Protein is one of the essential macronutrients. Most people recognize protein for its role in energy and muscle building, but its many functions throughout the body are often overlooked. In fact, protein is second only to water as a major component of the human body.(1) The word protein comes from the Greek proteios, meaning “first” or “primary,” which is fitting given how fundamental it is to human health.(2)
What Are Proteins?
Proteins are polypeptides made of amino acids linked together by peptide bonds. The human body uses 20 amino acids to build proteins. Of these, 9 are essential amino acids, meaning they must come from the diet. The remaining amino acids are considered non-essential because the body can synthesize them.
A single protein may contain anywhere from roughly 50 to 2,000 amino acids,(3) arranged in a precise sequence determined by DNA. That sequence matters. It shapes the protein’s three-dimensional structure, and that structure determines what the protein can do. Scientists are still working to pin down the exact number of distinct proteins in the human body, but estimates suggest there may be well over 100,000.(4)
Proteins are often classified by shape as either globular or fibrous. When a protein loses its natural structure, it becomes denatured. Heat and acid can both cause denaturation, which is why cooking and pickling change the structure of proteins in food. Once denatured, a protein usually loses its original function. For example, a denatured enzyme can no longer catalyze a chemical reaction. Even so, denatured proteins can still provide amino acids, which the body can use as building blocks for its own proteins and for important compounds such as neurotransmitters, creatine, and glutathione.
The Role of Proteins
Proteins made by the body perform an extraordinary range of functions. One of the most important is acting as enzymes. Enzymes are involved in far more than digestion. Nearly every chemical reaction in the body depends on an enzyme. Without them, metabolic reactions would proceed far too slowly to support life.
Proteins also act as transport and storage molecules. Many small molecules and ions need proteins to move through the bloodstream. Hemoglobin is a familiar example. Found inside red blood cells, it helps transport oxygen throughout the body. Transferrin transports iron, while myoglobin stores oxygen in skeletal and cardiac muscle. Ferritin stores iron inside cells, especially in the liver.
Structural support is another major role of protein. The most abundant protein in the human body is collagen, a fibrous protein that accounts for about 30% of total body protein.(5) Collagen is found in connective tissues such as bone, cartilage, tendons, ligaments, and the dermis of the skin. Vitamin C is essential for collagen synthesis, which helps explain why many symptoms of vitamin C deficiency are connected to poor collagen production.
The Role of Proteins
Albumin is another vital protein. It is the most abundant protein in blood plasma. Albumin helps transport substances such as bilirubin, contributes to pH buffering, and plays a central role in maintaining osmotic pressure. In simple terms, albumin helps keep fluid inside the bloodstream. When albumin levels are too low, fluid can shift into surrounding tissues and cause edema. Since the liver produces albumin, edema is one possible sign of liver dysfunction.
The best-known role of protein is in building and maintaining muscle. Muscle contraction depends on the interaction of two proteins, actin and myosin, which are the main components of myofilaments. Protein also supports other forms of movement in the body, including the beating of cilia in the respiratory tract, the motion of the sperm tail, and the separation of chromosomes during mitosis.
Other roles of protein include:
- Antibodies – These specialized proteins are produced by certain white blood cells and help defend the body against pathogens.
- Hormones – Many hormones, including insulin and growth hormone, are proteins.
- Receptors – Many receptors in cell membranes are proteins that receive signals from neurotransmitters, hormones, and other molecules.
- Gene regulation – Proteins help control which genes are expressed in a given cell, and genes themselves contain the instructions for making specific proteins.
Taken together, these examples show that protein is not just one nutrient among many. It is central to nearly every aspect of physiology.
Other roles of protein include:
Antibodies — these highly specific proteins are produced by some of our white blood cells and help protect us from invaders.
Hormones — many hormones such as insulin and growth hormone are proteins.
Receptors — in our cell membranes, there are many proteins that act as receptors for neurotransmitters, hormones and more.
Control of gene expression is mediated by proteins. Which genes are expressed within a particular cell is regulated in part by proteins. Furthermore, the information in the gene contains instructions to make a specific protein!
While there are other roles of protein this illustrates its importance. Given the obvious necessity of proteins within our bodies, it is clear that adequate protein intake is vital to our health. It is not clear what the ideal daily protein intake should be but low protein intake has definite adverse affects on our health. Symptoms of protein deficiency include brittle hair and nails, muscle weakness, getting sick often, feeling weak or hungry, mood changes and trouble thinking (6). Edema, anemia and slow growth in children are also signs of protein deficiency (7). Daily protein recommendations are typically 0.8 g/kg of body weight for most adults with active individuals, pregnant women, children and older adults needing a little more. While this may not seem to be a lot of protein, a 2018 study found that more than a third of adults over the age of 51 are not meeting this requirement (8). While it has been found that very high levels of animal protein intake may increase risk of kidney stones in at risk individuals (9, 10), most of us would probably benefit from increased levels of protein intake. Protein needs to be taken in on a daily basis since it is not stored in our bodies. Every time we eat protein, it is broken down to amino acids by our digestive system. These amino acids are absorbed and will fill up the “amino acid pool” that exists in our body fluids. Once that pool is full, any additional amino acids are burned for energy. In fact, the liver obtains a lot of its energy through the catabolism of amino acids.
Benefits of Increasing Protein Intake
For many people, increasing protein intake can offer meaningful benefits. One of the most widely discussed is support for healthy body weight. Protein can help reduce appetite and hunger levels,(11) and it may also decrease late-night snacking.(12) This is partly because protein influences hormones involved in satiety. It increases GLP-1, peptide YY, and CCK, while lowering the hunger hormone ghrelin.(13)
Protein is also important for bone health. Diets that are too low in protein may increase the risk of osteoporosis. Research has shown a positive relationship between protein intake and bone density,(14,15) particularly when overall diet quality is strong.
Muscle health is another key area. Strength training without adequate protein is far less effective when the goal is to build or maintain muscle mass. When resistance training is paired with adequate protein intake, gains in muscle mass and strength are more likely.(16) This is especially relevant for aging adults, since preserving lean mass supports mobility, balance, and long-term independence.
In practical terms, protein is not just about athletes or bodybuilders. It matters for immune function, tissue repair, hormone production, healthy aging, and day-to-day resilience.
As you savour your next protein-rich meal, it may be worth pausing to appreciate just how much that protein will go on to do in your body.
IHN students explore the functional roles of proteins in depth in our Body Metabolism course, one of the 19 unique courses in IHN’s Applied Holistic Nutrition Diploma Program. If you want a firsthand look at what this transformative program offers, you can be a student for a day and sit in on a class.
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References:
1. Hermann, J. R. (2021, April 1). Protein and the body – Oklahoma State University. Protein and the Body | Oklahoma State University.https://extension.okstate.edu/fact-sheets/protein-and-the-body.html?Forwarded=pods.dasnr.okstate.edu%2Fdocushare%2Fdsweb%2FGet%2FDocument-2473%2FT-3163web.pdf
2. Protein. protein noun – Definition, pictures, pronunciation and usage notes | Oxford Advanced Learner’s Dictionary at OxfordLearnersDictionaries.com. (n.d.).https://www.oxfordlearnersdictionaries.com/definition/english/protein
3. Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. ‘The Shape and Structure of Proteins.’ Available from:https://www.ncbi.nlm.nih.gov/books/NBK26830/
4. Lopez MJ, Mohiuddin SS. Biochemistry, Essential Amino Acids. [Updated 2023 Mar 13]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from:https://www.ncbi.nlm.nih.gov/books/NBK557845/
5. Collagen: What it is, types, function & benefits. Cleveland Clinic. (2022, May 23).https://my.clevelandclinic.org/health/articles/23089-collagen
6. Are you getting enough protein? here’s what happens if you don’t. UCLA Health System. (2022, November 4).https://www.uclahealth.org/news/are-you-getting-enough-protein-heres-what-happens-if-you-dont
7. Department of Health & Human Services. (2003, May 21). Protein. Better Health Channel.https://www.betterhealth.vic.gov.au/health/healthyliving/protein
8. Fetters, K. A. (2018, March 30). Older adults don’t get enough of this important nutrient – US news health. U.S. News & World Report.https://health.usnews.com/wellness/food/articles/2018-03-30/older-adults-dont-get-enough-of-this-important-nutrient
9. Ferraro PM, Bargagli M, Trinchieri A, Gambaro G. Risk of Kidney Stones: Influence of Dietary Factors, Dietary Patterns, and Vegetarian-Vegan Diets. Nutrients. 2020 Mar 15;12(3):779. doi: 10.3390/nu12030779. PMID: 32183500; PMCID: PMC7146511.
10. Pasiakos SM, McLellan TM, Lieberman HR. The effects of protein supplements on muscle mass, strength, and aerobic and anaerobic power in healthy adults: a systematic review. Sports Med. 2015 Jan;45(1):111-31. doi: 10.1007/s40279-014-0242-2. PMID: 25169440.
11. Halton TL, Hu FB. The effects of high protein diets on thermogenesis, satiety and weight loss: a critical review. J Am Coll Nutr. 2004 Oct;23(5):373-85. doi: 10.1080/07315724.2004.10719381. PMID: 15466943.
12. Leidy HJ, Tang M, Armstrong CL, Martin CB, Campbell WW. The effects of consuming frequent, higher protein meals on appetite and satiety during weight loss in overweight/obese men. Obesity (Silver Spring). 2011 Apr;19(4):818-24. doi: 10.1038/oby.2010.203. Epub 2010 Sep 16. PMID: 20847729; PMCID: PMC4564867.
13. Gunnars, K. (2023a, March 30). How protein can help you lose weight naturally. Healthline.https://www.healthline.com/nutrition/how-protein-can-help-you-lose-weight
14. Bonjour JP. Dietary protein: an essential nutrient for bone health. J Am Coll Nutr. 2005 Dec;24(6 Suppl):526S-36S. doi: 10.1080/07315724.2005.10719501. PMID: 16373952.
15. Kerstetter JE, Kenny AM, Insogna KL. Dietary protein and skeletal health: a review of recent human research. Curr Opin Lipidol. 2011 Feb;22(1):16-20. doi: 10.1097/MOL.0b013e3283419441. PMID: 21102327; PMCID: PMC4659357.
Written by Linda Bazinet, BSc MSc
Science Educator & Biological Sciences Instructor
Linda Bazinet holds a Bachelor of Science from the University of Toronto, specializing in Zoology with a major in Biomedical Science, and a Master of Science in Cell Biology from McMaster University. Since 1988, she has taught extensively across post-secondary institutions including Seneca and Sheridan College, covering a broad range of science disciplines such as Biology, Chemistry, Microbiology, Histology, and Environmental Science. Linda currently teaches Biological Chemistry, Human Anatomy & Physiology, Body Metabolism, and Applied Endocrinology at the Institute of Holistic Nutrition, where she brings decades of academic and practical teaching experience to the next generation of holistic health practitioners.