A high fiber diet is good for your body in more ways than one, but it could spell trouble if you eat food that’s contaminated with pathogenic Escherichia coli bacteria. A new PNAS Early Edition paper concluded that mice on high fiber diets were more likely to die from an E. coli infection than mice on low fiber diets. Fiber, the researchers found, shifts the balance and number of bacteria in the gut, making the environment more conducive to the pathogen. The finding shouldn’t discourage people from eating lots of fiber in their diets, the authors say, but helps drive forward research on why different people have different risks for developing complications from foodborne illnesses.
Microbiologist Alison O’Brien of the Uniformed Services University of the Health Sciences started wondering about the link between diet and foodborne illness when she read about an E. coli outbreak in 2006.
“I noticed that in the spinach outbreak in 2006, there were a preponderance of adult women who got sick,” she says. “I wondered whether there was something about eating that kind of roughage that would make you more likely to get sick.”
Women are more likely to eat high fiber diets than men, which could lead to differences in their guts, she thought. But the numbers also could have been explained by women simply being more likely to eat spinach. Either way, most food-borne pathogens cause severe illness almost exclusively in elderly, very young, or immunocompromised patients, so O’Brien was curious why average adult women were getting so sick.
The E. coli strain associated with the spinach outbreak is called E. coli O157:H7, and it causes disease by producing a compound called Shiga toxin. Inside the gut, the Shiga toxin binds to receptors that allow it to enter cells, move into the bloodstream, and cause damage in not only the gut but the kidneys as well.
To test whether amounts of fiber in the diet influenced either the ability of E. coli O157:H7 to thrive in the body, or the way the body interacted with the Shiga toxin, O’Brien and her colleagues turned to mice. They infected mice fed high or low fiber diets with the E. coli strain, followed their reactions, and took samples of tissue from their intestines.
“It was pretty clear to us in this study that high fiber diets led to increased morbidity and mortality in animals infected with this pathogen,” says O’Brien. The mice ingesting more fiber lost more weight and were more likely to die from the E. coli. Moreover, when the team analyzed the intestinal tissue from the mice, they discovered higher levels of the Shiga toxin receptors in the mice on high-fiber diets. More receptors mean that Shiga toxin can more effectively and quickly enter the bloodstream, O’Brien says.
The link between fiber intake and the toxin receptor came when O’Brien looked at another chemical called butyrate, which is produced by a plethora of healthy gut microbes. Fibrous diets, lead the gut bacteria make more butyrate. And butyrate, she discovered, stimulates the production of the Shiga toxin receptors.
A 2010 PNAS paper comparing the gut microbiota of children from Europe and rural Africa found that those ingesting more fiber had higher levels of butyrate, O’Brien points out, providing evidence that this link likely holds true in humans as well as mice.
In addition to the differences in butyrate and receptors, the researchers also found that the mice on high fiber diets had fewer commensal (non-pathogenic) E. coli bacteria in their guts. This could open the door for the pathogenic E. coli to more effectively colonize the gut.
“In no way am I saying that people shouldn’t eat high fiber diets,” says O’Brien. “I think it means we have to be extra vigilant in keeping our produce free of contaminants.”
Contaminated vegetables could be even more risky, in terms of causing disease, than contaminated meat, she says, since they have higher amounts of fiber. So she hopes the research helps emphasize the need for better produce screening methods.
The researchers plan to continue exploring the interaction between the gut microbiome, pathogen infections, and complications of those infections, as well as study how the mice results apply to human populations.