During prolonged fasting, the mucosal lining of the digestive tract can thin or become disrupted, and many gut microbiota species may die off.

“Well, my colleague from Yale and Dr. Sonnenburg both confirmed that during periods of fasting, especially prolonged periods of fasting, we actually start to digest away much of our digestive tract. Now, the whole thing doesn't start to disappear but there's thinning of the mucosal lining or the least disruption of the mucosal lining. A lot of the microbiota species can start to die off.”

“However, some forms of stress that can quote unquote negatively impact the microbiome include fasting, long periods of fasts, which makes sense because a lot of microbiota need food in order to thrive.”

“people have fasted for over a year there was a guy that fasted for I believe 370 days straight and he did not develop inflammatory bowel disease nor any clinical evidence that the mucous layer in his gut... degrades when you fast is crazy that does not happen clinically”