In a differentiated organism, each tissue must be provided with fuels that it can use, in amounts sufficient to meet its own energy needs and to perform its specialized roles. Energy production must meet needs that vary widely, depending on level of exertion, composition of fuel molecules in the diet, time since last feeding, and so forth. In humans, for example, the daily caloric intake may vary by 4-fold, depending in part on the level of exertion-from 1500 to 6000 kcal/day in an average-sized human or, in the thermodynamic units used in the text, from 6000 to 25,000 kJ/day.

The major fuel depots are triacylglycerols, stored primarily in adipose tissue; protein, most of which exists in skeletal muscle; and glycogen, which is stored in both liver and muscle (Figure 23.1 and Table 23.1). In general, an organ specialized to produce a particular fuel lacks the enzymes to use that fuel. For example, ketone bodies are synthesized in the liver, so little catabolism of ketone bodies occurs there.