October 04, 2017
Shift workers, especially those who work permanent night shift, are more likely to be overweight or obese than people who don't work shifts, and most frequently, their obesity is in the form of abdominal fat, a meta-analysis of the literature reaffirms.
"Shift work has recently been identified as an important occupational hazard, with a growing body of evidence showing an association between shift work and adverse health effects such as metabolism abnormalities that include obesity," Miaomiao Sun of the Chinese University of Hong Kong, People's Republic of China, and colleagues write in their paper, published online October 4 in Obesity Reviews.
"We showed that night shift work increases the risk of obesity/overweight by 23%, and the excess risk with abdominal obesity was even higher, at 35%," they state.
Risk More Prominent Among Long-term Night-Shift Workers
The systemic review and meta-analysis included 28 articles, 22 of which were cross-sectional studies and the remaining six of which were cohort studies.
Almost half of the studies, at 11 in total, were conducted among healthcare workers. As the study authors note, they pooled risk estimates from individual studies to arrive at a clearer picture of the association between different types of shift work and specific types of obesity.
Relative to nonshift workers, "the overall odds ratio [OR] of night shift work was 1.23 ... for risk of obesity/overweight," the authors report (P = .001).
Among those studies that defined night shift work as the interlude between midnight and 5 AM, night-shift workers were 32% more likely to be overweight or obese, at an OR of 1.32 (P < .001), than those who did not work overnight but who were still broadly defined as "shift workers."
For this latter group of workers — where day shifts might be involved — the risk of being overweight or obese was much smaller at an OR of 1.14.
A few studies also revealed that employees working permanent night shift were more than 40% more likely to be overweight or obese than rotating shift workers, at an OR of 1.43 (P < .001).
So employees working a permanent night shift were 29% more likely to be overweight or obese than rotating shift workers (OR, 1.43 vs 1.14).
Interestingly, ethnicity did not appear to affect the risk of being overweight or obese among shift workers, as compared with nonshift workers, as studies from different geographic regions indicted.
Circadian Disruption May Be to Blame
In trying to explain the greater propensity for shift workers to be heavier, the researchers point to disruption in circadian rhythms as a potential underlying cause. Circadian rhythms are disrupted upon exposure to light at night.
"Melatonin plays a key role in synchronizing central and peripheral circadian rhythms and regulates the secretion of hormones such as cortisol, insulin, and leptin," they explain.
"[And] such misalignments may lead to a disturbance of body homeostasis and produce abnormal metabolic profiles," they add.
This hypothesis was reinforced by the observation that shift workers who had rotating shifts — such that their circadian systems continuously adapted to their shift work — were not more likely to be overweight or obese compared with permanent shift workers.
"Our study revealed that much of the obesity and overweight among shift workers is attributable to such a job nature," Dr Tse said.
"Modification of working schedules to avoid prolonged exposure to long-term night-shift work might be an efficient administrative control to reduce the risk of obesity," she adds.
Worldwide, an estimated 20% of the overall workforce is engaged in some type of shift work.
This work was supported by National Natural Science Foundation of China. The authors report no relevant financial relationships.