Page 86 - Small Animal Clinical Nutrition 5th Edition

86 Small Animal Clinical Nutrition

Table 5-11. Glucogenic and ketogenic amino acids. degradation and synthesis cycles of specific proteins, a measure
VetBooks.ir Exclusively ketogenic Leucine, lysine of protein turnover is only a “snapshot” in time of protein
metabolism. In addition, rates of protein synthesis and degra-
Alanine, serine, glycine, cysteine,
Exclusively glucogenic
aspartate, asparagine, glutamate,
glutamine, arginine, histidine, valine, dation for any particular protein can change under different
threonine, methionine, proline physiologic conditions.
Ketogenic and glucogenic Isoleucine, phenylalanine, tyrosine, The body is able to synthesize new proteins and enzymes
tryptophan provided all the necessary amino acids are available. The
source of amino acids is not important. Cells use amino acids
from a variety of sources including those derived from food
Table 5-12. Factors for converting nitrogen to crude protein.
proteins, single amino acids added to the food and amino
Food protein Nitrogen (g/kg) Conversion factor acids synthesized by the body. In addition, cells synthesizing
Barley, oats, wheat 171.5 5.83 new protein cannot distinguish between amino acids from
Corn, eggs, meat 160.0 6.25
Milk 156.8 6.38 grains (e.g., corn and rice) and those from meats (e.g., chick-
Soybeans 175.1 5.71 en and beef). The only criterion is that all the amino acids
needed to synthesize a particular protein be present in suffi-
cient quantities when necessary. Protein synthesis will be lim-
intake and the recovery of endogenous protein and amino acids ited when certain amino acids are not present or available in
(Nissen, 1992).The third method for determining the recovery the quantities needed.
of endogenous protein is the N-isotope dilution technique. During protein turnover, a fraction of amino acids enter
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The endogenous protein is labeled, secreted into the GI tract catabolic pathways that lead to their permanent loss. The
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via a continuous infusion of N-leucine. This method directly amount of nitrogen lost every day as a result of the body’s con-
measures the contribution of endogenous to total protein tinuous breakdown process is called obligatory nitrogen loss.
recovered at the distal ileum in animals fed protein-containing Dietary protein must be consumed each day to replace amino
foods (Grala et al, 1998; Nissen, 1992). acids lost to catabolism. Trauma, infection, severe sepsis and
True, rather than apparent, ileal amino acid digestibilities burns increase protein turnover and nitrogen losses, whereas
should be used when formulating diets, but this requires further nitrogen losses are reduced during long-term fasting and star-
development of methods for routine estimation of endogenous vation. Nitrogen is normally lost from the body in feces
nitrogen losses. Improvements in protein usage should be (nitrogen, proteins, cells), in urine and through skin desqua-
sought via reducing endogenous nitrogen losses and improving mation and loss of hair.
true ileal amino acid digestibilites (Nyachoti et al, 1997). Nitrogen balance is the net difference between nitrogen con-
sumed and lost; however, the determination is fraught with
Protein Usage technical difficulties. Most commonly, nitrogen losses are
Absorbed amino acids and small di- and tripeptides are underestimated through incomplete collection of feces, urine,
reassembled into “new” proteins by the liver and other tissues of hair and sloughed cutaneous cells; whereas, nitrogen intake is
the body. Amino acids from food are transported from the liver routinely overestimated. Thus, nitrogen balance should only be
to other tissues by serum albumin or as free amino acids. The regarded as a crude estimate of body protein status and not be
fate of amino acids after absorption falls into three general cat- used to distinguish among subtleties in protein and nitrogen
egories: 1) tissue protein synthesis, especially in muscles and metabolism.
liver, 2) synthesis of enzymes, albumin, hormones and other
nitrogen-containing compounds and 3) deamination and use of Protein Storage
the remaining carbon skeletons for energy. Although, in effect, there is some storage of excess amino
A high rate of protein synthesis occurs in production of red acids, they are not stored to the same degree that extra fat and
and white blood cells, epithelial cells of the skin and those lin- carbohydrates are stored. Structural proteins in all tissues,
ing the GI tract (i.e., intestinal mucosa, which produces especially in muscles and liver and serum albumin, can be
exocrine secretions, such as digestive enzymes and mucus) and considered as amino acid stores. Muscle protein represents the
the pancreas. In addition, all body proteins are continuously largest reserve from which amino acids may be drawn in times
broken down and resynthesized, a process known as protein of need. Too much loss of body protein impairs muscle func-
turnover. Some proteins (muscle proteins and some plasma tion. Liver and muscle protein and serum albumin synthesis
proteins such as albumin) have a relatively long lifetime (days increase after consumption of a protein-containing meal.
to weeks). Other proteins (cytokines and enzymes) have rela- After protein synthesis is maximized, excess amino acids are
tively short lives (minutes to hours). deaminated and transaminated to yield amino groups and
Muscle protein composes nearly 50% of total body protein, carbon skeletons. The carbon skeletons can be used for many
but only accounts for 30% of new protein synthesized, whereas purposes including glucose precursors, which can be stored as
visceral and organ proteins compose a smaller portion of total glycogen or converted to fatty acids and acetyl-CoA, which
body protein but account for 50% of new proteins synthesized. can be used for fuel immediately. In the hours after consump-
Because protein turnover is the sum total of the continuous tion of a meal containing protein, body protein synthesis