A study has found that young adults who smoke face an increased risk of developing fatty liver disease.
A joint research team led by Professor Shin Hyun-young of the Department of Family Medicine at Seoul St. Mary’s Hospital and Professor Ji Yong-ho of the Ewha Womans University Seoul Hospital Advanced Biomedical Research Institute announced the findings on July 13.
The researchers analyzed data from the National Health Insurance Service’s database, tracking the development of fatty liver disease through 2022 among 3.49 million Korean adults aged 20 to 39 who underwent health screenings between 2004 and 2007. The study population consisted of 62% men and 38% women.
Fatty liver refers to a condition in which excessive fat accumulates in liver cells. Major causes include alcohol consumption, obesity, diabetes and dyslipidemia. In some cases, fatty liver can progress to steatohepatitis, liver fibrosis and cirrhosis.
The fatty liver index is scored from 0 to 100. A score below 30 indicates a low likelihood of fatty liver, scores between 30 and 59 represent an intermediate range, and scores of 60 or higher indicate a high likelihood of fatty liver.
The study found that men who smoked 20 or more cigarettes per day had a 41% higher risk of reaching a fatty liver index score of 60 or above. Men who had smoked for 10 to 19 years also showed a 15% increased risk of fatty liver.
Among women, 94.4% were non-smokers, resulting in a significantly lower smoking rate compared with men. However, women who had smoked for 10 to 19 years showed a 55% higher risk of reaching a fatty liver index score of 60 or above, representing a larger increase than observed among men.
The association was more pronounced among individuals with a body mass index below 25 or those consuming less than 25 grams of alcohol per day. In contrast, the relationship was weaker among individuals with obesity or higher alcohol consumption.
The research team explained that smoking may promote fat accumulation in the liver by worsening insulin resistance and disrupting lipid metabolism. Systemic inflammation, oxidative stress and tissue hypoxia caused by smoking are also known to contribute to the development and progression of fatty liver disease.
The team also noted that smoking can affect visceral fat distribution, while nicotine-induced stimulation of the sympathetic nervous system may accelerate the development of fatty liver and liver fibrosis.
However, the researchers acknowledged limitations of the study, including reliance on self-reported smoking levels and measurements taken only at baseline, which may not fully reflect changes in smoking behavior or possible underreporting.
The findings are consistent with previous research linking smoking to liver disease. Smoking is considered a significant risk factor not only for metabolic liver disease but also for liver cancer. The International Agency for Research on Cancer classifies both alcohol consumption and smoking as Group 1 carcinogens for liver cancer.
According to the Korea Disease Control and Prevention Agency’s 2023 report on the harms of tobacco use, smokers have approximately 1.5 times higher risk of death from liver cancer compared with non-smokers. A meta-analysis combining 81 epidemiological studies also found that current smokers had a 55% higher risk of developing liver cancer, while heavy smokers had up to a 90% higher risk.
Smoking was found to have a synergistic effect with obesity and alcohol consumption, further increasing liver cancer risk. The study also found that liver cancer risk does not return to levels seen among non-smokers until more than 20 years after quitting.
Professor Shin said, “Smoking independently increases the risk of fatty liver disease among young adults regardless of obesity or alcohol consumption status. We expect these findings to serve as evidence for strengthening smoking cessation policies aimed at improving public health.”
Professor Ji said, “The significance of this study lies in its comprehensive analysis of national health screening data among healthy adults. Preventing smoking and implementing cessation strategies during young adulthood could 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 international journal Journal of Gastroenterology and Hepatology. Co-authors included Lee Sun-joo, associate research fellow at the National Health Insurance Service’s NHIS Artificial Intelligence Division, and Professor Chun Ho-soo of the Department of Gastroenterology at Ewha Womans University Seoul Hospital.
