Nitric oxide explained: common questions about your body’s chemical messenger

Nitric oxide often comes up in conversations about circulation, exercise and ageing. But what is it, and why does your body make it?

This guide answers common questions in plain language, explaining the biology without turning it into a promise about how you should feel.

What is nitric oxide, and what does it do?

Nitric oxide, usually shortened to NO, is a small molecule that acts as a chemical messenger. Your body produces it naturally, and it helps cells communicate.

One of its best-understood roles involves blood vessels. Cells lining the vessels produce nitric oxide, which signals the surrounding smooth muscle to relax. This allows the vessel to widen and is one of the mechanisms involved in regulating blood flow.

Nitric oxide also participates in nervous system signalling and aspects of immune defence. Its role depends on where it is produced and the circumstances in which it is released.

How does the body make nitric oxide?

Two important pathways are involved.

The L-arginine pathway

Enzymes called nitric oxide synthases use the amino acid L-arginine to produce nitric oxide.

L-citrulline is connected to this process because the body can convert it into L-arginine. The distinction is useful: L-arginine is used directly by these enzymes, while L-citrulline contributes through its conversion into L-arginine.

The nitrate–nitrite pathway

Nitrate can also contribute to nitric oxide formation. Some nitrate is concentrated in saliva, where bacteria in the mouth convert it into nitrite. Further reactions can then convert nitrite into nitric oxide.

This pathway illustrates how the body’s chemistry and the bacteria living in the mouth can work together.

Does nitric oxide biology change with age?

Research has found age-related changes in nitric oxide production and availability.

For example, a small human study published in 2024 measured whole-body nitric oxide production and found lower production in older participants than in younger participants. This describes a pattern within the study group, rather than a rule that applies identically to everyone.

Production and availability are also different concepts. Production concerns how much is made; availability concerns how much remains able to carry out its biological role.

Research in younger and older adults suggests that physical activity and oxidative stress can influence nitric oxide availability and blood vessel responses. Age is therefore part of a wider picture.

Why is nitric oxide discussed in relation to exercise?

Working muscles require changes in blood flow, and nitric oxide contributes to the way blood vessels respond.

Human studies have found that regular aerobic exercise can improve nitric oxide-mediated blood vessel function. This is one reason researchers study nitric oxide when exploring how the body adapts to physical activity.

However, a molecule’s involvement in exercise does not automatically establish that increasing it will improve performance, energy or recovery. Each of those outcomes needs to be studied separately.

How do amino acids fit into everyday nutrition?

Many amino acids are building blocks of protein. During digestion, the body breaks dietary proteins down into amino acids and uses them to make its own proteins.

Nine amino acids are classed as essential because the body cannot make them in sufficient amounts and must obtain them from food. Protein-containing foods include eggs, dairy, fish, meat, legumes, soy products, nuts and seeds. Adequate quantities and variety across the day both matter.

L-arginine and L-citrulline are not among those nine essential amino acids. L-citrulline is also a non-protein amino acid: it is involved in metabolism rather than being incorporated into proteins during their synthesis.

Does a change in nitric oxide mean someone will feel different?

Not necessarily.

Researchers may measure nitric oxide production, related chemical markers or blood vessel responses. These measurements help explain biological processes, but they do not always translate into a noticeable change in how someone feels.

A study finding needs to be understood alongside what was measured, who participated and whether meaningful health or performance outcomes were assessed.

What is the useful takeaway?

Nitric oxide helps explain how the body coordinates blood vessel responses and other forms of cellular communication.

Its biology involves several pathways, and its production and availability can vary with age and other factors. Understanding these processes provides useful context for reading health information—while remembering that explaining a biological mechanism and demonstrating a health benefit are different tasks.