Then think about how the human body absorbs energy. You must know this, too. Eating is just, I haven't heard that you can maintain your life by basking in the sun.
Finally, it is more difficult to think about what the human body consumes. Your breathing, heartbeat, blood circulation, in short, the physiological movements of the heart, liver, spleen, lungs and kidneys all need energy to maintain, which is a part of human energy consumption, as well as sports, running, playing ball games, making love and so on.
What happens when you don't consume enough energy to maintain fat and muscle? Your body will break them down. It's simple, because you can't feed such meat, your body will throw it away. You will lose weight.
Now you know, as long as you control your diet, even if you sleep in bed, dream of having sex and do whatever you want, as long as you keep your mouth shut and don't eat anything you shouldn't eat, you will be thin if you want.