How many calories are in 1 gram of protein?

• A. 3
• B. 4
• C. 4.2
• D. 3.5

Answer: (B)

Proteins provide about 4 kilocalories per gram because that's the energy value assigned to protein in the standard Atwater factors used for nutrition labeling and energy calculations. When amino acids from dietary protein are metabolized, the amount of usable energy derived is about 4 kcal for each gram, placing protein's energy contribution alongside carbohydrate (also ~4 kcal/g) and below fat (about 9 kcal/g).

This value is the convention researchers and clinicians use in everyday practice, so it’s the best answer. Some sources discuss slightly lower metabolizable energy for protein (around 3.5 kcal/g) or minor rounding like 4.2, but those are not the standard values used in typical CNSC exam contexts.