Question

Chemical and biological methods to evaluate protein nutritive value

Answer 1

The biological evaluation of protein quality is usually
carried out using rats.
In 1919, Osborne et al. (1919) introduced the term Protein
Efficiency Ratio (PER), a biological method based on the
expression of growth-promoting value of protein numerically.
This hypothesis proposed by Osborne and his co-workers was
the “optimum of the protein which is determined not only by
the absolute amount furnished, but also by its quality”. From
this work, it was observed that protein sources could be
evaluated by simply comparing the relative weight gain of an
animal fed the material in relation to the actual amount of
protein consumed. According to Bender (1956) this was
considered the simplest and most convenient method of
measuring the nutritive value of proteins.
The protein efficiency ratio has been frequently criticized
and Mitchell (1924) stated that simplicity was its only
recommendation. There are two main drawbacks to the
method. Firstly, protein efficiency ratio as measured on any
protein increases with the quantity consumed (Mitchell, 1924;
Barnes et al., 1945). It should be noted that a certain amount of
protein is required for maintenance of body weight, and only
the increment above this amount is available for growth.
Consequently, when larger quantities are consumed, more is
available for growth and a higher protein efficiency ratio (PER)
results. Secondly, the method is based on the assumption that
the increase in the body weight on the protein containing diet
is proportional to the protein retained. It has been repeatedly
shown that the composition of the weight increases varies with
the type of diet during 6-8 weeks of the usual experiments
(Hamilton, 1939).
The protein efficiency ratio is open to other criticisms.
Bender and Doell (1957) pointed out that gain in body weight
is constant in composition is not valid. They argued further
that the result may vary with the level of protein in the diet,
and that the results are influenced by food intake. Moreover,
no allowance is made for maintenance; it is assumed that all
the protein consumed is used for growth. The method ascribes
a value of zero to protein that do not that permit growth. To
correct the dependence of protein efficiency ratio on food
intake Bender and Doell (1957) proposed the use of a slope
ratio method whereby weight loss of a control group of
animals given a protein free diet as well as the weight gain of
the test group are considered. This slope-ratio method was put
forward in an attempt to overcome some of the problems in the
protein efficiency ratio. It requires the testing of protein at
several levels in an effort to give an estimate of the precision
and validity of assay. However weight loss of the rat fed a non
protein diet id equivalent to the protein needed for
maintenance. In an argument put forward by Mclaughlan and
Keith (1975) to criticize the slope ratio method of Bender and
Doell (1957), they assumed a straight line relationship
involving the weight loss of the non-protein group of rats
proposed by Bender and Doell (1957), and other levels of
dietary protein. The results of the slope-ratio method,
otherwise known as the net protein retention value (NPR), at
any point of the curve, remained constant, while on the othe