NAD+ Explained: What It Does and Why Levels Fall

NAD+ is not a peptide and not a supplement in the usual sense. It is a coenzyme that sits in the middle of energy metabolism in every cell you have, and your body makes and consumes enormous amounts of it every day.

Understanding why it gets consumed rather than just recycled is the part that explains all the interest.

What NAD+ actually is

NAD+ stands for nicotinamide adenine dinucleotide. The plus sign matters: it indicates the oxidized form, which is the one able to accept electrons.

Its main job is carrying electrons. When a cell breaks down glucose or fat for energy, it strips electrons off those molecules, and NAD+ is what picks them up. Loaded with electrons it becomes NADH. NADH then delivers them to the electron transport chain in the mitochondria, hands them over, and reverts to NAD+ ready to go again.

That cycle runs constantly through glycolysis, the citric acid cycle and oxidative phosphorylation. Without enough NAD+ in circulation inside the cell, the whole chain of energy production backs up, because there is nothing available to carry electrons away.

The part people miss: it also gets used up

If NAD+ only cycled between two forms, supply would never be a problem. The molecule would be reused indefinitely.

But several enzyme families consume NAD+ as a substrate, meaning they break it apart rather than borrow it. That creates real demand for replacement.

• Sirtuins. A family of seven enzymes, SIRT1 through SIRT7, that remove acetyl groups from proteins including histones. They regulate gene expression, mitochondrial function and stress responses. Every reaction they run consumes a molecule of NAD+. This is the direct link between NAD+ and the longevity research literature.
• PARPs. Poly ADP-ribose polymerases, which respond to DNA damage. When DNA breaks, PARPs activate and consume NAD+ heavily while they work. More DNA damage means more NAD+ spent on repair.
• CD38. An enzyme that degrades NAD+ directly. Its expression increases with age, which means the drain gets larger over time.

So NAD+ is being spent on repair and regulation at the same time as it is being cycled for energy. Those demands compete.

How the body makes it

There are two routes.

The de novo pathway builds NAD+ from the amino acid tryptophan. It works, but it is long and low volume.

The salvage pathway does most of the work. It takes nicotinamide, the fragment left over when an enzyme consumes NAD+, and rebuilds it. Nicotinamide is converted to NMN by an enzyme called NAMPT, then NMN is converted to NAD+ by NMNAT. NAMPT is the rate limiting step, so the capacity of the entire salvage system depends largely on how much NAMPT activity you have.

This is why the precursor compounds people discuss all slot into the same pathway. NR, nicotinamide riboside, enters one step along. NMN enters one step further along again. Niacin comes in through a related route. They are all different entry points into the same rebuilding process.

The decline with age

NAD+ levels fall as people get older. That much is well documented across tissues.

The interesting part is that it happens from both directions at once. Production capacity drops, with NAMPT activity declining. Consumption rises, with CD38 expression increasing and accumulated DNA damage keeping PARPs busier. Supply falls while demand grows.

Because sirtuins depend on NAD+ to function at all, less available NAD+ means less sirtuin activity, and that is the mechanistic thread connecting NAD+ to most of the aging research it appears in.

Why it is injected rather than swallowed

NAD+ is a large, charged molecule. Charged molecules cross cell membranes poorly, and the digestive tract breaks much of it down into components before absorption anyway.

That is exactly why the precursor approach exists. NR and NMN are smaller and better absorbed, and the cell rebuilds NAD+ from them internally. The alternative is bypassing the gut entirely, which is why injectable and intravenous routes are the ones used in practice.

Stocked in 100 mg, 500 mg and 1000 mg vials. Handling follows the usual reconstitution and storage practice.

Where it connects to other compounds

NAD+ turns up repeatedly once you know to look for it.

5-Amino-1MQ blocks NNMT, an enzyme that methylates nicotinamide and takes it out of circulation. Blocking it leaves more nicotinamide available for the salvage pathway, so it acts on NAD+ supply from the side.

MOTS-c works on AMPK, the cell’s energy sensor, and SS-31 supports the structure of the inner mitochondrial membrane where NADH delivers its electrons. Different levels of the same energy system, and the ERR pathway that SLU-PP-332 targets builds the capacity NAD+ carries electrons through.

And NAD+ has a direct relationship with glutathione, covered separately, because glutathione recycling depends on NADPH, which is made from NAD+.

Common questions

What is the difference between NAD+ and NADH?

They are the same molecule in two states. NAD+ is the oxidized form, ready to accept electrons. NADH is the reduced form, carrying them. The cell converts between the two constantly.

Is NAD+ a peptide?

No. It is a dinucleotide, built from nicotinamide, adenine and two sugar-phosphate units. It contains no amino acids and is not a peptide at all.

What is the difference between NAD+, NMN and NR?

NMN and NR are precursors, entering the salvage pathway at different points, and the cell converts them onward into NAD+. NAD+ is the finished coenzyme.

Why do sirtuins keep coming up?

Because sirtuins cannot function without consuming NAD+. They regulate gene expression, mitochondrial function and stress responses, so NAD+ availability directly limits how active they can be.

TL;DR

NAD+ is a coenzyme that carries electrons through energy metabolism, cycling between NAD+ and NADH. It is also consumed outright by sirtuins, PARPs during DNA repair, and CD38. Most of the supply is rebuilt through the salvage pathway, with NAMPT as the rate limiting enzyme. Levels fall with age from both directions: production capacity drops while consumption rises. It is a large charged molecule with poor oral absorption, which is why precursors and injectable routes exist.

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Featured image by knowing3, public domain.