Introduction Previous studies in Asia developed areas showed inconsistent findings on the association between nightshift work and obesity occurrence, and there has been no study in Chinese. This study aims to examine the association between nightshift work, body weight gain and obesity occurrence in a Chinese night shift cohort after 3 years of follow-up.

Methods We recruited 5256 male workers from 6 companies in China in 2013. We used standardised questionnaire to collect participants’ information on occupational history of shift work. Night shift work was defined as ever worked in a working schedule during 00–5 am at least once per month for no less than one year. Anthropometric parameters were measured using standard medical protocols. All participants were followed up till the end of 2017. Multiple logistic regression analysis was conducted to evaluate the association between night shift work and overweight (BMI ≥25), obesity (BMI ≥30) and BMI gain status.

Results We only included 492 male workers in this report, as the data of other participants have not been input whilst the overall results from entire cohort will be presented in ICOH 2018. The mean age of night shift work and daytime workers is comparable (30 vs 29 years). Compared with the daytime workers, slightly more night shift workers were current smokers (25.1% vs 23.7%) and alcohol drinkers (28.1% vs 22.2%), but fewer had vigorous physical activity (29.9% vs 59.4%). More nightshift workers than daytime workers slept less than 8 hours per night (60.5% vs 34.8%) but the proportion of working longer than 55 hours per week was substantially higher (6.6% vs 3.7%). After three years of follow-up, night shift workers showed more BMI gain than the daytime workers (1.45±1.10 vs 1.32±1.09). More night shift workers with large body mass index at baseline tended to retaining in the same category of overweight (BMI ≥24 kg/m²) status with odds ratios of 1.49 (95% CI: 0.95 to 2.33), and stayed in the same category of abdominal obesity with OR of 1.34 (95% CI: 0.86 to 2.11). More night shift workers developed abdominal obesity from normal body size during 3 year period of follow-up and the risk of abdominal obesity was 1.43 (95% CI: 0.75 to 2.73), but there was no statistical significance.

Conclusions This study provided preliminary evidence to on a possible link between nightshift work and obesity occurrence or body weight gain in Chinese male workers; however, these findings would be verified in a larger dataset of all 5256 workers.

Funding This work was supported by National Natural Science Foundation of China (Project number 81273172 and 81372964).