The study, published in BMC Endocrine Disorders by a team led by Kuoqing Zhang and Jianhui Wu of North China University of Science and Technology, draws on the Beijing-Tianjin-Hebei Health Effects Cohort. Between 2017 and 2018, the researchers enrolled 2,296 employees of a petroleum enterprise in Hebei Province, collecting detailed questionnaire data on work schedules and smoking history through trained interviewers, alongside baseline health examinations. From 2019 through 2025, the participants returned for annual follow-ups, allowing the team to track who developed new-onset hypertension-diabetes comorbidity over time. Because the study was prospective, meaning exposures were measured before disease appeared, it avoids much of the recall bias and reverse-causation ambiguity that weaken cross-sectional occupational studies.

To quantify shift exposure, the investigators calculated a Weighted Shift Index, a composite measure that captures not merely whether someone works shifts but how intense those schedules are, weighting factors such as the frequency of night rotations and the cumulative burden of circadian disruption. Smoking was similarly graded with a Smoking Index that reflects both intensity and duration of the habit. Rather than forcing these continuous measures into arbitrary cutoffs alone, the team also used restricted cubic splines, a flexible statistical technique that lets the data reveal the true shape of the dose-response relationship. That analysis proved revealing: the association between the Weighted Shift Index and comorbidity risk was non-linear, meaning risk did not climb in a straight line with increasing shift burden but followed a curved trajectory, with certain thresholds of exposure marking steeper rises.

Over the seven-year follow-up window, 8.6 percent of the cohort developed hypertension-diabetes comorbidity. That figure may sound modest, but in occupational health terms it represents a substantial burden of preventable cardiovascular and metabolic disease in a working population, many of whom are in the prime of their productive lives. When the researchers applied multivariate-adjusted modified Poisson regression, a method well suited to estimating relative risks in cohort data where outcomes are not rare, the picture sharpened considerably. Workers with high-intensity shift schedules faced a 69 percent higher risk of developing the comorbidity compared with their peers, with a relative risk of 1.693 and a confidence interval that excluded the null value with strong statistical confidence.

Smoking told an equally sobering story, and with an unexpected twist in the dose-response curve. Moderate smokers carried roughly double the risk of never-smokers, with a relative risk of 2.046, while heavy smokers were not far behind at 1.937. The fact that moderate smoking appeared slightly riskier than heavy smoking in this cohort is a curiosity the authors report transparently; it may reflect survivor effects, behavioral compensation, or chance variation within the confidence intervals, which overlap substantially between the two categories. What matters most is the consistent signal: both moderate and heavy smoking significantly elevated comorbidity risk, each with a p-value of 0.001, reinforcing the well-established vascular toxicity of tobacco smoke while adding a specific new outcome to its ledger.

The study’s most consequential finding, however, lies in the interaction analysis. Using the classic epidemiological framework of the relative excess risk due to interaction, the attributable proportion, and the synergy index, the team tested whether shift work and smoking act on the same disease pathway in a way that multiplies harm. For high-intensity shift work combined with heavy smoking, the attributable proportion reached 0.573, with a confidence interval of 0.025 to 1.121. In practical terms, this suggests that more than half of the comorbidity risk observed in this doubly exposed group could be attributed to the interaction itself, the part of the risk that exists only because the two exposures co-occur. It is the statistical signature of a genuine synergistic effect, the kind of biological amplification that occupational health guidelines rarely account for.

What biological mechanisms could underlie such synergy? The authors and the broader literature point to overlapping pathways of metabolic and vascular injury. Shift work disrupts the circadian regulation of cortisol, melatonin, blood pressure dipping during sleep and glucose homeostasis, promoting insulin resistance, sympathetic overactivation and chronic low-grade inflammation. Smoking, meanwhile, delivers nicotine and combustion products that impair endothelial function, increase oxidative stress and worsen insulin sensitivity. When a circadian-disrupted metabolism meets a chemically stressed vasculature, the combined insult may exceed the sum of its parts, accelerating the convergence of hypertension and type 2 diabetes into a single comorbid state. Intriguingly, the study also found that moderate-intensity shift work showed a certain protective effect, hinting that the relationship between schedule disruption and metabolic health may be more nuanced than a simple more-is-worse model, though the authors urge caution in over-interpreting that single result.

The choice of study population is not incidental. Petroleum workers represent a large, globally distributed workforce exposed to a distinctive cluster of occupational hazards: irregular schedules dictated by continuous operations, physically demanding tasks, psychological stress, and in many regions, high smoking prevalence shaped by workplace culture. Hypertension-diabetes comorbidity is particularly insidious in this context because each condition worsens the other; elevated blood pressure accelerates the vascular complications of diabetes, while hyperglycemia damages the kidneys and vessels that regulate blood pressure. Workers who develop the pairing face compounded risks of atherosclerotic cardiovascular disease, stroke, chronic kidney disease and retinopathy, often at ages well before such diseases typically strike.

For employers and regulators, the findings translate into a concrete intervention logic. Because the synergy between heavy smoking and high-intensity shift work accounts for a large share of the excess risk in the most vulnerable group, reducing either exposure should yield outsized benefits for that population. Workplace smoking cessation programs, circadian-aware rostering that limits the intensity and frequency of night rotations, and routine screening for blood pressure and fasting glucose among shift workers emerge as evidence-backed priorities. The study was funded by China’s National Key Research and Development Program and approved by the ethics committee of North China University of Science and Technology, with all participants providing written informed consent, underscoring the rigor behind the data.

As night-shift economies expand worldwide, from energy extraction to logistics to healthcare, this study adds a compelling data point to a growing scientific consensus: the timing of work is not a neutral variable in human health. When circadian disruption and chemical exposures such as tobacco collide in the same body, the resulting disease risk can be far greater than occupational safety frameworks currently assume. For the petroleum workers of Hebei Province, and for shift workers everywhere who smoke, the message from this seven-year cohort is unambiguous. The combination is not merely additive. It is a multiplier, and it is modifiable.

Subject of Research: Effects of shift work and smoking on new-onset hypertension-diabetes comorbidity in Chinese petroleum workers

Article Title: Association of shift work and smoking with new-onset hypertension-diabetes comorbidity in Chinese petroleum workers: a prospective cohort study

Article References: Zhang, K., Zhang, H., Liu, L., Wang, D., Zhang, M., Ma, X., Li, X., Xue, L., Wang, G., Yu, M., & Wu, J. (2026). Association of shift work and smoking with new-onset hypertension-diabetes comorbidity in Chinese petroleum workers: a prospective cohort study. BMC Endocrine Disorders. https://doi.org/10.1186/s12902-026-02570-x

Image Credits: AI Generated

DOI: 10.1186/s12902-026-02570-x

Keywords: hypertension-diabetes comorbidity, shift work, smoking, petroleum workers, prospective cohort study, occupational health, circadian disruption, cardiovascular risk, type 2 diabetes, interaction effect, relative risk, China