If you fast too often or too long, your body thinks it's starving and slows down how fast it burns energy to save energy.

“Once you extend the fast beyond 36 hours, but this also ties in with fasting too frequently, the body recognizes a consistency pattern. And it starts to say, 'Oh, oh, oh, we're starving. We should conserve energy.' So what this is telling us is that a well-timed long fast speeds your metabolism up. But doing it too often, even with shorter fasts, or stacking calorie restriction on top of it on the days you're not fasting, will slow it down.”