I wonder how much of the fast increasing prevalence of gut-related diseases like IBS can be explained by the changes in gut microbes, and how much is just about

I wonder how much of the fast increasing prevalence of gut-related diseases like IBS can be explained by the changes in gut microbes, and how much is just about lack of exercise and changes in diet. Whenever I go work at relative's farm my stomach seems to work way better, even though I do plenty of exercise in my home city as well. Perhaps the type of manual labor is better for stomach than gym/running, but I wouldn't rule out the impact of microbes either.