# Gut Microbiome in Patients with Obstructive Sleep Apnoea
## 4 Discussion
Of note, a high-fat diet can increase _Proteobacteria_ populations in the colon [32]. Patients with OSA tend to consume a high-fat diet [7] which could eventually be associated with dyslipidemia. Supporting this, a significant association between _Proteobacteria_ and markers, as well as the prevalence of dyslipidemia, was found in our study. This is further supported by the fact that following adjustment for metabolic confounders, the association between OSA and _Proteobacteria_ became insignificant. We reported a higher abundance of _Lactobacillus_ in the stool samples of patients with OSA. Most of the studies on animal models showed an inverse relationship between _Lactobacillus_ and intermittent hypoxiaemia [6]. In contrast, a significant relationship between stool _Lactobacillus_ and serum homocysteine levels were found in patients with OSA, suggesting that _Lactobacillus_ may play a role in the development of cardiovascular diseases [11]. _Lactobacillus_ could also produce GABA [29] and sleep fragmentation could lead to decreased _Lactobacillus_[5]. In line with this, we found a significant relationship between _Lactobacillus_ and sleep efficiency. Gut _Lactobacillus_ are related to obesity and metabolic disease, such as diabetes and dyslipidemia [33]. Similarly, we found a relationship between _Lactobacillus_, BMI, and deranged lipid parameters. Taking into account that animal models suggest that intermittent hypoxemia and sleep fragmentation lead to lower _Lactobacillus_ levels, we hypothesise that the observed direct relationship between _Lactobacillus_ and OSA in the current study is significantly driven by obesity rather than OSA. Further studies comparing obese and non-obese patients with OSA are warranted in order to explore this relationship in detail. Of note, the association between _Lactobacillus_ and OSA became insignificant following adjustment for metabolic confounders. Similarly to the results on _Proteobacteria_, this suggests that metabolic associations (i.e., diet) and potential consequences (i.e., diabetes and dyslipidemia) drive the relationship, rather than intermittent hypoxemia. Finally, we found a higher abundance of _Roseburia_ in OSA; interestingly, however, the abundance was not related to disease severity but to dyslipidemia. _Roseburia_ is strongly associated with a protein-free, high-carbohydrate diet [8], which is linked to OSA [7]. Therefore, we believe our findings reflect dietary differences between patients and controls more than OSA-related factors per se. Taking medications may affect the gut microbiome [34]. Although there was no difference between the OSA and control groups for most medications (except ACE inhibitors), some medications indeed affected the gut microbiome in our study. Of note, there was no difference in the prevalence of subjects taking benzodiazepines, proton-pump inhibitors, antihistamines, laxatives, or clopidogrel; therefore, the observed differences were unlikely due to different medication habits. Further studies need consider the potential effect of medications when interpreting gut microbiome data. Our study has limitations. First, although the sample size was comparable to most human studies [6], some differences may have not been significant due to type II error. Second, in general, the disease severity in the OSA group was relatively mild, and analysing patients with a broader range of severity could reveal further associations between AHI and the microbiome. Third, this was a cross-sectional study, and the causality between OSA, gut microbiome, metabolism, and cardiovascular consequences cannot be investigated. Forth, some subjects had cardiorespiratory polygraphy as a diagnostic test; therefore, indices of sleep quality were estimated in a limited number of subjects. In addition, the AHI in subjects who had polygraphy may be underestimated, as hypopnoeas related to arousals were not counted. Finally, the diet was not controlled in the current study; therefore, some of the results might have been driven by differences in diet rather than OSA.