## 2 Materials and Methods
### Electronic Search Strategy and Eligibility Assessment  
The electronic literature searching strategies for PubMed and Web of Science databases are presented in Table 1. The initial searching was conducted to find manuscripts published in English that presented the original studies.  
Afterwards, the screening and inclusion were conducted based on the following inclusion criteria:  
* Study conducted in adult participants;
* Study conducted in a population of patients with obstructive sleep apnea diagnosed;
* Study conducted in patients treated with PAP;
* Study of any kind of dietary intervention;
* Effect of dietary intervention compared with the effect observed for control group with no dietary intervention or with any other kind of dietary intervention;
* Obstructive sleep apnea monitored using any objective or subjective method of assessment;
* Study presenting results of RCT.  
The following exclusion criteria were applied:  
* Study conducted in population of pregnant or lactating women;
* Study conducted in population with any concurrent disease diagnosed, other than excessive body mass (overweight/obesity);
* Study conducted in population with any mental health problems, eating disorders, or intellectual disabilities;
* Study conducted in animal model.  
\begin{table}
\begin{tabular}{l l} \hline \hline
**Database** & **Electronic Literature Searching Strategy** \\ \hline \multirow{4}{*}{PubMed} & ((obstructive sleep apnea[Title/Abstract]) OR (obstructive sleep apnea[Title/Abstract]) OR (OSA[Title/Abstract]) OR (OSA[Title/Abstract]) OR (OSA[Title/Abstract]) OR (OSAHS[Title/Abstract]) OR (sleep-disordered breathing[Title/Abstract]) AND (diet OR nutrition OR nutrient OR nutrients) \\ \hline \multirow{4}{*}{Web of Science} & (Topic)=((obstructive sleep apnea) OR (obstructive sleep apnea) OR (OSA) OR (OSAHS) OR (sleep-disordered breathing)) AND (All fields)=((diet) OR (nutrition) OR (nutrient) OR (nutrients)) \\ \cline{1-1}  & \\ \end{tabular}
\end{table}
Table 1: Electronic literature searching strategies for PubMed and Web of Science databases applied within this study.  
The population, intervention/exposure, comparator, outcome, and study design (PICOS) criteria were applied within the study as presented in Table 2.  
The duplicated records were manually removed after searching. Two researchers independently analyzed the titles of articles, abstracts of articles (records included based on titles), and full texts of articles (records included based on abstracts) on the basis of inclusion and exclusion criteria. Until consensus was achieved, any disagreement was discussed if necessary. The full texts of the articles defined as potentially eligible based on titles and abstracts were searched within electronic databases and libraries and, if necessary, the corresponding authors were contacted.  
Moreover, in order to identify the RCTs with diet therapy for obstructive sleep apnea syndrome patients conducted only for participants treated with PAP, the corresponding authors were contacted, if necessary. They were asked about the studied group in order to include only those studies in which all participants were treated with PAP; those with participants treated without PAP, with mixed populations, or with no defined mode of therapy were excluded [20; 22; 24; 27; 28; 29; 30; 31; 32; 33; 34; 35].  
The procedure of identification, screening, and inclusion of RCTs applied within the study is presented in Figure 1.