From the article:
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,” […] The new study reveals, in old mice, that butyrate inhibits production of damaging chemicals by inflamed microglia.