Fats contain 9 kilocalories per gram, while carbohydrates average about 4 kilocalories per gram. Fats are useful for storing energy, but sugars are easily broken down during metabolism. Nutritionists and diet gurus are debating which source of energy is better. The absolute answer lies in individual situations. For example, babies require fat in their diets for both energy and to build a healthy nervous system. Middle-aged adults quite often take in too much fat for their metabolic requirements. As one ages, average energy needs decrease by 5 percent per decade.