Proteins are the building blocks of life and every cell in your body contains protein. Protein is the glue that holds each cell in the body together, from your hair to your toenails, from your muscles and organs to the bones in your body. You need protein in your diet to assist your body to repair cells and make new ones. It is important to get enough protein in your daily diet, as the body doesn’t store it the way it stores carbohydrates or fats.
What is protein?
Protein is an essential macronutrient and is made from basic building blocks called amino acids. Put differently, proteins are macromolecules composed of amino acid subunits. The body needs 20 different amino acids to function correctly, and these 20 amino acids combine in different ways to make the many different proteins in your body. Because the body doesn’t store amino acids, the body either makes some of them from scratch or by modifying others. However, the body cannot make nine of the amino acids, known as essential amino acids, as they must come from the food you eat.
The basic structure of a single protein is a chain of individual amino acids, linked together in a special way, referred to as its unique sequence. The body has thousands of different proteins, each with its own unique sequence of amino acids and with different functions in the body.
Depending on the amino acid sequence, proteins have different shapes and molecular weights. For example, hemoglobin in blood is referred to as a globular protein, as it folds into a globe-like structure, while collagen in the skin is referred to as a fibrous protein due to it being folded into a long fiber-like chain. As form determines function, any slight change in shape may result in the protein becoming dysfunctional and small changes in the amino acid sequence can cause devastating genetic diseases.
Interestingly, you probably look similar to family members as you share similar proteins, but you look different from strangers because the proteins in your eyes, hair, and the rest of your body are different.
The thousands of proteins in the cells in the body must come from somewhere, especially the essential amino acids that the body can’t make, and then the dietary protein from the food we eat comes into play. Once eaten and digested, dietary protein is broken down into its individual amino acids, which are then absorbed into the blood stream and transported around the body. These amino acids are then used to make all the various proteins that the cells in the body need. Twenty different naturally occurring amino acids are required by the body for this process.
The different types of amino acids and the way they are put together determine the function of each protein and it can be compared to the letters of the alphabet. When letters are combined in various ways, different words are made. When amino acids are combined in different ways, different proteins are made.
The nine essential amino acids and some of the roles, each plays:
- Histidine helps to make a brain neurotransmitter called histamine, which plays an important role in the immune function, sleep, and sexual function.
- Isoleucine is involved with muscle metabolism and immune function, as well as helping to make hemoglobin and to regulate energy.
- Leucine helps to make protein and growth hormones, while also helping to grow and repair muscle tissue, heal wounds, and regulate blood sugar levels.
- Lysine is involved in the production of hormones and energy and is important for calcium and immune function.
- Methionine helps with tissue growth, detoxification, and metabolism. It also helps with the absorption of essential minerals, including zinc and selenium.
- Phenylalanine is needed for the production of chemical messengers in the brain, such as dopamine, epinephrine, and norepinephrine, while also playing a role in the production of other amino acids.
- Threonine plays an important role in collagen and elastin, which are proteins that provide structure to skin and connective tissue. They also help with forming blood clots, which help to prevent bleeding. It also plays a role in immune function and fat metabolism.
- Tryptophan helps withmaintaining the correct nitrogen balance and helps with making serotonin, a brain neurotransmitter which regulates mood, appetite, and sleep.
- Valine plays a role in muscle growth, tissue regeneration, and the production of energy.
Sources of dietary protein:
High quality dietary protein, also known as complete protein, contains all the essential and non-essential amino acids and in the right ratio as needed by the body to make new protein. Lower quality protein, also known as incomplete protein, lacks one or more essential amino acids. Complete proteins are of animal origin, while incomplete proteins are sourced from plants and lack one or more of the essential amino acids.
Sources of complete protein include meat, poultry, fish, milk, and eggs. Although protein from plants is incomplete in terms of essential amino acids, eating a variety of plant foods such as vegetables, fruit, and grains makes it possible to obtain all the essential amino acids. Grains, nuts, and legumes are good sources of plant protein.
All plants are deficient in one or more of the essential amino acids and the most critical of these are methionine (tissue growth, detoxification, and metabolism) which occurs limited in legumes, lysine (production of hormones and energy) which occurs limited in grains, and leucine (vital for muscle repair) which is also limited in grain products.
Milk alternatives such as oat milk or almond milk are low in lysine and don’t provide the same high-quality, balanced protein as real milk.
The source of the protein, rather than the amount of protein, makes the difference to our health, because foods contain a lot more than just protein. For example, steak is an excellent source of protein, but contains saturated fat (best to opt for lean meat), ham has less saturated fat, but is loaded with sodium (salt), while fish is naturally low in sodium and saturated fat, while cooked lentils provide less protein, but have fiber and virtually no saturated fat or sodium. Eating healthy protein sources such as fish, poultry, beans, and nuts instead of red meat (eat in moderation) and processed meat (best to avoid) can lower the risk of several diseases such as heart disease.
It is the protein “package” that makes a difference to our health, depending on what it contains apart from protein, such as saturated fat, fiber, sodium, vitamins and minerals, or the healthy omega-3 fats in oily fish such as salmon. Both animal and plant products are excellent sources of vitamins, for example vitamin B12 from meat and folate from leafy green vegetables, while animal products generally provide more minerals than plant products.
Plant protein would have to come from a variety of sources to ensure all the essential components of protein are included and should be mixed and matched from various sources high in protein:
- Legumes: Lentils, beans, chickpeas, peas, soybeans, and peanuts.
- Nuts and seeds: Almonds, pistachios, cashews, walnuts, hazelnuts, pecans, pumpkin seeds, sunflower seeds, sesame seeds, and chia seeds.
- Whole grains: Wheat, quinoa, rice, wild rice, oats, and buckwheat.
- Other plant sources: While fruits and vegetables are generally low in protein, some have higher protein quantities such as brussel sprouts, broccoli, corn, and artichokes.
Animal protein are complete sources of protein, but the protein “package” needs to be considered when it comes to animal-based foods.
- Good choices are poultry, seafood, and eggs.
- Dairy foods should be taken in moderation and yogurt is probably a better choice than getting all your servings from milk or cheese.
- Red meat such as beef, pork, lamb, mutton, and goat meat are rich in complete proteins, but should be consumed in moderation due to the saturated fat content.
- Processed meats such as bacon, sausages, and cold cuts should best be avoided, due to high sodium and other preservatives. (Processed meat refers to any meat that has undergone processes such as salting, curing, smoking, fermentation, or any other processes to add flavour or improve preservation.)
Animal-source foods contain higher quantities and more balanced proportions of amino acids which are more related to human tissues than plant-sourced foods, says the Royal Society of Chemistry and gives the example of beef containing 63-68 % protein on the dry matter basis, while most staple foods of plant origin have a protein content of 12% or less on the dry matter basis, and are deficient in most amino acids. Meat consumption can substantially reduce the need for plant-based foods to meet adequate amino acid requirements. (Dry matter of animal and plant material consists of all its constituents excluding water.)
Meat is also a rich source of taurine and carnosine, which plants do not contain. Taurine is a sulfur containing amino acid that is essential for protecting the eyes, heart, muscle, and other tissues from oxidative damage and degeneration. Carnosine is an antioxidant that helps to maintain neurological and muscular functions.
Animal protein has a higher digestibility than proteins in whole plant foods. Evidence has also shown that animal protein has a greater nutritional value than plant protein to sustain skeletal-muscle mass, which is vitally important in aging adults.
Distribution of proteins in the body:
Proteins are found in all cells in the body where they form part of the cell membrane and the inside of every cell. Due to its large mass, about 65% of the total body protein is found in the skeletal muscle. A single brain cell contains around 10% protein, while red blood cells and muscle cells each contain about 20% protein.
Functions of proteins in the body:
Different types of proteins have different functions, for example the most abundant protein in red blood cells is globin, which is part of hemoglobin that gives blood the characteristic red colour.
Hemoglobin is composed of four protein subunits and as part of the respiratory system, transports oxygen from the lungs to the cells, for use in cellular metabolism.
Other proteins in the blood plasma and lymph carry nutrients and metabolic waste products throughout the body.
Nucleoproteins are found in the nucleus of cells, and they are responsible for the transport of genetic material within the nucleus of the cell as well as the assembly and stability of the nuclear lumina (inside space). They also play a role in the control of protein synthesis in cells from the 20 amino acids.
The protein keratin forms the structural support for the cells that become hair and nails.
A protein called collagen is found in abundance in bones, tendons, and ligaments.
Proteins called actin and myosin allow muscles to contract.
The protein albumin is involved with the nourishment of an embryo.
Special proteins in the blood stream clot the blood when needed and prevent clot formation the rest of the time.
Globular proteins in cells are called enzymes. Enzymes are proteins that act as a catalyst in thousands of biochemical reactions in cells in the body and regulate the rate at which these reactions proceed. The two basic classes of enzymes are catabolic enzymes, which break down their substrate, and anabolic enzymes, which build more complex larger molecules from their substrates. Substrates are substances that enzymes bind and convert to a different product.
Some proteins function as hormones, which are used for cell signaling and communication. These chemical-signaling molecules are secreted by cells in the endocrine system, that act to control or regulate certain physiological processes in the body. One such example is insulin, a protein hormone that helps to regulate blood glucose levels. Not all hormones are proteins, for example estrogen and testosterone are lipid steroids and not proteins.
Protein deficiency:
The effects of protein deficiency already start with inadequate protein intake during gestation and postnatal periods, when this nutritional problem may not only result in impaired growth of fetuses and infants, but also increase the risk of metabolic syndrome with symptoms such as hypertension, obesity, and diabetes as adults.
Among elderly people, protein undernutrition will compromise skeletal-muscle functions and worsen sarcopenia, the age-related progressive loss of muscle mass and strength. Studies have shown that consumption of animal protein of least 65% of total dietary protein can prevent protein deficiency in the elderly. The findings from studies have indicated that plant proteins alone may not be adequate for maximal growth in infants and children, or for optimal health in adults.
Conclusion
:
Protein functions in every cell in our bodies, and it is important to get enough dietary protein, not just at dinner but to eat protein rich foods throughout the day. Adequate consumption of high-quality protein has been proven to be essential for optimal growth, development, and health.
An ideal human diet would consist of both animal protein and plant-based protein in appropriate amounts and proportions, to ensure the intake of sufficient quantity and quality proteins for a balanced provision of dietary amino acids.
References:
Types and functions of proteins. Published online. Libre Texts. Biology. (www.bio.libretexts.org)
Amino acids. Published online and reviewed 22 December 2021. Cleveland Clinic. USA. (A nonprofit multi-specialty medical centre that integrates clinical and hospital care with research and education.) (www.clevelandclinic.org)
Protein in diet. Published online and reviewed 13 April 2023. Medline Plus. National Library of Medicine. National Institutes of Health. USA. (www.medlineplus.gov)
Dietary protein intake and human health. Published 11 January 2016. The Royal Society of Chemistry. (www.pubs.rsc.org)
Protein. Published online. Harvard School of Public Health. Harvard Medical School. (www.hsph.harvard.edu)
How much protein do you need to eat every day? Published online and updated 15 September 2023. Eating Well. (www.eatingwell.com)
Protein 101: Why we need more high-quality protein as we age. Iowa Farm Bureau. USA. (iowafarmbureau.com)
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