Why do some foods have more caloric content per unit mass than other foods?

Caloric content is the amount of calorie (energy nutrients) present in a food. Carbohydrates, proteins and fats are the three macronutrients that make up all foods and they are the body’s sources of calorie. Some macronutrients contain more calories per gram than others and this is because proteins and carbohydrates foods contain 4 calories per gram while fatty foods contain 9 calories per gram. Potatoes and yam have the same amount of carbohydrate, so they contain the same number of calories. Alcohol contains 7 calories per gram.