Why do we eat

No records exist who first began to think about why we eat or about various effects of foods, but ancient Greek philosophers and doctors commented these lines. Socrates said the purpose of food is to replace the water lost through the skin and the loss of heat from the body. Lavoisier thought that the combustion that produces body heat should occur in the lungs.
Understanding of what happens to food from the time it is eaten until it is oxidized to produce heat and mechanical energy, could not be learned until the chemical nature of foods was discovered. They knew that all animals inhale oxygen, combine it with the food to produce carbon dioxide, heat, and the energy with which they could move. The chemistry of that time could not tell more.
Foods are composed of organic substances too complex to be understood from the state of chemistry as it was in the first half of the nineteenth century. Modern work on digestion and nutrition began about a century ago.
Now we know that we should eat to have energy. When a nutritionist uses the word «energy» he means the capacity to do work. To him «work» is movement; the more a person moves, the more energy he requires. Even when a man is asleep he is still partly in motion because his heart, lungs and most of the other organs are working.