Conventional teachings regarding eating too little and losing too quickly

Conventional teachings tell us that if you don’t eat enough, your metabolism will slow down and weight loss stalls. What’s your take? As long as I stay on protocol and manage my hunger with thoughts and feel good, is it possible to eat too little?

Conventional teachings also say that you shouldn’t lose weight TOO quickly, because it’s unhealthy and not sustainable. Again, as long as I’m following protocol and managing my thoughts and feel good, is there really such a thing as losing too quickly?