Problems on evaluation of nutritive value and safety of protein source by body weight of chicks.

Abstract

Recently two papers1, 2) has been published to evaluate nutritive value of various kinds of leaf protein concentrates (LPC) by comparing body weight gain of chicks fed either soyprotein as a standard or the sample as a sole dietary protein source. The body weight gain of the chicks fed the samples was unbelievably low.For example, the chicks fed LPC A took 151g of the diet and gained only 17.2g for 10 days, while those fed soyprotein took 152g of the diet and gained 81.8g for 10 days. The original data2) indicated metabolizable energy (ME) of the diet containing LPC A was determined as 1.63 kcal/g, although ME level of all of the diets was adjusted to be 2.89 kcal/g.It is a basic concept in formulating a diet that nutritive value such as ME of the diet is the sum of values contributed by each of the ingredients in the diet. This is the addition theorem which is almost true within a certain range of dietary levels of the ingrediedients5), but not true with the ingredient of extremely high dietary level beyond the range mentioned above.It should be pointed out that over 70% of LPC A was mixed in the test diet to make dietary protein level at 20%. With this high level of LPC A, the addition theorem may not be effective. This is certainly one of the main reasons of extremely low dietary ME than that calculated, resulting in extremely low growth rate of the chicks fed the diet. No detrimental effect in LPC A was expected from the data.Slope-ratio assays in evaluating gross protein value8), in which maximum contribution of the sample protein is 56% of total protein, and available energy10), in which maximum contridution of the sample ME is about 25% of total ME, are recommended in evaluation of both nutritive value and safety of the sample. Taking 2-4 levels of either the standard or the sample, linearity of the response of the chicks, therefore, reliability of the addition theorem can be confirmed statistically.As described by PAG, UN9), a test diet containing the sample as a sole protein source is not recommended without enough nutritional evidonce indicating the validity to do so.