Does it matter which way you breathe during exercise?
Anecdotal evidence and year of experience working with athletes, breath coaches Brian Mackenzie (Shift Adapt & HHP Foundation) and Patrick McKeown (Oxygen Advantage) suggest that nasal breathing is a more efficient way to breathe during exercise. Yet, exercise physiologists are to accept the notion, given that scientific literature is limited. In sports science, the study of metabolism is via breath-by-breath analysis and indirect calorimetry. This method has always used equipment such as the Hans Rudolph full face mask (imaged) or a valve and mouthpiece. Therefore, this scientific community has assumed that humans can produce the most capacity work by oral breathing only. Still, the assumption could be wrong, or at least should there be questioning the possibility.
As scientists, our aim is to be sceptical; we put across a null hypothesis and look for evidence to prove that wrong (despite most people wanting to prove themselves correct). However, another role of the scientist is to be curious so that we can continue to develop new theories and discoveries for the betterment of man and the world around us.
Dallam (2020) reviewed the literature and concluded that whilst data is limited, there is a body of evidence to suggest nasal breathing during exercise may lead to improved respiratory health and greater ventilatory efficiency. Especially in those adapted to nasal restriction breathing (> 6 months) with almost no impact on overall exercise capacity. 
One particular study by the same group gives a compelling case to suggest that a nasally restricted breathing pattern is more efficiently at all levels of running intensity in recreational runners. However, as with all scientific papers, we cannot just take the conclusions for granted without a critical analysis of the data and methodology.
Professor Dallam and his team  used a well thought out procedure. Over 2.5 years, recruited 5 males and 5 females to take part in the data blinded crossover trial. Following a familiarisation, each participant performed two maximal increment exercise tests, followed by 6 minutes steady state at 85% VO2 max (10-minute rest between), either using mouth breathing (pegged nose) or via nasal breathing (mouth taped).
Results were as followed. Both groups had similar VO2 max, Lactate, and Respiratory Exchange Ratio (suggestive of similar aerobic vs anaerobic work) for both exercise tests. However, minute ventilation was 22% less during nasal breathing, respiratory rate was between 16-20 % slower. So was ventilatory equivalent for O2 and C02 were both of lower value suggesting greater gas exchange efficiency when breathing through the nose. Well, at least at a glance.
The author does say that the study is not without limitations such as low participant numbers, a self-selection bias with their breathing choice, and the use of nasal strips to counteract pressure alterations by wearing a mask when running. Notwithstanding those, there are a few questions I would like to raise.
Given these individuals were nasally adapted, and there is no evidence of such exercise values before the 6 months of nasal training. Might we suggest that when oral breathing, this subject group, are hyperventilating due to poor breathing mechanics from lack of mouth breathing during training?
What about evidence?
Results suggest that VE/VCO2 during steady-state exercise, oral breathing had a ratio of 37, at 85% of VO2 max intensity. This seems high compared to typical values  near the anaerobic threshold (close to RER 1.1). More so at such a young age group. When paired together with a PETCO2 value of 40 mmHg, which is relative to a resting value at near max exercise, it could suggest excessive CO2 blow off. Therefore, I have requested the Wassermann nine graphs to view the ratio of physiologic dead space over tidal volume (Vd/Vt), another good indicator of hyperventilation  a powerful addition to the data set. I ask why it wasn’t presented.
In terms of methodology, I appreciate how challenging it is to recruit individuals, respect for 2.5 yrs worth. Still, it is well established that there are both gender differences in respiratory function  and ventilatory efficiency . While the ages of males of the two groups, males and females, are not significantly different, there is a decade of difference 35 vs 23 yrs for males and females. How would this contribute to skewing confidence in data?
Overall, I applaud that this is a solid start and very compelling case that once adapted, nasal breathing is a more efficient method during exercise. This is something that coaches, and scientists cannot just ignore. Though I do hope to see, some longer duration-controlled trials, taking non adapted athletes through the process of nasal training and measuring the parameters using a similar procedure with a group of individuals of similar age and the same gender.
1. Dallam, G., & Kies, B. (2020). THE EFFECT OF NASAL BREATHING VERSUS ORAL AND ORONASAL BREATHING DURING EXERCISE: A REVIEW. Journal of Sports Research, 7(1), 1-10.
2. Dallam, G. M., McClaran, S. R., Cox, D. G., & Foust, C. P. (2018). Effect of nasal versus oral breathing on Vo2max and physiological economy in recreational runners following an extended period spent using nasally restricted breathing. International Journal of Kinesiology and Sports Science, 6(2), 22-29.
3. Mezzani, A. (2017). Cardiopulmonary exercise testing: basics of methodology and measurements. Annals of the American Thoracic Society, 14(Supplement 1), S3-S11.
4. Nora, T. O. M. A., Bicescu, G., Enache, R., Dragoi, R., & Cinteza, M. (2010). Cardiopulmonary exercise testing in differential diagnosis of dyspnea. Maedica, 5(3), 214.
5. LoMauro, A., & Aliverti, A. (2018). Sex differences in respiratory function. Breathe, 14(2), 131-140.
6. Kilbride, E., McLoughlin, P., Gallagher, C. G., & Harty, H. R. (2003). Do gender differences exist in the ventilatory response to progressive exercise in males and females of average fitness?. European journal of applied physiology, 89(6), 595-602.