Dr. George Chapman

A new study shows that what we eat can affect our gut, both inside and out. The community of bacteria that colonize our intestines may shift depending on the makeup of our overall diet.

Why is that important? Increasingly, research in the field of gut flora shows that these bugs have a big impact on some crucial bodily functions: they aid digestion and metabolism, affect immunity and determine how many calories we extract from food, possibly contributing to obesity and diabetes, among other health effects.

Now researchers in the U.S. and Brazil report online in the journal Science that the dominant species of bacteria in the gut microbiome, as it’s known, may change in accordance with your diet — at least if you stick with a particular diet for a long enough period of time.

Gary Wu, a professor medicine in the division of gastroenterology at the University of Pennsylvania, and his colleagues studied the eating habits and gut flora (the latter is most easily examined by looking at stool samples) of 98 participants over two time periods. In the long-term comparison, the researchers looked at how frequently people ate certain kinds of foods over the course of a year; in the short-term analysis, they looked at what people ate over three consecutive 24-hour periods.

Analyzing the longer term data, the researchers found that two types of diets were linked with certain dominant species of guts bugs. The gut microbiomes of people who ate higher-fat and low-fiber diets rich in animal proteins predominately contained bugs of the Bacteroides genus, while those who ate more fiber and less animal fat and protein showed higher concentrations of Prevotella microbes.

In the shorter-term analyses, the researchers did not find the same patterns emerging between diet and gut flora, suggesting that gut bugs are relatively consistent in terms of their composition over the long term, says Wu.

The research team also looked at whether they could alter the makeup of the microbiome by changing people’s diets — whether, for example, a low-fat diet could lead to a drop in Bacteroides microbes and boost the population of Prevotella. The scientists sequestered 10 people in a hospital setting and controlled every bit of food they ate for 10 days to see what would happen.

“Almost all of the participants started out in the Bacteroides group — high animal protein, high fat — which is not surprising since the study took place in the U.S. where the western diet is more prevalent,” says Wu. “None of them switched permanently into the other when they changed their diet.”

Wu notes that the participants were sequestered for only 10 days, which is a relatively short time period of time. And while there wasn’t a switch from one dominant microbe to the other, the scientists did see an overall change in the composition of the gut flora within 24 hours of the participants’ change in diet. It wasn’t enough to boost populations of the fiber-loving Prevotella, but the shift does hint that diet may, over the long term, help change the gut environment to one that may be linked to healthier eating.

It’s not clear yet whether the relationship also works the other way around — whether different types of gut flora influence your eating habits. For example, could having more Prevotella make it easier or even more appealing to eat high-fiber, low-fat foods? It’s also not clear whether one gut bug profile is better for your health than another — that’s the subject of ongoing research. But the current study certainly gives new meaning to the idea that you are what you eat.