<P> Food energy is chemical energy that animals (including humans) derive from food through the process of cellular respiration . Cellular respiration may either involve the chemical reaction of food molecules with molecular oxygen (aerobic respiration) or the process of reorganizing the food molecules without additional oxygen (anaerobic respiration). </P> <P> Humans and other animals need a minimum intake of food energy to sustain their metabolism and to drive their muscles . Foods are composed chiefly of carbohydrates, fats, proteins, water, vitamins, and minerals . Carbohydrates, fats, proteins, and water represent virtually all the weight of food, with vitamins and minerals making up only a small percentage of the weight . (Carbohydrates, fats, and proteins comprise ninety percent of the dry weight of foods .) Organisms derive food energy from carbohydrates, fats and proteins as well as from organic acids, polyols, and ethanol present in the diet . Some diet components that provide little or no food energy, such as water, minerals, vitamins, cholesterol and fiber, may still be necessary to health and survival for other reasons . Water, minerals, vitamins, and cholesterol are not broken down (they are used by the body in the form in which they are absorbed) and so cannot be used for energy . Fiber cannot be completely digested by most animals, including humans . However, ruminants can extract food energy from the respiration of cellulose because of bacteria in their rumens . </P> <P> Using the International System of Units, researchers measure energy in joules (J) or in its multiples; the kilojoule (kJ) is most often used for food - related quantities . An older metric system unit of energy, still widely used in food - related contexts, is the calorie; more precisely, the "food calorie", "large calorie" or kilocalorie (kcal or Cal), equal to 4.184 kilojoules . (Contrast the "small calorie" (cal), equal to 1 / 1000 of a food calorie, that is often used in chemistry and in physics .) Within the European Union, both the kilocalorie ("kcal") and kilojoule ("kJ") appear on nutrition labels . In many countries, only one of the units is displayed; in Canada and the United States labels spell out the unit as "calorie" or as "Calorie". </P> <P> Fats and ethanol have the greatest amount of food energy per gram, 37 and 29 kJ / g (8.8 and 6.9 kcal / g), respectively . Proteins and most carbohydrates have about 17 kJ / g (4 kcal / g). The differing energy density of foods (fat, alcohols, carbohydrates and proteins) lies mainly in their varying proportions of carbon, hydrogen, and oxygen atoms: For food of elemental composition C H O N, the heat of combustion underlying the food energy is 100 kcal / g (c + 0.3 h − 0.5 o) / (12 c + h + 16 o + 14 n) to a good approximation (± 3%). Carbohydrates that are not easily absorbed, such as fiber, or lactose in lactose - intolerant individuals, contribute less food energy . Polyols (including sugar alcohols) and organic acids contribute 10 kJ / g (2.4 kcal / g) and 13 kJ / g (3.1 kcal / g) respectively . The amount of water, fat, and fiber in foods determines those foods' energy density . </P>

How do we measure the energy content of food