In order to first have energy to spend as outlined above, we must first acquire it. But how? The earth’s great bank of energy is the sun; its currency is is light and heat. These man cannot “cash in” directly. They have to go through the great clearing house, the plant world, before they become available for the human economy. Plant cells transmute light and heat into chemical energy and bind this with elements from the air and soil to make three great classes of energy-bearing substances, which man can use for his activities, know has protein, fat, and carbohydrate. These are the “fuels” which supply energy for the human machine.
Pg. 5. Rose, Mary Davies Swartz. Feeding the Family. New York: The Macmillan Company, 1916.