NAD vs NAD+ vs NADH: What the Notation Means

NAD, NAD+ and NADH are not three different substances. They are one coenzyme in different notations and redox states. “NAD” is the general name, “NAD+” specifies the oxidised form, and “NADH” is the reduced form. The confusion is common enough that catalogue listings and search queries treat them as rivals, so it is worth setting out plainly.

For laboratory research use only. The compounds discussed are not approved for human or veterinary use and are not drugs, foods or cosmetics. This article summarises published research and is not medical advice.

Key points

NAD = the coenzyme in general; NAD+ = oxidised; NADH = reduced.
The plus sign refers to the charge on the nicotinamide ring, not to a different compound.
NADH is 2 Da heavier and absorbs at 340 nm; NAD+ does not.
The NAD+/NADH ratio, not the absolute amount alone, reflects cellular redox state.
NAC (N-acetylcysteine) is an unrelated molecule.

The notation explained

Written as	Means	Formula	Mass
NAD	The coenzyme, state unspecified	—	—
NAD+	Oxidised form	C₂₁H₂₇N₇O₁₄P₂	663.4 g/mol
NADH	Reduced form	C₂₁H₂₉N₇O₁₄P₂	665.4 g/mol
NADP+ / NADPH	Phosphorylated versions, used in biosynthesis	—	—

The plus sign is a formal charge notation. In NAD+, the nitrogen of the nicotinamide ring is quaternary and positively charged. When the ring accepts a hydride ion (H⁻, a proton with two electrons), that charge is neutralised and the ring becomes a dihydropyridine. The product is NADH.

The redox cycle

NAD+ and NADH interconvert constantly:

Direction	What happens	Typical context
NAD+ → NADH	Accepts a hydride; is reduced	Glycolysis, the citric acid cycle, fatty acid oxidation
NADH → NAD+	Donates the hydride; is oxidised	The mitochondrial electron transport chain; lactate dehydrogenase

Because the molecule is recycled, cells do not need to synthesise new NAD+ for each redox reaction. Synthesis matters because of the other set of enzymes, sirtuins, PARPs and CD38, which consume NAD+ rather than recycling it (Covarrubias et al., 2021). This consumption is why NAD+ levels can fall even though the redox cycle itself is conservative.

Why the ratio matters

Researchers usually care about the NAD+/NADH ratio rather than either value alone, because the ratio reflects the cell’s redox state and drives the direction of many reactions. A cell with plenty of NAD+ relative to NADH can run oxidative reactions readily; the reverse state favours reductive chemistry. Reviews of NAD+ in ageing and neurodegeneration discuss how modulating NAD+ availability affects both the redox balance and the consuming enzymes (Verdin, 2015).

Distinguishing them in practice

The 340 nm absorbance of NADH is the most useful practical difference. NAD+ absorbs at 260 nm (from adenine) but not at 340 nm; NADH absorbs at both. This is the basis of standard enzyme assays, where a rising or falling 340 nm signal tracks the reaction.

It is also how degradation is detected. A long-term stability study noted that cofactor degradation can be monitored by UV-Vis, because it proceeds through oxidation and de-aromatisation of the dihydropyridine ring (Wolfe et al., 2024). An earlier study of NADH stability ranked the main effects as pH first, then temperature, then buffer, with faster degradation in phosphate buffer attributed to adduct formation with the pyridine ring (Rover Júnior et al., 1998).

Condition	Effect on nicotinamide cofactors
Tris buffer	Most stable of the three tested (Tris, phosphate, HEPES)
Phosphate buffer	Faster NADH degradation; adduct formation reported
Higher temperature	Significantly reduces long-term stability
Acidic pH	Unfavourable for NADH
Alkaline pH	Unfavourable for NAD+

What NAD+ is not

Not NAC. N-acetylcysteine is a modified amino acid and glutathione precursor, unrelated to NAD+ in structure or function.
Not a peptide. NAD+ contains no amino acids; see what is NAD+?
Not NMN. NMN is a precursor, about half the size; see NMN vs NAD+.

Our NAD+ research material is supplied in 100 mg, 500 mg and 1000 mg vials. For preparation, see reconstituting NAD+.

Open questions

How best to measure the NAD+/NADH ratio in intact tissue without perturbing it during extraction?
How do compartment-specific ratios (cytosol, mitochondria, nucleus) differ, and how independently are they regulated?

Related research

Frequently asked questions

What is the difference between NAD and NAD+?

They normally refer to the same molecule. ‘NAD’ is the general name for the coenzyme; ‘NAD+’ specifies the oxidised form, whose nicotinamide ring carries a positive charge. In most writing the two are used interchangeably.

Is NAD+ oxidised or reduced?

NAD+ is the oxidised form. NADH is the reduced form, carrying two extra electrons and one proton relative to NAD+.

What is the difference between NADH and NAD+?

One hydride transfer. NAD+ accepts a hydride ion to become NADH; NADH donates it to return to NAD+. NADH is 2 Da heavier and absorbs light at 340 nm, which NAD+ does not.

Is NADH oxidised or reduced?

Reduced. Remember it by the H: NADH carries the hydride it can donate.

Is NAC the same as NAD?

No. NAC is N-acetylcysteine, a modified amino acid used as an antioxidant and glutathione precursor. It is chemically unrelated to NAD+ despite the similar abbreviation.

References

Covarrubias AJ, et al. NAD(+) metabolism and its roles in cellular processes during ageing. Nat Rev Mol Cell Biol. 2021. PMID: 33353981 https://pubmed.ncbi.nlm.nih.gov/33353981/
Verdin E. NAD+ in aging, metabolism, and neurodegeneration. Science. 2015. PMID: 26785480 https://pubmed.ncbi.nlm.nih.gov/26785480/
Wolfe KD, et al. Long-Term Stability of Nicotinamide Cofactors in Common Aqueous Buffers: Implications for Cell-Free Biocatalysis. Molecules. 2024. PMID: 39598842 https://pubmed.ncbi.nlm.nih.gov/39598842/
Rover Júnior L, et al. Study of NADH stability using ultraviolet-visible spectrophotometric analysis and factorial design. Anal Biochem. 1998. PMID: 9648652 https://pubmed.ncbi.nlm.nih.gov/9648652/
PubChem Compound Summary for CID 5892, NAD. National Center for Biotechnology Information. https://pubchem.ncbi.nlm.nih.gov/compound/5892