Question

Why are carbohydrates called the "first energy source of the body", but not "main energy source of the body"?

Answer 1

Main energy source is glucose. "First," refers to the fact the carbohydrates are easier to process to get to some form of "glucose" or its intermediates.

Explanation:

There are three main mechanism to get a carbohydrate to a usable state: glycolysis, citric acid cycle and the electron transport chain. Further, the carbohydrates represent easier chemical formulations to decompose than say fats, which are made up of many more Carbon, Hydrogen and Oxygen molecules. So, the energetics of the biology make carbohydrates the "first" choice for a cell when given the choice between Carbohydrates, Fats and Proteins to decompose and produce ATP for the cell to use.