The lack of fiber in our modern-day Western diets has been much bemoaned — it correlates with lower diversity of gut microbes, higher risk of heart disease, and heavier body weight.
But a study in mice adds a new layer: That lack of fiber may not just be hurting our health, but that of our children and great-grandchildren, by way of their gut bacteria.
Researchers found that mice's depleted bacterial diversity as a result of low-fiber diets gets passed on to offspring, and that loss of diversity cannot be easily reversed in future generations.
For the study, the researchers transplanted gut microbes from a person into mice bred without their own microbiomes. Some of the mice had diets full of fiber, while some were deprived of fiber.
The diversity of the gut bacteria in the second group of mice soon waned, with some strains seeming to disappear, the researchers found. But when those mice started eating a fiber-rich diet, most of the bacteria recovered, a finding that echoed what other scientists have found.
When those mice were bred while on the low-fiber diets, their offspring had even lower bacterial diversity than their parents. This got worse with every generation the scientists studied, producing what the authors called a "diet-induced ratcheting effect" in the paper published recently in Nature.
Erica Sonnenburg, the paper's lead author and a senior research scientist at Stanford, explained that if a mouse did not have a lot of one type of bacteria in its gut when it had pups, those bacteria had less of a chance of being passed on. That process happened anew with each generation.
And when the offspring mice were eventually given high-fiber diets, the diversity of their gut bacteria recovered less and less with each generation. By the fourth generation, some of the species of bacteria from the first generation of mice had gone extinct in this family line.
"Those microbes aren't there to rebound," Sonnenburg said.
What did help rejuvenate the bacteria in fourth generation: fecal transplants. The researchers transferred fecal matter from control mice into the study group, and found that that, combined with a high-fiber diet, restored full diversity in their gut bacteria.
Because the study was in mice, it means the same findings might not be occurring in people. But researchers said at the least the study provided yet another reason people should be incorporating fiber into their diets.
"Everybody accepts that we pass on our human genes to our children, but what we really need to think about and consider is our children inherit a microbial set of genes from us," Sonnenburg said.
Kelly Swanson, who has studied how fiber supports the gut microbiome, said the new paper dovetails with the recently updated nutritional guidelines that continue to recommend eating a variety of fruits and vegetables and whole grains — all fiber-rich foods.
"To me it adds another reason to eat your fiber," said Swanson, a professor of animal and nutritional sciences at the University of Illinois at Urbana-Champaign. He added that a fiber-rich diet was all the more important if it is indeed true that microbial diversity is so difficult to repopulate once gone.
In a commentary also published in Nature, University of Michigan microbiologist Eric Martens wrote that scientists are getting a clearer picture of how diet shapes gut bacteria, and the role those "microbial partners" play in digesting complex carbohydrates.
"It remains to be determined whether some of this functionality has already been lost in some people and, if so, to what extent," Martens wrote.
But, he added, "You just might consider choosing a salad at lunch today or an extra serving of beans at dinner. Future generations may thank you, too."
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