Dietary
fiber reduces brain inflammation during aging
University of Illinois
College of Agricultural, Consumer and Environmental Sciences
As mammals age, immune
cells in the brain known as microglia become chronically inflamed. In this
state, they produce chemicals known to impair cognitive and motor function.
That's one explanation for why memory fades and other brain functions decline
during old age. But, according to a new study from the University of Illinois, there may be a remedy to delay the inevitable: dietary fiber.
Dietary fiber promotes
the growth of good bacteria in the gut. When these bacteria digest fiber, they
produce short-chain-fatty-acids (SCFAs), including butyrate, as byproducts.
"Butyrate is of interest because it has been shown to have anti-inflammatory properties on microglia and improve memory in mice when administered pharmacologically," says Rodney Johnson, professor and head of the Department of Animal Sciences at U of I, and corresponding author on the Frontiers in Immunology study.
Although positive
outcomes of sodium butyrate -- the drug form -- were seen in previous studies,
the mechanism wasn't clear. The new study reveals, in old mice, that butyrate
inhibits production of damaging chemicals by inflamed microglia. One of those
chemicals is interleukin-1?, which has been associated with Alzheimer's disease
in humans.
Understanding how
sodium butyrate works is a step forward, but the researchers were more
interested in knowing whether the same effects could be obtained simply by
feeding the mice more fiber.
"People are not
likely to consume sodium butyrate directly, due to its noxious odor,"
Johnson says. "A practical way to get elevated butyrate is to consume a
diet high in soluble fiber."
The concept takes
advantage of the fact that gut bacteria convert fiber into butyrate naturally.
"We know that
diet has a major influence on the composition and function of microbes in the
gut and that diets high in fiber benefit good microbes, while diets high in fat
and protein can have a negative influence on microbial composition and
function. Diet, through altering gut microbes, is one way in which it affects
disease," says Jeff Woods, professor in the Department of Kinesiology and
Community Health at U of I, and co-author on the study.
Butyrate derived from
dietary fiber should have the same benefits in the brain as the drug form, but
no one had tested it before. The researchers fed low- and high-fiber diets to
groups of young and old mice, then measured the levels of butyrate and other
SCFAs in the blood, as well as inflammatory chemicals in the intestine.
"The high-fiber
diet elevated butyrate and other SCFAs in the blood both for young and old
mice. But only the old mice showed intestinal inflammation on the low-fiber
diet," Johnson says. "It's interesting that young adults didn't have
that inflammatory response on the same diet. It clearly highlights the
vulnerability of being old."
On the other hand,
when old mice consumed the high-fiber diet, their intestinal inflammation was
reduced dramatically, showing no difference between the age groups. Johnson
concludes, "Dietary fiber can really manipulate the inflammatory
environment in the gut."
The next step was
looking at signs of inflammation in the brain. The researchers examined about
50 unique genes in microglia and found the high-fiber diet reduced the
inflammatory profile in aged animals.
The researchers did
not examine the effects of the diets on cognition and behavior or the precise
mechanisms in the gut-brain axis, but they plan to tackle that work in the
future as part of a new, almost-$2 million grant from the National Institute on
Aging, part of the National Institutes of Health.
Although the study was conducted in mice, Johnson is comfortable extending his findings to humans, if only in a general sense. "What you eat matters. We know that older adults consume 40 percent less dietary fiber than is recommended. Not getting enough fiber could have negative consequences for things you don't even think about, such as connections to brain health and inflammation in general."