While I prefer doing Core, a couple of times this summer I've chosen to do Flex for a week. Each of those times I gained weight (less than 1/2 lb. usually, but a gain is a gain!), even though I was following Flex to the letter, using the same amount of WPA and exercising. When I'd switch back to Core, then I'd lose what I'd gain and more than I usually lose in a week.
I'm curious if anyone else has experienced this? If so, why do you think it happens? Thanks!