Proteins are complex organic nitrogenous compounds. They are composed of carbon, hydrogen, oxygen, nitrogen and sulphur in varying amounts. Some proteins also contain phosphorus and iron and occasionally other elements. Proteins differ from carbohydrates and fats in that they contain nitrogen, this usually amounts to about 16 percent. Proteins constitute about 20 per cent of the body weight in an adult. Proteins are made up of smaller units, called amino acids. Some 20 amino acids are stated to be needed by the human body, of which 9 are called "essential" because the body cannot synthesize them in amounts corresponding to its needs, and therefore, they must be obtained from dietary proteins. They are: leucine, isoleucine, lysine, methionine, phenylalanine, threonine, valine, tryptophan, and histidine.
Evidence is now accumulating that histidine is essential even for adults. Non-essential amino acids include arginine, asparagine acid, serine, glutamic acid, praline and glycine. Both essential and non-essential amino acids are needed for the synthesis of tissue proteins, the former must be supplied through diet, whereas the latter can be synthesized by the body provided other building blocks are present. Some of the essential amino acids have important biological functions, e.g., formation of niacin from tryptophan; the action of methionine as a donor of methyl groups for the synthesis of choline, folates, and nucleic acids. There is evidence that cysteine and tyrosine are essential for premature babies. New tissues cannot be formed unless all the essential amino acids (EAA) are present in the diet. A protein is said to be "biologically complete" if it contains all the EAA in amounts corresponding to human needs. When one or more of the EAA are lacking, the protein is said to be "biologically incomplete". The quality of dietary protein is closely related to its pattern of amino acids. From the nutritional standpoint, animal proteins are rated superior to vegetable proteins because they are "biologically complete".
Proteins are needed by the body for bodybuilding; - this component is small compared with the maintenance component, except in the very young child and infant, repair and maintenance of body tissues, maintenance of osmotic pressure, and synthesis of certain substances like antibodies, plasma proteins, hemoglobin, enzymes, hormones, and coagulation factors. Proteins are connected with the immune mechanism of the body. The cell-mediated immune response and the bactericidal activities of leucocyte have been found to be lowered in severe forms of protein-energy malnutrition. Proteins can also supply energy (4 kcal per one gram) when the calorie intake is inadequate, but this is not their primary function. It is considered wasteful if proteins were used for such a purpose.
Humans obtain protein from two main dietary sources. Proteins of animal origin are found in milk, meat, eggs, cheese, fish, and fowl. These proteins contain all the essential amino acids (EAA) in adequate amounts. Egg proteins are considered to be the best among food proteins because of their high biological value and digestibility. They are used in nutrition studies as a "reference protein". Vegetable proteins are found in pulses (legumes), cereals, beans, nuts, oil-seed cakes, etc. They are poor in EAA. In developing countries, cereals and pulses are the main sources of dietary protein because they are cheap, easily available, and consumed in bulk. Cereal proteins are deficient in lysine and threonine; and pulse proteins in methionine. These are known as "limiting" amino acids. With proper planning, it is possible for a vegetarian to obtain a high-grade protein, at low cost, from mixed diets of cereals, pulses, and vegetables. This is known as the supplementary action of proteins and is the basis of counseling people to eat mixed diets.
Since proteins are not stored in the human body in the way that energy is stored in adipose tissue, they have to be replaced every day, the body proteins are constantly being broken down into their constituent amino acids and then reused for protein synthesis. The rates of turnover vary from tissue to tissue. The reutilization of amino acids is a major contributory factor to the economy of protein metabolism. The overall rate of turnover in adult man is equivalent to replacement between 1-2 percent of body protein each day; it is not only the amount of protein that is maintained constant but also the pattern of specific protein in the body. For maximum utilization of dietary proteins, the calorie intake should be adequate.
Knowledge of the amino acid content of protein is not sufficient for the evaluation of protein quality. Information is also required about the digestibility and suitability to meet the protein needs of the body. The parameters used for such an evaluation include the estimation of the biological value, digestibility coefficient, protein efficiency ratio, and net protein utilization. The net protein utilization (NPU) is considered of more practical value because it is the product of biological value and digestibility coefficient divided by 100. In exact terms, it is the "proportion of ingested protein that is retained in the body under specified conditions for the maintenance and/or growth of the tissues. In other words, growth is an important yardstick for ascertaining the essentiality of a nutrient.