Animals do not have enzyme systems which enable them to make use of the energy from :

Although fat is a common way of storing energy, in vertebrates such as HUMANS the fatty acids in these stores cannot be converted to glucose through gluconeogenesis as these organisms cannot convert acetyl-CoA into pyruvate; plants do, but animals do not, have the necessary enzymatic machinery. As a result, after long-term starvation, vertebrates need to produce KETONE bodies from fatty acids to replace glucose in tissues such as the brain that cannot metabolize fatty acids. In other organisms such as plants and bacteria, this metabolic problem is solved using the glyoxylate cycle, which bypasses the DECARBOXYLATION step in the citric ACID cycle and allows the transformation of acetyl-CoA to oxaloacetate, where it can be USED for the production of glucose.