You should pack foods high in to provide the most energy or kilocalories per gram of food

Fats (lipids)

You should pack foods high in fats (lipids) to provide the most energy or kilocalories per gram of food.

Fats is one of the three main macronutrients (others are proteins and carbohydrates), and is made up of fatty acids and glycerol. The body requires fats for growth and energy to perform its activities. Fat is the most energy efficient form of food. Each gram of fat supplies about 9 calories of energy to the body which is about times two of calories supplied by one gram of the other macronutrients.  Each gram of proteins and carbohydrates supplies 4 calories of energy.