Researchers at the University of Chicago found that high-fat diet changes the way food was digested and causes a "boom in bad bacteria" responsible for inflammatory bowel diseases (IBDs) such as Crohn's and ulcerative colitis.
Incidences of IBDs, which are often marked by chronic abdominal pain and diarrhoea, are increasing rapidly worldwide.
In the study, published in the journal Nature, the team used genetically modified mice which were more likely to develop IBDs. One in three developed colitis when fed either low-fat diets or meals high in polyunsaturated fats. This jumped to nearly two in three in those fed a diet high in saturated milk fats, which are in many processed foods.
These saturated fats are hard for the body to digest and it responds by pumping more bile into the gut. This, the researchers said, changes the gut environment and leads to a change in the bacteria growing there, the BBC reported.
One bacterium in particular, Bilophila wadsworthia, was identified. It thrives in the extra bile produced to break down the fats. It went from being incredibly rare to nearly six per cent of all bacteria in the gut in the high-fat diet.
"Unfortunately, these can be harmful bacteria. Presented with a rich source of sulphur, they bloom, and when they do, they are capable of activating the immune system of genetically prone individuals," Prof Eugene Chang, who led the study, said.
However, he said this could lead to possible treatments as the gut bacteria could be "reshaped" without "significantly affecting the lifestyles of individuals who are genetically prone to these diseases".
Commenting on the research, Dr Roy Sleator, from the Cork Institute of Technology, said: "The authors provide the first credible explanation as to how Western diet contributes to the unusually high incidence in inflammatory bowel disease; they also suggest an effective means of dealing with such diseases, by simply reshaping the microbial balance of the gut."