As we said above, food intake and energy expenditure for animals is approximately equal. If energy expenditure exceeds food intake, then the excess energy is taken up by utilisation of body fat. However, if food intake is excess, then the surplus is stored as fat irrespective of the kind of food eaten. Excess carbohydrates are changed to fats and accordingly RQ exceeds 1. This is because fats contain relatively less oxygen and the excess oxygen of carbohydrates is used in the metabolism. This reduces the oxygen uptake and the respiratory carbon dioxide, oxygen ratio is increased. For this reason fat is ideal storage material for energy. It is much lighter and yields twice as much energy as carbohydrates.
Migratory birds that may have to fly more than 1000 km non-stop, carry fat as 40% to 50% of their body weight. Nonetheless, some carbohydrates are important in energy storage. Glycogen a starch-like carbohydrate polymer is stored as granules in the skeletal muscles and liver of vertebrates. During heavy muscular exercise when blood does not deliver sufficient oxygen to meet demands, glycogen provides the energy. It is broken down directly into glucose-6-phosphate, providing fuel for carbohydrate metabolism more directly than does fat, On the other hand, many animals that do not move about, also store glycogen as excess energy source. For example, clams, oysters and many intestinal parasites like Ascaris use glycogen as the storage material. These animals have to face anaerobic conditions and in such situations glycogen breaks down to acetic acid to yield energy.