Characteristics of included studies of smoking and metabolic syndrome risk included in the meta-analysis.

1. WHO Guidelines, World Health Organization-West Pacific Region Guidelines; NCEP-ATP III, National Cholesterol Education Program's Adult Treatment Panel III; JCDCG, Chinese Joint Committee for Developing Chinese Guidelines on Prevention and Treatment of Dyslipidemia in Adults definition; ACE/AACE, American College of Endocrinology/American Association of Clinical Endocrinologists criteria; AHA/NHLBI, American Heart Association/National Heart, Lung and Blood Institute criteria.2. BMI, body mass index; HOMA-IR, homeostasis model assessment of insulin resistance.3. Age of all participants (include those with metabolic syndrome at the baseline); NA, not recorded or available.4. We used the risk estimates from NCEP III definition in this study.5. Kawada used Japanese criteria which is similar to NCEP criteria except for lower waist circumference.