I started 20:4 fasting last week and I have been having lower back pain during the time I fast, not everyday but few of the days. I have fasted 16:8 before and it never happened. Generally it goes away when I eat but today it didn’t. Does it happen generally or is it just me?
This could be many things, many of which have alredy been posted.
Another thing is, do you eat enough protein? If you generally dont, and then start IF, your body might try to compensate by reducing muscle mass for the Protein in the muscle. This happened to me after 4 month of Diet, so it might be unlikely that it is happening to you, but its still a thing to look out for.