Objective: With increased interest in non-alcoholic steatohepatitis, its common co-morbid condition, obesity, has recently attracted much attention as a risk factor for liver cancer. Recent studies also suggest that obesity may play a role in the development of liver cancer in alcoholic cirrhosis or viral hepatitis and in the general population.
Methods: We systematically reviewed epidemiologic studies on overweight/obesity and liver cancer among Japanese populations. Original data were obtained by searching the MEDLINE (PubMed) and Ichushi databases, complemented by manual searches. The evaluation was performed in terms of the magnitude of association in each study and the strength of evidence ('convincing', 'probable', 'possible' or 'insufficient'), together with biologic plausibility.
Results: Among nine cohort studies identified, five (four on patients with chronic liver disease and one on local residents) reported a weak to strong positive association, while four (one on patients with hepatitis B and three on local residents) found no association [summary relative risk for one unit increase in body mass index (kg/m(2)) 1.07, 95% confidence interval 1.03-1.10]. All three case-control studies identified (two on cirrhotic patients and one on atomic bomb survivors) reported a strong positive association (summary relative risk 1.31, 95% confidence interval 1.12-1.53). Overall, the summary relative risk was estimated at 1.13 (95% confidence interval 1.07-1.20), and overweight/obese individuals had a relative risk of 1.74 (95% confidence interval 1.33-2.28) compared with those who had normal/low weight.
Conclusions: We conclude that overweight or obesity 'probably' increases the risk of primary liver cancer, to a moderate degree, among the Japanese population.