THE SMALL INTESTINE AND HOMEOSTASIS
Within enterocytes a portion of the monosaccharides absorbed are converted to lactate by glycolysis. Excess nonessential amino acids, especially glutamine, are used to synthesize alanine and ammonia (Figure 11.23). These products are then delivered to the liver in the hepatic portal vein. Converting some of the absorbed nutrients to lactate and alanine reduces the metabolic load on the liver because it can easily regenerate pyruvate from them. Pyruvate is a versa-tile liver metabolite; it is a substrate for the TCA cycle, allowing the formation of ATP during oxidative phosphorylation but it can be used for the biosynthe-sis of glucose and glycogen, ketone bodies, fatty acids and all but two of the nonessential fatty acids and cholesterol. The GIT is a significant contributor to nutrient homeostasis both during and after nutrient absorption because the formation of lactate and alanine continues even when absorption ceases.