Q.How do proteins differ from fats in their energy value and their role in the body?

Step 1. Compare energy values: lipids have a caloric value of about 9.45 kcal/g and a physiological fuel value of about 9 kcal/g, while proteins have a caloric value of about 5.65 kcal/g but a much lower physiological fuel value of only about 4 kcal/g — so gram for gram, fat supplies roughly twice the usable energy of protein.

Step 2. Compare their role in energy storage: lipids are described as the body's best-reserved food, stored in adipose tissue and drawn on for energy production when needed.

Step 3. Compare their biological role: proteins are primarily a source of amino acids needed for the growth and repair of body cells and tissues, not an energy reserve — the body stores protein only to a limited extent, and any dietary excess is broken down and excreted as nitrogenous waste rather than being stockpiled like fat. …

Compare the caloric/physiological fuel values of fats and proteins side by side, then contrast their functional r …

• Quoting only the caloric value (5.65 kcal/g for protein) without also noting the lower physiological fuel value (4 kcal/g), which is the energy the body can actually use. …