Can more fiber restore microbiome diversity?

Insufficient nutrients for our gut microbes have been linked to a loss of certain beneficial bacterial species in industrialized societies and are likely impacting our immunological and metabolic health, although more data is needed. For example, most Westerners consume half of the amount of dietary fiber recommended by dietary guidelines, which nutritionists refer to as the "fiber gap," which is a problem because dietary fiber is the primary source of nutrition (e.g., carbohydrates) accessible to gut bacteria in humans.

"The idea to boost fiber levels is not new," says Jens Walter of the University of Alberta, Canada. "However, depletion of the microbiome adds a new perspective to this low-fiber Western diet that we are currently eating."

Earlier this year, Stanford University's Justin Sonnenburg found that mice fed a typical Western diet (high in fat and carbohydrates and low in fiber) transferred a lower diversity of beneficial microbial species to future generations. The re-introduction of the microbes' preferred fiber at that stage did not result in a return of some (good) species, indicating that extinctions had occurred in only a few generations.

Walter and co-author Edward Deehan, his PhD student, are concerned that a dramatic shift away from a diet similar to the one under which the human-microbiome symbiosis evolved is a key factor in the rise of non-communicable disorders like obesity.

"There is a lot of epidemiological evidence that fiber is beneficial, and food products containing dietary fiber have FDA-approved health claims for both colon cancer and coronary heart disease. There is also quite a bit of clinical evidence (although it is less consistent)," Walter says. "The most pressing issue at the moment that neither consumption of fiber in society nor the doses used in clinical research are high enough."

Walter has noticed that often researchers evaluating fiber doses in diets and health outcomes do so with "doses of fiber that [he] would consider physiologically irrelevant. Most of these studies use 5-15 grams of fiber; I would not think that these amounts would be actually beneficial," he says.

People living in non-industrialized societies have an average intake of fiber that is much higher than the low norms of Western societies. The authors note the recent work from the Stephen J.D. O'Keefe lab in Nature Communications (doi:10.1038/ncomms7342) in which modern African-Americans were given a traditional South-African diet that contained 55 grams of daily dietary fiber and had improved markers for colon cancer within two weeks.

In their Commentary, the authors propose a concerted effort by scientists, food producers, policy makers, and regulatory groups to address the fiber gap. They emphasize that clinical assessments of different fiber types and fiber-enriched foods on microbiome outcomes are needed.

Jens Walter also asserts that the challenge of restoring diverse gut inhabitants will be best met with regulatory policies that are specific to food, and not just the same as those for drugs. "To have a regulatory environment that makes it extremely hard to obtain health claims for food substances is very detrimental," says Walter. He is hopeful that regulatory policies will change to encourage innovative research on disease prevention by modulating the diverse microbial communities humans have evolved with and the ways our diet shapes them and by extension, all of us.