Table 2.

Quality assessment of the studies

First author, year	Study design	Selection	Comparability	Exposure/Outcome	Total scores
Abou-Donia et al. 2019	Cross-sectional	★★★★	★★	★★★	9
Al-Ayadhi and Mostafa 2012	Cross-sectional	★★★★	★★	★★	8
Ayaydin and Kirmit 2020	Cross-sectional	★★★	★★	★★	7
Esnafoglu et al. 2017	Cross-sectional	★★★	★★	★★★	8
Guloksuz et al. 2017	Cross-sectional	★★★	★★	★★	7
Ma et al. 2019	Cross-sectional	★★★★	★★	★★	8
Peng and Zhu 2017	Cross-sectional	★★★★	★★	★★	8
Shaker 2016	Case–control	★★★	★★	★★	7
Tomova et al. 2019	Cross-sectional	★★★★	★★	★★	8
Zhou 2015	Cross-sectional	★★★	★★	★★	7