Protein Energy Density. Protein contains 4 calories per gram. It provides you with the essential amino acids your body needs for forming new cells and repairing damaged cells. Most adults get plenty of protein in their diet as long as they eat at least two to three servings of protein-rich foods, according to MedlinePlus. Lean protein sources, such as beans, seafood and skinless poultry are healthier than fatty cuts of meat and lower in energy density.