Most people who are interested in the health of their microbiome have heard about resistant starch. This article is a pretty good primer on the subject:
For those who are new to this subject, resistant starch does not contribute to the true carb count of the food it comes from. It behaves similar to fiber in that it is only digestible by microbiota.￼ The really cool thing is that resistant starches are available from both pasta and white potatoes. If you cook them and then cool them in the refrigerator, Part of the carbohydrate content of the foods turns into resistant starch.
Don’t like to eat your pasta cold? Don’t worry. Apparently, reheating resistant starches actually creates even more resistant starch in the food.￼ This does not mean that you can stop eating high fiber foods and switch back to refined carbohydrates, though. Each type of fiber or resistant starch makes a unique contribution to the microbiome. See my earlier article on the desirability of eating 30 more or different plant types each week. So, you are going to want eat some fiber and some resistant starch.
It does mean that you can continue to enjoy some white potatoes and pasta( if you don’t have either a nightshade or a gluten sensitivity, that is). As an aside, unpeeled potatoes are a very high fiber food, and most semolina pasta has as high a fiber content as brown rice per 100g.