Nutritional Characteristics of a Substance | Protein Value

The following points highlight the three main nutritional characteristics of a substance. The nutritional characteristics are: 1. Digestibility of Proteins 2. Availability of the Digested Products 3. Presence and Amounts of the Various Essential Amino Acids.

Nutritional Characteristic # 1. Digestibility of Proteins:

It is ex­pressed as the percentage of the food ni­trogen which has been absorbed.

Absorbed N = Food N – (fecal N- metabolic N).

Proteins of animal origin have the highest digestibility (95 to 100 per cent) with the wastage in digestion only 5% or less. The less is the digestibility, the less the bio­logic value.

Nutritional Characteristic # 2.Availability of the Digested Products:

The greater the proportion of amino acids in the dietary protein which can serve for the construction of tissue protein, the greater will be its potential nutritive value. A smaller proportion of the amino acids will then be discarded.

Nutritional Characteristic # 3. Presence and Amounts of the Various Essential Amino Acids:

An essential amino acid is defined as one that cannot be synthesized by the animal organism at a speed commensurate with the demands for normal growth and which must, therefore, be supplied in the diet. There are nine essential amino acids for man. These are: methionine, threonine, tryptophan, valine, leucine, isoleucine, phenylalanine, histidine and lysine.

Zein is almost completely free from tryp­tophan and lysine. Growth is continued if zein is given as diet along with tryptophan and lysine. Gelatin is also lacking tryptophan. Casein lacks methionine. Normal growth will con­tinue if these amino acids are given along with these diets.

A biologically good protein is one which contains all the essential amino acids in proportions not far different from those needed by the body. A much smaller in­take of such a protein is required to pro­duce nitrogen equilibrium than that of a protein which has a limited amount of one or more of the essential amino acids.

The most suitable proteins for growth are those of animal origin and especially those which nature has provided for the nour­ishment of the growing animals, namely, Lactalbumin (of milk), ovalbumin (of hen’s egg), ovovitellin (of hen’s egg). These support growth when given at a level of about 9 or 10 per cent in the diet.

Proteins in the order of biologic value:

Proteins of meat, glutenin (wheat), casein (milk), glutelin (maize), glycinin (soya-bean).

These support growth when given in a higher concentration.

The following vegetable proteins are incapa­ble of supporting growth but are suitable for main­tenance:

Gliadin (wheat), legumin (pea), legumelin (soya bean), hordein (barley).

The following incomplete proteins are unsuit­able for either growth or maintenance.

Zein, gelatin.