The glycerol released by lipase action is phosphorylated by glycerol kinase in the liver (the only tissue in which this reaction can occur), and the resulting glycerol 3-phosphate is oxidized to dihydroxyacetone phosphate. The glycolytic enzyme triose phosphate isomerase converts this compound to glyceraldehyde 3-phosphate, which is oxidized via glycolysis, or converted to glucose via gluconeogenesis. Fatty acids as an energy source Fatty acids, stored as triglycerides in an organism, are a concentrated source of energy because they contain little oxygen and are anhydrous. The energy yield from a gram of fatty acids is approximately 9 kcal (37 kJ), much higher than the 4 kcal (17 kJ) for carbohydrates. Since the hydrocarbon portion of fatty acids is hydrophobic, these molecules can be stored in a relatively anhydrous (water-free) environment. Carbohydrates, on the other hand, are more highly hydrated. For example, 1 g of glycogen binds approximately 2 g of water, which translates to 1.33 kcal/g (4 kcal/3 g). This means that fatty acids can hold more than six times the amount of energy per unit of stored mass. Put another way, if the human body relied on carbohydrates to store energy, then a person would need to carry 31 kg (67.5 lb) of hydrated glycogen to have the energy equivalent to 4.6 kg (10 lb) of fat. Hibernating animals provide a good example for utilization of fat reserves as fuel. For example, bears hibernate for about 7 months, and during this entire period, the energy is derived from degradation of fat stores. Migrating birds similarly build up large fat reserves before embarking on their intercontinental journeys. The fat stores of young adult humans average between about 10–20 kg, but vary greatly depending on gender and individual disposition. By contrast, the human body stores only about 400 g of glycogen, of which 300 g is locked inside the skeletal muscles and is unavailable to the body as a whole. The 100 g or so of glycogen stored in the liver is depleted within one day of starvation. Thereafter the glucose that is released into the blood by the liver for general use by the body tissues has to be synthesized from the glucogenic amino acids and a few other gluconeogenic substrates, which do not include fatty acids. Nonetheless, lipolysis releases glycerol which can enter the pathway of gluconeogenesis. Carbohydrate synthesis from glycerol and fatty acids Fatty acids are broken down to