Nicotinamide adenine dinucleotide (NAD+) is an essential coenzyme required by every living cell in the body to support cellular energy production, as well as cellular repair and defense processes. Because NAD+ is constantly consumed and recycled within the body, and can naturally decline with age, cells rely on a steady supply of building blocks called NAD+ precursors to maintain NAD+ levels.
A “precursor” is a smaller molecule that cells convert through a series of chemical reactions into a larger, more complex compound—in this case, NAD+. Dietary NAD+ precursors, including forms of vitamin B3, such as niacin (nicotinic acid), nicotinamide (niacinamide), nicotinamide riboside, and, to a lesser degree, tryptophan, are the building blocks that cells use to create more NAD+. Each enters NAD+ metabolism through distinct routes that determine how efficiently they can raise NAD+ levels.
Niacin, also known as nicotinic acid, is one of the earliest discovered forms of vitamin B3. Discovered in the late 1930s, it quickly gained recognition for its remarkable ability to both prevent and treat pellagra—a severe disease resulting from vitamin B3 deficiency. During the early 1900s, pellagra was widespread across the southern United States, where poverty and corn-based diets were common.¹ʹ² The condition is characterized by the “three Ds”: dermatitis, diarrhea, and dementia. If left untreated, it can ultimately lead to death.³
Niacin occurs naturally in a wide range of foods. Good dietary sources include meat (especially liver), poultry, fish, milk, eggs, yeast, legumes, green vegetables, and cereal grains.⁴ The recommended daily allowance (RDA) for niacin equivalents (NE) is 14 mg per day for women and 16 mg per day for men. One milligram of NE is equivalent to either 1 mg of niacin or 60 mg of tryptophan. These intake levels were originally set to prevent pellagra and to ensure adequate vitamin B3 status in the general population.⁵
Beyond its essential nutritional role, niacin has a long history of use for cardiovascular health and is one of the most widely recognized vitamins used to help manage elevated cholesterol.⁶ When combined with statins, niacin has been shown to increase levels of “good” high-density lipoprotein (HDL) cholesterol,⁷ while producing a more modest reduction in “bad” low-density lipoprotein (LDL) cholesterol.⁸
However, niacin supplementation is not without side effects. One of the most common and uncomfortable reactions is flushing—a temporary sensation of warmth, redness, and tingling of the skin caused by dilation of blood vessels.⁹ Although harmless, this reaction often discourages consistent use. To mitigate this, extended-release niacin formulations were developed, which can lessen flushing in some individuals.⁹ Nevertheless, prolonged use of high-dose extended-release niacin has been linked to liver toxicity,⁵ underscoring the need for careful medical supervision during therapeutic use.
At the cellular level, niacin contributes to the synthesis of NAD+ through the Preiss-Handler pathway.¹⁰ In this three-step process, niacin is first converted to nicotinic acid mononucleotide (NaMN), then to nicotinic acid adenine dinucleotide (NaAD), and finally to NAD+. Although this pathway efficiently produces NAD+ from niacin, it is more energy-intensive than other routes because it involves multiple enzymatic steps that consume ATP (the cell’s primary energy currency) to drive the reactions.
Nicotinamide, also known as niacinamide, is another form of vitamin B3 that was identified in the late 1930s, around the same time as niacin (nicotinic acid). Soon after its discovery, nicotinamide became the preferred form of vitamin B3 for nutritional supplementation and food fortification.¹ Its popularity largely stems from the fact that it does not cause the flushing reaction commonly associated with niacin intake.¹¹
Although nicotinamide is generally better tolerated and more widely used, its biological effects differ from those of niacin in important ways. At high doses, nicotinamide has been shown to inhibit sirtuins—a family of NAD-dependent enzymes that play crucial roles in cellular metabolism, DNA repair, and stress response.¹² Sirtuins are important NAD-dependent enzymes critical in cellular metabolism and cellular repair processes, overall helping to maintain and regulate cellular homeostasis.¹³ Consequently, excessive nicotinamide intake may theoretically offset some of the potential longevity benefits associated with elevated NAD+ levels.
From a metabolic standpoint, nicotinamide supports NAD+ synthesis primarily through the salvage pathway, a highly efficient two-step process that recycles nicotinamide back into NAD+.¹⁰ This pathway serves as the human body’s main mechanism for maintaining intracellular NAD+ pools, ensuring a steady supply of this essential coenzyme for energy metabolism, cellular repair, and numerous enzymatic reactions.
Nicotinamide riboside (NR) is the third and most recently identified form of vitamin B3, first discovered in the 1940s. Its biological importance, however, was not fully recognized until 2004, when researchers demonstrated that NR could effectively elevate intracellular levels of NAD+.¹⁴ The same research group also detected trace amounts of NR in milk, establishing it as a naturally occurring nutrient.¹⁴ However, the amount is extremely low: one would need to drink roughly 87 gallons of milk to obtain the same 300 mg dose of NR typically used in supplements.
Although NR belongs to the vitamin B3 family, it exhibits unique metabolic and physiological properties that distinguish it from both niacin and nicotinamide. Unlike niacin, NR does not cause flushing, even at high doses, and unlike nicotinamide, it does not inhibit sirtuin activity.¹⁵ Sirtuins are a family of NAD-dependent enzymes crucial for DNA repair, metabolic regulation, and longevity. In fact, preclinical studies suggest that NR may enhance sirtuin function,¹⁵ supporting cellular health. The specific advantages of NR are explored further in the section below titled “Why Nicotinamide Riboside Stands Apart from Niacin and Nicotinamide.”
At the cellular level, NR supports NAD+ production through the nicotinamide riboside kinase (NRK) pathway.¹⁴ Once inside the cell, NR is converted in two steps—first into nicotinamide mononucleotide (NMN), and then into NAD+. This pathway bypasses the rate-limiting steps of other NAD+ biosynthetic routes, making NR one of the most direct and metabolically efficient precursors for restoring intracellular NAD+ levels.
To improve stability and bioavailability, NR is commonly formulated as a crystalline salt.¹⁶ Two of the most well-characterized forms are nicotinamide riboside chloride (NRCl) and nicotinamide riboside hydrogen malate (NRHM, or also known as nicotinamide riboside malate). Both deliver the same active NR molecule upon ingestion, which is subsequently converted into NAD+ within cells. However, because the two salts have different molecular weights, with NRHM being heavier, the actual amount of NR delivered per dose varies. For example, a 300 mg dose of NRCl provides roughly 263 mg of NR (the remainder being chloride), whereas the same 300 mg of NRHM provides only about 197 mg of NR, with the rest being hydrogen malate.
Nicotinamide riboside chloride was the first NR salt form to be successfully reviewed twice under the Food and Drug Administration’s (FDA) New Dietary Ingredient (NDI) notification program (NDI 882; NDI 1062) and successfully notified to the FDA as generally recognized as safe (GRAS).
Nicotinamide riboside hydrogen malate,¹⁷ a newer salt form, has likewise been successfully notified under the NDI program (NDI 1243) and may offer enhanced stability. However, it has not yet been evaluated as a standalone ingredient in published clinical studies. To date, only one clinical study, published as a preprint, has investigated NRHM in combination with other compounds—such as magnesium beta-hydroxybutyrate, glutathione, and coenzyme Q10 (CoQ10)—to assess potential effects on brain function.¹⁸ As a result, its independent safety and efficacy in humans remain undetermined.
A third option, nicotinamide riboside hydrogen tartrate (NRHT)—also referred to as NR tartrate—represents another recently introduced salt form.¹⁷ Unlike NRCl and NRHM, NR tartrate has been described primarily in technical and production-oriented literature,¹⁷ and no human clinical studies have evaluated its safety, bioavailability, or efficacy. It also lacks any known NDI notifications or GRAS submissions, and no regulatory reviews specific to NR tartrate are publicly accessible. Because of this, there is currently no evidence that NR tartrate is a safe or effective way to raise NAD+ levels in humans. Its higher molecular weight also means that a 300 mg dose delivers even less NR than NRHM—approximately 189 mg—with the rest consisting of hydrogen tartrate.
For a brief comparison of all NR salt forms, see Table 1.
Tryptophan is an essential amino acid, meaning it must be obtained through the diet since the body cannot synthesize it.
