Though most body tissues can use a variety of energy substrates, the brain, kidney medulla, testes, erythrocytes and central nervous system require glucose as their sole or primary carbon source (Figure 16.2). Moreover, the brain requires 120 grams of glucose per day, or about 75% of the body's total daily need. To put these amounts in additional perspective, the amount of glucose that can be generated from the body's glycogen reserves at any one time is about 190 grams and the total amounts of glucose in body fluids is about 20 grams. Thus, the readily available glucose reserves amount to about one day's supply.

To maintain sufficiently high levels of glucose, the body employs the glucose biosynthesis pathway called gluconeogenesis. The process takes three and four carbon precursors (generally non-carbohydrates) and converts them into glucose.

The liver is the primary gluconeogenic organ in the body, along with the kidney cortex.

The major fates of glucose as a metabolic intermediate are catabolism by nervous tissue and utilization by skeletal muscle. In addition, glucose is the primary precursor for all other carbohydrates, including amino sugars, complex polysaccharides, and the carbohydrate components of glycoproteins and glycolipids.

The need for glucose as a biosynthetic intermediate means that gluconeogenesis is an important pathway in animals, plants, and micoorganisms. The pathway is essentially identical in all organisms.