Smoking raises fatty liver risk by up to 55% in young adults, Seoul St. Mary's study finds

A joint research team from Seoul St. Mary's Hospital of the Catholic University of Korea and Ewha Seoul Hospital has found that smoking significantly raises the risk of fatty liver disease in young adults, the hospital announced Monday.

The team, led by Shin Hyun-young, a professor in the Department of Family Medicine at Seoul St. Mary's Hospital, and Ji Yong-ho, a professor at the Advanced Biomedical Research Institute of Ewha Seoul Hospital, analyzed big data from the National Health Insurance Service. They tracked fatty liver development through 2022 among 3,496,144 Korean adults aged 20 to 39 — 61.97 percent men and 38.03 percent women — who had undergone health checkups between 2004 and 2007.

The researchers assessed fatty liver status using the fatty liver index, a composite score derived from body mass index, waist circumference, triglyceride levels and gamma-glutamyl transferase values. The index runs from 0 to 100: scores below 30 indicate a low likelihood of fatty liver, scores between 30 and 59 fall in an indeterminate range, and scores of 60 or above suggest a high probability of fatty liver disease.

Among men, smoking 20 or more cigarettes a day was associated with a 41 percent higher risk of scoring 60 or above on the fatty liver index. Smoking for 10 to 19 years was linked to a 15 percent increase in fatty liver risk. Among women, 94.4 percent were non-smokers, making the overall smoking rate far lower than among men — but women who had smoked for 10 to 19 years showed a 55 percent higher risk of scoring 60 or above, a steeper increase than seen in men.

The association was more pronounced among people with a BMI below 25 or daily alcohol consumption below 25 grams. Conversely, the link weakened among those who were obese or drank heavily.

The research team said smoking can worsen insulin resistance and disrupt lipid metabolism, promoting fat accumulation in the liver. Systemic inflammation, oxidative stress and tissue hypoxia caused by smoking are known to contribute in combination to the onset and progression of fatty liver disease. The team also said smoking affects visceral fat distribution, and that nicotine's stimulation of the sympathetic nervous system may accelerate the development of fatty liver and liver fibrosis.

The findings show a clear link between smoking and fatty liver in people who are neither obese nor heavy drinkers, underscoring the need for smoking cessation efforts from an early age regardless of weight or alcohol consumption. However, the team acknowledged that the study relied on self-reported smoking data and a single baseline measurement, leaving open the possibility of behavioral changes over time and underreporting.

The results are consistent with the established connection between smoking and liver disease. Smoking is recognized as a significant risk factor not only for metabolic liver disease but also for liver cancer. The International Agency for Research on Cancer classifies smoking, along with alcohol, as a Group 1 carcinogen for liver cancer.

"Smoking has been confirmed to raise the risk of fatty liver disease in young adults independently of obesity or alcohol consumption," Shin said. "We hope these findings will serve as evidence to support stronger smoking cessation policies aimed at improving public health."

"The significance of this study lies in its comprehensive analysis of national health screening data covering healthy adults," Ji said. "We expect that practicing smoking prevention and cessation strategies from a young age will help prevent metabolic liver disease."

The study was supported by the National Research Foundation of Korea under the Ministry of Science and ICT and was recently published in the Journal of Gastroenterology and Hepatology. Co-authors include Lee Seon-ju, an associate research fellow at the NHIS AI Division of the National Health Insurance Service, and Cheon Ho-su, a professor in the Department of Gastroenterology at Ewha Seoul Hospital.