Nitric oxide (NO): what it is, what it does in the body, and how you can support it

In 1998, the Nobel Prize in Medicine was awarded for the discovery of a single molecule. A gas. One your body produces itself. You may never have heard of it.

Robert Furchgott, Louis Ignarro and Ferid Murad received the highest scientific honour for showing that nitric oxide — NO for short — is the key regulator of your blood vessels. Not a hormone. Not a nutrient from food. A gas your body makes on demand.

Why does that matter? NO determines how widely your blood vessels open — and therefore how much oxygen and nutrients reach your muscles, heart and brain. The body produces less of it as we age. There are two independent ways to support it from outside the body.

What NO actually does in your body

Nitric oxide is not a nutrient you take in; it is a messenger your body makes itself when needed. Once present, it sends a clear signal to the muscle cells in your blood vessel walls: relax.

The blood vessels widen. More blood flows through. This has several direct effects:

Oxygen supply: Working muscles receive more blood — and therefore more oxygen and nutrients. Studies show that after nitrate supplementation, athletes can achieve the same physical performance with measurably less oxygen consumption. Specifically, endurance athletes lasted 16% longer in controlled tests — not because they became stronger, but because their muscles worked more efficiently ^[1].

Blood pressure: When blood vessels widen, resistance to the heart’s pumping decreases. Meta-analyses show a consistent reduction in systolic blood pressure of 3–5 mmHg with regular nitrate intake, particularly when baseline readings are elevated ^[7].

Blood flow to the brain: NO does not act only in the large muscles — it regulates circulation throughout the body, including the brain. Recent research is examining whether improved blood flow to the brain from dietary nitrate supports cognitive function, particularly with age. The evidence is still developing, but the mechanism is plausible ^[5].

Muscle efficiency: NO improves the way your muscles produce energy. The result: you can do more for the same effort — or manage the same effort with less fatigue. You may notice this particularly in the final minutes of a long bout of exercise.

The first pathway: what you eat

Some vegetables contain high concentrations of inorganic nitrate — beetroot above all, followed by spinach, rocket and chard. When you eat them, a simple cascade takes place in your body.

First, something happens in your mouth: bacteria on your tongue convert dietary nitrate into nitrite. This is the first step, and you cannot skip it — which is why using antibacterial mouthwash shortly before or after taking nitrate can greatly reduce its effect ^[3]. The actual NO is then produced in the stomach and tissues.

The key advantage of this pathway is that it also works during exercise, when the body is short of oxygen. Other routes to NO production need sufficient oxygen; this one does not. It is, in a sense, a reserve system that takes over precisely when it is needed most: in the second half of a long run, the final third of a cycling race or a sprint to the finish.

This explains why dietary nitrate shows such consistent effects in endurance studies. It comes into play exactly where performance would otherwise falter ^[8].

Timing: Levels peak around 2–3 hours after taking it. If you want to use dietary nitrate before training, plan accordingly.

The catch with retail products: nitrate content varies considerably between varieties, growing regions and batches. One fresh beetroot may contain 50 mg of nitrate, another 150 mg. To know what you are taking, choose a standardised source that states the nitrate content in mg. The article on beetroot powder and standardisation.

The second pathway: L-citrulline as a building block for more NO

The second route to NO production starts inside the body and needs a different building block: L-citrulline.

L-citrulline is an amino acid that your body converts to L-arginine after you take it. L-arginine is what the cells in your blood vessel walls need to produce NO directly. The more L-arginine is available, the more NO can be produced — especially during physical exertion, when demand is greatest.

Why L-citrulline instead of L-arginine directly? Because supplemental L-arginine rarely reaches the places where it is needed. Most of it is broken down in the gut and liver before reaching the blood vessels. L-citrulline, by contrast, passes through largely intact and is converted to arginine only once it reaches the tissues. Studies show that L-citrulline raises blood arginine levels 2 times as effectively as the same amount of L-arginine taken directly ^[4]. That is why L-citrulline has long since replaced L-arginine as the standard in research.

This pathway is particularly relevant in two situations. First, during intense training, when short-term demand for NO is high and the body needs more raw material than it gets from a normal diet. Second — and this applies to many people — as we age. The body’s own NO production measurably declines from the 4th decade of life ^[6]. L-citrulline supplies more raw material, helping the body continue to produce NO despite declining capacity of its own.

Regular endurance training remains the strongest stimulus for this pathway — no supplement replaces it. But L-citrulline can supplement what training alone no longer fully provides.

Timing: L-citrulline acts faster than dietary nitrate: the relevant levels rise after around 60–90 minutes.

Why two pathways are better than one

Both pathways produce the same result, but under different conditions and at different times. That makes them complementary.

Combining both pathways gives you a broader window of action and covers different situations in life. For endurance athletes, L-citrulline provides NO early in a training session; dietary nitrate takes effect 2–3 hours later and remains active when exertion continues and oxygen is scarce ^[5]. For anyone over 40, as the body’s own NO production declines with age, using both pathways together may help make up for what the body can no longer produce on its own — supporting steadier blood vessel function and exercise capacity without relying on a single system.

The two pathways do not compete with each other. They cover different time windows and situations. That is the logic behind a product designed to support both at once.

What happens to NO as we age — and why it matters in practice

This is the part many people overlook.

The body’s own NO production measurably declines from the 4th decade of life. Blood vessels become less flexible and production falls ^[6]. This helps explain some of the changes many people notice after 40: higher blood pressure, reduced exercise capacity and slower recovery after sport.

This is where both pathways come in, for different reasons. The citrulline–arginine pathway supplies more of the raw material the blood vessel walls need to keep producing NO as the body’s own capacity declines. The nitrate pathway goes a step further: it works independently of the body’s own production capacity and may compensate for what that production no longer supplies.

A 12-week study of standardised beetroot extract in older participants found measurable increases in blood markers relevant to NO, with good tolerability throughout the study ^[2]. That is why dietary nitrate is not just a sports supplement for people over 40, but also a daily tool for vascular health.

How to support both pathways

Regular training remains the foundation for the citrulline–arginine pathway. No supplement replaces it. But L-citrulline as a supplement supplies more raw material — particularly when training alone is not enough: during intense exercise, with age or when recovery takes longer than it used to. Studies have used effective doses of 3 g L-citrulline and above ^[4].

Vegetables are the natural source for the nitrate pathway — but to reach the amounts used in studies, you would need to eat 200–300 g of rocket or several large beetroots every day. Nobody can do that reliably. Nitrate content also varies considerably in the same type of vegetable, depending on its origin, season and processing — so you never know exactly how much you are getting.

If you want a defined, repeatable daily effect, you need a standardised source that states the nitrate content in mg per serving. The article on beetroot powder and standardisation.

One more practical point: antibacterial mouthwash shortly before or after consuming nitrate significantly inhibits the first conversion step in the mouth — this alone can greatly reduce the overall effect ^[3]. Brushing your teeth normally is not a problem. Strong mouthwashes containing chlorhexidine or similar ingredients are.

Frequently asked questions

What is nitric oxide, and what does it do in the body?

Nitric oxide (NO) is a gas your body produces itself. It signals blood vessels to relax and widen — more blood flows through, and more oxygen and nutrients reach muscles and organs. It is the central regulator of blood vessel function.

How can I increase my nitric oxide levels?

Through two pathways: nitrate-rich vegetables (beetroot, spinach and rocket), and regular endurance training, which directly stimulates the body’s own NO production in the blood vessel walls. The pathways work independently and can be combined.

What is the difference between the nitrate pathway and the citrulline pathway?

The nitrate pathway comes from outside the body, through food. It works particularly well during exercise and does not need functioning blood vessel muscle — it is the reserve system. The citrulline–arginine pathway starts inside the body: L-citrulline supplies the building block the body uses to produce NO in the blood vessel walls. It acts faster, but depends more on how well the vascular system is working overall — and that function declines with age.

Why does nitric oxide decline with age?

The body’s own NO production in the blood vessel walls declines with age. The vessels become less flexible and their own production measurably falls. This is one reason blood pressure and exercise capacity change over the decades. The dietary nitrate pathway, by contrast, works largely independently of this ageing process.

Why is L-citrulline a better supplement than L-arginine?

A large proportion of oral L-arginine is broken down in the gut and liver before it reaches the blood vessels. L-citrulline, by contrast, reaches the places where it is needed largely unchanged and is converted to arginine there. Studies show that L-citrulline raises blood arginine levels more effectively than the same amount of L-arginine taken directly.

How long do NO supplements take to work?

Dietary nitrate from beetroot takes around 2–3 hours to reach its strongest effect. L-citrulline acts faster — relevant levels rise after around 60–90 minutes. Taking both before training gives you a broader window of action.