There may be a way to delay the inevitable decline of cognitive function as we age: dietary fiber, according to a new study from the University of Illinois. 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, explaining why memory fades and other brain functions decline during old age. Although the study was conducted in mice, the researchers are comfortable extending the 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,” 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
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 Johnson.
Although positive outcomes of sodium butyrate - the drug form - were seen in previous studies, the mechanism wasn't clear. The new study reveals that butyrate inhibits production of damaging chemicals by inflamed microglia. One of those chemicals is interleukin-1β, 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. Since people are not likely to consume sodium butyrate directly, due to its noxious odor, a diet a diet high in soluble fiber may be practical way to get elevated butyrate. This 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.
“This is exciting research from the University of Illinois,” Anke Sentko, Vice President Regulatory Affairs & Nutrition Communication at BENEO, tells.
“The study’s findings demonstrate the importance of the prebiotic fermentation of inulin in the colon and its outreach to support other functions of the body, moving beyond the support of gut health and digestive wellbeing. Research related to the gut-brain axis explains the mechanism behind this, with the breakdown of products resulting from the prebiotic fermentation of inulin being absorbed and reaching other parts of the body,” she continues.
“Counteracting systemic inflammation processes by making dietary alterations such as including prebiotic chicory root fiber, is a very important and exciting research field with enormous global relevance.”
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 grant from the US National Institute on Aging, part of the National Institutes of Health.
The research findings come at a time when fibers are gaining increased interest within the food and beverage industry. In June, the US Food and Drug Administration (FDA) ended a two-year period of uncertainty for manufacturers with its long-awaited guidance on what can be classified as “dietary fibers.”
The official guidance includes eight specific fibers that can be specified as such on the upcoming Nutrition Facts Label, set to commence in the US on January 1, 2020. A range of manufacturers supply functional fiber – soluble and insoluble – that can be used across a variety of applications, in turn tackling low fiber levels in populations. The space of fibers for gut health is evolving along with its growing suite of markets including gluten-free, meat-free and weight management.