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All About Protein

What is protein and why does it get all the good press?

Read about why current dietary recommendations for protein are too low.

Protein always gets to be the A-list celebrity and featured first in the menu. Do you ever get asked if you want a steak with your potato? Nobody plans their dinner around the carb side dish. But why is protein so important?

You need protein. Every living thing is made of protein. Proteins are large complex biomolecules made up of chains of different amino acids joined by peptide bonds. Amino acids (AAs) fit together like links in a chain. You can hook the basic AA blocks together in many combinations to build different proteins.

Protein (from the Greek word prōteios) means holding first place. Proteins contain nitrogen, carbon, hydrogen, and oxygen.

Plants and animals require protein to live. There are over 700 amino acids in nature but people normally only use 20 AAs. Proteins have many jobs in the body.

Vladimir Makovsky — Lunch (1875).

Vladimir Makovsky, Public domain, via Wikimedia Commons

Protein's jobs:

Structural engineer: Proteins create structure within cells and between cells. About 25% of your protein mass in the body is extracellular collagen. Amino acids proline and lysine are needed to make collagen. Collagen. literally holds your body together.

Bioengineer: Proteins are enzymes. Enzymes catalyze all the reactions essential for living organisms. This means that they bind/squeeze molecules (substrates) together and help facilitate a chemical reaction. This make it faster and less expensive in terms of energy. Enzymes typically work with coenzymes, mostly formed from vitamins, and cofactors, mostly formed from inorganic minerals.

Warehouse manager: Proteins are used to store amino acids and metal ions like iron. The protein ferritin forms a nanocage around iron to keep it nontoxic and available for use.

Broadcast engineer: Proteins are signaling molecules that allow cells to communicate with each other and respond to changes in the environment. Signaling molecules attach to receptors on cells.

Movement coach: Proteins enable muscle movement.

Long haul trucker: Proteins transport molecules through channels, pumps, and pores as well as across membranes. Some proteins carry molecules through the body.

Regulatory affairs manager: Proteins regulate cellular energy levels, cell functions, signalling pathways and metabolic pathways. Regulatory proteins control gene expression. Proteins make growth hormone, insulin, thyroid hormones and other hormones involved with metabolic regulation.

National Guard: Immune cells produce protein antibodies (immunoglobulins) for defense against invading pathogens.

Physician: Proteins help heal wounds and repair tissue.

Therapist and behavior coach: Protein helps produce neurotransmitters. Amino acids such as tryptophan and tyrosine are the precursors to neurotransmitters serotonin and dopamine, respectively. Neurotransmitters influence cognition, mood and behavior.

Discussion Morris et al. 2022

Woodbine Kendall Hinchliff (1874-1915) - Girls Threading Beads.

Woodbine Kendall Hinchliff, Public domain, via Wikimedia Commons

How to make proteins

Proteins level up to evolve into a functional structure

1) Basic level: primary protein structure. Proteins start as a chain of amino acids bonded together using peptide bonds in a specific sequence. Think of them as stringing different colored beads on a necklace. Each pattern makes a different amino acid.

2) Secondary protein structure. The amino acid chain is folded into regular patterns like paper origami. the patterns are mostly alpha helices (a right handed spiral) and beta sheets (strands of proteins lay along side each other to form a sheet like structure). Both structures are stabilized by hydrogen bonds.

Hydrogen bonds are a weak bond caused by a hydrogen ion (positive pole) being attracted to the negative pole of a molecule such as oxygen. It is commonly seen in water molecules (H2O) where the hydrogen of one water is attracted to the oxygen of another water.

Adolphe Piot (1831–1910) Origami Dreams oil.

Adolphe Piot, Public domain, via Wikimedia Commons

3) Tertiary protein structure. The secondary structure folds in a 3-D globular shape. It is stabilized using hydrogen bonds, hydrophobic interactions (nonpolar molecules clump together to minimize contact with water), ionic bonds (electrostatic attraction between a negative and a positive molecule), and disulfide bridges. Small molecules called chaperones assist to ensure correct folding.

The resulting folded structure is vital for the protein's specific function. The complex folds create unique active sites and binding pockets used for interacting with other molecules.

4) Quaternary protein structure. Two or more proteins bond together to form a functional subunit.

Your body breaks protein down into individual amino acids (AAs)

Your body uses a process called proteolysis to break down proteins. Enzymes; such as pepsin, trypsin, chymotrypsin, carboxypeptidase, dipeptidase, aminopeptidase, and elastase; deconstruct and break proteins into individual amino acids, dipeptides, and tripeptides in the stomach and small intestine. Additional breakdown occurs near the microvilli (finger like cells) in the the brush border of the intestine.

Individual amino acids, dipeptides, and tripeptides are actively taken up by various transporters in the small intestine. About 2-7% of amino acids people use are taken up from the large intestine. This is mostly from internal protein sources like mucus, shed epithelial cells, and microbial amino acids (discussion Bröer 2023). Interestingly, gut microbiota can influence protein breakdown and protein intake can influence gut microbiota populations (discussion Torres et al. 2023).

Amino acid breakdown in the body is called amino acid catabolism or amino acid degradation. It involves breaking down AAs into simpler molecules using amino acid degrading enzymes (AADEs). For most AAs, a lot of amino acid catabolism occurs in the liver. The exceptions are branched-chain amino acids (BCAA), leucine, valine, and isoleucine.

Besides the liver, AADEs are found in adipose tissue, skeletal muscle, kidney, intestine, and immune cells. Branched-chain amino acids (BCAA) is catalyzed by an enzyme, nicely called branched-chain aminotransferase (BCAT). BCAT is found in skeletal muscle, the heart, kidney, brain, intestine, and adipose tissue.

Molecules created from amino acid catabolism are involved in metabolism. They play a role in epigenetic gene regulation (discussion Torres et al. 2023).

Amino acids enter and leave cells through openings in the cell membrane called amino acid transports (AATs). Dysregulation of AATs in cancer cells and diabetes can cause metabolic reprogramming which changes the concentrations of various amino acids within the cell. Changes in the expression and function of AATs are associated with many disorders including obesity, cancer, diabetes, neurodegenerative diseases, inborn errors of metabolism and chronic kidney diseases. (discussion in Kandasamy et al. 2018).

There are twenty amino acids used in the human body:

✤ Nine essential amino acids (AAs): histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine

✤ Six conditionally essential AAs: arginine, cysteine, glutamine, glycine, proline, and tyrosine

✤ Five nonessential AAs: alanine, aspartic acid, asparagine, glutamic acid, and serine

Remember: so-called nonessential and conditionally essential amino acids are actually needed in the diet and used by your body.

Your body needs a surplus of protein to make the so-called nonessential amino acids

People can make some nonessential amino acids but they need adequate protein to do so (Weiler et al. 2020). Current research indicate that neither animals or humans can adequately synthesize enough nonessential amino acids to meet ideal metabolic and functional needs under either stress OR normal conditions (Hou and Wu 2017). This means you need to get them in your diet.

Nonessential and conditionally essential amino acids have been unfairly named; they are not actually nonessential. These amino acids are used for many functions including neurotransmission; folate conversion in one carbon metabolism; activating cell signaling pathways, renal regulation of acid/base balance; antioxidant reactions in organs and muscle; and making glutathione, nucleotides, dopamine, creatine, melanin, thyroid hormones, serotonin and more (see Hou and Wu 2017 for more details).

Recent research indicates that animals have defined dietary requirements of nonessential and conditionally essential amino acids to optimize their genetic potential for peak health and well-being (Hou and Wu 2017).

Jacques-Raymond Brascassat - A Bull Fight c1855 oil on canvas.

Jacques Raymond Brascassat, Public domain, via Wikimedia Commons

By Susan Fluegel PHD Nutritional Biochemistry. One of my first interests in nutrition was how macronutrients such as proteins, fats and carbohydrates worked together.

Proteins like to pair up with fats to help build your body. Learn more about how fats are named and built. Eating proteins and fats together is satisfying. It helps slow digestion and stabilize blood sugar.

*Names and some minor identifying details in all stories in this website are changed to protect people's privacy.

This information in this website is for informational purposes only and does not constitute medical advice, diagnosis, or treatment.

References:

Bröer S. Intestinal Amino Acid Transport and Metabolic Health. Annu Rev Nutr. 2023 Aug 21;43:73-99. doi: 10.1146/annurev-nutr-061121-094344. Epub 2023 Full pdf.

Burd NA, McKenna CF, Salvador AF, Paulussen KJM, Moore DR. Dietary Protein Quantity, Quality, and Exercise Are Key to Healthy Living: A Muscle-Centric Perspective Across the Lifespan. Front Nutr. 2019 Jun 6;6:83. doi: 10.3389/fnut.2019.00083. Full article.

Campbell WW, Trappe TA, Wolfe RR, Evans WJ. The recommended dietary allowance for protein may not be adequate for older people to maintain skeletal muscle. J Gerontol A Biol Sci Med Sci. 2001 Jun;56(6):M373-80. doi: 10.1093/gerona/56.6.m373. Abstract.

Churchward-Venne TA, Pinckaers PJM, Smeets JSJ, Betz MW, Senden JM, Goessens JPB, Gijsen AP, Rollo I, Verdijk LB, van Loon LJC. Dose-response effects of dietary protein on muscle protein synthesis during recovery from endurance exercise in young men: a double-blind randomized trial. Am J Clin Nutr. 2020 Aug 1;112(2):303-317. doi: 10.1093/ajcn/nqaa073. Full article.

Coelho-Junior HJ, Marzetti E, Picca A, Cesari M, Uchida MC, Calvani R. Protein Intake and Frailty: A Matter of Quantity, Quality, and Timing. Nutrients. 2020 Sep 23;12(10):2915. doi: 10.3390/nu12102915. Full article.

Elango R, Humayun MA, Ball RO, Pencharz PB. Evidence that protein requirements have been significantly underestimated. Curr Opin Clin Nutr Metab Care. 2010 Jan;13(1):52-7. doi: 10.1097/MCO.0b013e328332f9b7. Full pdf.

Holwerda AM, Paulussen KJM, Overkamp M, Goessens JPB, Kramer IF, Wodzig WKWH, Verdijk LB, van Loon LJC. Dose-Dependent Increases in Whole-Body Net Protein Balance and Dietary Protein-Derived Amino Acid Incorporation into Myofibrillar Protein During Recovery from Resistance Exercise in Older Men. J Nutr. 2019 Feb 1;149(2):221-230. doi: 10.1093/jn/nxy263. Full article.

Hou Y, Wu G. Nutritionally Nonessential Amino Acids: A Misnomer in Nutritional Sciences. Adv Nutr. 2017 Jan 17;8(1):137-139. doi: 10.3945/an.116.012971. Full article.

Kandasamy P, Gyimesi G, Kanai Y, Hediger MA. Amino acid transporters revisited: New views in health and disease. Trends Biochem Sci. 2018 Oct;43(10):752-789. doi: 10.1016/j.tibs.2018.05.003. Summary.

Mamerow MM, Mettler JA, English KL, Casperson SL, Arentson-Lantz E, Sheffield-Moore M, Layman DK, Paddon-Jones D. Dietary protein distribution positively influences 24-h muscle protein synthesis in healthy adults. J Nutr. 2014 Jun;144(6):876-80. doi: 10.3945/jn.113.185280. Full article.

Morris R, Black KA, Stollar EJ. Uncovering protein function: from classification to complexes. Essays Biochem. 2022 Aug 10;66(3):255-285. doi: 10.1042/EBC20200108. Full article.

Norton C, Toomey C, McCormack WG, Francis P, Saunders J, Kerin E, Jakeman P. Protein Supplementation at Breakfast and Lunch for 24 Weeks beyond Habitual Intakes Increases Whole-Body Lean Tissue Mass in Healthy Older Adults. J Nutr. 2016 Jan;146(1):65-9. doi: 10.3945/jn.115.219022. Full article.

Papanikolaou Y, Phillips S, Fulgoni V 3rd. Animal and plant protein usual intakes are not adversely associated with all-cause, cardiovascular disease-, or cancer-related mortality risk: an NHANES III analysis. Appl Physiol Nutr Metab. 2025 Jan 1;50:1-8. doi: 10.1139/apnm-2023-0594. Full article.

Torres N, Tobón-Cornejo S, Velazquez-Villegas LA, Noriega LG, Alemán-Escondrillas G, Tovar AR. Amino Acid Catabolism: An Overlooked Area of Metabolism. Nutrients. 2023 Jul 29;15(15):3378. doi: 10.3390/nu15153378. Full article.

Wall BT, Gorissen SH, Pennings B, Koopman R, Groen BB, Verdijk LB, van Loon LJ. Aging Is Accompanied by a Blunted Muscle Protein Synthetic Response to Protein Ingestion. PLoS One. 2015 Nov 4;10(11):e0140903. doi: 10.1371/journal.pone.0140903. Full article.

Weiler M, Hertzler SR, Dvoretskiy S. Is It Time to Reconsider the U.S. Recommendations for Dietary Protein and Amino Acid Intake? Nutrients. 2023 Feb 6;15(4):838. doi: 10.3390/nu15040838. Full article.

Yasuda J, Tomita T, Arimitsu T, Fujita S. Evenly Distributed Protein Intake over 3 Meals Augments Resistance Exercise-Induced Muscle Hypertrophy in Healthy Young Men. J Nutr. 2020 Jul 1;150(7):1845-1851. doi: 10.1093/jn/nxaa101. Full article.