Nitric oxide is a gas your body produces naturally to relax blood vessels, improve blood flow, and support dozens of essential functions. It’s one of the most important signaling molecules in human biology, acting as a messenger that tells smooth muscle cells in your artery walls to loosen up and let blood pass through more easily. Despite its simple chemical structure (one nitrogen atom, one oxygen atom), nitric oxide influences everything from blood pressure and exercise performance to immune defense and sexual function.
How Your Body Makes Nitric Oxide
The cells lining your blood vessels, called endothelial cells, are the primary producers of nitric oxide. They contain a specialized enzyme that converts the amino acid L-arginine into nitric oxide through a chemical reaction. Once produced, nitric oxide is a gas, so it simply diffuses out of the endothelial cell and into the neighboring smooth muscle cells that wrap around your arteries.
Inside those muscle cells, nitric oxide triggers a chain reaction. It activates an enzyme that produces a second messenger molecule called cGMP, which tells the muscle fibers to relax. When the smooth muscle around an artery relaxes, the vessel widens, blood flows more freely, and blood pressure drops. This entire sequence, from nitric oxide release to vessel relaxation, happens in seconds.
Your body also produces nitric oxide through a second pathway that doesn’t depend on enzymes at all. Bacteria on your tongue convert dietary nitrates (found in vegetables) into nitrites, which your stomach and bloodstream then reduce into nitric oxide. This “backup” pathway becomes increasingly important as you age, because the enzyme-driven route gradually loses efficiency.
Why It Matters for Blood Pressure
Because nitric oxide directly widens blood vessels, it plays a central role in blood pressure regulation. In a clinical study of hypertensive patients, a single dose of a nitric oxide-boosting supplement reduced systolic blood pressure by an average of 6 mmHg and diastolic pressure by 6 mmHg within 60 minutes. That may sound modest, but sustained reductions in that range are associated with meaningful decreases in heart attack and stroke risk over time.
The flip side is equally telling. When nitric oxide production falters, blood vessels stiffen and constrict, raising blood pressure and forcing the heart to work harder. Impaired nitric oxide signaling is one of the earliest detectable changes in cardiovascular disease, often showing up years before symptoms appear.
Nitric Oxide and Aging
Your capacity to produce nitric oxide declines significantly with age. Research shows that impairment in the endothelial cells responsible for nitric oxide production becomes clearly evident by the fourth decade of life, typically in your 40s. The decline accelerates from there. Studies measuring forearm blood flow found greater than 50% loss in endothelial function in the oldest age groups tested. In the coronary arteries specifically, research published in the Journal of Geriatric Cardiology reported a 75% loss of endothelium-derived nitric oxide in patients aged 70 to 80 compared to healthy 20-year-olds.
This age-related decline helps explain why cardiovascular risk climbs so sharply after middle age. It also explains why the dietary nitrate pathway, which bypasses the aging enzyme system, becomes a more important source of nitric oxide for older adults.
Its Role in Erectile Function
Nitric oxide is the primary chemical trigger for erections. Sexual arousal sends neural signals that cause nerve endings and endothelial cells in the penis to release nitric oxide directly into the surrounding tissue. The nitric oxide activates the same cGMP pathway that relaxes arterial walls, but here it relaxes the smooth muscle of the penile blood chambers, allowing them to fill with blood.
The erection is maintained as long as cGMP levels stay elevated. A specific enzyme called PDE5 naturally breaks down cGMP, which is how erections resolve. Common erectile dysfunction medications work by blocking PDE5, allowing cGMP to accumulate longer and strengthening the erectile response. Crucially, these medications only work when nitric oxide is already being released through arousal. Without that initial nitric oxide signal, blocking PDE5 has no effect.
Exercise Performance Effects
Nitric oxide’s ability to widen blood vessels and improve oxygen delivery has made it a popular target for sports supplements. The research, however, is mixed. Studies show that nitrate-based supplements taken a few hours before aerobic exercise can decrease oxygen consumption and increase exercise tolerance at moderate intensities. In practical terms, this means the same effort costs your body less oxygen, potentially letting you go longer before fatigue sets in.
These benefits appear limited to recreational and moderately trained athletes. In elite athletes, the effects largely disappear. A study of elite cross-country skiers with very high aerobic capacities found no differences in performance or physiological responses between nitrate supplementation and placebo. Nitric oxide supplements also do not appear to increase VO2 max, your body’s ceiling for oxygen use during intense exercise.
Top Food Sources of Nitrate
The most efficient way to boost nitric oxide through diet is by eating nitrate-rich vegetables. Your oral bacteria convert these nitrates into nitrites, which then become nitric oxide in your body. The highest-nitrate vegetables, measured in milligrams per kilogram, are:
- Arugula: roughly 4,355 mg/kg, by far the richest source
- Spinach: approximately 2,013 mg/kg
- Lettuce: approximately 1,265 mg/kg
- Kale: approximately 1,181 mg/kg
- Chard: approximately 1,025 mg/kg
Beets and beetroot juice are also well-studied sources, and they form the basis of most nitric oxide supplements marketed to athletes. Nitrate content in vegetables varies by season, with autumn-harvested greens generally containing higher concentrations than spring-harvested ones.
Supplements: Citrulline vs. Arginine
Two amino acid supplements are commonly sold as nitric oxide boosters: L-arginine and L-citrulline. Since L-arginine is the direct raw material your body uses to make nitric oxide, it seems like the obvious choice. But your gut and liver break down about 70% of supplemental arginine before it ever reaches your bloodstream. Only around 31% of what you swallow makes it into general circulation.
L-citrulline, on the other hand, passes through the gut almost entirely intact. Roughly 105% of supplemented citrulline appears in plasma (the number exceeds 100% because citrulline also stimulates release from other tissues). Your kidneys then convert citrulline into arginine, effectively raising blood arginine levels 35% more than taking arginine directly. For this reason, L-citrulline is generally considered the more efficient supplement for supporting nitric oxide production.
Nasal Breathing and Nitric Oxide
Your paranasal sinuses continuously produce nitric oxide and release it into your nasal airways. When you breathe through your nose, you carry this sinus-derived nitric oxide down into your lungs, where it acts as a local vasodilator, helping your lungs absorb oxygen more efficiently. Studies show that blood oxygen levels run about 10% higher during nasal breathing compared to mouth breathing in healthy subjects. This is one reason breathing experts and athletic coaches increasingly emphasize nasal breathing during lower-intensity exercise and sleep.
Interactions With Medications
Because nitric oxide lowers blood pressure by relaxing blood vessels, supplements that boost it can interact with blood pressure medications and diabetes drugs, potentially causing blood pressure to drop too low. People who already have low blood pressure face the highest risk. Nitric oxide supplements should also be stopped before surgery, since the combination with anesthesia-related blood pressure changes can be problematic.