Constant caloric restriction is hard on the body. Fung’s point is that prolonged caloric restriction together with frequent insulin spikes denies the bodies natural ability to burn fat, so it has to go to other sources. What’s missing from the rat example is the frequency, insulin measurements, and length of the caloric restriction.
Some people can calorically restrict and eat somewhat typically but have enough insulin sensitivity that they still get the required periods insulin free needed to work off of body fat. The other point that seems not to be made frequently enough is the one I started with:
caloric restriction (even 0) is hard on the body.
So, I don’t think its sound advice to say fast and stay at such for literal months at a time.
Here is another important point, since I’m deep in this. Every single day, you are either at a caloric restriction or a caloric surplus. We shouldn’t fear either of these things. When fasting, couple with breaks (even week long breaks) at caloric surplus, you are not going to put on body fat quickly.
I have made this on so many posts, but I’ll do it again. Let’s say you eat 1000 calories over your ideal maintenance. It takes energy to push that 1000 calories into fat. That means that fat you stores is less than 1000 calories worth. For simplicity, if your idealized TDEE is 1000 calories exactly, and fast for 1 day and then you have pulled more body fat than you stored.
When the body is constantly burning the supply and never getting the signal that more food will come, as well as constantly spiking insulin so not as easy to dip into the fat stores, it has to go to other methods for fuel. Sub 10 % body fat really isn’t natural at all. Hence, its entirely possible the rats having smaller organs because of the way, duration, of the caloric deficit as well as the body fat percentages along the way. The body isn’t digital, it doesn’t hit a switch. So as the body fat decreases, the game changes continuously!