Table 2. Newcastle-Ottawa Scale (NOS) assessment of the quality of the studies.

Study	Selection (MAX 4)	Comparability (MAX 2)	Exposure (MAX 3)	All (MAX 9)
Benoist et al. 1998 France	3	2	2	7
Carrèr et al. 2006 France	4	2	2	8
Choi et al. 2012 Korea	4	2	2	8
Feng et al. 2013 China	3	2	2	7
MA et al. 2011 China	3	2	1	6
Mekeel et al. 2011 USA	4	2	2	8
Richardson et al. 1989 USA	3	2	1	6
Rodríguez et al. 2006 USA	4	2	1	7
Shoup et al. 2002 USA	4	2	2	8
Yamaguchi et al. 2001 Japan	3	2	1	6
Zhao et al. 2012 China	4	2	2	8