This is something that comes up repeatedly in a variety of studies and anecdotal reports from fellow Colitis patients (many of which are featured on this blog):
“Fiber-rich foods act as fuel for the “good” bacteria that live in the gut and produce short-chain fatty acids, which are critical for an efficient immune response.
Eating high-sugar diets and decreasing intake of fiber feeds “bad” microbes, such as E. coli, that are associated with inflammation and a defective immune response.
Madsen’s study showed that the mice on the high-sugar diet had greater intestinal tissue damage and a defective immune response. These problems were alleviated when their diet was supplemented with short-chain fatty acids normally produced by good bacteria.”
Source: https://www.sciencedaily.com/releases/2019/11/191114115949.htm
The Colitist 