Researchers are continually discovering more about the role gut microbes play in human health. Now a new study suggests that modern diets which are low in fiber may be irreversibly depleting gut microbes with each generation.
The study comes from microbiologists Justin and Erica Sonnenburg at Stanford University and was published in the journal Nature January 13th. Because the gut microbiomes differ so much between humans who eat a Western-style diet and those have a more traditional diet, the researchers set out to determine whether the lack of fiber could be the culprit.
To test their theory, the scientists introduced fecal samples from an American man into two sets of mice, so that the microbes would establish themselves in the gastrointestinal tracts of the mice. This gave both sets of mice similar gut microbes at the start of the experiment, similar to the gut microbes of a human on a Western diet.
The researchers then went on to feed each mouse group a different diet. The control group ate a plant fiber-rich diet, but the other group ate a diet low in fiber, similar to a Western processed-food diet. The mice continued on these diets for seven weeks then the researchers re-tested the gut microbes.
After enjoying their fibrous diet, the one mouse group still lost about 11 percent of their introduced gut microbe species. However, in the group that ate a low-fiber diet, 60 percent of the gut microbe species were lost. This seemed to indicate that a lack of fiber could cause the mice to lose microbes, with unknown health consequences.
The researchers then tested whether improving the diets would help the gut microbes bounce back. When the low-fiber mice were fed a higher fiber diet, they regained much of their lost diversity. This seemed to indicate that the species had still been present, just not in high enough levels to detect. However, even when the diets were improved, 33 percent of the microbe species were still at low or undetectable levels.
For the next step of their experiment, the researchers wanted to know if the gut microbes can bounce back after generations of low-fiber diets. The second generation of mice in the experiment were fed either low or high fiber diets for ten weeks, but at the eleventh week, all were eating a high-fiber diet. After the fourth generation, the fiber-deprived mice seemed to lose most of their microbial diversity. Only a few recovered with an increase in fiber content in their diet.
All was not lost, however. The researchers found that they could use a fecal transplant to reintroduce the gut microbes to the mice that had lost them.
Although this study involved mice, there are implications for humans. After generations of eating processed foods low in fiber, people in the Western world could be prone to losing their microbiome diversity. More research would be needed to confirm if the same process is going on in humans as in the mice.
Microbes and their human hosts have co-evolved for thousands of years, but relatively recent changes in diet, lifestyle, and environment seem to be shifting the human microbiome just as researchers are beginning to understand the complex relationship between gut microbes and health. Gut microbes can affect anything from food digestion to the immune system. Researchers have suggested links between changes in the gut microbiome and health issues like obesity, allergies, cancer, and inflammatory bowel disease.
Fiber refers to carbohydrates from plants that humans are unable to digest naturally. Microbes do digest these carbohydrates, providing nutrients to the body and reducing inflammation. Because different microbes specialize in breaking down different carbohydrates, maintaining diversity is important for digesting a wide variety of fiber types.
Dietary guidelines recommend 25 and 38 grams of dietary fiber a day for women and men, respectively. However, the average American east only 15 grams of fiber per day.