Abstract Body (Do not enter title and authors here): Background and Aims
Night shift work increases the risk of coronary artery diseases, contributing to worsened prognosis. However, the association between night shifts and coronary microvascular dysfunction (CMD) remains unclear. This study aimed to investigate whether night shift workers have a high incidence of CMD, and the potential mechanisms linking night shifts and CMD.

Methods
We collected cross-sectional clinical traits of traditional cardiovascular risk factors and sleep-related parameters in a security guards cohort at Fudan University. 101 subjects with no history of coronary artery disease were enrolled, of which 50 were night shift workers. Coronary flow reserve (CFR) < 2.0 evaluated by single-photon emission computed tomography (SPECT) myocardial perfusion imaging was recognized as CMD. Integrated analysis of fecal 16s sequencing and serum untargeted-metabolomics was conducted for mechanistic exploration. Microbial features were centered log-ratio(CLR)-transformed for differential enrichment analysis, while metabolic traits were inverse-normal transformed.

Results
After adjusting for age, BMI, remnant cholesterol, TyG (triglyceride-glucose index), NT-pro BNP, and melatonin, night shift work was associated with increased occurrence of CMD (OR 5.26, 95% CI 1.41-28.71; P = 0.012). One year of prolonged night work was 29% more likely to have declined CFR, with an increasing trend between shift durations and CMD (P for trend 0.005). Instead of evident abnormalities in traditional cardiovascular risk factors, night shifts favored a microbial and metabolic profile of perturbed lipid metabolism. Specifically, reduced abundances of probiotic Lactobacillus species and an array of Bacteroides species were the main contributors to the dysbiosis of night-shift gut microbiota. Circulating metabolites of increased 2-Hydroxyhexadecanoylcarnitine and reduced LysoPC(16:0/0:0), a potential feature of impaired palmitic acid utilization, partially mediated the association between night shifts and CMD. Species- and metabolites-based classifiers of CMD status added predicting power combined with night shift durations.

Conclusions
The current study found working night shifts is associated with CMD independent of traditional cardiovascular risk factors. Identifying gut microbiota and circulating metabolites linked to CMD will help to recognize high-risk night shift workers. Probiotics or metabolite supplementation may be promising in conducting a healthier night shift.