Vitamin B3 Niacin — Pellagra

• NAD+ Depletion and Impaired Energy Metabolism: Niacin serves as the precursor for NAD+, the critical electron acceptor in glycolysis, the citric acid cycle, and oxidative phosphorylation. In the mitochondrial electron transport chain, NAD+ accepts electrons from substrates via Complex I (NADH dehydrogenase), directly enabling ATP production. Niacin deficiency reduces NAD+ availability, severely compromising ATP synthesis in highly metabolically active tissues including skin, gastrointestinal epithelium, and the central nervous system. Skin cells, which undergo rapid turnover and require substantial ATP for homeostasis, become particularly vulnerable to energy depletion, manifesting as photosensitive dermatitis in sun-exposed areas.

• Impaired DNA Repair and Cellular Turnover: Beyond energy production, NAD+ serves as a substrate for poly-ADP-ribose polymerase (PARP), which catalyzes DNA repair and genomic stability. Additionally, sirtuins—NAD+-dependent deacetylases critical for cellular stress responses and genomic integrity—depend on adequate NAD+ availability. With niacin deficiency, reduced PARP activity impairs the capacity to repair UV-induced and oxidative DNA damage in keratinocytes. This defect, combined with the photosensitive environment of exposed skin, explains the characteristic development of dermatitis in sun-exposed areas (face, neck, hands, and intertriginous regions). The accelerated turnover of epithelial tissues (skin, gastrointestinal mucosa) without adequate ATP or repair capacity results in tissue breakdown and inflammation.

• Altered Metabolic Redox State and Increased Oxidative Stress: NADP+ functions as a critical electron donor in the pentose phosphate pathway and in biosynthetic reactions. Reduced NADP+ availability impairs the regeneration of NADPH, limiting antioxidant defenses including glutathione reduction and catalase function. This creates a state of relative oxidative stress, particularly in tissues already stressed by energy depletion. The cumulative effect is increased lipid peroxidation, protein oxidation, and mitochondrial dysfunction, further compromising cellular viability and perpetuating inflammatory responses in affected tissues.

• Neurochemical and Neuropsychiatric Manifestations: The brain's extraordinarily high metabolic demands (consuming ~20% of total body ATP at rest) make it exceptionally vulnerable to NAD+ depletion. Niacin deficiency impairs monoamine neurotransmitter synthesis and metabolism, affecting dopamine, serotonin, and norepinephrine signaling. Additionally, reduced NAD+ availability compromises the NAD+-dependent sirtuins and PARP function in neurons, impairing stress responses and neuronal repair mechanisms. The result is a constellation of neuropsychiatric symptoms including depression, cognitive decline, psychosis, and dementia, reflecting both direct neuronal dysfunction and impaired neuroendocrine signaling. Prolonged deficiency may cause irreversible neuronal death.

• Gastrointestinal Mucosal Barrier Dysfunction: The gastrointestinal epithelium, one of the most metabolically active tissues, depends heavily on NAD+-driven ATP production for maintenance of tight junctions, active nutrient absorption, and mucus production. Niacin deficiency leads to impaired enterocyte mitochondrial function, loss of epithelial integrity, and increased mucosal permeability. This culminates in chronic diarrhea, malabsorption, and a vicious cycle where gastrointestinal dysfunction further reduces absorption of niacin and other nutrients. Secondary bacterial overgrowth may occur.

• Tryptophan-Niacin Conversion Pathway Dysfunction: Humans can synthesize niacin endogenously from the amino acid tryptophan through the kynurenine pathway, via the intermediate quinolinic acid, which is converted to nicotinic acid mononucleotide by quinolinate phosphoribosyltransferase. One mole of tryptophan yields approximately 1 mg of niacin (the "niacin equivalent"). This conversion requires several vitamin cofactors including pyridoxal-5-phosphate (B6), riboflavin (B2), and iron. Deficiencies in these cofactors impair the conversion pathway, reducing endogenous niacin synthesis even when tryptophan intake is adequate. This partially explains why pellagra often occurs in populations with multiple micronutrient deficiencies.

• Severe Dietary Restriction and Malnutrition: Historically, the most common cause was populations dependent on corn (maize) as a primary carbohydrate source without adequate dietary diversity. Corn contains niacin primarily in bound, non-bioavailable forms; traditional nixtamalization (alkaline treatment) of corn releases bound niacin and increases bioavailability, but this practice was not universal. Endemic pellagra affected populations in the American South, Southern Europe, and parts of Asia from the late 19th through mid-20th century. Contemporary primary dietary deficiency occurs in regions with extreme poverty, food insecurity, or limited dietary diversity, and in developed nations among socioeconomically disadvantaged populations with severely restricted diets.

• Alcoholism and Alcohol Use Disorder: Chronic alcohol abuse is now the most common cause of pellagra in developed countries. Ethanol damages the gastrointestinal mucosa, reducing niacin absorption; ethanol metabolism and its associated oxidative stress increase metabolic demand for NAD+ and other antioxidants; alcoholics frequently have poor nutritional intake and comorbid deficiencies in B vitamins; and ethanol interferes with the tryptophan-to-niacin conversion pathway through impaired hepatic function. Alcoholic cirrhosis may specifically impair the conversion pathway. Pellagra in the setting of alcoholism often presents acutely in the context of acute illness, infection, or other metabolic stress.

• Gastrointestinal Disorders with Malabsorption: Chronic inflammatory bowel disease (Crohn's disease, ulcerative colitis), celiac disease, tropical sprue, and short bowel syndrome reduce niacin absorption. Active inflammation increases mucosal permeability and impairs active nutrient transport. Surgical resection of the small intestine reduces absorptive surface area. These conditions often present with multiple micronutrient deficiencies complicating the clinical picture.

• Carcinoid Syndrome: Neuroendocrine tumors, particularly carcinoid tumors of the small bowel or bronchus, produce serotonin and other vasoactive substances. The increased production and metabolism of serotonin via the kynurenine pathway dramatically increases tryptophan shunting toward serotonin synthesis rather than niacin synthesis. This can precipitate pellagra despite adequate dietary niacin and tryptophan intake. Carcinoid-induced pellagra classically presents with flushing, diarrhea (from increased serotonin), and dermatitis—symptoms that may be attributed solely to the carcinoid syndrome itself.

• Isoniazid and Related Drugs: Isoniazid, a first-line antituberculous drug, inhibits pyridoxal-5-phosphate (B6) function and impairs the tryptophan-to-niacin conversion pathway, increasing the risk of pellagra in patients with marginal niacin status. Other drugs that impair B6 metabolism or metabolism of tryptophan, including fluorouracil (chemotherapy) and 6-mercaptopurine, can similarly increase risk.

• Hartnup Disease: This rare autosomal recessive disorder of amino acid transport causes defective absorption of tryptophan in the intestine and impaired renal tubular reabsorption, leading to marked tryptophan loss. Despite normal or adequate niacin intake, patients cannot efficiently convert tryptophan to niacin. Hartnup disease presents with a pellagra-like syndrome triggered or worsened by sun exposure, poor nutrition, or infection.

• Genetic and Metabolic Disorders: Defects in the kynurenine pathway enzymes or in NAD+ synthesis machinery are exceptionally rare but can cause niacin-dependent disorders. Maple syrup urine disease and other branched-chain amino acid disorders can impair tryptophan metabolism.

• Increased Metabolic Demands: Hyperthyroidism, fever, malignancy, sepsis, and high physical demands increase overall metabolic rate and thus requirements for NAD+-dependent processes. In the setting of marginal niacin stores, these conditions can precipitate pellagra.

• Dialysis and Renal Disease: Chronic kidney disease with uremia and hemodialysis cause loss of water-soluble vitamins including niacin. Additionally, uremia itself increases oxidative stress and NAD+ consumption.

Cardinal Dermatologic Features

• Photosensitive Dermatitis (Most Characteristic Finding): The hallmark dermatologic manifestation is a symmetric, photosensitive dermatitis affecting sun-exposed areas. The classic distribution follows the "glove and stocking" pattern on the hands and "necklace" distribution on the neck and anterior chest. The face, ears, and V-zone of the chest are commonly involved. The dermatitis is sharply demarcated from adjacent normal skin. Initially, affected skin appears erythematous and edematous, resembling severe sunburn. With progression, the ermatitis becomes hyperpigmented, hyperkeratotic, and eventually lichenified or atrophic. Secondary infections are common. The photosensitivity is explained by the vulnerability of sun-exposed skin to oxidative damage (UV radiation) when NAD+-dependent antioxidant defenses are compromised. In chronic cases, desquamation, scaling, and eventual atrophy and pigmentation changes give the skin a weathered, aged appearance.

• Non-Photosensitive Dermatitis in Flexural Zones: In addition to sun-exposed areas, dermatitis may involve intertriginous areas (axillae, groin, under breasts), where moisture, friction, and secondary yeast infection worsen inflammation. This flexural involvement may progress to maceration and erosions if untreated.

Gastrointestinal Manifestations

• Chronic Diarrhea: Often the second most prominent feature, diarrhea is typically watery or loose, non-bloody, and may be profuse, sometimes occurring 10+ times daily. It results from impaired intestinal epithelial function with loss of tight junction integrity, reduced absorption capacity, and increased secretion. Mucosal inflammation contributes. Prolonged diarrhea leads to dehydration, electrolyte depletion (potassium, magnesium, phosphate), and secondary malabsorption of other nutrients.

• Stomatitis and Glossitis: The oral mucosa becomes inflamed, erythematous, and edematous. The tongue may be swollen (glossitis), beefy red, and painful, with loss of papillae. Oral lesions are painful and may impair eating, further reducing nutritional intake. Angular cheilitis (cracks at the corners of the mouth) is characteristic.

• Esophagitis and Dysphagia: Inflammation extends to the esophagus, causing dysphagia and substernal pain with swallowing, which further reduces oral intake.

• Achlorhydria: Some patients develop reduced gastric acid secretion, impairing protein digestion and absorption of other micronutrients. This may be partially reversible with niacin repletion.

Neuropsychiatric Manifestations (The Most Disabling and Prognostically Significant)

• Depression and Mood Disorder: Often the earliest neuropsychiatric manifestation, depression is profound and may be resistant to initial psychiatric intervention. Patients describe anhedonia, hopelessness, and loss of motivation. This stage is sometimes termed the "dementia precox" or primary depressive phase of pellagra.

• Confusion, Disorientation, and Delirium: Cognitive dysfunction may develop acutely or insidiously. Patients become disoriented to time and place, demonstrate poor concentration, and may have delirium with hallucinations, particularly in acute presentations or when concurrent systemic illness is present.

• Personality Changes and Behavioral Disturbances: Irritability, emotional lability, social withdrawal, and inappropriate behavior may precede more obvious cognitive decline. Families often report a marked personality change preceding hospitalization.

• Psychosis and Hallucinations: In acute presentations or advanced deficiency, patients may develop frank psychosis with visual, auditory, or tactile hallucinations, delusions, and paranoia. Historically, some patients with pellagra were institutionalized for psychiatric illness before the nutritional etiology was recognized. The psychosis in pellagra may be described as "organic psychosis" and is fully reversible with niacin repletion if caught early.

• Dementia and Cognitive Decline: With prolonged deficiency, progressive memory loss, global cognitive decline, and dementia develop. This may be partially or fully irreversible if the deficiency is severe or prolonged. Neuropsychological testing may reveal deficits in attention, processing speed, and executive function.

• Tremor, Ataxia, and Movement Disorder: Some patients develop a coarse tremor or ataxia. Hyperreflexia or mild pyramidal signs may occur. These manifestations suggest CNS structural or functional damage.

• Spasticity and Pyramidal Signs: In severe chronic cases, spasticity, hyperreflexia, and pathologic reflexes (Babinski sign) may develop, suggesting demyelination or neuronal loss in motor tracts.

Systemic and Constitutional Manifestations

• Fatigue and Asthenia: Generalized fatigue and weakness result from impaired ATP production and mitochondrial dysfunction. Energy depletion is profound in metabolically demanding tissues.

• Fever: Low-grade fever or constitutional symptoms may accompany acute presentations, particularly when secondary infections (dermatitis, diarrhea-associated) are present.

Physical Examination Findings

• Symmetric dermatitis in characteristic photodistribution with sharp demarcation
• Hyperpigmentation, hyperkeratosis, and lichenification of chronic dermatitis
• Edema and erythema of lips, tongue, and oral mucosa
• Possible secondary infections of skin lesions (bacterial or candidal)
• Evidence of weight loss or cachexia
• Neuropsychiatric abnormalities ranging from depression to frank psychosis
• Mild cognitive impairment or dementia
• Possible tremor or ataxia
• Signs of volume depletion (orthostatic vital signs, dry mucous membranes) from diarrhea

Important Clinical Variants

• Acute Presentation in Alcoholism: Acute decompensation with severe psychiatric symptoms, disorientation, or delirium may occur in alcoholic patients with marginal niacin status who experience acute illness, infection, surgery, or alcohol withdrawal. The presentation may mimic delirium tremens or hepatic encephalopathy.

• Carcinoid-Associated Pellagra: Presentation includes flushing and diarrhea from the carcinoid syndrome plus pellagra's dermatitis and neuropsychiatric manifestations. Recognition of carcinoid as the underlying cause is critical for appropriate tumor management.

• Drug-Induced Pellagra: Patients on isoniazid or other medications affecting B6 or tryptophan metabolism may present insidiously with dermatitis and neuropsychiatric symptoms if overall nutritional status is marginal.

• **Atyp

Initial approach

• Recognize the triad in context: symmetric photodistributed dermatitis (Casal necklace, glove-and-stocking pattern) plus diarrhea plus neuropsychiatric change in a patient with alcohol use disorder, malabsorption, carcinoid, or isoniazid exposure is sufficient to start therapy.
• Baseline labs: CBC (frequently macrocytic anemia from coexisting folate/B12 deficiency), CMP for electrolyte losses and hepatic function, albumin/prealbumin as markers of protein-energy malnutrition, and volume status assessment.
• Screen for concurrent deficiencies: thiamine, pyridoxine, riboflavin, folate, B12, zinc, and iron. Multiple micronutrient deficiency is the rule, and B2/B6/iron are cofactors for tryptophan→niacin conversion.

Confirmatory biochemical testing

• Urinary niacin metabolites are the reference standard: reduced 24-hour excretion of N1-methylnicotinamide (and its 2-pyridone derivative) indicates deficiency. A low 2-pyridone:N1-methylnicotinamide ratio is the more sensitive early marker.
• Whole-blood NAD/NADP ratio (the "niacin number") can be used where available; serum niacin levels are unreliable and are not recommended.
• These assays are send-out tests with slow turnaround, so they confirm retrospectively rather than guide initial management.

Adjunctive and cause-directed testing

• Urinary 5-HIAA if flushing or secretory diarrhea suggests carcinoid; urine amino acid chromatography showing neutral aminoaciduria establishes Hartnup disease.
• Skin biopsy is nonspecific (hyperkeratosis, epidermal atrophy, vacuolar change) and serves mainly to exclude mimics.

Immediate stabilization

• Volume and electrolyte resuscitation: isotonic crystalloid for hypovolemia from profuse diarrhea; replete potassium, magnesium, and phosphate before and during refeeding.
• Give thiamine before any glucose-containing fluid in alcohol-associated or severely malnourished patients — a glucose load in thiamine deficiency can precipitate Wernicke encephalopathy.

First-line therapy

• Nicotinamide (niacinamide) is the agent of choice, given orally in divided doses for several weeks; the WHO guidance on pellagra in emergencies specifies nicotinamide roughly 300 mg/day in divided doses in adults, with lower dosing in children. Nicotinamide is preferred over nicotinic acid because it does not cause prostaglandin D2–mediated cutaneous flushing.
• Parenteral nicotinamide when there is severe diarrhea, vomiting, obtundation, or documented malabsorption.
• Co-administer a B-complex/multivitamin plus zinc and a high-protein, tryptophan-adequate diet; isolated niacin repletion in a polydeficient patient will unmask other deficiencies.

Cause-directed and escalation therapy

• Alcohol use disorder: withdrawal management plus nutritional rehabilitation; niacin repletion alone will fail if drinking continues.
• Isoniazid-associated: add pyridoxine, which CDC/ATS recommend routinely for at-risk patients on isoniazid.
• Carcinoid syndrome: somatostatin analog (octreotide) and surgical resection/debulking of the neuroendocrine tumor are definitive; niacin supplementation is otherwise lifelong while tumor-driven tryptophan shunting persists.
• Hartnup disease: nicotinamide supplementation plus a high-protein diet and photoprotection.

What to avoid

• High-dose nicotinic acid, especially sustained-release formulations, is not the treatment of choice — it causes flushing, hepatotoxicity, hyperglycemia, and hyperuricemia. Note that current ACC/AHA cholesterol guidance no longer endorses niacin for cardiovascular risk reduction after AIM-HIGH and HPS2-THRIVE.
• Do not delay repletion pending confirmatory assays, and do not treat with tryptophan alone.

Complications of untreated disease

• Hypovolemic shock and severe electrolyte derangement — from high-volume secretory/malabsorptive diarrhea. Signaled by orthostatic hypotension, tachycardia, oliguria, and hypokalemia/hypomagnesemia with arrhythmia. Emergency.
• Secondary skin and soft-tissue infection with sepsis — the eroded, macerated dermatitis breaches the epidermal barrier; purulence, spreading erythema, fever, and hemodynamic instability signal cellulitis progressing to bacteremia. Emergency.
• Delirium and organic psychosis — acute NAD+ depletion in a highly ATP-dependent brain; agitation, hallucinations, and disorientation may be misattributed to delirium tremens or hepatic encephalopathy. Reversible with early repletion.
• Irreversible dementia and spastic paraparesis — prolonged deficiency causes neuronal loss and dorsolateral cord degeneration; persistent memory deficits, hyperreflexia, and Babinski sign after repletion indicate structural damage.
• Death — the fourth D. Untreated severe pellagra is fatal, typically within a few years, from sepsis, dehydration, or inanition.
• Progressive malnutrition and coexisting deficiencies — glossitis, esophagitis, and diarrhea create a self-perpetuating cycle of reduced intake and absorption.

Complications of treatment

• Refeeding syndrome — reintroduction of carbohydrate drives insulin-mediated intracellular shift of phosphate, potassium, and magnesium; signaled by falling serum phosphate with weakness, arrhythmia, or heart failure within days of nutritional repletion. Emergency; advance calories slowly and monitor electrolytes daily.
• Wernicke encephalopathy — precipitated by glucose administration before thiamine in an alcohol-dependent patient; ophthalmoplegia, ataxia, confusion. Emergency.
• Nicotinic acid flushing — prostaglandin D2–mediated cutaneous vasodilation with warmth and pruritus; avoided by using nicotinamide, blunted by aspirin pretreatment.
• Hepatotoxicity — a hazard of sustained-release nicotinic acid; transaminase elevation, jaundice, or fulminant failure.
• Hyperglycemia, hyperuricemia/gout flare, and myopathy (the latter increased when nicotinic acid is combined with a statin) are dose-related nicotinic acid effects, not effects of nicotinamide repletion.

• The 3 Ds — dermatitis, diarrhea, dementia — plus death as the fourth. The photodistributed dermatitis is the discriminator; a rash confined to sun-exposed skin with a Casal necklace over the anterior chest is pellagra until proven otherwise.
• Single best next step: start nicotinamide empirically. Do not wait for urinary N1-methylnicotinamide results. Nicotinamide, not nicotinic acid, because it does not flush.
• The association examiners test most: carcinoid syndrome. Serotonin overproduction shunts tryptophan away from the kynurenine pathway, so a patient with flushing, secretory diarrhea, and a photodistributed rash has pellagra from tryptophan diversion — check urinary 5-HIAA.
• Isoniazid → pellagra, because INH depletes pyridoxal-5-phosphate, a required cofactor for tryptophan→niacin conversion. CDC/ATS recommend pyridoxine with isoniazid in at-risk patients.
• Hartnup disease is the genetic mimic: autosomal recessive neutral amino acid transporter defect, neutral aminoaciduria, ataxia and pellagra-like rash in a child, treated with nicotinamide and a high-protein diet.
• Maize-based diets cause pellagra because niacin is bound as niacytin; nixtamalization (alkali treatment) liberates it — the reason pellagra was endemic in the American South but not in Mesoamerica.
• In an alcoholic patient, give thiamine before glucose, and watch for refeeding hypophosphatemia — both are more likely to kill the patient in the first 72 hours than the niacin deficiency itself.

Common distractors

• **Flushing is a feature of niacin *excess*/pharmacologic nicotinic acid, not deficiency** — mediated by prostaglandin D2 and blunted by aspirin.
• Zinc deficiency rash is periorificial and acral, not photodistributed; riboflavin deficiency gives angular cheilitis and corneal neovascularization without dementia.
• Niacin is no longer recommended for lipid lowering by current ACC/AHA cholesterol guidance — do not pick it as add-on therapy for residual risk.