What is protein?
Proteins are biomolecules made up of nitrogen-containing amino acids. It is one of the essential macronutrients for the development of the human body. Proteins play a major role in the synthesis of muscles, tissues, hormones, enzymes and haemoglobin. In addition to this, they serve as the energy source but not as the prime energy source. Proteins are composed of 20 amino acids among them 9 are essential and remaining are non-essential. Essential amino acids are named so because they are the ones that are not synthesised by our body and have to be consumed from external sources. While non-essential ones can be synthesised by our body. The deficiency of any of the amino acids can result in an improper development of the body i.e. the inability of any tissue to grow, repaired or maintained.
Sources of proteins
Proteins can be obtained from either plant sources or animal sources. Plant-source is considered as an incomplete protein as they lack in one or more essential amino acids. For example, cereals are extremely low in lysine but contains other amino acids like tryptophan, methionine and cysteine in a generous amount. On the other hand, legumes are deficient in methionine but high in lysine. So, this deficiency of amino acids can be overcome by consuming cereals-legumes combination as they complement each other and proffer you all the essential amino acids.
The best source of good quality proteins are animals. As animal source (like milk, white meat, fish, egg etc) provides you all the essential amino acids that are required for the human development, muscle growth, protein muscle synthesis, repairing of damaged tissues, synthesis of neurotransmitters, hormones, enzymes, etc. So, vegan people can consume milk along with cereal-legumes to accomplish their protein requirement.
Protein Requirement
The protein requirement of a healthy individual varies according to their sex, age, physical activity, source, etc. In Nutrient Requirement and Recommended Dietary Allowances For Indians (2010) it was mentioned that in children adequate dietary protein is essential for the synthesis of new tissues while in adults, dietary protein is important for the repairing of tissues that are broken down during exercise. During pregnancy, additional dietary protein is required to meet the increased demand of growing fetus. During lactation, extra dietary protein is essential for the synthesis of proteins found in breast milk. As per the physical activity, dietary protein requirement of a sedentary worker is different from that of an athlete’s as they need more of proteins compared to a sedentary worker. According to Kato H et al.(2016) protein requirement of strength/power athletes is higher than a healthy non-exercising individual to maintain the positive nitrogen balance. The increased protein requirement is essential for muscle protein synthesis, repairing and remodelling of skeletal muscle fibres that are damaged during a resistance training session. In addition, high protein diet is continuously gaining popularity among weight watchers because of its beneficial effects. But we all know that, the excess of anything is hazardous. Therefore, protein evaluation has become necessary to determine its appropriateness in the human diet. As all the food components in our diet do not provide the same quality and quantity of proteins. Like proteins found in vegetables (cereals, legumes, etc.) are limited to one or more essential amino acids due to which it is also known as incomplete protein. While protein present in animals is considered a complete protein because of its amino acid profile. An important factor for calculating the human protein requirement depends upon the digestibility and absorption of the released amino acids. For this purpose eggs in particular serve as the reference protein to judge the quality of the other proteins. There are various methods that we use to estimate the protein requirement.
Timings of protein consumption
The timing of protein consumption appears to play a major role in accomplishing the goal by positive training adaptation. The ideal time for protein intake is immediately after the training session. Therefore, it is important to consume a high-quality protein diet immediately after an intense workout to promote the tissue protein synthesis, repairing action, prevent fatigue, etc. Besides this, adequate calorie intake according to physical activity is also crucial in order to spare the amino acids for muscle protein synthesis.
Class of Proteins and Their function in the body
“9 Important Functions of Protein in Your Body” states some of the crucial functions performed by proteins in the human body are :
- Structural proteins: They provide structural strength such as collagen and keratin protein. Collagen protein is found in different parts of the body like muscles, tendons, ligaments and bones. Collagen and elastin both are present together in the lungs, walls of blood vessels and skin where collagen fibres provide strength while elastin fibre provide flexibility. Keratin is another structural protein which is present in our skin, hair and nails.
- Transport Proteins: They help in transportation of essential substances like oxygen, nutrients, etc., throughout the body. These include haemoglobin, plasma, lipoproteins. Red blood cells present in haemoglobin imparts the red colour to the blood cells and aids in the transportation of oxygen from lungs to the body tissues. Plasma contains a protein called fibrinogen which favours blood clotting. Albumin is another protein found in plasma that maintains the fluid balance in the blood vessels and it also transfers bilirubin to the liver.
- Storage: As the name suggests they store nutrients in our body. Actin, myosin, myoglobin, ferritin and casein are some of the storage proteins. Actin and Myosin are muscle filaments which aid in muscle contraction. Myoglobin is a red pigment found in muscles where it binds to oxygen and releases oxygen whenever muscle cells require. It is the only protein that has the efficiency to store oxygen and abundantly found in diving animals. Ferritin is a protein that stores iron in the skeletal muscle, liver, spleen, bone marrow and in other regions of the body. Blood also contains ferritin in a small amount.
- Hormones: Amino acids or peptides helps form the hormones which aid in regulating the metabolism of the body and nervous system. These hormones also act as a messenger which transmits information throughout the body. For example, Human Chorionic Gonadotropin (HCG) is produced during the early pregnancy by embryo and placenta where it supports the pregnancy by maintaining the correct level of estrogen and progesterone in women’s body. Erythropoietin is synthesised by the kidney and it stimulates the production of red blood cells in the bone marrow. Insulin is a polypeptide that regulates blood glucose level.
- Enzymes: It is a biological catalyst which catalyses all the biochemical reactions in the cells. These involve trypsin (hydrolyzed proteins), pepsin, lipase (breakdown lipids), salivary amylase, etc.
- Protection: The immune system of our body is composed of proteins that protect us from various diseases. For example white blood cells, immunoglobulins, T-lymphocytes which recognises and destroys foreign substances.
Evaluation of protein quality
The proteins obtained from daily diet varies in their composition and has a direct influence on the human physiological growth. Therefore, the main objective while assessing a protein is to determine the ability of food protein sources to meet the human protein requirements. Moreover, an enhanced knowledge of protein quality of human food aids in reducing the protein malnutrition, prevent childhood stunting, anaemia, etc. The protein quality can be determined by assessing the profile of essential amino acids, digestibility and bioavailability of amino acids (FAO/WHO,1989). So, there are various fundamental approaches used to evaluate the protein quality.
Protein Efficiency Ratio
The protein efficiency ratio (PER) helps asses the efficacy of protein by measuring the weight gained by an animal in grams per gram of protein consumed. The limitation of this approach is that the calculation does not show a strong association with the growing needs of the human body.
Biological Value
It determines the protein quality by measuring the amino acids utilised for protein synthesis divided by the amino acids absorbed from food. The biological value shows how efficiently body utilises protein ingested through the diet. The high biological value of food correlated with the increased supply of all indispensable amino acids. The problem associated with this rating system is that it does not account several key factors which affect the protein digestibility and interference with other food prior to consumption.
Protein Digestibility Corrected Amino Acid Score (PDCAAS)
PDCAAS measures the concentration of limiting amino acids in the test protein with the concentration of same amino acid in the reference protein. The PDCAAS value estimates the overall efficacy of protein utilisation on the basis of protein digestibility and its biological value (BV) where BV is directly correlated to its amino acids score. The PDCAAS values of Cereal-Legume-milk based diet is 82.5%
Protein deficiency
Bauman College Wellness Program Lean Protein and Clean Fats (2011) conveys that the protein deficiency occurs when our daily diet is inadequate in providing all the essential amino acids required for human development. Protein malnutrition is common throughout the world and children are most vulnerable to it as protein deficiency at this stage affects their growth and development.
Symptoms of protein deficiency:
- Loss of muscle tone
- Slows down the wound healing process
- Retention of fluid
- Fatigue and muscle weakness
- Weight loss
High Protein Diet
High protein diet or protein supplementation is usually consumed by athletes or by fitness enthusiast as their physical activity level is higher than the sedentary worker. Due to which athletes require protein in a higher amount than what is recommended by RDA for repairing of damaged muscle fibres, muscle protein synthesis and many more. St Jeor ST (2001) suggests that high-protein diets are not usually recommended as they impede the inclusion of healthy foods that give indispensable nutrients. Moreover, a diet high in proteins does not proffer the variety of foods required to meet the nutritional demands. A person who includes a lot of protein-rich foods in their meal is at higher risk of developing potential cardiac, renal, bone and hepatic abnormalities. Besides that, high protein diet also increases the urinary excretion of calcium and may even lead to dehydration.
Conclusion
Protein is an essential nutrient that should be consumed as per the requirement of the body. We should know about the constituents of what we eat. A balanced diet is the key to a healthy life.
References
- Bauman College Wellness Program Lean Protein and Clean Fats (2011). Retrieved from https://slideplayer.com/slide/6184421/
- Food and Agriculture Organization/World Health Organization (1989) Protein quality evaluation; report of the joint FAO/WHO expert consultation. FAO Food and Nutrition Paper 51, Rome, Italy
- Kato, H., Suzuki, K., Bannai M., and Moore. D.R. (2016). Protein Requirements Are Elevated in Endurance Athletes after Exercise as Determined by the Indicator Amino Acid Oxidation Method. Public Library of Science, 11(6): e0157406. https://doi.org/10.1371/journal.pone.0157406
- St. Jeor, ST., Howard, BV., Prewitt, T.E., Bovee, V., Bazzarre, T., and Eckel, R.H. (2001) Dietary protein and weight reduction: a statement for healthcare professionals from the Nutrition Committee of the Council on Nutrition, Physical Activity, and Metabolism of the American Heart Association. Nutrition Committee of the Council on Nutrition, Physical Activity, and Metabolism of the American Heart Associatio, 104(15):1869-74
- Nutrient Requirements and Recommended Dietary Allowances for Indians. (2010). New Delhi: Indian Council of Medical Research
- 9 Important Functions of Protein in Your Body(n.d.).Retrieved from https://www.healthline.com/nutrition/functions-of-protein#section3