What are Proteins?

Protein is found in each cell of our body.  No Protein → No Existence → No Survival. The moment a person makes plans to be healthier, he or she thinks of adding protein in the diet. Let us dwell more into the world of dietary proteins.

Proteins, like carbohydrates and fats, is a macronutrient. They are large, complex molecules that play important roles in our bodies. Proteins are actually long chains of amino acids that are linked together through peptide bonds. In nature there are protein molecules of various sizes depending on the amino acids present in it. The size of the protein molecules and their unique amino acid composition makes them structurally and functionally distinguishable.
Lets dive into the general functions of proteins:

Growth and Development:  The word ‘Growth’  itself implies an increase in number of cells by multiplication and expansion, thereby forming the structure of the body during foetus development, followed by infancy, childhood, and adolescence. Proteins are essential for growh and development in infants, teenagers and pregnant or lactating women. Growth usually ceases by adulthood.

Physical Activity: Physical activity involves actions of our muscles. Movements occuring via contraction and expansion of muscles consumes proteins.  Collagen is another protein that provides strength to our skeletal system which are involved in physical activities.

Maintenance: There is constant wear and tear of the cells in the body while performing myriad biological functions such as breathing by lungs, digestion of food in GI tract, etc. Many cells die and new ones replace them, for which protein is very essential, particularly in adulthood, old age and if there is any injury.

Formation of Enzymes, Hormones, Neurotransmitters, and Antibodies: All the biological reactions occurring in the body need these compounds. Enzymes are specific proteins, that act as catalysts and help in speeding up biological reactions, whereas hormones are our body regulators, made up peptides or polypeptides.

Neurotransmitters are peptides which facilitate the transmission of nerve signals within the body while regulating numerous body functions like mood, appetite, sleep etc., and antibodies are the compounds produced by our immune system to fight infections. Protein is required for the synthesis of the above compounds and is hence highly essential.

Cell differentiation and Cellular Functions: The body is made up of trillions of cells but all cells do not perform the same functions. Proteins also are important for forming different types of cells in the various organs and tissues and plays a significant role in cell development.

Provide Energy: One-gram protein provides 4 kcal. Body uses protein as a source of energy particularly when our carbohydrate intake is insufficient.

Transport of Nutrients: Different proteins confer health and homeostasis in the body. For example, Transferrin transports iron; Haemoglobin assists in carrying oxygen in the blood to all body cells and tissues; Retinol Binding Protein (RBP) that binds and transports vitamin A and lipoproteins signify the concentration of fat in the blood.

Building Blocks of Proteins- Amino Acids

The healthy human body has the capability to synthesize non-essential (dispensable) amino acids whenever they are required in sufficient quantities but aren’t able to synthesize essential (indispensable) amino acids. Therefore, we have to rely on external sources, namely the human diet for this.

It is also very important to ensure that the protein should be of good quality. Protein synthesis in the body depends on three factors (i) the amount and the balance between the different amino acids (ii) the digestibility of the protein and (iii) availability of the amino acids for protein synthesis.

Protein digestibility is the proportion of the protein that has been consumed, which is absorbed after the dietary protein has been digested in the gastrointestinal tract. In mixed vegetarian diet, the protein digestibility is approximately 85%, which is much lower than the reference protein egg, the digestibility of which is 97%.

Inadequate intake of essential (indispensable) amino acids may result in poor appetite, fatigue, slowing down or faltering in growth, stunting (low height for a given age) in children, particularly young children. These are clinical signs of protein malnutrition.

All foods do not contain all the 8 essential (indispensable) amino acids as per our body’s requirements.  However, all are present in egg. Therefore, expert groups throughout the world use regard egg as a “reference protein”. For vegetarians, some amino acids are present in cereals and fruits and vegetables while others are present in pulses, legumes, nuts, oilseeds.

How much protein do we need?

Recently the Indian Council of Medical Research (ICMR) – National Institute of Nutrition (NIN) who gives the recommendations and guidelines for India has published new recommended dietary allowances (RDA) in 2020.

There we can find the recommendations for each subgroup of population, age, gender, physiological status. On an average with a safety margin, 0.83g/kg body weight/per day of protein is recommended for Indian population. (Nutrient Requirements for Indians. (2020).

Adult Man	54 g of protein per day
Adult Woman	45.7 g of protein per day
During Pregnancy	2nd trimester- additional 9.5g of protein per day 3rd trimester- additional 22g of protein per day
0-6 months	8.1 g of protein per day
6-12 months	10.5 g of protein per day
1-3 years	11.3 g of protein per day
4-6 years	15.9 g of protein per day
5-9 years	23.3 g of protein per day
10-12 years	Boys – 31.8 g of protein per day Girls – 32.8 g of protein per day
13-15 years	Boys – 44.9 g of protein per day Girls – 43.2 g of protein per day
16-17 years	Boys – 55.5 g of protein per day Girls – 46.2 g of protein per day

Let’s take a closer look at amino acids:

There are only 20 amino acids, among which 8 are essential and 12 are non-essential or dispensible amino acids. Permutation and combinations of these 20 amino acids results in the myriad proteins that are present in our bodies and responsible for all our life processes.

The Essential (indispensable) Amino Acids:

No.	Essential Amino acid	Foods sources
1.	Phenylalanine – helps in synthesis of neurotransmitters and skin pigment melanin.	Legumes and rice, sesame seeds/til
2.	Tryptophan- helps the body adjust to changes in environment and improves immunity.	Though present in all protein foods, high amounts are found in dairy foods
3.	Methionine- reduces oxidative stree, improves DNA sysnthesis and immunity.	Cereals and millets
4.	Lysine- helps in growth and tissue repair as well as increases calcium absorption.	Kidney bean, chickpea, soy, potato, pea, avocado, pumpkin seeds,
5.	Leucine- source of energy for muscle cells and Shows growth-stimulating effects of insulin	Jowar, Pulses, Soybean and dairy product
6.	Isoleucine- source of energy, stimulates protein synthesis and reduces plasma glucose levels.	Soy, cashew nuts, cereals, meat, egg, milk and piyal seeds.
7.	Valine- helps in muscle synthesis an absorption of amino acids.	Rice, beans, legumes, dairy and mushrooms
8.	Threonine- helps in fat metabolism and collagen formation	Sesame seeds, dairy products
9.	Histidine- helps synthesis hemoglobin, reduces oxidative stress and is good for growth and repair.	Legumes and buckwheat

The Non-Essential or Dispensible Amino Acids:

• Glycine: This simple amino acid repairs cell damage and delays aging by reducing oxidative stress. It is needed for synthesis of proteins like heme, a part of hemoglobin that is present in blood. It is present in legumes and dairy products.
• Alanine: Alanine is important for glucose metabolism, release of energy from glucose, protein breakdown and helps in production of antibodies for better immunity. It supports utilization and synthesis of B vitamins.
• Serine: This amino acid helps break down of proteins during digestion process, aids in fat and protein syntheis and production of glucose by the body. It is important for formation of several amino acids and is present in soybean, peanuts, almonds, walnuts, lentil and chickpea.
• Cysteine: Cysteine is a powerful antioxidant. It protects the body by binding with heavy metals. It is essential for low birth weight infants and hence it is considered to be conditionally essential. It is also important in liver disease.  Good sources include wheat, rice, pea, chickpea, sunflower seeds.
• Tyrosine: It is needed for synthesis of neurotransmitters that are important for the nervous system. It helps in boosting memory, alertness in stress and for production of proteins that are natural pain relievers and in the treatment of depression and anxiety. It is present in all food protein sources particularly soybean, peas, beans, etc.
• Glutamate or Glutamic acid: It plays a role in energy production and is especially important for the body’s response during infections, inflammation, and muscle trauma. Glutamate is a component of one of the popular flavour-enhancing ingredient monosodium glutamate (MSG) or Ajinimoto. It helps to maintain the integrity of the gastrointestinal tract. It is found in seeds, beans and buck wheat.
• Glutamine: Glutamine maintains acid base balance and helps in removing waste products like ammonia. Under physical or physiological (in illness) stress, or when body feels depleted, glutamine needs replacement. Plant based sources can be almonds, peanuts, hazelnuts and beans including soybean. It plays very important roles in energy and protein metabolism.
• Arginine: This conditionally essential amino acid has an important role in urea formation and in releasing hormones like growth hormone, insulin, glucagon, etc. It is important for immunity and is used to synthesize glutamine by the body. It plays a role in wound healing, immune functions and removing ammonia from the body. It is important during the growing years and in patients with burns. It is also involved in regulating immune system. Good sources of this amino acid are rice, oats, pulses/beans, soy, etc.
• Proline: Proline is a part of collagen, and is needed to maintain texture of the skin, wound healing, preventing loss of collagen in old age and also strengthens the cardiac muscles. It plays a significant role in antioxidant functions. It is important in growth and development and assumes importance for foetal and neonatal growth. In human colostrum, proline has antiviral activity. It is part of proteins that play a role in neurodegenerative diseases like Alzheimer’s and Parkinson’s diseases. Proline is present in all foods that are plant sources of protein like, cabbage, green leafy vegetables, and whole grains, pumpkin seeds, sesame seeds, etc.
• Aspartic acid: It aids in synthesis of compounds that are part of DNA and RNA. Like all other amino acids it is also needed for protein synthesis. Its role is linked with liver enzymes and bile acids, is needed for urea formation (waste product of protein and nitrogen metabolism that is excreted in urine) and in energy metabolism. It is present in all legumes including soy, rice, etc. Aspartic acid is also a source for energy and helps us to overcome fatigue.
• Asparagine: It is crucial in formation of many nitrogenous compunds in the body. Asparagine if combined by reducing sugar at sufficiently high temperature forming acrylamide which is toxic to health. Food sources are potato, legumes, nuts, seeds, soy, asparagus, and whole grains.

Conequences of High Protein Intake

High protein intake (more than 2g per Kg/day) is recommended for high performance elite athletes but not advisable for healthy adults. People who are looking to enhance their performance build stamina or loose body weight often indulge in high protein diet either from dietary sources or protein supplements. High protein intake may increase the nitrogen load in the body and tax the kidney to detoxify ammonia.

Excess of protein intake may show initial symptoms dehydration, fatigue, intestinal discomfort, nausea and weight gain, and after prolonged period of time it may  adversely affect the vital organs like liver, heart and kidney. It may cause increased loss of calcium from bones causing bone related issues.

Adverse effects are more severe when carbohydrate intake is low like in keto diet owing to excessive production of urea and ammonia in the body which may not be released from the body. Hence persons with renal dysfunction and gout are advised to use selective protein sources in limits quantity.

High intake from meat based diet is not healthy over plant based protein foods and may cause certain health issues over a period of time. Undoubetedly animal protein provides all the essential acids and complete protein which is essential for growth but excess of them may lead to inflammation.

Additionally heme iron in meat, may  produce more free radicals that tend to damage the structure of the cell. Animal protein rich foods lack dietary fiber and are rich in saturated fats thus facilitate cholesterol and increase the risk of cardiovascular diseases. On contrary plant protein may lack some of the essential amino acids but contain more nutrients including fiber, carbohydrates, vitamins and health promoting phytochemicals.