The relationship between food and a gastro-intestinal disease might sound simple. But new research is revealing that what we put into our bodies can cause a cascade of complex interactions among various systems—from metabolism to the immune system—that keep us well or make us sick.

And it appears that a popular component of the classic Western diet—saturated fats—has likely contributed to the increase in colitis cases.

Colitis, swelling of the large intestine that can cause pain and diarrhea, seems to run in families, but not everyone with the genetic risk gets it. So scientists have presumed that an environmental trigger initiates the disease. "Moving from elevated risk to the development of the disease seems to require a second event, which may be encountered because of our changing lifestyle," according to Eugene Chang, a professor of medicine at the University of Chicago and co-author of a new study published online June 13 in Nature (Scientific American is part of Nature Publishing Group).

Chang and his colleagues traced how saturated fats, particularly those from dairy, which are also present in many baked goods and processed foods, can change the composition of naturally harmless bacteria communities in the gut. As the balance of species shifts, it can trigger an immune response that results in inflammation and tissue damage.

"This is the first plausible mechanism showing step-by-step how Western-style diets contribute to the rapid and ongoing increase in the incidence of inflammatory bowel disease," Chang said.

The team first found that mice with a genetic predisposition to get a colitis-like condition were more likely to get the disease when they were fed on a diet that contained concentrated milk fats (in similar proportion to that found in a common Western diet: about 37 percent of calories). A diet high in these fats encourages the growth of the harmful microbe Bilophila wadsworthia, which has also often been found in high levels in patients who have an inflammatory bowel and other intestinal inflammations. But how is diet fostering the growth of these microbes?

Saturated fats are difficult to break down, so the intestinal tract gets help from sulfur-rich bile provided by the liver. Many bacteria find sulfur an unpleasant element. But the B. wadsworthia bacteria happen to love sulfur. As the gut gets more sulfurous, this species thrives. Indeed, the researchers found that mice that were fed low levels of fat or unsaturated fats had an infinitesimal amount of these bacteria in their guts (for healthy humans the proportion is somewhere around 0.01 percent). But for mice that were getting the saturated milk fat, B. wadsworthia made up about 6 percent of the total gut microbiome. This unwelcome prevalence can trigger the immune system to attack in those mice with a genetic propensity for the disease, initiating a state of inflammation and subsequent damage. The result for humans can be lasting bowel troubles.

To make matters worse, the bacteria's waste can weaken the intestines' walls, making it easier for the immune system to do more unintended damage to the tissue.

So far treatments for colitis have been hit and miss. Doctors often prescribe anti-inflammatory medication and sometimes an immune suppressant, although these can be dangerous, leaving a patient more vulnerable to infection. But the new findings suggest that addressing the balance of the microbiome might help quiet the condition. "The balance between host and microbes can be altered back to a healthy state to prevent or treat these diseases," Chang said. "In essence, the gut microbiome can be 'reshaped' in sustainable and predictable ways that restore a healthy relationship between host and microbes, without significantly affecting the lifestyles of individuals who are genetically prone to these diseases."

What can the average person do to achieve a better balance? "We are testing that right now," Chang said. There are of course "probiotic" products advertised to improve gut flora and more experimental treatments, such as parasitic worms and fecal transplants, but the research is still in its early phases. It seems like cutting back on saturated fat wouldn't hurt as a first step.