What is the main source of heat?

The energy sources of human body are carbohydrates, lipids and protein in food. These three nutrients are ubiquitous in all kinds of foods. Cereals and potatoes are the most economical food sources because they are rich in carbohydrates. Oil crops are rich in fat; Animal food generally contains more fat and protein than plant food; Except soybeans and hard fruits, they are rich in oil and protein; Vegetables and fruits are generally less in content.