Nicotinamide adenine dinucleotide (NAD) is a molecule found in every cell, crucial for cellular functions and energy production. The body relies on precursor molecules from diet or supplements to synthesize NAD. There is increasing interest in these precursors for their potential health influence.
NAD: The Cell’s Essential Coenzyme
NAD exists in two primary forms: NAD+ (oxidized) and NADH (reduced). This coenzyme is involved in redox reactions, transporting electrons within cells. NAD also participates in other cellular processes, acting as a substrate for enzymes that modify proteins.
NAD is a cofactor for enzymes in metabolic pathways, including glycolysis, fatty acid oxidation, and the citric acid cycle, contributing to energy production like ATP synthesis. Beyond energy, NAD is involved in critical cellular processes like DNA repair and cellular signaling. Enzymes like sirtuins and poly(ADP-ribose) polymerases (PARPs) use NAD to regulate functions such as cell survival, DNA repair, and gene expression.
Meet the Precursors
The body uses several precursors to synthesize NAD, primarily falling into vitamin B3 derivatives and an amino acid. These precursors include Nicotinamide Riboside (NR), Nicotinamide Mononucleotide (NMN), Nicotinic Acid (NA), Nicotinamide (NAM), and Tryptophan.
Nicotinamide Riboside (NR) is a form of vitamin B3. It consists of nicotinamide and ribose as its fundamental components. NR is converted by the body into NAD+ and is recognized for its role in supporting cellular energy and repair.
Nicotinamide Mononucleotide (NMN) is an intermediate compound in the pathway to NAD+. It is a nucleotide derived from ribose and nicotinamide, a form of vitamin B3. NMN is one step away from becoming NAD+, making it efficient for NAD+ production.
Nicotinic Acid (NA), also known as niacin, is another form of vitamin B3. It was identified in the late 1930s and historically used to address pellagra, a condition resulting from severe niacin deficiency. NA plays a role in converting carbohydrates into glucose and metabolizing fats and proteins.
Nicotinamide (NAM), also called niacinamide, is a water-soluble form of vitamin B3. It is an amide of nicotinic acid. NAM is involved in the function of fats and sugars in the body and helps maintain healthy cells.
Tryptophan is an essential amino acid, meaning the body cannot produce it and must obtain it from the diet. While not a vitamin B3 itself, tryptophan can be converted into NAD, primarily in the liver. This conversion makes tryptophan a dietary source of niacin.
Turning Precursors into NAD
The body converts these precursors into NAD through several metabolic pathways. These include the Preiss-Handler pathway, the salvage pathway, and the de novo pathway. These pathways ensure a continuous supply of NAD within cells.
The de novo pathway synthesizes NAD from the amino acid Tryptophan. This process mainly occurs in the liver and kidneys. Tryptophan undergoes a series of steps to form quinolinic acid, which is then converted into nicotinic acid mononucleotide (NAMN) and nicotinic acid adenine dinucleotide (NAAD) before becoming NAD.
The Preiss-Handler pathway utilizes Nicotinic Acid (NA). In this pathway, NA is converted into NAMN by an enzyme called NAPRT. NAMN is then transformed into NAAD, which is subsequently converted to NAD.
The salvage pathway is a significant route for NAD production and recycling. It primarily uses Nicotinamide (NAM) and Nicotinamide Riboside (NR), and Nicotinamide Mononucleotide (NMN). When NAD is consumed by cellular processes, it breaks down into NAM, which the salvage pathway recycles back into fresh NAD. NR and NMN are converted to NAD by forming NMN through specific enzymes before entering the salvage pathway.
Finding NAD Precursors
Many foods contain NAD precursors, making it possible to obtain these compounds through a balanced diet. Animal-based foods often provide NAD and NADP directly or various forms of niacin. Plant-based foods also contribute to precursor intake.
Nicotinamide Riboside (NR) is found in trace amounts in certain foods, including cow’s milk and yeast. Nicotinamide Mononucleotide (NMN) is present in various vegetables and fruits, such as broccoli, cabbage, cucumber, avocado, and tomatoes. Raw beef and shrimp also contain NMN.
Nicotinic Acid (NA) and Nicotinamide (NAM) are both forms of vitamin B3 found in a wide variety of foods. Good sources include meat, poultry, fish, eggs, and fortified cereals. Nuts, legumes, and grains also provide these forms of niacin.
Tryptophan, as an essential amino acid, is found in protein-rich foods. Common sources include poultry like turkey and chicken, fish such as salmon, eggs, and dairy products like milk and cheese. Plant-based sources include nuts, seeds, and legumes.
Dietary supplements containing NR, NMN, or other precursors are also available. Some individuals consider supplementation to support NAD levels, particularly as natural levels may decline with age. These supplements offer a concentrated way to increase precursor intake.