New research published in Nature is among the first to identify why obesity is a risk factor for cancer. Specifically, researchers found that individuals who are obese are more likely to develop liver cancer than their normal weight counterparts.
The culprit: microbes that live in our guts.
Obesity has long been associated with a higher risk of many cancers, including breast, colorectal, pancreatic, prostate and esophageal; however, the link between obesity and cancer has been unclear.
To identify the mechanism that leads to cancer, researchers at the Japanese Foundation for Cancer Research injected laboratory mice with a carcinogen known to spur various types of tumors. Half the mice were fed a normal diet while the other half consumed a high-fat diet.
All the obese mice developed liver cancer after 30 weeks while none of the lean mice did. Of the lean mice, 5 percent developed lung cancer. For the obese mice, why liver cancer?
The researchers looked at blood serum from the mice in each group and found that the obese mice had significantly higher amounts of the bile acid deoxycholic acid (DCA). Certain microbes in the gut turn bile acids to DCA, which is a more harmful acid.
In general, microbes are considered beneficial, aiding in digestion. But research in the past few years has linked microbes to conditions such as asthma and autism. In individuals who are obese, these microbes activate bacteria to secrete chemicals that damage DNA and lead to tumor growth.
In their study, the researchers found that DCA damaged the DNA of obese mice and made their cells senescent, which means the cells shut down and were no longer capable of dividing. Senescent cells emit chemical signals that can cause inflammation and lead to tumor growth.
Antibiotics were given to some of the obese mice to clear intestinal bacteria. Those mice had fewer liver tumors than the obese mice that did not receive antibiotics.
Writing in Nature, the research team concluded, "It is clear that the increased levels of DCA produced by gut bacteria play key roles in the promotion of obesity-related" liver cancer.