Page 430 - Anatomy and Physiology of Farm Animals, 8th Edition

Nutrition and Metabolism / 415

following a meal, during which nutrients (75% of the total) is formed and stored in
skeletal muscle. Blood glucose is also avail-
are being absorbed from the gastrointesti-
VetBooks.ir nal tract (absorptive state), with a period able for use by all cells of the body for
energy during this period, but no other
during which there is no net absorption
(postabsorptive state). During the absorp- organ is capable of significant glycogen
tive state, blood levels of glucose, amino storage.
acids, and triglycerides (as part of chylomi- Absorbed amino acids are immediately
crons) increase. The overall goals of the available to all body cells for protein
metabolic processes during this period synthesis. Because all of the amino acids
appear to be to increase the use of these necessary for synthesis of a given protein
nutrients by cells of the body or store them must be available at the time of synthesis, it
so that they can be used later. Metabolic is imperative that animals have a balanced
periods of ruminants differ from those of diet that contains all of the essential
other animals, because nutrients are amino acids. Protein synthesis in many
constantly being absorbed from the fores- organs, including liver and skeletal muscle,
tomach and passing from the forestomach is stimulated by insulin, so the increase in
through the remainder of their gastrointes- insulin following a meal also promotes
tinal tract. This chapter discusses some protein synthesis during this period.
specifics of ruminant metabolism after a However, this stimulatory effect on protein
more general overview. synthesis is minor compared to the effects
of insulin on glucose metabolism (e.g., the
rate of plasma protein production by the
Absorptive State: Anabolism liver increases by only a small percentage
after a meal).
Figure 22‐1 summarizes the overall fate of The amounts of amino acids absorbed
the major nutrients absorbed during the after a typical meal are more than can be
digestion of a meal, and these are described efficiently used by the body for protein
in more detail in the following paragraphs. synthesis. However, no metabolic path-
Glucose is the predominant product of ways permit the various amino acids to be
carbohydrate digestion in most animals, stored for later use the way glucose is
and following a typical meal, blood glucose stored as glycogen. Many of the excess
levels may rise to 150% of fasting levels. amino acids are taken up by hepatocytes
The increase in blood glucose is a major and enter metabolic pathways that result in
stimulus for the release of insulin from the triglyceride (lipid) formation. These path-
pancreas, but increases in plasma amino ways remove nitrogen‐containing amino
acids during the digestion of a high‐protein groups from the amino acids (deamina-
meal can also stimulate insulin release. tion). Most of the resulting lipids are
Insulin affects carbohydrate, amino acid secreted by hepatocytes into the blood as
(protein), and lipid metabolism during the part of lipoproteins (discussed later).
absorptive period, and it is considered the Deamination of amino acids is also part
primary endocrine regulator of metabo- of a different metabolic pathway by which
lism during anabolism. liver cells use amino acids to produce glu-
Insulin stimulates the uptake of glucose cose. However, the hormone glucagon
by skeletal muscle cells, where it can be must be available to stimulate this pathway,
used for energy or stored as glycogen and glucagon release from the pancreas is
(essentially a polymer of glucose mole- reduced by increases in blood glucose.
cules). The liver also stores glucose as Thus, during the period that blood glucose
glycogen during the absorptive period, and is elevated following a meal, the use of
this is also stimulated by insulin. Primarily amino acids to produce glucose is sup-
because the mass of skeletal muscle is pressed. Gluconeogenesis is the term for
greater than the liver, much more glycogen the collective metabolic processes by