## 3 Results  
The general characteristics of the randomized controlled trials included in the study [23; 39; 40; 41; 42] and studied populations are presented in Table 3. Only studies clearly defined as those conducted in a population of patients with PAP treatment applied were intended to be included; thus, numerous studies with mixed populations with and without PAP treatment were excluded [22; 32; 33; 34]. Similarly, if not obstructive sleep apnea was monitored, but the concurrent diseases and symptoms or other factors only, it was not included [43; 44; 45]. All the included studies were conducted in developed European countries: Spain [40; 41], Sweden [23], the UK [39], and Italy [42]. The studies were conducted in groups with patients treated with CPAP [23; 39; 40; 41] or intended to be treated with CPAP [42]; patients with moderate-to-severe [23; 39; 41] or severe obstructive sleep apnea were included [40; 42]. The additional elements taken into account within the general characteristics of the studied groups were male gender [23; 41], obesity [40; 42], and being  
Figure 1: Procedure of identification, screening, and inclusion of randomized controlled trials applied within this study. scheduled for bariatric surgery [42]. The populations studied within the included trials were of a medium size of either less than 50 [40] or more than 50 participants [23; 39; 41; 42]. The studied participants were mainly in their 40s [23; 40; 42] or 50s [41]. Among the additional inclusion criteria, there was a criterion based on body mass index (BMI); only overweight and obese participants [41], obese participants [23; 39; 40], or obesity class two and three participants were qualified [42]. Similarly, for some studies the inclusion criteria included having CPAP treatment conducted for at least 6 months [12; 41] or not smoking [42]. The interventions applied within the randomized controlled trials included in the study and accompanied by the conclusions of the study regarding the influence of diet therapy are presented in Table 4. The conclusions of the study and the influence of diet therapy were formulated based on the results and conclusions formulated within the randomized controlled trials included in the study (Supplementary Table S1). Within all included studies, an experimental diet was compared with the control group; however, various interventions were applied, including: dietary intervention, which was compared with no intervention [23; 42], and dietary and physical activity intervention, which was compared with general advice [39; 40; 41]. At the same time, the applied intervention included various dietary approaches, such as various energy deficits [23; 40; 42] or dietary health-promoting behaviors [39; 41]. The monitored variables included apnea-hypopnea index (AHI) [23; 40; 41; 42], sleep time [23; 40; 41], sleep efficiency [40; 41], sleep latency [41], oxygen desaturation episodes [39; 41], oxygen saturation [23; 41], obstructive sleep apnea type (positional/not) [40], Epworth Sleepiness Scale score [23; 40; 41], Pittsburgh Sleep Quality Index [41], and quality of life (EuroQol EQ5D-3L VAS) [39]. The assessment of the risk of bias, conducted based on the revised Cochrane risk-of-bias tool for randomized trials, within the randomized controlled trials included in the study, is presented in Table 5. For the risk of bias, the majority of studies defined some concerns [23; 39; 41; 42] resulting from the randomization process (D1 domain) [39; 42], deviations from the intended interventions (D2 domain) [23], or selection of the reported result (D5 domain) [23; 39; 40; 42]. For one study, the risk of bias was assessed as high [40]; this risk resulted from the randomization process (D1 domain) and deviations from the intended interventions (D2 domain).