Breathing through the nose vs. the mouth has been a topic of significant interest and discussion in the last few years, this subject being treated thoroughly by Patrick McKeown and James Nestor in their books, The Oxygen Advantage, and Breath, respectively. Nasal breathing as a general health principle is important for a number of reasons including filtration of incoming air, this filtration serving to eliminate particles and foreign debris, warming and humidifying, and importantly, exposing incoming air to the immune system, the respiratory epithelium covering of the turbinates offering a first line of defense against airborne pathogens entering the nose.

The nasal turbinates are comprised of folds of tissue that have a complex surface structure, this complexity yielding the surprising surface area, where if the tissue were to be flattened, is estimated at a surprising 120 square centimeters or ~18 square inches. (1)

The eye is not capable of seeing the turbinates, much less their microscopic surface structure, so it’s something that we find difficult to wrap our minds around.

[As a point of interest, activated carbon is possibly the most mind blowing example of microscopic surface where 1 gram is estimated to have an area of several thousand square meters. This is the reason why activated charcoal is so effective as a filtering medium, be it the filtration of chemical contaminants out of air, e.g. air filters for the home, powered air purifying respirators used in industry, etc., and the purification of water for drinking, aquariums, manufacture, etc. When either foreign object debris or chemicals make their way into the complex microscopic structure of activated carbon they get trapped.]

The nasal turbinates do not consist of convoluted microscopic tunnels as does activated carbon but they do present the nasal epithelium, a complex surface that is covered with mucus over which air must flow, thereby trapping matter, both large and microscopic, where incoming pathogens get exposed to antibodies which serve to neutralize threats. The role of the epithelium throughout the body is to “protect” it from threats posed by the “external environment”.

The spatial environment of the nose is rich in the gas nitric oxide, NO originally thought to be generated and concentrated in the paranasal sinuses, making its way into the nose in less concentrated form therefrom. There is still scientific back and forth on where NO in the upper respiratory tract originates, a reason being that it is a difficult thing to measure.

Nitric oxide is both a signaling molecule and an anti-pathogen, the discovery of NO’s signaling role in the cardiovascular system winning the 1998 Nobel Prize for physiology, jointly awarded to Robert Furchgott, Louis Ignarro, and Ferid Murad. (Watch this brief 2012 interview with Louis Ignarro.) There, it functions as the mechanism by which the endothelium of the arterial tree is signaled to expand and contract, maintaining viable/optimal circulatory capacity from the aorta down to the smallest arteriole in real time, throughout the entirety of the body. NO is also known to play an important role in defending against bacteria, fungi, viruses, and parasites in the respiratory tract (2), accomplishing this via nitrosative and oxidative damage to incoming pathogens. Another immune system role of NO is that it modulates T cell-mediated immune response. (3) A toxic gas and a free radical, it is one of few “environmental toxins” that play an almost universal role in the physiological processes of mammalian life.

Once synthesized, nitric oxide has a half-life up to 6 seconds (4), so it must be synthesized locally by cells throughout the body including the brain where it acts as a neurotransmitter. It is synthesized by a group of enzymes referred to as “nitric oxide synthases”, which convert the amino acid arginine into citrulline, a byproduct being nitric oxide. A major source of arginine is the kidneys which take citrulline produced by the small intestine arriving in plasma and convert it into arginine which is then released into the renal vein and inferior vena cava where it finds its way back into the circulation at large. (5)

Healthline’s “5 Ways to Increase Nitric Oxide Naturally” (6) explains that diet plays a critical role in nitric oxide production because NO is derived from nitrate, the only source of nitrate being dietary intake. (As author, I recommend reading this Healthline article in its entirety.) To this end, the article suggests eating vegetables high in nitrate principally including green leafy ones, citing celery, chervil, lettuce, and of course beetroot, as beets are one of the vegetables that is highest in nitrate with >250mg of nitrate per 100g of fresh weight. 

The article also suggests increasing intake of antioxidants including vitamin C, vitamin E, polyphenols and glutathione, this to limit the number of free radicals that can serve to neutralize nitric oxide, itself a free radical and a reason it is susceptible to neutralization by other free radicals. 

A third recommendation is to use nitric oxide boosting supplements including L-Arginine and L-Citrulline, both essential amino acids that are ultimately involved in the production of nitric oxide. Both have been popular pre-work-out supplements for decades because they facilitate increased circulation and with it the necessary hydration, nutrition, and metabolic waste removal, from energy producing cells and working muscles. They accomplish this via boosting nitric oxide availability and consequent vasodilation.