Dietary Fiber in the Context of Gut Bacteria, Inflammation, and Aging

Gut microbes have some level of influence over the pace of natural aging. It isn't yet clear as to how large this influence might be, but it may well turn out to be of a similar magnitude to that of exercise. Identifying the most important mechanisms by which the microbiota of the gut affect aging is an ongoing process, still in its comparatively early stages. Many researchers are, quite reasonably, focused on inflammation as a primary concern. Inflammation rises with age, and accelerates the development of all of the common age-related conditions. Scientists are thus attempting to trace back the ways in which different bacterial populations and byproducts can spur the immune system into inappropriate chronic inflammation, and link those mechanisms with known dietary changes and bacterial population changes that take place in later life.

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. 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."

Although positive outcomes of sodium butyrate - the drug form - were seen in previous studies, the mechanism wasn't clear. A new study reveals, in old mice, 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.

The concept takes advantage of the fact that gut bacteria convert fiber into butyrate naturally. 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. 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. The researchers examined about 50 unique genes in microglia and found the high-fiber diet reduced the inflammatory profile in aged animals.