Winter 2016

Obesity and Colorectal Cancer

Certain gut bacteria could affect the chances of developing the disease

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Mice that developed colorectal tumors had low levels of the bacteria P. distasonis, which is thought to have anti-inflammatory effects. Photo: Dr. Gary Gaugler/Science Source

People who are extremely overweight have a higher risk for developing colorectal cancer, though why that is so remains unknown. There is growing evidence that people who have adenomas, the precursors to colorectal cancer, have different gut bacteria—or microbiota—than those who do not. Several studies have shown that mice are more likely to get colorectal cancer when given gut microbiota from humans or other mice with colorectal tumors, suggesting that microbiota are responsible, in part, for the cancer.

Researchers in the cancer cluster at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts sought to understand whether, when it comes to cancer risk and gut bacteria, it matters if the obesity is caused by genetics or by poor diet. Their study was done in two strains of mice genetically prone to developing colorectal cancer: mice bred to be of normal weight and a group with genes that cause extreme obesity.

Scientists fed half of the normal-weight mice a low-fat, low-calorie diet and half an obesity-inducing high-fat, high-calorie diet. All the genetically obese mice were fed a low-fat diet so researchers could assess the effects of obesity per se. After the experiment, they compared the gut microbiota of the groups of mice.

The findings, published in PLOS ONE, found that obesity caused by genetics and obesity resulting from greater calories both increased cancer risk, meaning that obesity itself is a risk factor for cancer, at least in mice. Additionally, mice that developed colorectal tumors had low levels of the bacteria P. distasonis, which is thought to have anti-inflammatory effects. Much more research is needed to see whether P. distasonis reduces inflammation in the colon, and thereby the risk for colon cancer. –Katherine Pett

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