Also, feel free to comment and correct me if anything is wrong or you want to add some type of correct term or definition or description if necessary!
As you should know, the sun and it's rays(?) = energy. Plants use the sun energy to create energy for themselves, then an animal eats the plants and gets its energy in that way and is then eaten by another animal etc etc The foodchain basically.
So food is energy (and the energy is made up of macronutrients) and our body NEEDS energy. Think of a car without oil/energy it doesnt get very far... and it is the same with our bodies. Of course we can last around 3 weeks without any food, but the amount of energy you have is limited and your organs and other functions begin to shut down or slow down as there is not enough energy to keep them going. As well as your body beginning to break down fat and muscle for energy... and this is what happens when you eat very little for a long time. Muscle is broken down for energy, the metabolism slows down and eventually the heart muscle wont get the energy it needs to keep pumping blood or it will begin to be used for energy. Though by that stage other organs have already shut down and caused problems or death.
- Your body size and composition.
- Your sex.
- Your age.
- Activity level