First, we would like to congratulate Carvalho et al. on their article published in the JBP entitled "Correlation of lung function and respiratory muscle strength with functional exercise capacity in obese individuals with obstructive sleep apnea syndrome" (OSAS),(1) a matter of extreme importance for all professionals working in this area; this clinical condition affects a large number of people, with a direct impact on their quality of life.

An important observation to be made about the aforementioned study(1) is that it has a cross-sectional design, which does not establish causality, that is, it is not possible to know whether the changes in lung function and respiratory muscle strength are due to either obesity or OSAS alone. Support for the first hypothesis may be evidenced in a study by Melo et al.,(2) who reviewed studies of lung function in obese individuals and observed reductions in total lung capacity and FVC, accompanied by a reduction in FEV1; those were the key findings across all samples, suggesting that the presence of a restrictive respiratory pattern is associated with obesity. What can be perceived is that decreased lung capacity is already a characteristic of obesity, even in the absence of comorbid OSAS.(2) A study by Tassinari et al.,(3) cited by the study in question,(1) reported that no lung function or respiratory muscle impairment was observed in normal-weight OSAS patients and that there were similarities between that group of patients and healthy subjects.

Another limiting factor are the comorbidities in the sample,(1) such as type II diabetes mellitus; it has been reported in a study by Punjabi et al.(4) that, regardless of adiposity, sleep-disordered breathing is associated with impairments in insulin sensitivity and that using body mass index, which does not discriminate between muscle mass and adipose tissue, as an assessment variable may affect the results found regarding the comparison of the degree of obesity of each individual.

A finding that deserves comment is the lack of correlation between lung function and six-minute walk distance in that population,(1) because the six-minute walk test has been validated in obese subjects, and found to be reproducible, and it has been demonstrated that an 80 m increase in six-minute walk distance is related to clinical improvement. A factor that could possibly explain this finding was reported by Ucok et al.,(5) who compared individuals with OSAS and healthy individuals: individuals with lower maximal oxygen consumption values had premature leg fatigue. This disorder of the muscle metabolism is associated with high levels of lactic acid in the blood and with decreased ability to reduce these levels during exercise in patients with sleep disorders.(5)

It is incumbent upon us to emphasize that the results obtained are likely to be improved by using a different methodological design and by redefining the study population by limiting it to individuals without comorbidities, thus avoiding a possible selection bias.

REFERENCES

1. Carvalho TMDCS, Soares AF, Climaco DCS, Secundo IV, Lima AMJ. Correlation of lung function and respiratory muscle strength with functional exercise capacity in obese individuals with obstructive sleep apnea syndrome. J Bras Pneumol. 2018;44(4):279-284. https://doi.org/10.1590/s1806-37562017000000031
2. Melo LC, da Silva MA, Calles AC. Obesity and lung function: a systematic review. Einstein (Sao Paulo). 2014;12(1):120-5. https://doi.org/10.1590/S1679-45082014RW2691
3. Tassinari CC, Piccin CF, Beck MC, Scapini F, Oliveira LC, Signori LU, et al. Capacidade funcional e qualidade de vida entre sujeitos saudáveis e pacientes com apneia obstrutiva do sono. Medicina (Rib Preto). 2016;49(2):152-9. https://doi.org/10.11606/issn.2176-7262.v49i2p152-159
4. Punjabi NM, Beamer BA. Alterations in Glucose Disposal in Sleep-disordered Breathing. Am J Respir Crit Care Med. 2009;179(3):235-40. https://doi.org/10.1164/rccm.200809-1392OC
5. Ucok K, Aycicek A, Sezer M, Genc A, Akkaya M, Caglar V, et al. Aerobic and anaerobic exercise capacities in obstructive sleep apnea and associations with subcutaneous fat distributions. Lung. 2009;187(1):29-36. https://doi.org/10.1007/s00408-008-9128-0