According to Elmhurst College, fat tissue contains a higher amount of energy stored per gram than carbohydrates. With higher energy value per gram, fat tissue takes up less space than glycogen, reducing the overall mass of the human body. Moreover, fat tissue serves as insulation.
With an adequate supply of water, fat energy reserves can sustain body functions for 30 to 40 days, while carbohydrates can only preserve body functions for a few minutes, states Elmhurst College. However, carbohydrates are a ready source of energy, while energy obtained through oxidation of fats is available much more slowly, according to Clackamas Community College.