After a few days on the low-fiber diet, mouse intestines developed chronic inflammation. After a few weeks, Dr. Gewirtz’s team observed that the mice began to change in other ways, putting on fat, for example, and developing higher blood sugar levels.
Dr. Backhed and his colleagues also fed another group of rodents the high-fat menu, along with a modest dose of a type of fiber called inulin. The mucus layer in their guts was healthier than in mice that didn’t get fiber, the scientists found, and intestinal bacteria were kept at a safe distance from their intestinal wall.
Dr. Gewirtz and his colleagues gave inulin to their mice as well, but at a much higher dose. The improvements were even more dramatic: Despite a high-fat diet, the mice had healthy populations of bacteria in their guts, their intestines were closer to normal, and they put on less weight.
One way that fiber benefits health is by giving us, indirectly, another source of food, Dr. Gewirtz said. Once bacteria are done harvesting the energy in dietary fiber, they cast off the fragments as waste. That waste – in the form of short-chain fatty acids – is absorbed by intestinal cells, which use it as fuel.
But the gut’s microbes do more than just make energy. They also send messages.
Intestinal cells rely on chemical signals from the bacteria to work properly, Dr. Geqirtz said. The cells respond to the signals by multiplying and making a healthy supply of mucus. They also release bacteria-killing molecules.
By generating these responses, gut bacteria help maintain a peaceful coexistence with the immune system.
While some species of gut bacteria feed directly on dietary fiber, they probably support other species that feed on their waste. A number of species in this ecosystem – all of it built on fiber – may be talking to our guts.
Going on a law-fiber diet disturbs this peaceful relationship, the new studies suggest. The species that depend on dietary fiber starve, as do the other species that depend on them. Some species may switch to feeding on the host’s own mucus.
With less fuel, intestinal cells grow more slowly. And without a steady stream of chemical signals from bacteria, the cells slow their production of mucus and bacteria-killing poisons.
As a result, bacteria edge closer to the intestinal wall, and the immune system kicks into high gear.
“The gut is always precariously balanced between trying to contain these organisms and not to overreact,” said Eric C. Martens, a microbiologist at the University of Michigan who was not involved in the new studies. “It could be a tipping point between health and disease.”
Inflammation can help fight infections, but if it becomes chronic, it can harm our bodies. Among other things, chronic inflammation may interfere with how the body uses the calories in food, storing more of it as fat rather than burning it for energy.
Hannah D. Holscher, a nutrition scientist at the University of Illinois who was not involved in the new studies, said that the results on mice need to be put to the test in humans. But it’s much harder to run such studies on people.
In her own lab, Dr. Holscher acts as a round-the-clock personal chef. She and her colleagues provide volunteers with all their meals for two weeks. She can then give some of her volunteers an extra source of fiber – such as walnuts – and look for changes in both their microbiome and their levels of inflammation.
Dr. Holscher and other researchers hope that they will learn enough about how fiber influences the microbiome to use it as a way to treat disorders. Lowering inflammation with fiber may also help in the treatment of immune disorders such as inflammatory bowel disease.
(Relevant NPR Program)
