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Biochemisty -- Study Notes

   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Protein Efficiency Ratio (P.E.R)


Protein Efficiency Ratio is the ratio of grams of body weight gain (in specified time) to the grams of protein consumed. The P.E.R of various foods has been found to decrease in the following order: Fish muscles, beef muscles, beef liver and kidneys, egg (whole), milk, soyabeans, oats, and wheat. The animal protein are called first class proteins because they can maintain nitrogen balance by supplying all the essential amino acids even if these proteins are the only source of nitrogen diet. Most plant proteins, on the other hand, are of low biological value (also called second class proteins) because they do not maintain nitrogen balance if they become the only source of nitrogen in diet; this is due to the fact that these proteins do not contain all the essential amino acids. In addition, plant proteins are generally not as well digested and absorbed as are animal proteins.


In addition to the fact that most animal proteins contain all the essential amino acids, another reason for the superiority of animal proteins over plant proteins is that the essential amino acids contained in animal proteins are present in an optimal ratio. This is quite an important factor because an abnormally increased intake of one of the essential amino acids can exert toxic effects in the body. Thus a protein will have a high biological value if it has the following characteristics.

  1. It should contain all the essential amino acids in sufficient amounts.
  2. The essential amino acids should be present in an optimal proportion.
  3. The protein should be easily digestible.

Most animal proteins fulfill the above mentioned conditions and are therefore of high biological value. Important exceptions to this general rule are gelatin, keratin, and hemoglobin.


The above discussion may undermine the importance of plants proteins and make the reader believe that plant protein are just useless as they are of low nutritive values. Plants products such as corn germ, wheat germ, yeast and soya beans yield approximately the same proportion of protein of amino acids as are supplied by animal proteins. Moreover, it is quite possible to increase the nutritive value of plant proteins and to maintain N balance by diet consisting of a mixture of vegetable proteins because the essential amino acids absent in one such protein may be supplied by the other proteins. One such formula has been developed by INCAP (Institute of Nutrition of Central America and Panama). It contains corn (50%), sesame meal (35%), cotton seed meal (9%), Torula yeast (3%) and Kikuyu grass (3%). It has a biological value equal to that of casein. Another such formula being used in India contains preparation of peanut meal, Bengal gram dhal, black gram dhal, and sesame oil meat. Deodorized fish meal is also added to plant proteins to increase their biological value. In addition, essential amino acids such lysine and methionine have been prepared through fermentation process from waste carbohydrates and are being added to foods of plant origin in order to increase their nutritive values. It has also become possible to synthesize lysine, the amino acid most likely to be low in plant proteins, from coal at the negligible cost of about shillings a lb, which is enough to fortify the food of three children for over a year.
Another approach to solve the problem of good quality of protein shortage is to grow crops which yield a much higher protein content. For example, mutant strains of wheat and maize have been evolved which yield up to twice the amount of protein found in customarily grown varieties. Certain cereals have been developed which yield proteins which are rich in lysine.
The recommended daily dietary allowances of proteins are given as below.


New-born Infant              --             2.2 grams/Kg body weight
Infant, 1 year old              --             1.8 grams/Kg body weight
Children 8—10 years      --             40 grams
Males 14—22 years         --             60 grams
Males 22—75 years         --             65 grams
Female 14—75 years      --             65 grams


It must be noted that the recommended daily dietary allowances of proteins given above are a bit higher than the minimum protein requirements which for an average man is approximately 35 grams per day. Individuals who are engaged in work involving severe muscular exertion need relatively more protein than others.