The effects of cigarette smoking on the associations between sitting time and all-cause mortality: a meta-analysis

Background: Sedentary behavior is recognized as an independent risk factor for mortality, but it remains unclear whether cigarette smoking will aggravate the detrimental effects of prolonged sitting on mortality. This study examined the impact of cigarette smoking on the relationship between sitting time and all-cause mortality in adults. Methods: Electronic database searches were conducted in PubMed, Web of Science, and the EMBASE up to 1 June 2017. Prospective studies that reported sitting time, percent of current smokers, and all-cause mortality were included. Data were extracted independently by two authors. Results: Ten prospective studies met the inclusion criteria. These studies included 850990 adults who were followed up for 2-15.7 years, during which 64 781 died (7.6%). Generally, during follow-up sitting time showed a dose-response relationship with all-cause mortality, with each 1 h increment of sitting time per day accounting for hazard ratio (HR) of mortality 1.02 (95%CI, 1.02-1.03). The relationship remained significant when stratified by the quartiles of smoking populations (<= 8.4%, 8.5%-12.6%, 12.7%-27.9%, and >= 28.0%), and the risk of sitting time-related mortality increased parallel to the increment of the percent of smoking populations, with HRs 1.02 (95%CI, 1.02-1.03), 1.03 (95%CI, 1.02-1.03), 1.04 (95%CI, 1.03-1.04) and 1.06 (95%CI, 1.06-1.06), respectively. The associations between the risk of prolonged sitting-related mortality and the percent of smoking populations were linear (P = 0.032). Conclusions: Cigarette smoking significantly aggravated the detrimental effects of sitting time on all-cause mortality. Our findings provided further evidence on the harmful effects of smoking combing prolonged sitting on adult health.