessive defect in neutral amino acid transporter (SLC6A19) — impairs intestinal absorption and renal reabsorption of tryptophan → reduced substrate for niacin synthesis → pellagra-like rash, cerebellar ataxia, psychiatric symptoms |
| Carcinoid Syndrome | Carcinoid tumour (enterochromaffin cells) massively overproduces serotonin from tryptophan → tryptophan diverted from niacin synthesis → secondary pellagra |
| Isoniazid (INH) therapy | INH is a Vitamin B6 (pyridoxine) antagonist → inhibits kynureninase (B6-dependent) → blocks tryptophan → niacin conversion → pellagra-like symptoms |
| Oral contraceptives | Impair Vitamin B6 metabolism → same kynureninase effect as INH |
| Prolonged corn diet + leucine excess | Leucine inhibits quinolinate phosphoribosyltransferase (QPRT), blocking niacin synthesis |
4D. Diagnosis and Treatment
Diagnosis:
- Clinical (4 Ds in appropriate context)
- Urine: reduced N-methylnicotinamide and its metabolite 2-pyridone (N-methyl-2-pyridone-5-carboxamide) — these are the major urinary excretion products of niacin catabolism; reduced in deficiency
- N-methylnicotinamide:creatinine ratio < 0.5 mg/g creatinine = deficiency
Treatment:
- Nicotinamide 100–300 mg/day orally (preferred over nicotinic acid as it lacks flushing side effects)
- High-protein diet (to provide tryptophan)
- B-complex supplementation (especially B6 if isoniazid is implicated)
- Treat underlying cause (stop offending drug, treat carcinoid tumour, correct diet)
💊 Section 5 — Pharmacological Uses of Niacin (High-Dose Nicotinic Acid)
At doses of 1.5–3 g/day (far above the RDA of ~15 mg/day), nicotinic acid has significant pharmacological effects on lipid metabolism:
| Effect | Mechanism |
|---|---|
| ↓ VLDL (and therefore LDL) | Inhibits hormone-sensitive lipase in adipose tissue → reduces FFA release → less substrate for VLDL synthesis in liver |
| ↓ Triglycerides | Same mechanism — reduced hepatic VLDL secretion |
| ↑ HDL | Inhibits HDL catabolism; reduces hepatic clearance of ApoA-I |
| ↓ Lp(a) | Unique among lipid-lowering drugs — reduces Lp(a) synthesis |
Nicotinic acid is the most effective drug for raising HDL cholesterol and the only drug that reliably lowers Lp(a) — both high-yield pharmacology facts.
Key Side Effects of Pharmacological Niacin:
- Flushing — the most common side effect; prostaglandin-mediated (PGD2); skin warmth/redness/tingling; prevented by aspirin 325 mg 30 min before dose (blocks PGD2 release) or extended-release formulations
- Hepatotoxicity — especially with sustained-release (SR) formulations; monitor LFTs
- Hyperglycaemia — inhibits insulin secretion at high doses; avoid in diabetes
- Hyperuricaemia/Gout — competes with uric acid for renal tubular secretion
- Acanthosis nigricans — at chronic high doses
Flushing pearl: Nicotinamide (the amide form) does NOT cause flushing and does NOT lower lipids — only nicotinic acid has the pharmacological lipid effects. This is a classic exam trap.
🎯 High-Yield Exam Facts
These are the facts that appear repeatedly across NEET PG, USMLE, AIIMS and FMGE papers.
- 🔴 Niacin is the ONLY B-vitamin synthesised in the human body — from tryptophan via the kynurenine pathway; 60 mg tryptophan = 1 mg niacin (60:1 ratio)
- 🔴 The 4 Ds of pellagra: Dermatitis, Diarrhoea, Dementia, Death — in a maize-eating, malnourished, or alcoholic patient = pellagra until proven otherwise
- 🔴 Casal’s necklace — pathognomonic photosensitive dermatitis in a collar distribution around the neck; bilateral, symmetric, photodistributed
- 🔴 Hartnup disease — AR defect in neutral amino acid transporter → tryptophan malabsorption → secondary pellagra + cerebellar ataxia; pellagra-like rash is intermittent and precipitated by sunlight and stress
- 🔴 Carcinoid syndrome → secondary pellagra — serotonin overproduction hijacks tryptophan; carcinoid = diarrhoea + flushing + wheezing + pellagra-like rash
- 🔴 Isoniazid (INH) causes secondary pellagra — INH inhibits B6 → impairs kynureninase → blocks tryptophan → niacin conversion; treated by giving Vitamin B6 (pyridoxine) prophylactically with INH
- 🟠 Vitamin B6 (pyridoxal phosphate) is essential for tryptophan → niacin conversion — specifically required by kynureninase; B6 deficiency → impaired niacin synthesis even with adequate tryptophan
- 🟠 Corn/maize diet → pellagra because niacin is bound (niacytin) — nixtamalisation (alkali treatment) releases niacin; jowar diet causes pellagra via excess leucine blocking QPRT
- 🟠 Active coenzyme forms: NAD⁺ (catabolism) and NADP⁺ (anabolism/antioxidant) — NAD⁺ in ETC/TCA/β-oxidation; NADPH from HMP shunt for fatty acid synthesis and glutathione reduction
- 🟠 Pharmacological niacin: best drug for raising HDL and lowering Lp(a) — unique properties not shared by statins, fibrates, or ezetimibe
- 🟡 Nicotinic acid causes flushing; nicotinamide does NOT — and only nicotinic acid lowers lipids; nicotinamide is used to treat pellagra but not dyslipidaemia
- 🟡 Flushing from niacin is prostaglandin-mediated — PGD2; prevented by aspirin pretreatment; this distinguishes it from opioid-mediated or histamine-mediated flushing
- 🟡 Urine N-methylnicotinamide ↓ in pellagra — the diagnostic biochemical marker; also reduced 2-pyridone excretion
🧠 Mnemonics & Memory Tricks
Mnemonic: “4 Ds of Pellagra“
Stands for: Dermatitis → Diarrhoea → Dementia → Death
Use it for: The complete clinical tetrad in sequence — they also tend to appear in this order as severity progresses
Mnemonic: “CHICK for pellagra causes”
Stands for: Corn (maize diet), Hartnup disease, Isoniazid (INH), Carcinoid syndrome, Kwashiorkor (severe protein deficiency)
Use it for: Remembering all causes of pellagra beyond simple dietary deficiency
Mnemonic: “60 Tries Per Niacin“
Stands for: 60 mg Tryptophan Produces 1 mg Niacin
Use it for: The 60:1 conversion ratio — the most tested number in this entire topic
Mnemonic: “NAD Breaks things Down; NADP Builds things Up“
Use it for: Remembering that NAD⁺ is the catabolic cofactor (accepts electrons during breakdown) and NADP⁺ (as NADPH) is the anabolic cofactor (donates electrons during biosynthesis and antioxidant reactions)
⚠️ Common Mistakes Students Make
❌ Mistake: “All B-vitamins must come entirely from the diet”
✅ Reality: Niacin (Vitamin B3) is unique — it can be synthesised in the liver from tryptophan. This makes it the only B-vitamin with endogenous synthesis. All other B-vitamins (B1/thiamine, B2/riboflavin, B6/pyridoxine, B12/cobalamin, folate, pantothenic acid, biotin) must be obtained entirely from diet or gut bacteria
📝 Exam trap: Questions specifically designed to test this “which vitamin is synthesised in the body” fact — niacin is ALWAYS the answer
❌ Mistake: “Pellagra only occurs in people who eat no niacin”
✅ Reality: Pellagra can occur despite apparent niacin in the diet (e.g., corn/maize diet where niacin is bound and unavailable) or despite adequate tryptophan intake (Hartnup disease, B6 deficiency, carcinoid). The secondary causes are frequently tested in clinical vignette questions
📝 Exam trap: Patient on INH for TB develops skin rash, diarrhoea, and confusion → secondary pellagra from B6 antagonism; also, patient with flushing + diarrhoea + wheezing → carcinoid → check for pellagra
❌ Mistake: “Nicotinamide and nicotinic acid are interchangeable in all situations”
✅ Reality: For treating pellagra — both work (nicotinamide preferred because no flushing). For treating dyslipidaemia — ONLY nicotinic acid works (nicotinamide has NO lipid-lowering effect). For causing flushing — ONLY nicotinic acid
📝 Exam trap: “Which form of niacin is used to treat dyslipidaemia?” = Nicotinic acid. “Which form is used to treat pellagra without side effects?” = Nicotinamide
❌ Mistake: “Hartnup disease affects tryptophan synthesis”
✅ Reality: Hartnup disease affects transport of neutral amino acids (including tryptophan) — not their synthesis. The transporter gene SLC6A19 is defective, impairing intestinal absorption AND renal tubular reabsorption of tryptophan (and other neutral amino acids). Tryptophan that reaches the colon is metabolised by bacteria to indole compounds (causing indicanuria — blue-diaper-like indigo pigments in urine)
📝 Exam trap: Hartnup disease → pellagra-like rash + ataxia + aminoaciduria (neutral amino acids in urine, NOT all amino acids) + indicanuria
❌ Mistake: “Flushing from niacin is an allergic reaction”
✅ Reality: Niacin-induced flushing is a pharmacological effect mediated by prostaglandin D2 (PGD2) — not an immune/allergic reaction. It can be reliably prevented by aspirin (325 mg) taken 30 minutes before the dose, which inhibits COX and reduces PGD2 production. Understanding this mechanism is key for pharmacology questions
📝 Exam trap: “What prevents niacin-induced flushing?” = Aspirin (not antihistamines, not corticosteroids)
🔗 How This Topic Connects to Others
- Tryptophan Metabolism — Tryptophan is also the precursor for serotonin (5-HT) and melatonin; competition between these pathways underpins both carcinoid-related pellagra and the mood effects of tryptophan-rich diets; connects to neurotransmitter biochemistry
- Vitamin B6 (Pyridoxine) — B6 is essential for the kynureninase step in niacin synthesis; isoniazid toxicity connects niacin biochemistry to pharmacology of anti-TB drugs; routinely tested together
- HMP Shunt (Pentose Phosphate Pathway) — NADPH produced by glucose-6-phosphate dehydrogenase in the HMP shunt is the primary source of NADPH for biosynthesis and antioxidant reactions; directly connects to niacin’s coenzyme function
- Carcinoid Syndrome — Serotonin overproduction by carcinoid tumours causes secondary pellagra; connects biochemistry to endocrinology/oncology; carcinoid tumours are also tested extensively in surgery and medicine papers
- Hartnup Disease — Connects to renal tubular amino acid transport disorders; also connects to amino acid absorption physiology and the clinical genetics of autosomal recessive enzyme/transporter deficiencies
- Dyslipidaemia Pharmacology — Nicotinic acid as a lipid-lowering agent is tested in pharmacology alongside statins, fibrates, bile acid sequestrants, and PCSK9 inhibitors; its unique effect on HDL and Lp(a) sets it apart
❓ The MCQ That Started This — Fully Explained
Question: Which of the following vitamins is synthesised in vivo, in the body by humans?
- A. Niacin
- B. Pantothenic acid
- C. Cyanocobalamin
- D. Folic acid
✅ Correct Answer: A. Niacin
Why correct: Niacin (Vitamin B3) is the only B-vitamin that can be synthesised endogenously in the human body. The liver converts the essential amino acid tryptophan to niacin via the kynurenine pathway, with the rate-limiting enzyme being tryptophan 2,3-dioxygenase (TDO). The conversion ratio is 60 mg tryptophan per 1 mg niacin. While this synthesis is insufficient to meet total niacin requirements in the absence of dietary intake, it is a genuine in vivo biosynthetic pathway — unlike any other B-vitamin.
Why B is wrong: Pantothenic acid (Vitamin B5) cannot be synthesised by humans and must be obtained entirely from the diet. It is a component of Coenzyme A (CoA) and is widely distributed in foods (hence “pantothenic” from the Greek for “from everywhere”). While gut bacteria synthesise some pantothenic acid, this is not considered significant human in vivo synthesis.
Why C is wrong: Cyanocobalamin (Vitamin B12) cannot be synthesised by the human body at all — neither in liver nor in any other tissue. Humans obtain B12 exclusively from animal products (meat, dairy, eggs) or from supplements. Note that gut bacteria in the large intestine can synthesise B12, but it cannot be absorbed there (absorption requires intrinsic factor in the terminal ileum, which is above the large intestine where bacteria reside) — so this bacterial synthesis is physiologically inaccessible.
Why D is wrong: Folic acid (Vitamin B9) must be obtained from diet. The human body cannot synthesise the folate ring structure — this is precisely why sulphonamides and trimethoprim work as antibiotics (they inhibit bacterial folate synthesis, which bacteria must perform, while humans must obtain folate from food and are unaffected by these drugs at the folate synthesis step)
📝 Test Your Understanding — 5 Practice MCQs
Q1. Which of the following is the rate-limiting enzyme in the synthesis of niacin from tryptophan?
- A. Kynureninase
- B. Quinolinate phosphoribosyltransferase (QPRT)
- C. Tryptophan 2,3-dioxygenase (TDO)
- D. Indoleamine 2,3-dioxygenase (IDO)
✅ **C. Tryptophan 2,3-dioxygenase (TDO)** — TDO is the first and rate-limiting enzyme of the kynurenine pathway, catalysing the oxidative cleavage of the tryptophan indole ring to form N-formylkynurenine. It is induced by glucocorticoids and tryptophan itself, and requires heme as a cofactor. IDO performs the same reaction but is an extrahepatic isoenzyme induced by interferon-gamma (relevant in immune contexts, not the primary rate-limiting step in hepatic niacin synthesis).
Q2. A 25-year-old patient on isoniazid (INH) for pulmonary tuberculosis develops a photosensitive rash, diarrhoea, and confusion. Supplementation with which vitamin would have prevented these symptoms?
- A. Vitamin B12 (Cyanocobalamin)
- B. Vitamin B6 (Pyridoxine)
- C. Vitamin C (Ascorbic acid)
- D. Vitamin B2 (Riboflavin)
✅ **B. Vitamin B6 (Pyridoxine)** — INH is a structural analogue of pyridoxine that competitively inhibits pyridoxal kinase, depleting active Vitamin B6 (pyridoxal phosphate). PLP is the essential cofactor for kynureninase, a key enzyme in the tryptophan → niacin pathway. B6 deficiency blocks this conversion, causing secondary niacin deficiency → pellagra (rash + diarrhoea + dementia). Standard practice is to co-prescribe pyridoxine 25–50 mg/day with INH to prevent this complication.
Q3. A 30-year-old woman presents with episodic flushing, watery diarrhoea, bronchospasm, and a pellagra-like skin rash. Urine 5-HIAA is markedly elevated. The pellagra-like symptoms in this patient are caused by:
- A. Direct toxicity of serotonin on the skin
- B. Tryptophan diversion to serotonin synthesis, reducing niacin production
- C. Intestinal malabsorption of niacin due to mucosal damage from serotonin
- D. Cross-reactivity of 5-HIAA with niacin receptors
✅ **B. Tryptophan diversion to serotonin synthesis, reducing niacin production** — This is carcinoid syndrome. The carcinoid tumour massively overproduces serotonin (elevated urine 5-HIAA = 5-hydroxyindoleacetic acid, the serotonin metabolite). This serotonin overproduction consumes large amounts of tryptophan, leaving insufficient substrate for the kynurenine pathway to produce niacin. The result is secondary pellagra, superimposed on the classic carcinoid triad of flushing, diarrhoea, and bronchospasm.
Q4. A patient is prescribed high-dose nicotinic acid (2 g/day) for hypertriglyceridaemia. Thirty minutes after the first dose, he experiences intense facial flushing, warmth, and tingling. Which of the following would MOST effectively prevent this reaction with subsequent doses?
- A. Switching to nicotinamide (same dose)
- B. Taking aspirin 325 mg 30 minutes before the nicotinic acid dose
- C. Antihistamine (diphenhydramine) prior to dosing
- D. Reducing the dose to 0.5 g/day
✅ **B. Taking aspirin 325 mg 30 minutes before the nicotinic acid dose** — Niacin-induced flushing is mediated by prostaglandin D2 (PGD2) released from skin Langerhans cells via the GPR109A receptor (the niacin receptor in adipocytes and skin). Aspirin inhibits COX-1 and COX-2, reducing PGD2 production and thereby preventing flushing. Option A (nicotinamide) would eliminate both flushing AND lipid-lowering effects — not a solution if the goal is to treat hypertriglyceridaemia.
Q5. A child presents with intermittent pellagra-like rash, cerebellar ataxia, and psychiatric symptoms. Urine amino acid analysis shows elevated neutral amino acids (tryptophan, alanine, leucine, isoleucine) with normal plasma amino acid levels. Which of the following is the most likely diagnosis?
- A. Classic phenylketonuria (PKU)
- B. Hartnup disease
- C. Maple syrup urine disease (MSUD)
- D. Homocystinuria
✅ **B. Hartnup disease** — The combination of pellagra-like rash (from reduced tryptophan → niacin conversion), cerebellar ataxia, psychiatric symptoms, and neutral aminoaciduria (tryptophan and other neutral amino acids in the urine with normal plasma levels) is pathognomonic of Hartnup disease. The defective SLC6A19 transporter impairs intestinal absorption AND renal tubular reabsorption of neutral amino acids. Plasma levels may be normal because unaffected transporters partially compensate; urine levels are elevated because renal reabsorption is impaired. Treatment includes niacin supplementation and high-protein diet.
📚 References & Further Reading
- Harper’s Illustrated Biochemistry — 32nd Edition; Chapter 44: Micronutrients: Vitamins & Minerals (Niacin section); Chapter 30: Conversion of Amino Acids to Specialised Products (Tryptophan → Niacin)
- Lippincott’s Illustrated Reviews: Biochemistry — 7th Edition; Chapter 28: Vitamins (Water-Soluble Vitamins — Niacin); Chapter 20: Amino Acid Degradation (Tryptophan catabolism)
- Goodman & Gilman’s Pharmacology — 13th Edition; Chapter 31: Drug Therapy for Hypercholesterolaemia and Dyslipidaemia (Niacin as a lipid-lowering agent)
- Robbins & Cotran Pathologic Basis of Disease — 10th Edition; Chapter 9: Environmental and Nutritional Diseases (Pellagra, Vitamin Deficiencies)
- Nelson Textbook of Pediatrics — 21st Edition; Chapter 60: Vitamin Deficiencies and Excesses (Niacin); Chapter 85: Hartnup Disease
🚀 Ready to Master Biochemistry?
You’ve just covered one of the most conceptually rich topics in Biochemistry — niacin synthesis, the kynurenine pathway, pellagra and its causes, and pharmacological niacin. This topic rewards candidates who understand mechanisms rather than memorise isolated facts.
medicalmcq.in has hundreds of free Biochemistry MCQs — each with detailed explanations just like this article.
When you’re ready to simulate real exam pressure, our Mock Test Series gives you timed, subject-wise and full-length tests with performance analytics — so you know exactly where to focus next.
