(Photo: Wikimedia Commons)

Colorectal cancer is a scourge of modern times, killing 50,000 Americans every year. It's responsible for a heavier death toll than any other cancer besides lung cancer and, when it comes to women, breast cancer.

And new research, which was published last week in Cell, has provided insights into the dangerous link between colorectal cancers and modern diets heavy in wheat, rice, and other complex carbohydrates—diets that became possible with the advent of agriculture.

University of Toronto scientists led research that suggests a common type of gut-dwelling bacteria breaks down carbs into certain metabolites that can lead to cancer. These metabolites appear to cause cells that line the colon to divide and proliferate rapidly, forming polyps. These polyps, which can grow into a cancer, are the abnormal growths that your doctor is probing for when they subject you to a colonoscopy.

"The microbiota in our gut play an important role in our physiology, so wiping out a specific phylum of bacteria is probably not a healthy thing to do."

The scientists found that they could protect specially bred mice from the cancers in two ways. In some mice, they used targeted antibiotics to kill off the clostridia bacteria that convert carbs into the metabolite butyrate. In other mice, they reduced the amount of carbs in their diets.

"We know it depends on bacteria, and we know it depends on carbs," says Alberto Martin, an associate immunology professor at the University of Toronto and one of the authors of the study. "This is the part of the study that’s still not solid, but we think that butyrate is somehow fueling the hyperproliferation of colon epithelial cells." Other metabolites of carbohydrates might also be involved, he says. "It would be naïve to think it's only butyrate."

The phyla of bacteria inhabiting mouse guts resembles that in human intestines, which suggests the findings could be relevant for human health. And that suggests that you could protect yourself from colon cancer by taking the same potent cocktail of targeted antibiotics, including ampicillin, metronidazole, neomycin, and vancomycin, that were fed to the mice.

But Martin definitely doesn't think you should do that. Colorectal cancers tend to crop up in older people, meaning 30 or 40 years' worth of antibiotics might need to be taken before they would do any good. Meanwhile, the clostridia would likely become resistant to the drugs, and the antibiotics could do more harm during those years than good.

"The microbiota in our gut play an important role in our physiology, so wiping out a specific phylum of bacteria is probably not a healthy thing to do," Martin says. "We’re investigating whether probiotics can modulate colon cancer, but we don’t have any answers on that yet."

A change in diet would be a better approach than taking prophylactic antibiotics.

"Reducing carb intake might be an approach to reducing the incidence of this type of colon cancer," Martin says. But "before we declare war on carbs," he warns that it would be important to undertake similar studies in humans—not just mice. "We're really only beginning to learn about these things."