Did you know that the horse, generally regarded as one of the most robust animals on the planet, breathes almost exclusively through its nose? It is physically incapable of mouth breathing unless it suffers from an anatomical abnormality. 
Though I learned that tidbit after completing a majority of the research for this article, I think it is a testament that nature has designed mammals with an intent for optimal respiration through the nostrils.
The purpose of this piece is to investigate whether conscious nasal breathing during exercise, specifically that of the anaerobic type, might be beneficial over oronasal or mouth breathing in terms of performance and recovery.
Aerobic exertion relates to my findings as well.
Working out creates acidity. Acidity creates fatigue. Oxidation neutralizes acidity. Nasal breathing is better at oxidation than mouth breathing. Therefore nasal breathing should in theory reduce fatigue and speed recovery better than mouth breathing.
One of the penalties of kinetically induced metabolic excitation (i.e. exercise) is H+ and lactate production (the accumulation of which being more marked in the case of intense physical activity).
The buildup of these two byproducts creates acidity which the body wants to balance by raising its pH back up to normal levels. Acidity also inhibits glycolysis, the process by which most energy is generated under anaerobic conditions. These factors contribute to the feeling of fatigue.
The path of least resistance for restoring pH is through hyperventilation, which by definition is when the body expels more carbon dioxide (CO2) than is produced. Hyperventilation typically occurs through the mouth (and not the nose).
However, a lower pH and higher concentration of CO2 foster more willing delivery of oxygen throughout the body, as per the Bohr effect. Oxidation by definition offsets reduction (i.e. acidity), and also converts lactate back into pyruvate, a building block of energy production.
By nasal breathing, CO2 is not dispelled as disparately and though airflow is constricted, limiting the rate at which oxygen can be assimilated into the bloodstream compared to mouth breathing, the oxygen that is inhaled is more efficiently distributed to fatigued tissues which should in theory improve athletic performance and recovery, with practice of the technique.
High intensity exercise, which often (but not always) recruits fast-twitch (aka type II) muscle fibers, stimulates glycolysis to synthesize ATP for energy in predominance over oxidative phosphorylation because glycolysis is able to produce ATP at a faster rate to fulfill acute energy demands, though oxidative phosphorylation is the preferential and more cost-efficient pathway of energy production within the body. 
Fermentation (i.e. reduction) of pyruvate to lactate oxidizes NADH back to NAD+ for reuse in glycolysis. (Pyruvate and NAHD themselves are products of glycolysis.) NAD+ is available in limitation, hence the need to regenerate it. 
Some of the H+ is buffered in the muscle and some diffuses into the blood in exchange for Na+ or along with lactate through monocarboxylate transporters (MCTs). This then decreases pH in the blood (because of the influx of H+ and plasma lactate, lowered HCO3– concentration, and thus increased amounts of CO2 from H2CO3 dissociation) and as a consequence, the body wants to raise its pH back up to maintain homeostasis.  This acidity specifically inhibits phosphofructokinase, an enzyme that catalyzes a key regulatory step of glycolysis, and also impairs the utilization of glucose.  This is partly what causes fatigue and in a way shows the self-regulation of these mechanisms to protect the body from overexertion.
The path of least resistance for raising pH is by eliminating plasma CO2, which is vaporized in the alveoli and exhaled by the lungs.  Its dismissal is hastened by hyperventilation, which happens primarily by breathing through the mouth.
This helps restore pH, though CO2 is essentially displaced as lactate is produced, which is undesirable as lactate is not as synergetic with oxygen in the way carbon dioxide is through the Bohr effect. 
Another method by which pH can be leveled is consumption of the glycolytic byproducts, which is advantageous because this produces energy. The protons can be used in cellular respiration and the lactate can be oxidized back to pyruvate for use in metabolic processes as well. 
As a side note, I find the interconnection here rather elegant; the heart is able to utilize the built up plasma lactate for energy, which thus allows it to pump harder and increase blood flow to tissues that have a pressing need for oxygen. 
Mouth breathing is advantageous over nasal breathing in that it allows for increased airflow, which lets an individual reach higher levels of exercise intensity presumably because of the combination of higher oxygen consumption and lower carbon dioxide retention, both of which help balance acidity.  The cost of this is that it is inefficient when compared to nasal breathing due to the Bohr effect, which means energy is wasted to achieve similar results of oxidation and subsequently I would imagine fatigue sets in sooner as this is a stressful state of physiology.  If maintained, CO2 concentrations will likely further deplete, making oxygen delivery even poorer, exacerbating the effect, suggesting this is a mechanism to be avoided when possible and used only for short durations.
Therefore nasal breathing is preferential for its energy efficiency which should in theory better promote oxidative metabolism of glycolytic byproducts, increase available ATP, and thus lessen fatigue and speed recovery from athletic endeavors.
I believe there are a few simple takeaways to be gleaned from this science that can easily be applied to improve the efficacy of one’s training.
- Consciously make an effort to breath through your nose at all times, as in 24/7, to develop mastery of the nasal breathing technique. 
- During high intensity activity, allow yourself frequent breaks to fully regain control of your breathing and allow your heart rate to reset before continuing. Don’t keep pushing while you are winded.
- Nasal strips can help improve airflow, which appears to be the limiting factor in the exercise intensity one can achieve solely through nose breathing.  (That limiting of intensity could be construed as a positive, however.) Nasal resistance does actually reduce on its own during exercise, too. 
First and foremost, this is undoubtedly a simplified view of energetic processes and I do not claim to have that deep a grasp on the subject matter. There may be mistakes in my understanding and presentation above.
Secondly, I think there is ample evidence that shows mouth breathing allows for a higher respiratory rate than nose breathing. Whether the influx of oxygen or exhalation of carbon dioxide is the more relevant factor, I am not sure, but the increased flow rate of mouth breathing does allow exercise to reach a higher intensity.
However, unless you are a professional athlete and your livelihood hinges upon you sucking for air while putting your body through extreme stress, then do it when necessary, but for the rest of the population, if you are reaching the point where you must breathe through your mouth, I think that’s a sign you are training too hard.
What I am unclear about here though is exactly how oxygen is utilized when mouth breathing becomes a necessity at maximal intensity. It is delivered less efficiently, and I would assume certain metabolic processes take priority over others in terms of needing that oxygen. I am guessing oxidative phosphorylation is preferential over lactate consumption in this situation, which might help explain the lactate paradox.  This warrants further investigation.
Thirdly, by forcing nasal breathing during high intensity exercise, I have a feeling the body might be exposed to a more acute period of acidity as compared to mouth breathing because of the lower but more efficient ventilatory rate of nasal breathing. By mouth breathing, my hunch is that the body’s pH is restored more gradually as it is the less effective but more voluminous technique. There may be consequences associated with this, if my assumptions are correct.
At high altitude, there is a belief that red blood cell production is stimulated to compensate for the relative scarcity of oxygen in the air, and that this is the primary cause for performance gains associated with altitude training. 
However, others postulate that the positive effects of altitude training are mostly due to other factors, such as an adaptation to a more economic utilization of oxygen.  This claim seems to be supported by the lactate paradox, which shows “reduced production of lactic acid at a given work rate at high altitude.”  Lactate levels should not be reduced if increased red blood cell mass was the predominant factor in performance increase because carbon dioxide plays such a role in oxygen delivery.
Building one’s tolerance of nasal breathing is probably comparable to physiological adaptations of high altitude.
It also increases PCO2, allowing O2 to be delivered more readily to fatigued muscles because of the Bohr effect, though the increase in pH may initially offset the increase in carbon dioxide concentration, limiting the phenomenon. 
As all this translates to aerobic exercise, the main principle still stands: nasal breathing improves the delivery of oxygen. Oxidative phosphorylation, the preferential metabolic pathway of the body, is more efficient than glycolysis and relies on O2 availability. Thus, sufficiently supplying an increased demand for oxygen during low intensity activity is important as well.
Those interested in endurance exercise may want to read about lactate threshold and note how it relates to oxidation.
Unmentioned here is that metabolic processes create heat. Thus when one exercises, extra energy is spent and body temperature rises. This typically is compensated for by the dissipation of heat through the skin to maintain a functional core temperature.  When one’s internal temperature becomes too high, performance suffers (and the risk of serious biological harm onsets). 
I have yet to delve deep into the literature on this subject matter, but as it relates to respiration, I think the goal is still ultimately to promote energy efficiency, and excessive heat retention should be viewed as the result of an obstruction, namely the temperature of the outside environment. 
It is unclear if there is an optimal ambient temperature for which to exercise, but marathon results show a progression of improved performance all the way down to 41 °F.  (Data presumably hasn’t been interpreted below that number.)
My guess would be that the lowest temperature one can tolerate without impediment of motor functioning is the best in terms of maximizing potential.
I am unsure about the relationship between respired air temperature and pulmonary gas exchange (it may again be influenced by the Bohr effect), but nasal breathing warms air better than mouth breathing, though tidal volume lessens with cold air.  Glycogenolysis is also reduced at lower temperatures, suggesting improved oxygenation. 
Alas, this is a topic for another day.
“Horses maintain nasal breathing, normally, throughout exercise and rely on capacitance vessel constriction and contraction of upper airway dilating muscles to minimize airflow resistance.”
“DDSP [(dorsal displacement of the soft palate)] creates flow-limiting expiratory obstruction and may be caused by neuromuscular dysfunction involving the pharyngeal branch of the vagus nerve. It may alter performance by causing expiratory obstruction and by altering breathing strategy in horses.”
“Based on clinical observation, it has been suspected that horses might open-mouth breathe during episodes of dorsal displacement of the soft palate. Transoral breathing would be a unique feature of this syndrome because horses generally are obligate nasal breathers.”
“Fast-twitch muscle fibers have fewer mitochondria (where cell respiration occurs as well as the uptake of protons) than slow-twitch, or aerobic endurance fibers. Thus, during high-intensity resistance training, because of the extensive use of the fast-twitch fibers (with few mitochondria and less uptake of protons) there is a greater accumulation of protons, causing acidosis.”
“Robergs et al. (2004) show through detailed chemical reactions that lactic acid is not produced in the body. Rather, lactate is the product of a side reaction in glycolysis.”
“The utility of anaerobic glycolysis to a muscle cell when it needs large amounts of energy stems from the fact that the rate of ATP production from glycolysis is 100 times faster than from oxidative phosphorylation.”
“All cells have plenty of ADP and Pi because these are the hydrolysis products of ATP. However, the amounts of NAD+ are limited, and therefore NADH must be oxidized back to NAD+.”
“Every time ATP is broken down to ADP and Pi, a proton is released. When the ATP demand of muscle contraction is met by mitochondrial respiration, there is no proton accumulation in the cell, as protons are used by the mitochondria for oxidative phosphorylation and to maintain the proton gradient in the intermembranous space. It is only when the exercise intensity increases beyond steady state that there is a need for greater reliance on ATP regeneration from glycolysis and the phosphagen system. The ATP that is supplied from these nonmitochondrial sources and is eventually used to fuel muscle contraction increases proton release and causes the acidosis of intense exercise. Lactate production increases under these cellular conditions to prevent pyruvate accumulation and supply the NAD+ needed for phase 2 of glycolysis.”
“Hydrolysis of ATP generated by glycolysis, rather than glycolysis per se, releases H+ in the muscle (Robergs et al., 2004).”
“A portion of the muscle H+ load is removed by metabolic and fixed physicochemical buffers, and by the reduction in muscle bicarbonate concentration, while another portion leaves the cell in exchange with Na+ or along with lactate through MCTs. Plasma lactate and H+ concentration thus increase. Although fixed physicochemical buffers in the blood (Cerretelli and Samaja, 2003) remove a portion of the H+ load, plasma pH decreases, reducing the concentration of bicarbonate in the blood, and the CO2 released appears in the expired gas.”
“A fall in pH also inhibits phosphofructokinase activity. The inhibition of phosphofructokinase by H+ prevents excessive formation of lactic acid (Section 16.1.9) and a precipitous drop in blood pH (acidosis).”
“To support an increase in glycolysis, NAD+ from the conversion of pyruvate to lactate, is required. The activity of phosphofructokinase (PFK) is rate limiting.”
“Impairment of oxidative pathways during lactate production results in a net gain of H+ and acidosis occurs. (Oxidative phosphorylation during severe exercise prevents acidosis despite massive lactate production.)”
“Mitochondria-rich tissues such as skeletal and cardiac myocytes and proximal tubule cells remove the rest of the lactate by converting it to pyruvate.”
“With severe exercise, type II myocytes produce large amounts of lactate […] This provides some of the increased cardiac energy requirements (Fig. 4).”
: Peak M, Al-habori M, Agius L. Regulation of glycogen synthesis and glycolysis by insulin, pH and cell volume. Interactions between swelling and alkalinization in mediating the effects of insulin. Biochem J. 1992;282 ( Pt 3):797-805.
“It is concluded that glycogen synthesis and glycolysis are both stimulated by cell swelling and inhibited by acidification, under certain conditions, but glycolysis is more sensitive to inhibition by acidification and glycogen synthesis to stimulation by swelling. Consequently, simultaneous swelling and acidification is associated with inhibition of glycolysis and stimulation of glycogen synthesis. Stimuli that cause swelling and alkalinization activate both glycogen synthesis and glycolysis, alkalinization being more important in control of glycolysis and swelling in control of glycogen synthesis. Both cell swelling and alkalinization are components of the mechanism by which insulin controls glycogen synthesis and glycolysis.”
“In skeletal muscle, metabolic acidosis stimulates protein degradation and oxidation of branched-chain amino acids. This could occur to compensate for impairment of glucose utilization induced by acid.”
“The maintenance of hydrogen ion concentration in blood samples at pH 5.3-5.9 immediately inhibits glycolysis. This effect is due to the inhibition of all glycolytic enzymes, as shown by measurement of various glycolytic intermediates.”
“The major factors that accounted for the glycolytic inhibition in the ischemic heart compared with the anoxic heart appeared to be higher tissue levels of lactate and H+ in the ischemic tissue. […] It is concluded that accumulation of lactate represents a major factor in the inhibition of glycolysis that develops in ischemic hearts.”
“Lactate paradox: The reduced production of lactic acid at a given work rate at high altitude. Muscle work efficiency may be 50% greater at high altitude. ATP wastage is decreased.”
“The idea of the “oxygen debt” produced by exercise or stress as being equivalent to the accumulation of lactic acid is far from accurate, but it’s true that activity increases the need for oxygen, and also increases the tendency to accumulate lactic acid, which can then be disposed of over an extended time, with the consumption of oxygen. This relationship between work and lactic acidemia and oxygen deficit led to the term “lactate paradox” to describe the lower production of lactic acid during maximal work at high altitude when people are adapted to the altiude. Carbon dioxide, retained through the Haldane effect, accounts for the lactate paradox, by inhibiting cellular excitation and sustaining oxidative metabolism to consume lactate efficiently.”
“The loss of carbon dioxide from the lungs in the presence of high oxygen pressure, the shift toward alkalosis, by the Bohr-Haldane effect increases the blood’s affinity for oxygen, and restricts its delivery to the tissues, but because of the abundance of oxygen in the lungs, the blood is almost completely saturated with oxygen.”
“At high altitude, the slight tendency toward carbon dioxide-retention acidosis decreases the blood’s affinity for oxygen, making it more available to the tissues. It happens that lactic acid also affects the blood’s oxygen affinity, though not as strongly as carbon dioxide. However, lactic acid doesn’t vaporize as the blood passes through the lungs, so its effect on the lungs’ ability to oxygenate the blood is the opposite of the easily exchangeable carbon dioxide’s. Besides dissociating oxygen from hemoglobin, lactate also displaces carbon dioxide from its (carbamino) binding sites on hemoglobin. If it does this in hemoglobin, it probably does it in many other places in the body.”
“Most (75%+) of the lactate formed during sustained, steady-rate exercise is removed by oxidation during exercise, and only a minor fraction (approximately 20%) is converted to glucose.”
“Of the lactate which appears in blood, most of this will be removed and combusted by oxidative (muscle) fibers in the active bed and the heart.”
“However, as the muscle respiratory rate declines in recovery, lactate becomes the preferred substrate for hepatic gluconeogenesis. Practically all of the newly formed liver glucose will be released into the circulation to serve as a precursor for cardiac and skeletal muscle glycogen repletion. Liver glycogen depots will not be restored, and muscle glycogen will not be completely restored until refeeding.”
“The concept of a ‘lactate shuttle’ is that during hard exercise, as well as other conditions of accelerated glycolysis, glycolytic flux in muscle involves lactate formation regardless of the state of oxygenation. Further, according to the lactate shuttle concept, lactate represents a major means of distributing carbohydrate potential energy for oxidation and gluconeogenesis. In humans and other mammals, the formation, distribution and disposal of lactate (not pyruvate) represent key steps in the regulation of intermediary metabolism during sustained exercise.”
“It was concluded that, in humans, 1) lactate disposal (turnover) rate is directly related to the metabolic rate, 2) oxidation is the major fate of lactate removal during exercise, and 3) blood lactate concentration is not an accurate indicator of lactate disposal and oxidation.”
“Lactate is actively oxidized at all times, especially during exercise when oxidation accounts for 70–75% of removal and gluconeogenesis for most of the remainder. Working skeletal muscle both produces and uses lactate as a fuel, with much of the lactate formed in glycolytic fibres being taken up and oxidized in adjacent oxidative fibres. Because it is more reduced that its keto-acid analogue, sequestration and oxidation of lactate to pyruvate affects cell redox state, both promoting energy flux and signalling cellular events.”
: Prestwich KN. Removal of Lactic Acid — Oxidation and Gluconeogenesis. 2003. Available at: http://college.holycross.edu/faculty/kprestwi/exphys/lecture/ExPhysEx2Lect_pdf/ExPhys_03_M08_lac_remove.pdf. Accessed November 25, 2013.
“It is as if aerobic glycolysis started in the muscle and finished in the heart.”
“In the recovery period after exercise there is an increase in oxygen uptake termed the ‘excess post-exercise oxygen consumption’ (EPOC), consisting of a rapid and a prolonged component.”
“The percentage decrease in maximal ventilation with nose-only breathing compare to mouth and mouth plus nose breathing was three times the percentage decrease in maximal oxygen consumption. The pattern of nose-only breathing at maximal work showed a small reduction in tidal volume and large reduction in breathing frequency. Nasal breathing resulted in a reduction in FEO2 and an increase in FECO2. While breathing through the nose-only, all subjects could attain a work intensity great enough to produce an aerobic training effect (based on heart rate and percentage of VO2 max).”
“Twenty of the 30 subjects (normal augmenters) switched from nasal to oronasal breathing at submaximal exercise[…]”
“Dead space and airway resistance were significantly greater during nose than during mouth breathing.”
“It is suggested that a loss of nasal functions, such as during nasal obstruction, may result in lowering of CO2, fostering apneic spells during sleep.”
“End-tidal PCO2 during nose-obstructed sleep was lower than that during nose-open sleep in all of the subjects.”
“Nose breathing was found to be more energetically efficient in most but not all subjects, but additional research is needed to explore this finding further.”
: Thomas S. A., Phillips, V., Mock, C., Lock, M., Cox, G. and Baxter, J. (2009) The effects of nasal breathing on exercise tolerance. Liverpool conference centre: Chartered Society of Physiotherapy Annual Congress 2009, Liverpool conference centre, 16th and 17th October 2009.
“Nasal breathing was possible at 85% of maximum workload suggesting that people are capable of nose breathing at much higher intensities than they would normally chose to do, suggesting a potential for nose breathing training interventions even with normal healthy individuals.”
: Geor RJ, Ommundson L, Fenton G, Pagan JD. Effects of an external nasal strip and frusemide on pulmonary haemorrhage in Thoroughbreds following high-intensity exercise. Equine Vet J. 2001;33(6):577-84.
“The external nasal strip appears to lower the metabolic cost of supramaximal exertion in horses.”
“Exercise time to exhaustion in NBFNS [(nasal breathing with fake nasal strip)] trial, which was 23.6+/-6.7% less than the CON [(oronasal breathing)] value, increased 31.9+/-12.3% under NBENDS [(nasal breathing with external nasal dilator strip)] condition. [….] Nasal breathing reduces the sustainability of moderate exercise measured under oronasal breathing condition. Nostril dilatation increases the capacity to sustain moderate exercise under nasal breathing condition.”
“Exercise causes a fall in nasal resistance that may be due to sympathetic vasoconstriction in the nasal mucosa.”
“The results suggest that during incremental exercise 1) changes in AN EMG activities are highly correlated with changes in nasal VI, 2) turbulent flow in the nose may be the stimulus for the switch to oronasal breathing so that total pulmonary resistance is minimized, and 3) the correlation between nasal airflow and neural drive to the AN muscles is probably mediated by mechanisms that monitor airway resistance.”
: Levine BD, Stray-gundersen J. Point: positive effects of intermittent hypoxia (live high:train low) on exercise performance are mediated primarily by augmented red cell volume. J Appl Physiol. 2005;99(5):2053-5.
“While the results of many early studies on the use of altitude training for sea level performance enhancement have produced equivocal results, newer studies using the ‘live high, train low’ altitude training model have demonstrated significant improvements in red cell mass, maximal oxygen uptake, oxygen uptake at ventilatory threshold, and 3000m and 5000m race time.”
: Gore CJ, Hopkins WG. Counterpoint: positive effects of intermittent hypoxia (live high:train low) on exercise performance are not mediated primarily by augmented red cell volume. J Appl Physiol. 2005;99(5):2055-7.
: Singer RB, Deering RC, Clark JK. The acute effects in man of a rapid intravenous infusion of hypertonic sodium bicarbonate solution. II. Changes in respiration and output of carbon dioxide. J Clin Invest. 1956;35(2):245-53.
“During the infusion of sodium bicarbonate, arterial pH, arterial and alveolar PCO2, total ventilation, and rate of elimination of CO2 were significantly increased above control levels.”
“Following the infusion, the rate of CO2 elimination returned to the control level, but arterial pH was still elevated despite a steady fall toward the control range.”
“During hard physical exercise, metabolic rate may rise 10 or 15-fold, and this rate of heat production may be sustained for several hours. For the exercising individual, therefore, cold exposure does not normally represent a serious challenge to the body’s homeostatic mechanisms, but the problems of heat loss when exercising at a high ambient temperature may be acute.”
“It is also important to remember that, although it is the core body temperature which is regulated, it is the temperature of the skin relative to that of the environment which determines whether heat is gained or lost.”
: González-alonso J, Teller C, Andersen SL, Jensen FB, Hyldig T, Nielsen B. Influence of body temperature on the development of fatigue during prolonged exercise in the heat. J Appl Physiol. 1999;86(3):1032-9.
“These results demonstrate that high internal body temperature per se causes fatigue in trained subjects during prolonged exercise in uncompensable hot environments. Furthermore, time to exhaustion in hot environments is inversely related to the initial temperature and directly related to the rate of heat storage.”
“Air temperature is the most important factor influencing marathon running performance for runners of all levels.”
“There is a progressive slowing of marathon performance as the WBGT [(Wet Bulb Globe Temperature)] increases from 5 to 25 degrees C. This seems true for men and women of wide ranging abilities, but performance is more negatively affected for slower populations of runners.”
“The air inspired through the nose and oral cavity is heated during respiration. For typical external conditions (T = 22 degrees C i RH = 50%) the nose heats inspired air 1,5 times better then oral cavity (short time range of measurement approximately 1 min.). Heat from expired air is recovered for both nasal cavities and oral cavity. Nasal cavities respiration ability for heat recovery from expired air is 3 times higher then oral cavity respiration.”
“The results confirm the previous observation that cold air breathed through the nose inhibits ventilation in normal subjects and show that this is not related to an increase in flow resistance.”
“Heating and humidification of the respiratory air are the main functions of the nasal airways in addition to cleansing and olfaction. Optimal nasal air conditioning is mandatory for an ideal pulmonary gas exchange in order to avoid desiccation and adhesion of the alveolar capillary bed.”
“These results suggest that glycogenolysis in contracting skeletal muscle is reduced during exercise when the rise in body core temperature is attenuated. These changes in carbohydrate metabolism appear to be influenced by alterations in muscle temperature and/or sympatho-adrenal activity.”
“Muscle glycogenolysis and percentage of type I muscle fibers showing glycogen depletion were greater (P < 0.05) in the PRE ACC [(40 degrees C and 20% relative humidity before acclimation)] than in the RTT [(20 degrees C and 20% relative humidity)] trial.”