Page 88 - Small Animal Clinical Nutrition 5th Edition

88 Small Animal Clinical Nutrition

culations and can be rationalized by keeping in mind that most complementation (feeding multiple protein sources) and/or
VetBooks.ir pet foods are mixtures of protein-containing ingredients. In amino acid supplementation. Thus, measuring protein quality
becomes necessary in formulating different feedstuffs to supply
addition, animal protein requirements are typically calculated
digestible amino acids or crude protein.
from determination of individual amino acid requirements that
are converted to nitrogen requirements and then expressed as a
protein requirement by multiplying nitrogen by 6.25. Although MEASURES OF PROTEIN QUALITY
the crude protein content of pet foods gives a measure of the Several methods have been used to determine the quality of
amount of nitrogen available to the animal, it does not provide protein sources for animals. Some methods involve testing the
information about protein quality or the nutritional value of the protein source by feeding it to animals (in vivo methods),
protein source(s). whereas other methods evaluate the protein’s quality by
chemically analyzing its amino acid composition (in vitro
Protein Quality methods). There are a number of ways to measure protein
Protein quality refers to the efficiency by which amino acids quality in vivo. Many of these tests are based on variations of
from food are converted into tissue. That efficiency in turn nitrogen balance experiments.
depends on the protein source, concentration of essential amino The first and crudest method is the protein efficiency ratio
acids in the food and their availability (Brown, 1989). Proteins (PER), which measures the ability of dietary protein to be con-
that provide optimal proportions of all essential amino acids are verted into tissue.The method works only with young, growing
referred to as high quality proteins. When a protein lacks one animals and may be influenced by the level of energy or fat in
or more of the essential amino acids (limiting amino acid), the the diet and by the level of dietary protein. The PER is calcu-
quality of the protein decreases and it is referred to as a poor lated as: PER = weight gain of animal ÷ protein intake of ani-
quality protein (Orok et al, 1975). Other amino acids may not mal (Brown, 1989; Burns et al, 1982). The typical PER value
be used when a limiting amino acid is absent. Poor protein qual- for animal proteins is 2.5 (Brown, 1989).
ity can also result from an excess of certain amino acids that may The most frequently used measure of protein quality is bio-
interfere with usage of other amino acids. (See Imbalance and logical value (BV), which is defined as the percentage of
Antagonism.) Digestibility can influence protein quality and absorbed protein retained. BV gauges the efficiency with which
can be affected by ash content, processing time and/or temper- the body is able to convert absorbed dietary amino acids into
ature. The amino acids of poor quality protein cannot be prop- tissues. The BV is calculated as: BV = (food nitrogen - [fecal
erly digested and used by the animal. For example, overcooking nitrogen + urinary nitrogen]) ÷ (food nitrogen + fecal nitrogen)
can form indigestible nutrient forms called Maillard complexes. (Brown, 1989; Burns et al, 1982; Davidson et al, 1979). There
Processing methods and heat treatment of various feedstuffs can are some practical problems using BV as a measure of protein
also affect digestibility. Researchers studied the effects of pro- quality. It is possible for a protein source to lack high digestibil-
cessing temperature and time on nutrient quality of moist dog ity, but have a high BV if the residue is absorbed and well used
food (Jamikorn et al, 2000).They demonstrated that raising the by the animal (Brown, 1989).
º
processing temperature (6 C) or increasing the processing time To account for these differences in digestibility, a new
(25 min.) may alter nutrient content and digestion. method (net protein utilization) was established for determin-
Other factors that can affect protein availability include fiber ing protein quality. This method, unlike BV, includes
content, tannins, pectins, oligosaccharides and phytase or allowances for nitrogen losses during digestion and is equal to
trypsin inhibitors, which can be found in plant protein sources. BV multiplied by true digestibility. Use of the coefficient allows
Proteins of plant origin generally have lower digestibility than for adjustments to determine differences in true and apparent
animal proteins because plant fiber and carbohydrates lower availability of dietary amino acids for the animal (Brown, 1989;
digestion, due to a reduced degradation rate of nutrients in the Burns et al, 1982; Davidson et al, 1979).
gut and increased bacterial activity (Murray et al, 1997; The amino acid score (AAS) is a chemical method for com-
Neirinck et al, 1991; Meyer, 1984). Feeding high-fiber diets paring the amount of the first limiting amino acid in a test pro-
decreases apparent protein digestibility (Kendall and Holme, tein with the level of the same amino acid found in a reference
1982), presumably from increased adsorption of amino acids protein. The AAS is calculated as: AAS = mg of amino acid/g
and peptides by fiber, and obstruction of digestive enzymes by of test protein ÷ mg of amino acid/g of reference protein. This
fiber components in cell walls (Knabe et al, 1989). method for measuring protein quality is very rapid and highly
Dogs use poor quality proteins less efficiently (Burns et al, reproducible. Because it is derived solely from amino acid com-
1982). Poor quality proteins can lead to profound nutritive fail- position, AAS does not compensate for differences in digestibil-
ure, accompanied by a rapid decline in weight, loss of appetite ity that can be caused by processing (Brown, 1989; Food and
and eventual death (Palika, 1996; Rose and Rice, 1939). Dogs Agriculture Organization of the United Nations, 1973).
and cats will increase food intake to meet their protein/amino The slope ratio method is a biologic equivalent of the AAS.
acid requirements for maintenance if the food contains low The slope ratio method compares the growth performance pro-
quality protein. By understanding the availability of limiting duced by a test protein with that of a reference protein such as
amino acids in a particular food or protein source, nutritionists defatted whole egg (Samonds and Hegsted, 1977). The slope
can improve the protein quality of the food through protein ratio (SR) is calculated as: SR = slope of response curve for ani-