Fiber has a reputation for being good for your gut, partly because it helps nourish the bacteria living there. So you'd expect people who eat plenty of fiber to have a greater variety of gut bacteria, right?
Not necessarily.
A new study from the University of Copenhagen found that vegans, who consumed considerably more fiber than people who ate both plant and animal foods, actually had fewer types of gut bacteria on several measures. But there was an interesting twist: The vegans also had healthier blood sugar and cholesterol levels and lower levels of certain inflammation markers. Published in the journal Microbiome, the observational study suggests that when it comes to gut health, more bacterial variety isn't always better.
Researchers examined 150 healthy Danish adults, including 75 long-term vegans and 75 people who ate both plant and animal foods. They analyzed stool, blood and urine samples and collected detailed records of what participants ate.
The differences in their diets were substantial. Vegans ate an average of 49 grams of fiber daily, compared with 29 grams among those who ate animal products. But the vegans also consumed less protein, getting about 12% of their daily calories from it compared with 17% among the other participants.
Despite their higher fiber intake, the vegans had less variety in their gut bacteria. However, they also had more of certain bacteria associated with better metabolic health, including how the body manages blood sugar and cholesterol.
That distinction matters because gut bacterial diversity is often treated as a measure of gut health. While having a wide variety of bacteria can be beneficial, the kinds of bacteria present may be just as important as how many different types there are.
The researchers also noticed something interesting about protein. Among the vegans, those who ate more protein tended to have a greater variety of gut bacteria. That raises the possibility that protein, along with fiber, may help shape the mix of bacteria living in the gut.
But the findings don't mean vegans need to eat more protein or that eating more protein would necessarily improve anyone's gut health. The study wasn't designed to answer those questions.
It also has limitations. With just 150 participants, all healthy Danish adults, the findings may not apply to everyone. And because researchers compared people's existing eating habits rather than assigning them different diets, they couldn't determine whether diet directly caused the differences they observed.
Still, the results offer an interesting reminder that gut health isn't simply a numbers game. A greater variety of bacteria doesn't automatically mean a healthier gut, and fiber isn't the only part of our diet that may influence the bacteria living there.
The study received support from the Danish Council for Independent Research and the Metagenopolis program. The participating Novo Nordisk Foundation Center for Basic Metabolic Research is partially funded by the Novo Nordisk Foundation.