The human gut is literally teeming with billions of "friendly" bacteria that aid digestion and help ward off dangerous pathogens.
But a new study suggests the modern Western diet, which is high in saturated fats, could disrupt this delicately balanced microbial community and set the stage for certain diseases.
"Our work provides an explanation why many diseases that were essentially uncommon or rare 100 years ago are now being seen with increased frequency," said the senior researcher, Eugene Chang at the University of Chicago.
The team focused on just one aspect of the diet: concentrated milk fat, a powdered substance that remains when fat has been separated and dehydrated. Milk fats are found in abundance in processed foods and sugary treats.
Previous studies have suggested that high levels of dietary fat could change the mix of bacteria in the intestines. So the researchers set out to determine the effects of different types of dietary fats on gut microbes and whether these changes could affect disease risk.
For the experiment, they used genetically modified mice that are prone to developing inflammatory bowel disease, or IBD, which includes Crohn's disease and ulcerative colitis. When fed a low-fat diet or one high in polyunsaturated fat, about 20 per cent of the mice developed IBD. But when given a diet high in saturated milk fats, the disease rate shot up to 60 per cent.
Additional analysis revealed the milk fats led to a huge increase in a type of bacteria known as Bilophila wadsworthia.
Dr. Chang noted that milk fats are difficult to digest. The liver must secrete a form of bile that is rich in sulphur – and B. wadsworthia thrives in this environment.
"This bacteria doesn't normally cause problems. But when you give it the right stimulus, it can overgrow and stimulate an immune response," explained Suzanne Devkota, the first author of the study published in the journal Nature. In particular, the microbes break down the mucosal barrier that normally protects the gastrointestinal tract. The immune system then goes into high gear, leading to further tissue damage associated with colitis.
Although the study investigated only milk fat, Drs. Chang and Devkota think other factors well represented in Western diets can alter the bacterial composition of the gut and boost the risk of a wide range of immune disorders – such as rheumatoid arthritis, Type 1 diabetes and asthma – in genetically susceptible individuals.
And, in a similar fashion, it may be possible to lower disease risk with the right mix of bacteria. "I think there is a lot we can do to reconstitute, or reshape, the gut bacteria," said Dr. Chang. "That's the major focus of our research right now."
Other scientists are also investigating the role that microbes play in our overall health. This week, both Nature and three Public Library of Science journals published a collection of studies as part of the Human Microbiome Project which sequenced the genetic material of bacteria taken from almost 250 people.