you mean… clinical depression?

You can undergo changes in your metabolism over time for many reasons. Usually this is caused by changes in eating habits, not the other way around. It has its own momentum, and understanding that momentum can be the key to functional dieting. The less you eat, the slower your metabolism will become. You will have less energy but your body will become more efficient with it. If you eat a lot, your metabolism will speed up and your body will burn energy less efficiently too. It also depends exactly what you eat; Your body doesn’t burn fat and carbs in quite the same way and you can feel the difference.