Nitric oxide (NO): a small molecule with a major role in the human body

Nitric oxide helps regulate vascular tone, mediates cellular signals and contributes to the body’s defence responses. Its effects, however, are not determined by quantity alone. They depend on where it is produced, how long it remains available and what it reacts with in a particular tissue. Understanding NO therefore means understanding the interplay between the endothelium, enzymes, the microbiome and redox regulation.

Why NO matters to our understanding of human physiology

When a tissue’s blood supply requirements change, blood vessels need to adjust their diameter accordingly. Nitric oxide is one of the signals that make this response possible. It is produced, for example, in the endothelial cells lining the inner surface of blood vessels and acts on neighbouring smooth muscle cells. Their relaxation can widen the vessel and alter resistance to blood flow. The vascular lining therefore actively influences the behaviour of the vessel wall. [1–4]

NO also participates in neural communication and certain immune mechanisms. It consequently performs different roles in different environments. Its physiology cannot be adequately described as a single reserve that simply needs replenishing. The body produces and uses NO locally, according to cellular activity and the chemical conditions in the surrounding environment. [5–11]

For vascular health, what matters is that an appropriate signal is produced, reaches its target and elicits a suitable response. Increased production in another tissue or during inflammation does not automatically achieve this. This article explains the fundamental mechanisms of NO and shows why assessing its effects requires us to distinguish between a biochemical process, a measurable marker and a health outcome.

How an unidentified vascular signal became a specific molecule

In 1980, Robert Furchgott and John Zawadzki showed that acetylcholine could relax isolated blood vessel preparations only when the endothelial lining remained intact. The endothelium had to be releasing a substance that acted on smooth muscle. This signal became known as endothelium-derived relaxing factor, or EDRF. The experiment changed how the endothelium was understood: removing it fundamentally altered the response of the entire vessel. [1]

In 1987, work by Richard Palmer, Salvador Moncada and their colleagues, and independently by Louis Ignarro and colleagues, identified this relaxing signal as NO. Earlier experiments by Ferid Murad and colleagues had already shown that NO activates guanylate cyclase and increases cyclic GMP production. Bringing these findings together provided a mechanistic explanation of how the endothelium communicates with the muscular layer of a blood vessel. [2–4]

These papers remain foundational sources today. Their value lies in the experimental separation of cause and effect: an intact endothelium, a released substance, an identified molecule and a response from the target enzyme. Subsequent research expanded this framework to include enzyme regulation, additional cellular targets and alternative routes of NO production.

What nitric oxide is and why its effects depend on the environment

NO consists of one nitrogen atom and one oxygen atom. It has an unpaired electron and is therefore classified as a free radical. The term radical, however, does not by itself tell us whether its effects are beneficial or harmful. The chemical properties of NO are precisely what enable the signalling functions used by the body. NO must be distinguished from nitrous oxide, N₂O; nitrogen dioxide, NO₂; nitrite, NO₂⁻; and nitrate, NO₃⁻. These substances have different properties and biological roles. [2–5, 16–18]

NO can diffuse from its site of production and bind to molecular targets. Its fate is influenced by the chemical environment. It may activate a receptor enzyme, participate in reactions involving proteins or be consumed in a reaction with superoxide. Its availability for one process can therefore change without a corresponding change in total production. [14, 15, 21, 22]

There is little value in assigning NO a single universal lifetime throughout the body. Conditions differ between a cellular microenvironment, the vessel wall and the bloodstream. Nor is there a single whole-body quantity that reliably expresses the quality of NO signalling on its own. A measurement must always be interpreted in relation to the specific sample, method and physiological question. [18, 30, 31]

How NO production, bioavailability and effect relate to one another

Production describes the generation of NO through a particular pathway. In this article, bioavailability refers to the extent to which NO remains accessible to its physiological targets before being consumed in other reactions. Its effect also depends on the condition of the target molecules and cells. Each of these levels can influence the resulting response. [12–14, 20–22]

This distinction has practical consequences. An increase in the concentration of a precursor does not necessarily demonstrate improved vascular function. A change in a vascular measurement, in turn, does not by itself establish fewer heart attacks or a longer lifespan. With NO, we need to know which part of this chain a study actually measured. [26, 27, 29, 30, 33]

Enzymatic NO production and the three forms of NO synthase

One of the main pathways produces NO from the amino acid L-arginine using enzymes called NO synthases, abbreviated NOS. The reaction also produces L-citrulline. Classic experiments using isotopically labelled L-arginine helped establish the origin of the nitrogen in NO produced by endothelial cells. Production requires coordinated enzyme activity, oxygen and electron transfer. [5–8]

Three main isoforms are recognised in humans. Their names largely reflect the setting in which they were originally described or the way they are regulated; they should not be interpreted as absolute boundaries of tissue distribution. [6–8]

Isoform	Gene	Main context
nNOS	NOS1	Neural signalling and additional tissue functions
iNOS	NOS2	Inducible production during certain inflammatory and defence responses
eNOS	NOS3	Endothelial signalling and regulation of vascular function

eNOS and vascular regulation

Endothelial NO synthase enables the vascular lining to produce NO continuously. The importance of this production has also been demonstrated in human experiments: local administration of a NOS inhibitor into the brachial artery reduced forearm blood flow. The finding supported the conclusion that endogenous NO contributes to the maintenance of resting vascular tone. [9]

Cellular regulatory mechanisms influence eNOS activity. Studies by Dimmeler, Fulton and their respective teams demonstrated the importance of phosphorylation mediated by the kinase Akt. The endothelium can thus translate a stimulus into a change in NO production. During blood flow, an important stimulus is shear stress: the tangential force acting on the endothelial surface. Blood flow also provides the endothelium with mechanical information to which it can respond by producing NO. [12, 13]

This creates a natural connection with TANVEA Biological Systems™. Physical activity provides an example of the relationship between a physiological stimulus, eNOS regulation and the vascular response. [25] For heat and for red and near-infrared light, later articles will examine whether, and under what conditions, they influence NO production, its bioavailability or the tissue response. Each of these outcomes requires its own evidence.

This mechanism explains why regular physical activity is relevant when considering support for vascular function. It also highlights why supplying a single amino acid does not address the whole regulatory process. Substrate availability, the condition of the enzyme and its cofactors, and the course of the activating signal are distinct requirements for enzyme function. [5, 12, 13, 20, 21]

nNOS and iNOS extend the role of NO beyond blood vessels

In the nervous system, NO can mediate communication between cells. Garthwaite and colleagues investigated this process in experimental brain tissue. Activation of NMDA receptors triggered the release of a diffusible signal with EDRF-like properties and a subsequent change in cGMP. Later cloning of brain NOS helped clarify its molecular basis. [6, 10]

iNOS contributes to certain defence responses. Experiments with activated macrophages demonstrated NO production from L-arginine and its involvement in cytotoxic activity. This is a different context from the ongoing regulation of vascular tone. Much of the foundational evidence comes from cellular and animal models. The extent of such a response cannot be extrapolated to all human immune cells without verification. [8, 11]

It would nevertheless be inaccurate to divide the isoforms into “good eNOS” and “bad iNOS”. NO production during a defence response may serve a useful purpose, while eNOS can lose its proper function under disturbed conditions. What needs to be assessed is the specific process, its regulation and its biological outcome. [8, 11, 20, 21]

How a cell reads the NO signal

One of the best-studied targets of NO is soluble guanylate cyclase, or sGC. This enzyme contains a haem group capable of binding NO. Activated sGC converts GTP into cyclic guanosine monophosphate, cGMP, which carries the signal onwards. [4, 14]

Structural studies of human sGC have shown how NO recognition can be translated into a change in enzyme activity. NO binding changes the spatial arrangement of the enzyme’s sensor region. This change is transmitted through another part of the protein to the catalytic region, supporting its enzymatic activity. [14]

In vascular smooth muscle, this signalling system promotes relaxation. Its existence does not mean, however, that every increase in NO anywhere in the body will dilate all blood vessels equally. The site of production and availability to specific target cells are decisive. [1–4, 9, 14]

NO signalling also involves protein modifications

NO signalling includes pathways beyond cGMP. An important area is S-nitrosylation, in which an S–NO group forms on the sulphur atom of a cysteine residue in a protein. This chemical modification can alter the protein’s properties. Its formation and effects depend on the specific chemical environment and target protein. [15]

This is therefore not a single universal reaction with the same outcome in every cell. We need to identify which protein was modified, under what conditions and with what functional consequences. A general measurement of “NO” does not distinguish these details. [15]

Nitrate and the oral microbiome provide another pathway

The body does not rely exclusively on direct NO production by NOS. A second important pathway involves the stepwise reduction of nitrate to nitrite and subsequently to NO. Nitrate comes from food and from the metabolism of the body’s own NO. Following absorption, some circulating nitrate can be taken up by the salivary glands and secreted into saliva. Oral microorganisms participate in its subsequent conversion. [16, 19]

Bacteria capable of reducing nitrate produce nitrite. After saliva is swallowed, nitrite enters further reactions in the digestive tract; in the acidic environment of the stomach, NO can be generated chemically. Nitrite can also contribute to systemic processes. Human experiments have demonstrated vasodilatory effects following its administration and a reduction mechanism involving deoxygenated haemoglobin. [16–18]

The conditions for nitrite reduction differ in part from those required for enzymatic synthesis by NOS. In certain environments with lower oxygen availability, this pathway may become more important. It is not, however, a completely independent system that automatically takes over every function of eNOS whenever a disturbance occurs. Individual reactions remain dependent on concentrations, the chemical environment and the site of action. [18]

The two main pathways of NO production

Pathway	Simplified sequence	Important considerations for interpretation
Enzymatic NOS pathway	L-arginine → NO, requiring NOS, oxygen and the necessary cofactors	Enzyme regulation, substrate and cofactor availability, and properly coupled electron transfer all matter.
Nitrate pathway	Nitrate → nitrite → NO, involving oral bacteria and transport between blood, saliva and the digestive tract	Microbial functions, the chemical environment and conditions for subsequent nitrite reduction all matter.

This is a simplified comparison. Both pathways participate in a wider metabolic system, and NO production alone does not determine the eventual effect. Preservation of the signal and the response of the target tissue are also essential. [5, 12–14, 16–22]

What the role of oral bacteria means for health

The oral microbiome is a functional component of this metabolic pathway. In a study by Kapil and colleagues involving 19 healthy volunteers, a control period was followed by seven days of chlorhexidine mouthwash use. The researchers observed marked suppression of oral nitrate reduction, lower plasma nitrite and a small increase in blood pressure. This was a short experiment using a specific antiseptic, supporting the biological importance of the oral pathway. [19]

The finding does not justify labelling all mouthwashes as harmful or diagnosing NO deficiency from common symptoms. The study did not evaluate every formulation, long-term dental treatment or every patient group. A prescribed antiseptic should therefore not be discontinued without professional advice. Preserving microbial functions belongs within the wider context of good oral hygiene and the treatment of oral disease.

Likewise, identifying a single bacterial genus in a microbiological result is not enough. What matters is the metabolic function of the community and its relationship with the host as a whole. More recent intervention studies show that the microbiome’s response to nitrate may differ between individuals and that a change in its composition does not necessarily improve every vascular marker. [27, 28]

Why NO bioavailability can decline

When NO signalling is impaired, reduced production needs to be distinguished from increased consumption of NO in other reactions. One important mechanism is its reaction with the superoxide anion radical, which forms peroxynitrite. This can remove NO from its physiological signalling role while generating a substance capable of oxidative and nitrating reactions. [22]

Another mechanism involves tetrahydrobiopterin, BH₄, an important NOS cofactor. Disturbances in its availability and redox state can lead to enzyme uncoupling. Electron transfer is then no longer properly coupled to NO production, and the enzyme can contribute to superoxide generation. Landmesser and colleagues demonstrated this mechanism experimentally in a mouse model of hypertension. [20]

Human data also support the importance of the local environment. Antoniades and colleagues examined blood and vascular tissues from 219 patients undergoing coronary bypass surgery. Greater BH₄ availability in vascular tissue was associated with a more favourable vasomotor response and signs of better eNOS coupling. Plasma values did not simply reflect vascular tissue values. These were associations in a specific patient group, rather than evidence that any given BH₄ supplement is effective. [21]

These findings explain why asking “how can NO be increased?” often overlooks an important part of the problem. A particular tissue may need proper enzyme function to be preserved or signal loss to be reduced. Simply increasing the amount of substrate may not correct these conditions. [20–22, 32]

NO can have protective and adverse effects

NO enables normal cellular communication, but under certain conditions it also participates in processes that damage cells. Its reaction with superoxide is one reason why pursuing the highest possible production regardless of the environment makes little sense. The resulting chemistry depends on the simultaneous presence of reactants and their location. [11, 22]

The relationship between NO and mitochondria is similarly conditional. In an experimental study, Brown and Cooper showed that NO can reversibly inhibit oxygen consumption at cytochrome c oxidase, with competition between NO and oxygen contributing to this effect. This is a mechanistic finding from an experimental system. It cannot support a general promise of increased ATP production from any intervention affecting NO signalling. [23]

Recovery likewise requires us to examine specific functions. Improved blood flow, altered mitochondrial activity and changes in the inflammatory response are distinct outcomes. Demonstrating one does not automatically establish the others or explain the entire process of recovery.

Ageing and physical activity change the conditions for vascular signalling

Human studies have observed weaker endothelial responses and an altered contribution of NO to vasodilation with increasing age. Taddei and colleagues examined forearm vascular responses in people with and without hypertension using vasodilators, NOS inhibition and locally administered vitamin C. Their findings supported the involvement of altered NO availability and oxidative mechanisms. They do not establish a universal percentage decline in “NO in the body” that applies to everyone of the same age. [24]

Physical activity provides a direct physiological stimulus for vascular adaptation. In a controlled study of patients with coronary artery disease, four weeks of exercise training improved the endothelium-dependent response in the artery under investigation. Tissue samples obtained during subsequent surgery showed differences in eNOS expression and phosphorylation between the exercise and control groups. The study included 35 patients and linked functional measurements with molecular markers. [25]

These results provide a specific reason to include physical activity among the important topics in supporting vascular function. They do not mean that everyone should follow the same training programme or that the NO pathway alone explains all the health benefits of exercise. Baseline health and the suitability of the exercise load remain important.

What human nitrate studies show

Clinical research allows us to test whether a mechanism translates into a measurable response in the body. Results need to be interpreted according to the population studied, the duration of the intervention and the specific outcomes assessed. An increase in a metabolite concentration, a change in blood pressure and an improvement in vascular function represent different levels of response. [26–29, 33]

When an intervention improves blood pressure and vascular markers

Kapil and colleagues enrolled 68 patients with hypertension in a randomised, double-blind study. For four weeks, participants received either nitrate-containing beetroot juice or a nitrate-depleted control juice. The intervention lowered blood pressure assessed by several methods and improved certain measures of vascular function. This demonstrates the effect of a specific intervention in a defined group over a limited period. [26]

When a metabolic response does not mean improved function

In their 2024 paper, Fejes and colleagues studied 15 older adults with treated hypertension. Four weeks of nitrate-containing beetroot juice increased nitrate and nitrite concentrations. Compared with placebo, however, the intervention did not significantly improve the forearm vascular responses assessed or blood pressure measured by 24-hour monitoring and home measurements. The study directly illustrates that a metabolic response need not be accompanied by improvement in the function being examined. [33]

The same team’s microbiome paper published in 2025 reports findings from the same clinical trial. Following the four-week intervention, it described changes in the abundance of certain oral bacteria, including an increase in the relative abundance of the genus Neisseria. Comparable changes in the composition of the intestinal microbiome were not found. These are therefore not two independent experiments, but publications assessing different outcomes of the same intervention. Changes in microbial composition need to be read alongside the functional results, rather than treated as a substitute for them. [28, 33]

A 2025 study by Vanhatalo and colleagues compared responses in 39 younger and 36 older adults during two-week intervention periods. Nitrate reduced mean arterial pressure in the older group, but the FMD parameters assessed did not improve in that group. The younger group did not show the same blood pressure response. The study therefore identified differences according to both age and the outcome measured. [27]

Why measuring a single metabolite is not enough

A randomised study by McLellan and colleagues published in 2026 also shows that individual markers do not necessarily respond in parallel. Following acute administration of the nitrate doses studied, plasma nitrate and nitrite concentrations increased, but plasma cGMP did not change. Vascular function and blood pressure findings differed according to dose and outcome. Such acute observations cannot be turned into a universal long-term dosing protocol. [29]

Taken together, these experiments support the biological and clinical relevance of the nitrate pathway. They do not demonstrate that every increase in nitrite means improvement across the entire NO system, or that an effect on blood pressure automatically establishes prevention of cardiovascular events or a longer lifespan. Those questions require studies with the appropriate outcomes and duration of follow-up.

How to investigate NO and what measurements cannot tell us

With NO, the research question needs to be stated first. Are we interested in nitrate reduction in the mouth, metabolite concentrations in the blood, the response of a particular artery or a health outcome? Each requires a different approach. Using the shared label “NO measurement” can obscure these distinctions. [18, 19, 30, 31]

FMD, or flow-mediated dilation, assesses the change in arterial diameter following an increase in blood flow and shear stress. Under standardised conditions, it measures an endothelium-dependent response with a substantial contribution from NO. The result nevertheless represents the function of the artery examined under those conditions, rather than a direct measurement of NO concentration throughout the body. Reproducibility depends on a robust protocol and the experience of the laboratory. [30]

Test or marker	What it helps assess	Main limitation of interpretation
Salivary nitrite	Part of oral nitrate metabolism	Does not automatically reflect plasma nitrite or endothelial function.
Blood nitrate and nitrite	Metabolic changes under defined conditions	The measured substance and the source of the change must be distinguished; this is not a direct map of tissue NO.
Arterial FMD	An endothelium-dependent vasodilatory response	Depends on the protocol and does not establish the condition of the entire microcirculation.
Blood pressure	A clinically important haemodynamic variable	Can change through many pathways and does not identify an NO mechanism on its own.

In a direct comparison of samples, Clodfelter and colleagues did not find a significant correlation between baseline nitrite concentrations in saliva and plasma. A salivary test strip therefore cannot automatically be regarded as a diagnostic test for endothelial dysfunction, NO deficiency or biological age. Its usefulness must be assessed against what it actually measures. [31]

Likewise, improved FMD in a larger artery does not establish restoration of every capillary function. The endothelium, vascular tone and microcirculation are related concepts, but they do not describe the same level of organisation. Precise interpretation protects against conclusions that extend beyond the measurement used. [30]

What these findings mean for supporting health

Physical activity and a diet containing vegetables provide a biologically meaningful context for the NO pathways studied. For exercise, human data link changes in vascular function to eNOS regulation; for nitrate from vegetables, intervention studies provide data on blood pressure and other outcomes. Results must nevertheless be related to the specific population and conditions. Individual studies do not provide a personalised treatment prescription. [25–29, 33]

The safety of a supplement cannot be inferred solely from the fact that the substance is a natural substrate. In the VINTAGE MI trial involving 153 patients after a myocardial infarction, L-arginine did not improve the main outcomes assessed, and a safety signal of increased mortality emerged in the treatment group. This finding concerns a specific clinical situation, but it also clearly demonstrates the limits of the assumption that “more substrate must mean better function”. [32]

For someone considering their vascular health, assessing overall health and risk factors with a qualified professional is therefore more useful than searching for a single NO number. Hypertension treatment, dental procedures and medicines have their own indications. Educational content about NO should not be a reason to change them without professional advice.

Why this topic belongs in Biological Systems™

NO connects enzyme regulation, microbial functions and vascular responses. Within TANVEA LIBRARY, it therefore belongs in BIOLOGICAL SYSTEMS™ as a biological topic in its own right. Individual technologies can only be evaluated in relation to a specific mechanism, measurement and quality of evidence.

Further articles in the series will examine the oral microbiome, eNOS and BH₄, oxidative mechanisms, and research into physical activity, heat, light, H₂ and oxygen. For each intervention, we will need to distinguish cellular or animal experiments, human mechanistic studies and clinical outcomes. An effect studied with one intervention cannot automatically be attributed to a device with different parameters or to a combination of technologies.

The central question of the series remains how to preserve physiological NO production and bioavailability, together with an appropriate response in target tissues. Answering it requires identifying the cause of a disturbance and verifying the outcome at the level we actually intend to influence.

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Frequently asked questions about nitric oxide

What is nitric oxide, NO?

Nitric oxide is a small signalling molecule composed of nitrogen and oxygen. It contributes to vascular regulation, neural communication and certain defence responses. Its effects depend on where it is produced and on the surrounding environment. [2–4, 10, 11]

Is greater NO production always beneficial?

No. Appropriate NO signalling is necessary, but its effects in an inflammatory or oxidative environment may differ from its role in physiological vascular regulation. When superoxide is also being produced, peroxynitrite can form. The goal is appropriate regulation, rather than the highest possible value without context. [20–23]

How is the oral microbiome connected to NO?

Certain oral bacteria convert nitrate into nitrite, which can subsequently contribute to NO production. This pathway complements enzymatic production by NOS. Impairment of the oral pathway does not mean that all NO production in the body has stopped. [5, 16–19]

Can a saliva test identify NO deficiency in the body?

A salivary nitrite test alone does not establish such a diagnosis. It measures part of oral metabolism, and its result cannot automatically be translated into the condition of the endothelium or the nitrite concentration in blood. [31]

Does exercise increase NO production?

Exercise can support endothelial signalling through changes in blood flow and shear stress. Human studies have demonstrated improvements in certain vascular responses and changes in eNOS regulation. The extent of the effect depends on the population and training programme. [12, 13, 25]

Does an increase in nitrite demonstrate improved vascular function?

Not automatically. An increase in nitrite demonstrates a metabolic response under the conditions studied. Improved vascular function needs to be verified using an appropriate functional measurement. Studies show that these outcomes do not necessarily occur together. [29, 33]

Does lower blood pressure prove that NO has increased?

A fall in blood pressure alone does not identify the mechanism. Combining a controlled intervention with metabolic and functional measurements allows a more convincing interpretation. Even then, an effect on one marker cannot automatically be extended to every health outcome. [26, 27, 29]

Is arginine supplementation suitable for everyone?

The physiological role of L-arginine as a NOS substrate does not justify recommending supplements to everyone. The outcome depends on the specific situation and safety considerations; the VINTAGE MI trial after myocardial infarction provided an important warning. [5, 32]

Does the involvement of NO establish that a wellness technology is effective?

The existence of a mechanism alone is not enough. Evidence is needed for the particular intervention, its parameters, the population studied and the relevant outcome. Demonstrating an individual effect also does not establish a synergistic effect from combining devices.

Continuing the series

Next article: The oral microbiome and NO production: why health also begins on the surface of the tongue.

It will explain the enterosalivary cycle, the functions of nitrate-reducing bacteria, the interpretation of more recent studies and the limits of current approaches to supporting this pathway.

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Scientific and safety note

This article is intended for education. It does not provide an individual diagnosis, treatment plan or dosing protocol and does not replace medical or dental assessment. It should not be used as a basis for changing prescribed treatment, discontinuing indicated antiseptics or starting concentrated nitrate, nitrite or supplements marketed as affecting NO without professional advice. Evidence concerning a mechanism or a single marker does not establish the effects of a specific TANVEA product.

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Scientific references

References are numbered according to their citations in the text. The evidence base consists primarily of original experimental and clinical studies; reference 30 is an expert methodological consensus. The selection includes foundational discoveries and verified publications from 2025 and 2026. This is an expert-curated narrative explanation, rather than a systematic review of all publications. References 28 and 33 report findings from the same clinical trial and do not represent two independent experiments.

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