Protein Efficiency Ratio – Animal Husbandry Notes- For W.B.C.S. Examination.

প্রোটিনের কর্মদক্ষতার অনুপাত – পশুপালন নোট – WBCS পরীক্ষা।

A number of researchers have uncovered an important conclusion: the protein value (protein efficiency ratio) of the raw amaranth grain does not reflect the amino acid pattern of the protein. Furthermore, amaranth grain processed by wet cooking gives a higher protein quality value than that of the raw grain in all amaranth species. When processed under conditions that do not damage the availability of essential amino acids, its protein quality is very close to that of casein or milk protein. The effect of processing is evident in both the consumption of the diet and the weight gains of test animals. This effect is still unexplained and deserves some research. It has been suggested that the presence of antiphysiological factors that are inactivated by heat may be responsible; well-known antigrowth factors such as trypsin inhibitors, tannins, and lectins are present at low levels in amaranth grain.Continue Reading Protein Efficiency Ratio – Animal Husbandry Notes- For W.B.C.S. Examination.

Some processes, such as expansion, flaking, and wet cooking, apparently do not affect protein digestibility. However, the product from toasting gives an equal or lower protein digestibility than the raw grain. Table 6 lists the protein quality values of amaranth grain protein and that of other cereal grains. With the exception of quality protein maize, also high in lysine, all other cereal grains have a protein quality below that of amaranth grain.

Initial Evaluations of Plant and Animal Protein Quality

Nearly 100 years ago, Mitchell described the basic concept used to estimate protein quality as being the fraction of a dietary protein that is lost during digestion plus the fraction that is lost in metabolism, i.e., not retained in the body (the so-called “biological value”), and proposed a method to measure the level of nitrogen losses beyond the obligatory losses that could be accounted for by consumption of the protein (Mitchell, 1923, 1924). Under this principle, the fundamental value of a dietary protein is to supply alpha-amino nitrogen to be used for anabolic purposes, thereby ensuring optimum growth or, in an adult, enabling the normal renewal of body protein with an equilibrated nitrogen balance (Sherman, 1920). This was judging protein quality on experimental grounds—a highly pragmatic viewpoint—based on the basic criteria of metabolic indispensability, a very fundamental viewpoint.

Much research has been based on this approach in rodents. Numerous studies have measured the “protein efficiency ratio” (PER, weight gain divided by the amount of protein consumed) of different protein sources in rodent diets, or other similar indexes such as the “relative protein efficiency ratio” or “(relative) net protein ratio,” to compare weight gain between groups receiving a reference protein, or to compare the weight loss of a group receiving a protein-free diet. For a review, readers can refer to the publication by Boye et al. (2012). Based on these criteria, a series of studies in rodents concluded that plant proteins were of poor quality. For example, protein efficiency ratios were found to lie within the 1.2–2.4 range for plant proteins (including pea flour, soy protein, beans) and could be as low as 0.95 for wheat flour, whereas animal proteins were in the 3.1–3.7 range (Sarwar et al., 1984; Cruz et al., 2003).

These studies were also useful in demonstrating the concept of a limiting amino acid. The protein efficiency ratio of soy flour in a rodent diet increases with the addition of methionine to reach a ratio similar to that of casein, showing that the low level of sulfur amino acid in soy protein limits the utilization of other amino acids for the most quantitatively important pathway, i.e., protein synthesis. Interestingly, while the PER of soy protein and casein increase with the amount of protein in the diet, a difference between the two sources remains whatever the level of protein, provided it is not too low. Therefore, in the case of a single source of protein, the limiting amino acid is the key factor regarding relative protein utilization. From this literature, cereals have been established as being deficient in lysine and legumes deficient in methionine, or more generally speaking in sulfur amino acids (Friedman and Brandon, 2001; Sarwar et al., 1978). In line with this concept, it has also been demonstrated by experiments in rats that proteins can complement each other (Sarwar et al., 1978).

Another key factor demonstrated by these approaches with respect to protein quality is digestibility. The antinutritional factors found in many plant proteins may limit the digestion of protein, resulting in a reduction in its final global efficiency of utilization. This is particularly important regarding the trypsin inhibitors found in many beans, the inhibitory effect of which is markedly decreased by heat treatment or chemical reduction, resulting in a structural change (via the disulphide bonds) (Friedman and Brandon, 2001; Friedman and Gumbmann, 1986; Faris et al., 2008; Sarwar Gilani et al., 2012; Gilani and Sepehr, 2003).

As argued by Mitchell a century ago, and based on series of classic growth experiments in rats, plant protein is more digestible when the protein fraction is purer, and poorer digestibility estimates are limited to legumes, but this depends on previous heat treatment. With respect to legumes, including white bean, lima bean, and velvet beans, he wrote “The value of these proteins in growth experiments on rats in large part depended upon whether or not the proteins were cooked or uncooked” (Mitchell, 1923).

Assessing protein quality by measuring the efficiency of protein utilization in a rat model has some important limitations. Protein requirements for rapid growth in animals are not the same as those in humans, which are mainly driven by maintenance, even in young children (Young, 1991). Furthermore, individual amino acid requirements are not the same in rodents and humans, due to the different metabolic demands of specific tissues. A higher sulfur amino acid requirement in rodents than in humans has long been suspected, and it has been evidenced by the fact that the PER of animal proteins (or a mixture of proteins) is not maximal and increases with the addition of methionine. These initial evaluations based on rat growth had many limitations regarding their application to the human diet, but together with the predominance of plant-based diets in a context of protein-energy malnutrition, they have shaped the lasting view that plant proteins have a very poor quality for human nutrition.

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