Metabolic-associated fatty liver disease (MAFLD) is a growing global health challenge and poses a substantial economic burden. A large-scale epidemiologic study in China (“Exposure to air pollution is associated with an increased risk of metabolic dysfunction-associated fatty liver disease”), published in the Journal of Hepatology, the official journal of the European Association for the Study of the Liver, has identified links between long-term exposure to ambient air pollution and MAFLD. These links are exacerbated by unhealthy lifestyles and the presence of central obesity.

The incidence of MAFLD has increased steadily since the 1980s, currently affecting a quarter of the global population and a majority of patients with adult-onset diabetes and poses a substantial global burden. In Asia, MAFLD increased to 40% between 2012 and 2017. Formerly known as nonalcoholic fatty liver disease (NAFLD), it may progress to end-stage liver diseases such as cirrhosis and liver cancer, liver transplantation and liver-related death.

Animal studies have shown that breathing air pollutants may increase the risk of MAFLD. For instance, fine particulate matter exposure may trigger a nonalcoholic steatohepatitis (NASH)-like phenotype, impair hepatic glucose metabolism, and promote hepatic fibrogenesis.

“Accumulating animal studies have demonstrated the harmful contribution of ambient air pollution (AP) to metabolic dysfunction-associated fatty liver disease (MAFLD), but corresponding epidemiological evidence is limited. We examined the associations between long-term AP exposure and MAFLD prevalence in a Chinese population,” write the investigators.

“We conducted a cross-sectional study of 90,086 participants recruited in China from 2018 to 2019. MAFLD was assessed based on radiologically diagnosed hepatic steatosis and the presence of overweight/obese status, diabetes mellitus, or metabolic dysregulation. Residence-specific levels of air pollutants, including particulate matter with aerodynamic diameters of ≤1 μm (PM1), ≤2.5 μm (PM2.5), and ≤10 μm (PM10), and nitrogen dioxide (NO2), were estimated by validated spatiotemporal models.

“We used logistic regression models to examine the AP–MAFLD associations and further evaluated potential effect modifications by demographics, lifestyle, central obesity, and diabetes status.

“Increased exposure levels to all 4 air pollutants were significantly associated with increased odds of MAFLD, with odds ratios (ORs) of 1.13 (95% CI 1.10–1.17), 1.29 (1.25–1.34), 1.11 (1.09–1.14), and 1.15 (1.12–1.17) for each 10 μg/m3 increase in PM1, PM2.5, PM10, and NO2, respectively. Further stratified analyses revealed that individuals who are male, alcohol drinkers, and current and previous smokers, those who consume a high-fat diet, and those with central obesity experience more significant adverse effects from AP exposure than other individuals.

A large-scale epidemiologic study in China has identified links between long-term exposure to ambient air pollution and metabolic-associated fatty liver disease. [Journal of Hepatology]
“This study provides evidence that long-term exposure to ambient PM1, PM2.5, PM10, and NO2 may increase the odds of MAFLD in the real world. These effects may be exacerbated by unhealthy lifestyle habits and central obesity.”

“The MAFLD epidemic corresponds to environmental and lifestyle changes that have occurred alongside rapid industrialization worldwide, especially in many Asian countries,” explained lead investigator Xing Zhao, PhD, West China School of Public Health and West China Fourth Hospital, Sichuan University, Chengdu, Sichuan, China. “A growing number of studies have suggested that ambient air pollution, which is the biggest environmental problem caused by industrialization, may increase the risk of metabolic disorders such as insulin resistance and dyslipidemia, and related diseases such as type 2 diabetes mellitus and metabolic syndrome.

“However, epidemiologic evidence for the association was limited, so we conducted this research to improve our understanding of the effects of air pollution on human health and also to help reduce the burden of MAFLD.”

Investigators conducted an epidemiologic study on the potential role of ambient air pollution in the risk of MAFLD in approximately 90,000 adults in China based on the baseline survey of the China Multi-Ethnic Cohort (CMEC), a prospective cohort that enrolled nearly 100,000 participants in southwest China from 2018 to 2019. The CMEC collected participant information including sociodemographics, lifestyle habits, and health-related history through verbal interviews performed by trained staff and subsequently assessed anthropometrics, biosamples (blood, urine, and saliva), and imaging data.

Researchers found that long-term exposure to ambient air pollution may increase the odds of MAFLD, especially in individuals who are male, smokers, and alcohol drinkers, and those who consume a high fat diet. Unhealthy lifestyle behaviors and an excess accumulation of fat in the abdominal area may exacerbate the harmful effects.

“Our findings add to the growing evidence of ambient pollution’s damaging effects on metabolic function and related organs,” commented Zhao and his co-investigators. “However, physical activity did not seem to modify the associations between air pollution and MAFLD. We suggest that future studies explore whether the timing, intensity, and form of physical activity can mitigate the harmful effects of air pollution.

The investigators propose that air pollution should be recognized as a modifiable risk factor for MAFLD. Populations at high risk should be aware of the air quality in the areas where they live and plan their activities to minimize their exposures to air pollution.