ANN 111: Principles of Animal Nutrition and Feed Technology (2+1)

BIOLOGICAL VALUE

• It is a direct measure of of the proportion of the food protein which can be utilised by the animal for synthesising body tissues and compounds and may be defined as the percentage of the nitrogen absorbed which is retained by the animal.
• A balance trial is conducted on albino rats in which nitrogen intake and urinary and faecal excretion of nitrogen are measured and the results are used to calculate the biological value as follows.

• Part of the faecal N is not derived from the feed but from endogenous losses and is called metabolic faecal N. Urinary n also contains a proportion of N known as endogenous urinary N .
• It is N derived from irreversible reactions involved in the break down and replacement of various proteins structures and secretions.
• MFN and EUN can be estimated in an animal fed a nitrogen - free diet.
• Since these fractions represent the already used up protein they have to be subtracted from faecal and urinary N lossses to arrive at a more precise BV.                    

While estimating BV the protein intake must be sufficient to bring about N retention. It should not be in excess of maximum retention.

• The diet must be adequate in all other nutrients.
• The amino acid mixtures absorbed by the animals are required for the synthesis of body proteins.
• The efficiency of this synthesis depends upon how close the amino acid proportions of the abosorbed mixture resemble those of body proteins, and partly on the extent to which these proportions can be modified.
• The biological value of a food protein, therefore, depends upon the number and kind of amino acids present in the molecule: the nearer the food protein approaches the body proteins in amino acid makeup, the higher will be the biological value.
• Animals have little ability to store amino acids in the free state and if an amino acid is not immediately required for protein synthesis it is readily broken down and either transformed into a dispensable amino acid which is needed by the animal, or used as an energy source.
• Since indispensable amino acids cannot be effectively synthesised in the animal body, an imbalance of these in the diet leads to a wastage.
• Food proteins with either a deficiency or an excess of any particular amino acid will tend to have a low biological value.
• If we consider two food proteins, one deficient in lysine and rich in methionine and the other deficient in methionine but containing an excess of lysine, then if these proteins are given separately to young pigs, they will both have low biological values because of the imbalance of these two indispensable amino acids.
• If however the two proteins are given together, then the mixture of indispensable amino acids will be better balanced and the mixture will have a higher biological value than either protein given alone. Such proteins supplement each other.
• In practice, and for a similar reason, it often happens that a diet containig a large variety of proteins has a higher biological value than a diet containing only a few.
• This also explains why biological values for individual foods cannot be applied when mixtures of foods are used, since clearly the resultant biological value of a mixture is not simply a mean of the individual components. For the same reason it is impossible to predict the value of a protein, as a supplement to a given diet, from its biological value.
• Animal proteins generally have higher biological values than plant proteins, although there are exceptions such as gelatine, which is deficient in several indispensable amino acids.