Our bodies and the bodies of all living things must maintain a state of metabolic equilibrium. This means there has to be a certain balance and control over our intake of fuel and its use. Thirst, hunger and appetite regulate our body weight.
In the brain there is a hunger centre. It acts as a brake on the activities of the stomach and intestines. When the blood has sufficient nutrients, the hunger centre stops the activities of the stomach and intestines, but when there is lack of nutrition, the intestines and stomach become active. That’s why you can hear your stomach rumble when you are hungry.
But hunger itself has nothing to do with an empty stomach. For example, a person who is feverish may have an empty stomach but may not feel hungry. His body uses up its protein supplies and feeds itself from within.
When you feel hungry, your body is crying out for fuel- any kind of fuel. A really hungry person will eat any kind of food. It is your appetite that sees to it that you choose the mixed diet that your body requires. For example, when a man sits down at dinner, one bowl of soup may be all the soup he wants. Then he goes on to meat and vegetables; when he’s had enough of these, he may go on to dessert: cake, coffee etc. But it would be pretty hard for him to eat this same quantity of food if it all consisted of potatoes.
How long a living creature can go without food depends on its metabolism. Warm-blooded animals have a more active metabolism, and so use up their store of fuel more rapidly. The smaller and more active the animal, the more rapidly it uses up its food supplies.