Protein is a general name of chemical compounds formed by combining the basic building blocks called amino acids in a highly varied and complex chemical manner. Even though proteins can be utilized by the body for energy production (1 gram of protein is equivalent to 4Calories or16.8 kilojoules), their role in our body is far more important than that. When proteins are mentioned, generally emphasis is given on the amino acid content rather than the calories. If you regard carbohydrates or fat as fuel, it’s ok, but same is not true for protein. We buy potatoes by weight, but not a microprocessor, for example! The role protein plays is very important in the body, which cannot be replaced by any other macronutrient, such as fat or carbohydrate. Likewise a protein’s amino acid content is even more important than the total amount of protein.
Amino acids are the building blocks of proteins. There are about 23 amino acids in all. Proteins have different combinations of these amino acids. There are essential and non-essential amino acids. Essential amino acids can not be synthesized by the body and should be consumed through food. Non-essential amino acids can be synthesized by the body from essential amino acids by a process called trans-amination. There are grades of protein based on the amino acid content and their proportion with respect to one another. The essential amino acids are: histidine, tryptophan, leucine, isoleucine, lysine, methionine, valine, phenylalanine and threonine.
Proteins differ in their ability to support growth, repair and maintenance of tissues in the body. Their quality is based on their ability to fulfill these purposes. Those proteins which mostly qualify on these criteria are of highest quality. Proteins are rated numerically for their quality by their so called biological value. Generally it is observed that protein quality depends on their food source.
Proteins derived from animal sources are regarded as the best quality proteins (highest biological value) than their plant based counterparts. They contain all essential amino acids. They can support growth and life.
Plant based proteins have lower proportions of some amino acids. This deficiency is not same for all plant sources. For example Cereals have lower proportions of some amino acids while pulses and legumes have higher proportions of these ones. In short, these two groups of grains (cereals and pulses) have amino acid contents which are complimentary to each other. Therefore combination of food articles made from these two groups will result in a protein of better quality. If you happen to be a pure vegetarian, then you should note this fact and apply always in daily life.
Those proteins that can support life but can’t support growth have intermediate biological value. Examples are proteins found in pulses, legumes, cereals, lentils, nuts etc.
Proteins that can support neither life nor growth are lowest quality proteins. Gelatin is an example of this. Animals will die if they are fed gelatin as the only source of protein.