<Dd> 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 . </Dd> <P> Fatty acids, stored as triglycerides in an organism, are an important source of energy because they are both reduced and anhydrous . The energy yield from a gram of fatty acids is approximately 9 kcal (37 kJ), compared to 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 can bind 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 storage 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 . </P> <P> Hibernating animals provide a good example for utilizing 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 . </P> <P> Thus the young adult human's fat stores average between about 10--20 kg, but varies greatly depending on age, 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 . Please note however that lipolysis releases glycerol which can enter the pathway of gluconeogenesis . </P>

Where do the catabolic products of fatty acid