How Nitric Oxide Works in the Body
A gas with a half-life of a few seconds controls blood pressure, exercise capacity and erectile function. Understanding the two routes to producing it explains most of what circulation supplements are trying to do.
A signalling molecule that won a Nobel Prize
In 1998 Furchgott, Ignarro and Murad shared the Nobel Prize in Physiology or Medicine for showing that nitric oxide functions as a signalling molecule in the cardiovascular system. It was a genuinely surprising result: a simple, unstable gas, produced on demand, acting as a messenger between cells.
NO has a half-life measured in seconds. It is not stored and not transported far — it is synthesised where it is needed, diffuses across a few cell layers, and is gone. That local, on-demand character is why its production capacity, rather than its concentration, is what matters physiologically.
Route one: L-arginine and eNOS
Endothelial nitric oxide synthase (eNOS) converts the amino acid L-arginine into NO and L-citrulline, using oxygen and several cofactors including tetrahydrobiopterin (BH4). The NO diffuses into adjacent vascular smooth muscle, activates soluble guanylate cyclase, and raises cyclic GMP. Cyclic GMP triggers relaxation, the vessel widens, and flow increases.
The pathway is regulated mainly by shear stress — the drag of blood moving across the endothelial surface. More flow means more shear, which upregulates eNOS. This is the mechanistic reason exercise improves endothelial function and inactivity degrades it within days.
Two things break this route. BH4 depletion under oxidative stress causes eNOS to "uncouple", producing superoxide instead of NO — turning the enzyme from protective to damaging. And superoxide from smoking or inflammation reacts with NO directly to form peroxynitrite, destroying the signal and generating a damaging oxidant in the same reaction.
Route two: dietary nitrate
An entirely separate route runs backwards through the nitrate–nitrite–NO pathway. Dietary nitrate from beetroot, rocket, spinach, celery and other leafy greens is absorbed, concentrated in saliva, and reduced to nitrite by bacteria on the tongue. Swallowed nitrite is then reduced to NO in the stomach and in tissue.
This route does not require oxygen or eNOS, which makes it complementary — it works best precisely where the enzymatic route struggles, in low-oxygen and acidic tissue.
The clearest demonstration that the bacterial step is essential: using antibacterial mouthwash abolishes most of the blood-pressure-lowering effect of dietary nitrate, because it kills the oral bacteria performing the first conversion. Few physiological findings are so easy to test and so counterintuitive.
Effective intake is roughly 300–600 mg of nitrate — one concentrated beetroot shot, or a large serving of rocket or spinach. Randomised trials consistently find systolic blood pressure reductions of about 3–5 mmHg and improved exercise economy.
Where PDE-5 inhibitors fit
Sildenafil and tadalafil do not produce nitric oxide. They inhibit phosphodiesterase type 5, the enzyme that degrades cGMP — so the signal that NO generates lasts longer and produces a larger effect.
This is why those drugs require sexual stimulation to work: without the NO release triggered by parasympathetic signalling, there is no cGMP for the drug to preserve. It is also why they are absolutely contraindicated with nitrate medication, which floods the system with NO donors — the combination can drop blood pressure catastrophically.
Supplements marketed for circulation act upstream: supplying substrate (citrulline, arginine), supplying nitrate (beetroot), or supporting eNOS activity (ginseng, polyphenols). Upstream support at a modest magnitude is a fundamentally different intervention from blocking the degradation enzyme.
What raises NO availability, in order of effect
Aerobic exercise — the largest and most durable effect, via sustained shear stress upregulating eNOS. Flow-mediated dilation improves measurably within 2–4 weeks.
Dietary nitrate — reproducible, food-based, cheap.
Stopping smoking — removes the single most direct antagonist of NO signalling.
L-citrulline, 3–6 g — raises plasma arginine more effectively than arginine itself.
Polyphenols — cocoa flavanols, pomegranate and grape seed all improve flow-mediated dilation in trials.
Sleep and blood pressure control — both act on the oxidative-stress side of the equation rather than the substrate side.
Frequently asked questions
Does mouthwash really reduce the benefit of beetroot juice?
Yes, and it is one of the more striking findings in this field. The first step converting nitrate to nitrite is performed by bacteria on the tongue. Antibacterial mouthwash kills them and measurably blunts both the nitrite rise and the blood-pressure effect.
Is L-arginine or L-citrulline better for nitric oxide?
Citrulline. Oral arginine is largely broken down by arginase in the gut and liver before reaching circulation; citrulline passes through intact and is converted to arginine in the kidneys, producing a higher and longer-lasting plasma level.
How fast does exercise improve endothelial function?
Flow-mediated dilation — a direct measure of NO-mediated vessel widening — improves within two to four weeks of regular aerobic training, and declines just as quickly during inactivity.
Can NO supplements be combined with ED medication?
Not without medical advice. Both act on the same pathway and the combination can lower blood pressure more than either alone. Combining anything vasodilatory with nitrate medication is contraindicated outright.