That was the question asked to Robb Wolf in the Paleo Solution Podcast stuck to my brain when I heard it on episode 158 and since then I’ve learned a few things about biochemistry and metabolism. For example, that gaining a bit of fat increases the size of existing adipocytes (fat cells) but gaining a lot of fat increases both their size and number. Losing fat makes them shrink but not necessarily gets rid of them, so it’s generally easy to “refill” them. There’s also a “set point” theory that claims that our body defends a particular weight and will eventually come back to it after periods of overfeeding or fasting followed by eating ad libitum. Lately, this has been renamed to “settling point”, to indicate that it can change under particular circumstances.
I don’t have much time to go into detail with this, but my n=1 experience tells me that whatever mechanisms are in place, once a fatty, always a fatty is mostly true. Apart from the biological changes (size and number of adipocytes), there are chemical changes (hormones, cytokines) and psychological changes that come with the extra chub. For a large percentage of FFP (former fat people) it’s not impossible to be lean but it is hard, as can be demonstrated by the 4+ kilos (!) I’ve put on the past few months. People assume that I’m naturally thin but my fat past has caused a dent in my ability to become/remain lean. A calorie is not a calorie, but at some points calories do matter, and as Mark Sisson wrote in a recent article, a cheat meal eaten often is no longer a cheat. Sometimes we do need to “paleo harder”.