A large-scale epidemiologic study in China has identified links between long-term exposure to ambient air pollution and metabolic-associated fatty liver disease (MAFLD).
The link to air pollution is exacerbated by unhealthy lifestyles and obesity, according to a study published recently in the Journal of Hepatology.
In an accompanying editorial, Massimo Colombo, M.D., with San Raffaele Hospital in Milan, Italy, and Robert Barouki, M.D., Ph.D., with the University of Paris noted that the assessment of the major determinants of mortality worldwide by WHO showed that global pollution topped the list, ranking higher than smoking, alcohol consumption and major infectious diseases.
The incidence of MAFLD — which was previously known as nonalcoholic fatty liver disease — has increased steadily since the 1980s and now affects about a quarter of the world's population. A majority of patients with adult-onset diabetesh have MAFLD, which can worsen and develop into cirrhosis and liver cancer.
A number of 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.
“The MAFLD epidemic corresponds to environmental and lifestyle changes that have occurred alongside rapid industrialization worldwide, especially in many Asian countries,” lead investigator Xing Zhao, Ph.D., with West China School of Public Health and West China Fourth Hospital, Sichuan University in Chengdu, Sichuan, China, said in a press release.
“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,” Zhao said. "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.”
The research involved an epidemiologic study encompassing around 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.
Zhao and colleagues found that long-term exposure to ambient air pollution increased the odds of MAFLD — especially in individuals who are male, smokers and alcohol drinkers, and those who consume a high fat diet. In addition, 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,” Zhao said. 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, Zhao and colleagues wrote.
The findings that ambient pollution could exacerbate MAFLD risk might offer clues for how to lower that risk, Colombo and Barouki wrote in their editorial. The findings, they wrote, might also provide “an additional incentive for decision makers to speed up the efforts to conform with the WHO guidelines and limits on air pollution, as many cities in Europe and worldwide are still well above those limits."
Physical activity did not seem to modify the associations between air pollution and MAFLD, according to Zhao and the study's co-authors: “We suggest that future studies explore whether the timing, intensity, and form of physical activity can mitigate the harmful effects of air pollution."