How many calories are in one gram of protein?

• A. 3.75 kcal
• B. 9 kcal
• C. 2 kcal
• D. 4 kcal

Answer: (D)

Protein yields about 4 kilocalories per gram when it is metabolized in the body. This value is part of the Atwater factors used to estimate the energy provided by different nutrients in foods: protein and carbohydrate ≈ 4 kcal/g, fat ≈ 9 kcal/g, and alcohol ≈ 7 kcal/g. The reason is that amino acids released from protein oxidation produce energy, with some energy tied up in nitrogen excretion, but the usable energy to dietary calculations is about 4 kcal per gram. Therefore, the correct amount of calories in one gram of protein is 4 kcal. The other figures don’t fit the standard energy values for protein—9 kcal/g is for fat, and 2 kcal/g or 3.75 kcal/g are not the typical, widely used values for protein.