Carbohydrates
Carbohydrates is the main energy source of energy and thus comprises the main bulk of the average human diet. It is require for proper functioning of your tissues, cells, brain, heart and nervous system, a constant supply of carbohydrate is require.
One gram of carbohydrate provides four calories of energy.

Protein
protein provides energy when carbohydrates are not available. Protein helps the body build, maintain, and replace the body’s tissue. The body’s organs, muscles, and immune system are made up of protein. Protein can also be broken down into sugar, although less efficiently than carbohydrates.
One gram of protein provides four calories of energy.

fat
we need fat for
– normal growth and development
– helps absorption of fat-soluble vitamins (such as A,D,E,K AND CAROTENOIDS)
– protecting organs (such as heart) by cushioning them
– provide energy

One gram of fat provides nine calories of energy.