Proteins

Introduction

• The word “protein” by derivation means that which is of first importance.
• Indeed they are of the greatest importance in human nutrition.
• 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 per cent.

Proteins constitute about 20 per cent of the body weight in an adult.

Essential vs Non-essential amino acids

Essential amino acids

• Proteins are made up of smaller units, called amino acids.
• Some 20 amino acids are stated to be needed by the human body,
• Of 20 which 9 are called “essential”
  • because the body cannot synthesize them in amounts corresponding to its needs,
  • they must be obtained from dietary proteins.
• They are : [[TV MILL PTH]]
  • threonine,
  • valine,
  • methionine,
  • isoleucine,
  • leucine,
  • lysine,
  • phenylalanine,
  • tryptophan
  • histidine.

Evidence is now accumulating that histidine is essential even for adults (6).

Non-essential amino acids

• These include [[PG SAAG]]
  • proline
  • glycine.
  • serine,
  • arginine,
  • asparaginic acid,
  • glutamic acid,
• Both essential and non-essential amino acids are needed for synthesis of tissue proteins,
  • the former must be supplied through diet,
  • 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
      • nucleic acids.
  • There is evidence that cystine and tyrosine are essential for premature babies (7).
• New tissues cannot be formed unless all the essential amino acids (EAA) are present in the diet.
• The quality of dietary protein is closely related to its pattern of amino acids.
• Biologically complete vs incomplete
  • 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”.
    • From the nutritional standpoint, animal proteins are rated superior to vegetable proteins because they are “biologically complete”.

For example milk and egg proteins have a pattern of amino acids considered most suitable for humans.

Functions

• Proteins are needed by the body for
  • (a) body building, – this component is small compared with the maintenance component, except in the very young child and infant;
  • (b) repair and maintenance of body tissues;
  • (c) maintenance of osmotic pressure; and
  • (d) synthesis of certain substances like
    • antibodies,
    • plasma proteins,
    • haemoglobin,
    • enzymes,
    • hormones
    • coagulation factors.
• Proteins are connected with the immune mechanism of the body.
  • The cell mediated immune response and the bactericidal activity 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.

Sources

• Humans obtain protein from two main dietary sources;
  • (a) ANIMAL 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”,
  • (b) VEGETABLE SOURCES :
    • Vegetable proteins are found in pulses (legumes), cereals, beans, nuts, oil-seed cakes, etc.
    • They are poor in EAA.
    • In developing countries such as India, cereals and pulses are the main sources of dietary protein because they are cheap, easily available and consumed in bulk.

Protein content of some foods are as given in Table ![[Pasted image 20221020101943.png]]

Supplementary action of proteins

• Man derives protein not from a single source, but from a variety of food sources, animal and vegetable.
• limiting amino acids.
  • Cereal proteins are deficient in lysine and threonine; and
  • pulse proteins are deficient in methionine.
  • These are known as limiting amino acids.
• When two or more of vegetarian foods are eaten together (as for example, rice-dhal combination in India) their proteins supplement one another and provide a protein comparable to animal protein in respect of EAA.
• Thus 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 supplementary action of proteins, and is the basis of counselling people to eat mixed diets.

Protein metabolism

• There are three features of protein metabolism :
  • (a) 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;
  • (b) 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 (6).
    • The overall rate of turnover in adult man is equivalent to replacement between 1-2 per cent of body protein each day (7);
  • (c) it is not only the amount of protein that is maintained constant, but also the pattern of specific protein in body.

For maximum utilization of dietary proteins, the calorie intake should be adequate.

Evaluation of proteins

• A 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
  • net protein utilization (NPU).
    • Def, 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’
    • The net protein utilization (NPU) is considered of more practical value
    • it is the product of biological value and digestibility coefficient divided by 100 .

. In other words, growth is an important yardstick for ascertaining the essentiality of a nutrient.

Assessment of protein nutrition status

• A battery of tests have been suggested to assess the state of protein nutrition.
• These include :
  • arm muscle circumference,
  • the creatinine – height index,
  • serum albumin and transferrin,
    • Serum albumin and transferrin assess the ability of the liver to synthesize proteins.
  • total body nitrogen, etc.
  • At the present time the best measure of the state of protein nutrition is probably serum albumin concentration.
    • Normal should be more than 3.5 g/dl,
    • a level of 3.5 g/dl is considered mild degree of malnutrition;

a level of 3.0 g/dl severe malnutrition.

Protein requirements

• it is customary to express protein requirements in terms of body weight.
• The Indian Council of Medical Research in 2020 (11) recommended 0.83 g protein/kg body weight per day as a safe intake for an Indian adult, assuming a median obligatory nitrogen loss of 48 mg/kg body weight.
• Daily allowances recommended by the ICMR for various population groups are as given in Table 28.![[Pasted image 20221020103857.png]]