What is Vitamin B3 (Niacin)?
Vitamin B3, also known as niacin, is an essential water-soluble vitamin that exists in multiple forms—nicotinic acid, nicotinamide (niacinamide), and nicotinamide riboside—and serves as the precursor to the critical coenzymes NAD+ and NADP+, which are fundamental for energy metabolism, DNA repair, and cellular function throughout the body. 1, 2, 3
Chemical Forms and Nomenclature
Vitamin B3 encompasses several related compounds:
- Nicotinic acid (also called niacin in the strict sense) 1, 4
- Nicotinamide (also called niacinamide) 1, 4
- Nicotinamide riboside 2
- All forms can be converted to the active coenzyme forms NAD+ and NADP+ 3, 5
The body can also synthesize niacin from the amino acid tryptophan at a conversion ratio of approximately 60:1 (60 mg tryptophan yields 1 mg niacin), though this conversion depends on adequate intake of other cofactors like vitamin B6. 6
Biochemical Function
Niacin functions primarily as a component of the coenzymes NAD+ (nicotinamide adenine dinucleotide) and NADP+ (nicotinamide adenine dinucleotide phosphate), which can be reversibly reduced to NADH and NADPH. 6
These coenzymes are essential for:
- Energy metabolism: Transfer of hydride ions (H–) in dehydrogenase-reductase systems, driving ATP production in mitochondria 6
- DNA repair: Serving as substrates for poly(ADP-ribose) polymerases (PARP) that perform DNA base excision repair 6
- Gene expression and cell signaling: Acting as substrates for sirtuins (SIRT1, SIRT2) that regulate cellular processes 6
- Calcium homeostasis, mitochondrial function, antioxidation, and immune function 6
NAD+ participates in over 400 enzymatic reactions in the body, making it one of the most versatile coenzymes in human metabolism. 7
Dietary Sources
The richest food sources of vitamin B3 include yeasts, teas and coffees, whole-grain cereals, dark-green leafy vegetables, poultry and meats, red fish (especially tuna and salmon), nuts and legumes, and organ meats (particularly liver). 6
Important considerations about bioavailability:
- Animal-based foods contain approximately 30% nicotinic acid and 70% nicotinamide 8
- Plant-based foods (cereals and legumes) contain approximately 87% nicotinic acid and 13% nicotinamide 8
- Cereal grains (particularly wheat and corn) contain niacin bound to glycosides, which significantly reduces bioavailability 6, 1
- Fortified packaged foods are reliable sources with good bioavailability 1, 7
Recommended Daily Intake
The American College of Nutrition and UK guidelines provide the following recommendations:
- Adult men: 16 mg/day 6, 1
- Adult women: 12-14 mg/day 6, 1
- Pregnant women: 18 mg/day 1
- Lactating women: 17 mg/day 1
- Upper tolerable limit: 35 mg/day for general supplementation to avoid adverse effects 1
For patients requiring clinical nutrition support:
Deficiency and Clinical Manifestations
Severe niacin deficiency causes pellagra, classically characterized by the "3 Ds": diarrhea, dermatitis, and dementia. 9, 2
Neuromuscular symptoms of deficiency include:
- Muscle weakness and wasting 6
- Gait and truncal ataxia 6
- Peripheral neuritis 6
- Limb areflexia and myoclonus 6
Non-specific early symptoms include:
- Anorexia, weakness, inactivity, and fatigue 6
- Anxiety, irritability, and depression 6
- Decline in nerve transmission velocities 6
Risk factors for deficiency include corn-based diets, malnutrition, chronic alcoholism, and malabsorption states. 9, 2
Important Clinical Distinctions Between Forms
Nicotinic acid causes facial, arm, and chest flushing at doses as low as 30 mg, while nicotinamide does not cause flushing. 1, 7
This difference has important therapeutic implications:
- Nicotinic acid has lipid-lowering effects at pharmacological doses (500-2000 mg/day) but causes flushing 1, 3
- Nicotinamide does not affect lipid levels but has a better tolerability profile for general supplementation 7
- The upper safety limit for free nicotinic acid is only 10 mg/day due to flushing, while nicotinamide can be safely used up to approximately 900 mg/day in adults 7, 9
Therapeutic Applications
Beyond treating deficiency, niacin has been studied for:
- Lipid management: The American College of Cardiology recommends against routine use of pharmacological niacin (500-2000 mg/day) in statin-treated patients 1
- Dermatological conditions: Topical niacinamide for acne, hyperpigmentation, and promoting epidermal barrier function 4
- Neuroprotection: Potential roles in neurodegenerative diseases, though this remains investigational 5
Safety Monitoring for Therapeutic Doses
When using therapeutic doses (≥500 mg/day), the American College of Cardiology recommends monitoring:
- Hepatic transaminases (ALT, AST) at baseline and every 6 months 1, 7
- Fasting glucose or HbA1c at baseline and periodically 1
- Uric acid levels 1
- Discontinue if transaminases exceed 2-3 times upper limit of normal 1, 7
Niacin should not be used in patients with active liver disease, unexplained hepatic dysfunction, or significant transaminase elevations. 1