After initiating an intermittent fast that combines daily meals within eight hours, what’s the mechanism behind the two-phase effect in which some overworked exhausted people initially feel great on the fast but subsequently [feel] much worse?

I think it’s a question of allostatic load and metabolic reserve. What that means is what we always need to remember is that tools like intermittent fasting, sauna, cold therapy, and even exercise, for that matter, work on the basis of hormesis, and that means it’s essentially a stressor that causes a positive adaptation afterward. Now, if you have someone with pretty low levels of background stress—they’re taking care of themselves, they’re eating well, they’re sleeping, they don’t have a chronic illness—then their metabolic reserve is high. They’re doing well overall. They’re not under a lot of stress, you add these hormetic stressors, it’s beneficial. You have someone who’s burning the candle at both ends—they’ve got chronic inflammation, they’re not sleeping well, and they’re doing CrossFit three times a week—and then they add intermittent fasting, sauna or cryotherapy, they’re already at a very high level of stress, their metabolic reserve is low, and adding more hormetic stressors, even though, in theory, that could be positive, is going to push them over the edge. I think that’s what can happen with fasting or ketogenic dieting, or any of these other interventions.