Niacinamide (Nicotinamide) - Vitamin B3

Chemical Identity & Structure

Pharmacokinetics

• Absorption: Dietary NAD and NADP are hydrolyzed by intestinal NAD glycohydrolase to release nicotinamide, which is absorbed in the stomach and small intestine by Na+-dependent facilitated diffusion at low concentrations and passive diffusion at higher concentrations.
• Circulation: Nicotinamide is the main circulating form in plasma.
• Metabolism: Inside cells (liver, kidney, brain), nicotinamide is converted to NAD and NADP via phosphoribosyltransferase reactions. NADP is then formed by kinase-catalyzed phosphorylation of NAD.
• Excretion: Excess is methylated in the liver to N-methylnicotinamide (NMN) and excreted in urine, along with its oxidation products (N-methyl-2-pyridone-5-carboxamide and N-methyl-4-pyridone-carboxamide).
• Storage: Minimal storage occurs in the body.

Biochemical Functions

1. NAD/NADH & NADP/NADPH co-enzyme activity - Niacinamide is incorporated into NAD+ and NADP+, which act as hydrogen/electron carriers in hundreds of redox reactions: glycolysis, fatty acid synthesis and oxidation, the TCA cycle, and amino acid metabolism.
2. DNA repair via PARP-1 - NAD is a substrate for poly(ADP-ribose) polymerase-1 (PARP-1), an enzyme critical for base excision repair and genomic stability. This is why nicotinamide deficiency impairs DNA repair.
3. Cellular energy - Prevents ATP depletion, particularly under UV radiation stress in keratinocytes.

Dietary Sources & Requirements

• Rich sources: meat, fish, liver, eggs, mushrooms, nuts, legumes, dairy, whole grains.
• Tryptophan is a precursor: ~60 mg tryptophan yields 1 mg niacin equivalent (NE). Up to two-thirds of adult niacin requirements can be derived from tryptophan.
• Bound niacin in corn/maize (niacytin) is nutritionally unavailable without alkaline hydrolysis (e.g., lime-water treatment used in Mexican cuisine - "nixtamalization").

Deficiency: Pellagra

• Caused by inadequate niacin AND tryptophan intake, or impaired conversion.
• At-risk populations: populations subsisting on untreated maize (parts of South Africa, India, China); alcoholism; isoniazid therapy (blocks B6 needed for tryptophan conversion); malabsorptive GI diseases; carcinoid syndrome (tryptophan diverted to serotonin).
• Treatment: Nicotinamide 100 mg three times daily (oral or parenteral) to correct malnutrition; animal proteins, eggs, milk, and vegetables alongside supplementation.

Clinical Uses

1. Chemoprevention of Non-Melanoma Skin Cancer (NMSC)

• Dose: 500 mg orally twice daily for 12 months
• A large randomized Phase III trial showed a 23% lower rate of newly formed NMSC (both SCC and BCC) compared with placebo, with a favorable safety profile.
• Mechanism: prevents UV-induced NAD depletion in keratinocytes, restores cellular energy, enhances DNA repair, and inhibits UV-induced immunosuppression.
• Also significantly reduces actinic keratoses in high-risk patients.
• Note: in immunosuppressed organ-transplant recipients, benefit was not demonstrated.

2. Acne Vulgaris (Topical)

• 4% nicotinamide gel is available OTC and is as effective as clindamycin 1% gel (non-inferior) for moderate inflammatory acne in comparative studies.
• 2% nicotinamide cream reduces sebum production and improves facial shine/oiliness within 4-6 weeks.
• Mechanism: anti-inflammatory (reduces IL-1, IL-6, TNF-alpha release), sebostatic, and increases ceramide production to reinforce skin barrier.

3. Hyperpigmentation & Photoaging

• 5% nicotinamide cream applied twice daily for 12 weeks in a split-face study improved fine lines, wrinkles, hyperpigmented spots, redness, sallowness, and skin elasticity.
• Mechanism: inhibits melanosome transfer from melanocytes to keratinocytes (interferes with the Pak1 signaling pathway), without directly inhibiting tyrosinase (contrast with hydroquinone or vitamin C).
• Often combined with 2% N-acetyl glucosamine; a double-blind RCT (n=202) found this combination significantly reduced facial spots and pigmentation vs. vehicle.

4. Glaucoma (Investigational)

• Clinical trials have commenced to evaluate whether supplementary nicotinamide is neuroprotective in glaucoma patients, given its role in maintaining cellular NAD+ levels in retinal ganglion cells.

5. Dyslipidemia (Nicotinic Acid - Note the Distinction)

• At pharmacologic doses, nicotinic acid (not niacinamide) is used for dyslipidemia. Niacinamide does not cause flushing or have lipid-lowering effects because it does not activate the GPR109A (HM74A) receptor on adipocytes and Langerhans cells that nicotinic acid activates.

Key Distinction: Niacinamide vs. Nicotinic Acid

Safety Profile

• Well-tolerated orally at 500 mg twice daily (standard chemoprevention dose).
• Does not cause the cutaneous flushing seen with nicotinic acid.
• High oral doses (>3 g/day) can cause hepatotoxicity, nausea, and hyperglycemia, but these are uncommon at therapeutic dermatological doses.
• Topically (2-5%): excellent safety record, non-irritating, suitable for sensitive skin.