Adding soy to your diet may be the key to keeping liver cancer at bay, says an international team of scientists who studied the effect of this soybean-derived substance in a group of Japanese people at risk for the disease.¹

The most common form of liver cancer is known as hepatocellular carcinoma, accounting for over 5% of all human cancers, and is ranked as the fifth most common cancer around the world. In the Western hemisphere, infection with hepatitis C is the leading cause of hepatocellular carcinoma, particularly in people with liver cirrhosis. Only about 30 to 40 percent of those with this type of cancer respond to radical treatment, say estimates, such as surgical removal, liver transplant, or percutaneous ablation—an approach in which a form of alcohol is injected directly into the liver tumor.²

Does What You Eat Affect Your Cancer Odds?
The researchers led by Gerald Sharp, DrPH, of the National Institute of Allergy and Infectious Diseases at the National Institutes of Health, collected data on more than 700 Japanese survivors of the atomic bomb attacks at the end of World War II to find out if consuming soy-based foods reduces the liver cancer risk.

This group is part of a larger cohort that has been studied extensively for more than 50 years; specifically, the rates of various cancers that have arisen during this time, Sharp and his colleagues in Washington, DC, Hiroshima, Japan, and Oxford, England reported.

For their study, Sharp's group used only information that had been collected on people at least two years before liver cancer diagnosis or death. They divided the patient data by those diagnosed with liver cancer between 1965 and 1988, and those without liver cancer—included as a comparison.  Data were then collected on the incidence of hepatitis B and C among the study participants, as well as their types of diets, based on questionnaires that they had previously completed. The researchers focused specifically on the amount of tofu and miso soup consumed by the study population. Miso is a type of paste made from fermented soybeans, and tofu is a type of food made from curdled soy milk. Information on alcohol and smoking habits was also gathered.

There were no differences between both groups in terms of age, sex, city of residence at the time of the bombing, or radiation exposure. However, there was about a 50% difference in the risk of hepatocellular carcinoma between those who had consumed higher levels of tofu and miso soup and those who ate lower levels. The investigators found that those who had not been diagnosed with liver cancer had consumed about at least five servings of miso soup and tofu per week, on average. This was the result after taking the risk of hepatocellular carcinoma from hepatitis B or C infection, and smoking and alcohol consumption into account.

The risk of liver cancer was higher in women than in men in this study, particularly for women diagnosed with hepatitis C. "Although HCV infection rates among controls [those without cancer] in our study were similar for both sexes, 78 percent of female cases were HCV-infected compared to 37 percent of male cases, thereby increasing [the risk] of hepatocellular carcinoma for HCV among females," wrote the researchers.

Why May Eating Soy Work?
In conclusion, soy-based foods reduced the risk of hepatocellular carcinoma in this study population, claim the researchers. While it's not exactly known why, "epidemiologic studies have found that increased exposure to estrogen is significantly associated with elevated hepatocellular carcinoma risk among women and that isoflavones may reduce estrogen levels," they maintained.  (Isoflavones are phytoestrogens—estrogen-like compounds—found in soybeans.)

But since previous studies suggest that other estrogen antagonists (i.e., the breast cancer drug, tamoxifen) have no therapeutic value against hepatocellular carcinoma,³ it may be possible that isoflavones have some other anti-tumor effect, the investigators speculated.

Isoflavones may also inhibit levels of testosterone in the body. Higher levels of this hormone have been linked to an increased risk of hepatocellular carcinoma, they explained.

1. Sharp GB, Largarde F, Mizuno T et al. Relationship of hepatocellular carcinoma to soya food consumption: a cohort-based, case-control study in Japan. Int J Cancer 2005 Jun 10;115(2):290-5.
2. Pons-Renedo F, Llovet JM. Hepatocellular carcinoma: a clinical update. MedGenMed 2003 Jul 22;5(3):11.
3. Barbare JC, Bouche O, Bonnetain F et al. Randomized controlled trial of tamoxifen in advanced hepatocellular carcinoma. J Clin Oncol 2005 Jul 1;23(19):4338-46.

John Martin is a long-time health journalist and an editor for Priority Healthcare. His credits include overseeing health news coverage for the website of Fox Television's The Health Network, and articles for the New York Post and other consumer and trade publications.