Written by: Jesse Cook, Dietetic Intern
Reviewed by: Madison Frye, MS, RDN, LDN
What Is Protein? The Greek term for protein, Proteos, refers to “taking first place” and was given to reflect the importance of this macronutrient back in the 1800s. Proteins are found throughout our entire body with over 40% in skeletal muscle, 25% in our organs, and the rest within our skin and blood. Proteins are composed of amino acids, which are the building blocks for many functions in our body. Many people associate protein with muscle growth, but proteins are also key components of our immune system, hormone signaling, digestive enzyme production, structural development, and influence satiety.1The Basics of ProteinProteins are broken into two main categories of complete and incomplete proteins.Complete protein - contains all our indispensable or essential amino acids in the amounts roughly needed by humans. Indispensable amino acids, discussed below in more detail, cannot be synthesized by our bodies and must come from our diet. This source of protein is primarily found in animal sources such as meat, fish, poultry, dairy, eggs, etc. The exception to this is gelatin, which is of animal origin, but lacks the amino acids tryptophan and collagens.1Incomplete protein - lacks some indispensable or essential amino acids needed by humans. These are primarily found in plant food such as nuts, seeds, vegetables, grains, legumes, and lentils. The exception to this is soy and quinoa, which are complete proteins. 1 Additionally, it is important to remember that incomplete does not mean they’re of no use, but just have limiting amino acid profiles. We will discuss this more in the “Animal and Plant Proteins" section.What are Amino Acids?Amino Acids (AA’s) are the building blocks of protein. When amino acids begin to bond together into linear chains, they are called polymers. As polymers bond together, they become polypeptides. As the various polypeptide shapes and sizes are formed, they bond together and are referred to as proteins. There are a total of 20 amino acids that directly code with human genetics, each provide unique benefits in the body. These amino acids are categorized into three groups: essential, non-essential, and conditionally essential.
- Essential (indispensable) - can only be obtained via diet (our body cannot make these!) There are 9 in total.
- Non-essential - can be made by our body. There are 11 in total. Conditionally essential - normally our body can make these, but in weakened states, our body cannot make sufficient amounts. 7 of the 11 non-essentials fall into this category.
For our body to absorb protein, we must first break the polypeptide bonds down into smaller amino acid molecules. This process occurs in the stomach via stomach acid (HCl) and protein specific enzymes, known as protease. The absorption of any protein is done through our interstitial brush border (along the small intestine) via facilitative diffusion or active transport. However, large bonds are unable to be absorbed. This means that when we eat something, our body must break it down into tiny molecules and then build it again once absorbed.
Specific Amino Acid Benefits/ExamplesEach amino acid has a unique and pivotal role in our bodily functions. Discussed below are only a few of the 20 amino acids, each with unique functions. It is important to recognize that amino acids are the building blocks to every protein found in the human body. In order to improve our quality of life, a well-balanced diet that includes a variety of amino acids should be consumed.Valine, Isoleucine, Leucine (BCAAs)Valine, Isoleucine, and Leucine are referred to as Branch-Chain Amino Acids (BCAA’s) due to the structure of their molecule. These are essential amino acids. Each of these amino acids, especially leucine, are well known for their benefits relating to Muscle Protein Synthesis (MPS), inhibiting Muscle Protein Breakdown (MPB), lean muscle growth, muscle recovery, and reducing muscle fatigue/soreness. 2*BCAAs are found most abundantly in red meat, chicken, eggs, fish, dairy products, legumes, nuts, grains, and seeds.TryptophanSerotonin, melatonin, and other neurotransmitters are derivates of tryptophan. This essential amino acid is responsible for many functions that allow us to thrive! Serotonin regulates our mood, fights anxiety, improves our sleep, impacts our pain perception, improves thermoregulation, and regulates our blood pressure. Melatonin plays a large part in our circadian rhythm and sleep. As we age natural melatonin production diminishes, resulting in difficulty sleeping. 1*Tryptophan is found most abundantly in salmon, chicken, turkey, eggs, spinach, seeds, nuts, soy, and dairy products.GlutamineGlutamine is one of the most abundant free AA’s in the body and is responsible for many metabolic processes. Our bodies store large amounts of Glutamine, so it can be readily used. This conditionally essential amino acid is referred to as “glucogenic,” meaning that if and when needed, will readily convert into glucose (energy), via a process called gluconeogenesis. Glutamine is also essential in the production/replication of leukocytes (white bloods cells that fight infection). Additionally, glutamine may be recommended in someone who has increased intestinal permeability (commonly referred to as “Leaky Gut”), as it plays a significant role in healing the lining of our gut.1*Glutamine is found most abundantly in chicken, fish, cabbage, spinach, dairy, tofu, lentils, and beans.
Roles of protein in bodyMuscle Growth – Proteins are the building blocks of all muscle. Adequate intake of all essential amino acids alongside total protein intake is vital for muscle development, maintenance, and repair. Muscle growth occurs when MPS is stimulated, generally via resistance training. Without adequate protein, the rate of MPB will surpass that of MPS, and ultimately net in a loss of muscle. An important note, after the age of 30, our lean mass is estimated to decrease 3-8% per decade. 2 So, promoting the development and maintenance of lean muscle mass is something everyone should consider!Immune System Function - Antibodies, also known as immunoglobulins (IgG, IgM, IgA, IgD, an IgE), are large proteins used by our immune system to target and neutralize antigens (foreign pathogens such as bacteria and viruses). 1Hormone Signaling – Hormones are chemical signals released by our endocrine cells. These signals control a wide variety of physiological processes including growth, metabolism, reproduction, and many others. Some hormones (such as steroid hormones) are lipid based and others are protein based (such as peptide hormones). 1A great example of peptide hormones is insulin. When blood glucose rises (after a meal) our cells within our pancreas release insulin into the bloodstream. The insulin then binds to cells within the liver, skeletal muscle, adipose tissue, brain, and other areas the glucose can be used. Resulting in blood sugar returning to normal levels.Digestive Enzymes – Enzymes comprised of proteins are designed to help speed up chemical reactions in our body. The digestive enzymes amylase, lipase, and protease are the three most common digestive enzymes. These specific enzymes are found throughout our gastrointestinal system with the purpose of breaking down macronutrients (carbohydrate, lipids, and protein) into their smallest forms they can be absorbed and used. 1Structural Development – Proteins are the foundation for many structures in our body. Collagen, elastin, and keratin are the three well-known structural proteins.Collagen makes up 35% of the body’s total protein. Its purpose is to improve the elasticity and firmness of our skin, ligaments, tendons, and bone density. 1Elastin is found in nearly every tissue of the body, helping maintain the structural integrity even when stretched. 1Keratin gives our hair, skin, and nails their strength and structure! This type of protein has sulfur-rich amino acids (cysteine) that make it less prone to damage and tearing. 1Satiety – Protein is considered the most filling macronutrient. This is because ghrelin and leptin, our hunger and fullness hormones, are more impacted by foods that are more filling and digested slower.Ghrelin, a peptide hormone, increases our drive to eat as levels rise. This hormone is released by our stomach when it is empty. Improving our regulation of ghrelin commonly results in less overeating. 1Leptin, a peptide hormone, increases as we eat satiating foods and promotes the feeling of fullness. This hormone is released by white adipose tissue and inhibits ghrelin release. 1Ensuring that each meal contains protein is very beneficial for hunger/fullness regulation. However, this is only a piece of the puzzle as protein should always be part of a balanced meal! (do NOT skip the carbs, they are essential also!!!).Protein IntakeProtein and AA’s needs vary depending on the age, size, activity, and physiological state of the person. The Recommended Dietary Allowance (RDA) for protein in adults is currently set at .8g/kg/d. This, however, is outdated in all populations.1General Recommendation for HealthA recommendation of 1.2g/kg/d for active individuals seeking to maximize their general health. The benefits of protein discussed above cannot occur if adequate amounts are not obtained through the diet. For reference, someone who weighs 68 kg (150 lbs.) should consume a minimum of 82g of protein per day. This, however, is a generalized guideline and varies depending on the individual.65+ Years Old1.2g/kg/d is the minimum recommendation for those over 65 years of age that wish to promote muscle growth, maintain current muscle mass, and improve bone health. As we age, our lean muscle mass declines. It is important to maintain an ideal protein intake to help support this (and to consider introducing some resistance training). 1Endurance training Protein intake is put on the back burner for endurance athletes as their primary source of energy is carbohydrates. However, without adequate dietary protein intake, our bodies are unable to keep up with MPB that occurs during extended periods of vigorous exercise. The quantity of protein needed for endurance athletes varies depending on the training intensity and duration. The current recommendation for protein intake is 1.2-1.4 g/kg/d for muscle maintenance but is best individualized for each athlete via a registered dietitian! 2Resistance trainingA common misconception within the fitness community is the more protein consumed, the more muscle growth occurs. This is a massive oversimplification and lacks truth to it. There is, however, truth to larger individuals requiring more protein than smaller individuals. This leaves the question of how much protein is best to maximize muscle protein synthesis. Current recommendations for promoting hypertrophy are set at 1.6-2.0 g/kg/d. 2 In those who are attempting to maximize their lean mass development you may have heard “1 gram per pound”. This is overkill. 1.6-2.0g/kg/d is the highest amount you will need under normal circumstances (.7-.9g/lb/d.). The upper end of this recommendation is generally for those in extreme sports such as bodybuilding contest preparation. Talk to a Registered Dietitian to figure out what is best for you and your goals.Protein TimingThis discussion has been heavily debated in the fitness industry. The bottom line is that total protein intake, spread throughout the day, trumps everything else. This is because amino acids circulate through our blood steam for hours after a meal. However, if you are looking to maximize your muscle mass having a well-balanced meal with at least 3g leucine, 10g essential amino acids, 20g of protein, and 40g carbohydrates within 2-3 hours of resistance training may be beneficial. This is because our mTOR pathway (the pathway responsible for anabolic signaling) increases after resistance training and amino acids ingestion. 1Animal and Plant ProteinsThere is an ongoing battle between animal and plant protein that is best answered through understanding amino acid content. At face value, most plant products lack one or more of the essential amino acids making them an incomplete protein and animal products are mostly complete proteins. However, this does not mean animal products are superior to plant products.Let's use a basic example to understand why this does not matter if we understand amino acid content! Beans lack the essential amino acid methionine but have high amounts of the essential amino acid lysine making it an incomplete protein. Rice is also an incomplete protein but contains high amounts of methionine and lacks lysine. When we pair these incomplete proteins together the meal becomes complete as all the essential amino acids can be found in adequate amounts. *Similar to this is pita and hummus!While most animal products tend to be complete proteins and plant products tend to be incomplete protein, this doesn’t make one better than the other. All that matters is those who prefer to avoid animal products must be more diligent with their daily protein intake and balanced amino acid profiles. The only argument that can be truly made is it is more convenient to get your protein through animal products.No Tolerable Upper IntakeTo date, no adverse consequences have been identified from high-protein diets. For those without specific requirements advised by a doctor or Registered Dietitian, increasing your protein intake has no negative consequences! Concerns have been raised relating to increased risk of dehydration and negative effects on the kidneys and bones, but these do not have evidence to support or truth behind them. Dehydration is not a result of excretion of urea (and other nitrogenous waste) from protein catabolism. Additionally, a systematic review on renal function and high protein intake (up to 35% intake) concluded there were no negative outcomes. It has also been found that at least 1.2g/kg/d of protein should be considered to maximize bone health (alongside calcium, vitamin D, and other nutrients). 1,2References:
- Gropper SS, Carr TP, Smith JL. Advanced Nutrition and Human Metabolism. Cengage Learning; 2021.
- Karpinski C, Rosenbloom C. Sports Nutrition: A Handbook for Professionals. Academy of Nutrition and Dietetics; 2017.
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