I don’t think we can expect to retain 100% of muscle mass, and losing just 1/5th sounds like a good outcome.

I’ve understood that generalizing anything in today’s time is a losing game. I know many people with IBS/GI issues and I am also sure they have different underlying causes. Our gut biome and how digestion works in general needs to be researched much more.

I don’t know why progress has generally been so slowly on that front. For instance, GLP-1 was discovered in the 1970s. It took us another 40 years to commercialize it in the form of Semaglutide and another 10 years to get it ready for human consumption.