Going along with vin's post, essentially the "detoxification" we see in fasting is related to autophagy and increased insulin sensitivity. Brad's book (especially his second book) has a lot of good research, but there isn't a lot of research yet about autophagy and human timetables
A good tell is changes in skin. Does your skin get softer? Do age spots and blemishes start to disappear? That's usually a sign of fasting promoting positive effects.
Directory of Tennis Warehouse Clubs (courtesy of Mountain Ghost)