Protein : A Deep Dive

Protein : A Deep Dive

Topics Covered:

I. What is Protein?

II. Protein Digestion, Absorption and Metabolism

III. History of Protein

IV. Pros and Cons of Animal-Derived & Plant-Based Proteins

V. High Protein Diets & Weight Loss

A. Thermogenesis

B. Satiety

C. Subsequent Energy Intake

D. Area of Controversy: High Protein Diets and Renal Function

E. Area of Controversy: Blood Lipids and Risk of Cardiovascular Disease

VI. Muscle Protein Synthesis


I.              What is Protein?

Proteins have many important functions in the body. They come in all shapes and sizes, functioning as enzymes, antibodies and more. Scientist have discovered over 100 different structural proteins in the body, but collagen is the most abundant. Collagen can be found in tendons, ligaments, cartilage, skin, and muscle. For the purpose of this article, we will only discuss protein digestion as it relates to nutrition.

II.           Protein Digestion, Absorption and Metabolism

A.            Consumption

Your eggs are done. Whether its meal 1 or meal 7, you chow down! So what happens after you eat your eggs? Well, hopefully you’ve managed to chew and swallow the delicious source of protein. Now, assuming your eggs were cooked; digestion starts in your mouth! You mechanically break down the food using your teeth then your saliva helps glide your food down your esophagus into your stomach. Your acidic stomach juices (also known as gastric juices) begin to break down your food. The key player in this process is the enzyme, pepsin*.

            As your stomach churns, the egg/pepsin extravaganza begins to create something called chyme. The process of breaking down protein takes much longer than the breakdown of carbohydrates, but slightly shorter than the breakdown of fat. This is why you feel much more full after a high protein meal.

B.            Onto the Small Intestine

The chyme mixture is deposited into the small intestine where the magic truly happens. The pancreases will toss its own juices that contain more protein enzymes* and break down the chyme mix into smaller pieces. The small intestine joins the party and adds its own enzymes* as well.

At this point they chyme mixture is now protein fragments called amino acids and are now small enough for absorption. As the small intestine contracts, these amino acids are pushed to their absorption sites. From the lower small intestine, the amino acids are sent to intestinal lumen through intestinal blood cells. However, this trip requires a toll be paid in the form of adenosine triphosphate (ATP). From the blood, the next stop is the liver. The liver acts as a check point and might breakdown the amino acids even further, if necessary.

Amino acids contain nitrogen so when the liver beaks it down even further, nitrogen is released by way of ammonia which the liver turns into urea. From here the urea is sent to the kidneys and excreted as urine. Only about 10% of protein ingested gets broken down further so this amino acid monomer is where we’ll end the process.

C.            Absorption

Protein is broken down into amino acids, tripeptides and dipeptides in the duodenum or proximal jejunum of the small intestine. From here the peptides or amino acids pass through interstitial brush border via active transport. Active transport Na and ATP then take the molecule and transport it through the cell membrane. From here onto the blood stream they go on the hepatic portal vein highway and onto the rest of the body.

It is important to mention that nitrogen is a unique component of amino acids. The nitrogen component plays an important role to where the amino acid + nitrogen combo is sent and if B6 is needed. If removed from the duet, nitrogen is primarily excreted in the urine, but may also be excreted in the feces, skin, hair, and nails.

D.            Amino Acid Pool

Amino acids are also recycled and used to make new proteins. In order to efficiently do this, ingested amino acids are stored in a “pool.” Amino acids are pulled from this pool when other biological process require it (e.g., build DNA and RNA).

Consuming protein is important, but so is the quality and source of your protein. If high-quality proteins are not consumed, protein destruction from tissue may occur. This unique storage of amino acids is different from how the body stores carbohydrates (as glycogen in the muscles and liver) and lipids (as triglycerides in adipose tissue).

III.        History of Protein Consumption

We can thank our primitive ancestors for consuming protein and fostering cognitive development. While there were many factors that helped facilitate our evolution, one of the most noteworthy was hominin increase in meat consumption. Using tools to hunt and butcher, our ancestors increased their efficiency and reduced the time required to find food.

Finding prey was just half the battle. Having the advanced individual and social cognition required our ancestors to think outside of the box. This means powering our ancestral brain required more fuel than what we could forage. With key micronutrients such as vitamin B3 / nicotinamide, we begin to see an increase in organ development. Tryptophan is a great source of nicotinamide, which can be found in animal protein. Nicotinamide is an essential component of NAD/NADH which drives the electron chain converting free energy of the electromotive force into a proton gradient across the mitochondrial inner membrane driving ATP production and controlling pH and other voltage-coupled processes. The NAD is also a master controller of such metabolism necessary for running large bodies and brains and restoring stem cells.

IV.        Pros and Cons of Animal-Derived & Plant-Based Proteins

Time and time again studies have endorsed animal-derived proteins over the plant-based alternative. Whey protein, and more specifically whey hydrolysate, has been found to stimulate mixed muscle protein synthesis quicker than casein or soy. While a majority of the research in support of this were experiments conducted on males, new research is emerging with similar results for women.

So how does this work? After you ingest whey protein, there are more essential amino acids (EAA) and branched-chain amino acids (and leucine concentrations) in the blood before and after resistance exercise when compared to other types of protein.

It has also been suggested that animal-derived protein “promotes better whole body nitrogen retention at rest and greater skeletal muscle protein accretion after resistance exercise” (Kujbida, G., et al., 2009). The importance of nitrogen and more specifically, the role it plays in the cardiovascular system will be discussed in a later post.

This graph compares various protein isolates and their EAA content. While it may look like plant based proteins aren’t too far behind, they have varying amino acid profiles (i.e., leucine, isoleucine, valine, etc., content). The other drawback to plant-based proteins like pea proteins is that they are not as bioavailable as animal-derived proteins. That being said, plant-based proteins tend to be gluten and lactose-free and a suitable protein source for meat-sensitive diets.

Overall, most research has shown that animal-derived proteins tend to have higher amino acid and leucine content than plant-based proteins. Animal-derived proteins have also been found to have higher postprandial muscle protein synthesis rates. So why the big push for plant-based proteins? Other than diet restrictions and lifestyle choices we have to look at the growing world population. It requires much less land and water to produce plant-based foods as well as less greenhouse gas emissions when compared to animal-based foods.  

At some point in your protein-selecting career you’ve been faced with the daunting task of choosing an EAA and/or a BCAA supplement. Most of your protein sources already have an EAA and BCAA profile and the easiest way to choose which one you prioritize is simple; what is your goal? EAAs are primarily responsible for postprandial stimulation of muscle protein synthesis. Bierau et al., (2018) states that “there is a dose-dependent relationship between the amount of essential amino acids ingested and the postprandial muscle protein synthetic response until a plateau is reached.” Secondly, it is important to look at the leucine content specifically as research suggest that leucine content directly impacts the muscle protein synthetic machinery after protein ingestion. Some plant-based proteins provide an EAA profile that supports muscle growth, but tend to fall short in other amino acid profiles and compromise other aspects of the muscle protein synthesis. My suggestion: determine exactly what EAA profile you need and find the protein source that matches that.

While there are tradeoffs when deciding between plant-based or animal-derived proteins, they are all great choices. It is important to also note the fat and carbohydrate profiles of the specific protein you choose! Those macronutrients will be covered in another blog.

So what’s my recommendation? In my opinion and my opinion only; I’m team whey isolate. I know there are diet restrictions, religious factors or medical issues or medication that may prevent some people from choosing whey, that’s ok! There are other great options that will keep you on track to meeting your goal and/or sustaining your achievements.

V.          High Protein Diets & Weight Loss

Many of us are used to seeing the glorification and promotion of high protein diets. While this is the preferred diet and recommendation of nutritionists and many physicians, this type of diet needs careful consideration and balance. A high protein diet may cause a significant acid load on your kidneys (as explained in Section II). It is important to note that the effects of a high protein diet on weight loss and the long-term effects a high protein diet has on weight maintenance and overall health are now being researched and studies are beginning to emerge.

The Recommended Daily Allowance (RDA) for protein is 0.8 g/kg or around 10-15% of dietary energy. A high protein diet generally recommends a protein intake that meets or exceeds 25% of dietary energy. Since high protein intake increases dietary thermogenesis, increased satiety and thus a decrease in subsequent energy intake, it is believed to be the magic recipe for weigh loss. Translation: high protein diets increase calories burned at rest and make you feel full so you don’t eat stuff not in your diet plan aka the “magic recipe” for weigh loss. Now this makes a lot of sense on the surface so let’s dive into some recent scientific evidence to support or reject this claim.

A.            Thermogenesis

Thermogenesis is also referred to as a thermic effect. This is defined as the energy required for digestion, absorption and disposal of the nutrient. This effect is also influenced by food consumed. Protein typically has a thermic effect of 20-35% of energy consumed whereas carbohydrates have roughly 5-15%. Fats have been found to have the same thermic effect as carbohydrates or slightly lower. The main explanation for the variation in thermogenesis of protein is due to the fact that the body has no storage capacity for protein and therefore it needs to be metabolized immediately. Another reason for the higher thermic effect of protein is also believed to be due to the synthesis of protein, high ATP cost of peptide bond synthesis and high cost of urea production and gluconeogenesis.

B.            Satiety

Satiety relates to how full you feel after a meal. Most research suggests that a preloaded meal (meal consisting of 50% or more of protein) increases satiety. Satiety is more than just macronutrient content; palatability, food mass, energy density, fiber and glycemic index also play a role. If you try to manipulate all of these factors while ingesting a certain amount of protein, you will find it extremely difficult. Satiety is a complex concept that has psychological, behavioral, and physiological mechanism that we won’t discuss in this post… but I’m thinking about making it its own post… we’ll see.

C.            Subsequent Energy Intake

Subsequent energy intakes simply means; what is the likelihood you’ll eat something after having a preloaded meal. This is positively correlated to satiety in that individuals who consumed preloaded meals were far less likely to snack and consumed less calories after a protein rich meal.

D.            Area of Controversy: High Protein Diets and Renal Function

With all of the information presented so far we are left with a couple questions. The first question being; “are high protein diets safe?” The answer to this question depends on the individual. In individuals or populations with established renal disease it has been shown that a low protein diet may slow the progression of the disease. In healthy populations, it is unclear. Now before you call your trainer or jam-pack your diet with red meat and protein, there have been studies that show high protein diets can cause hyperfiltration and may increase renal mass. There are also studies that suggest higher protein diets may increase the risk of kidney stones, uric acid stones, and calcium stones. Overall, there isn’t a substantial amount of evidence that suggest high protein diets cause a serious risk to kidney function in healthy individuals. Regardless of your beliefs or where you fall or what your Instagram personal trainer told you, see your primary care physician. Request labs regularly and make sure you are healthy inside AND out.

E.             Area of Controversy: Blood Lipids and Risk of Cardiovascular Disease

Battle number two is high protein diets vs. blood lipids. One study conducted found that obese patients on a high protein diet lost more weight and had significantly lower triglycerides than obese patients on a lower protein diet. One study also found lower LDL cholesterol levels in patients on a high protein diet. Another study even found that a higher protein diet decreased the risk of coronary heart disease during a 14 year follow-up. A study in Japan suggests that low protein intake can increase the risk of intraparenchymal hemorrhagic stroke. And our last research study found a significant inverse association between high protein diets and blood pressure (higher protein: lower blood pressure).

A review of multiple studies as presented above suggests that high protein diets might actually be beneficial to cardiovascular health! As amazing as that sounds, I refer back to my previous statement: talk to your doctor! Find out if a high protein diet is something that is reasonable for you, specifically. These studies might not represent you and your demographic and therefore might mean absolutely nothing… and yes, I will be publishing another post that discusses how to determine if a research study relates to you.

VI.        Muscle Protein Synthesis After Exercise

As you might have already assumed, there is in fact a positive correlation to muscle protein synthesis (MPS) after exercise. Resistance training stimulates MPS and muscle protein breakdown (MPB) which can result in muscle growth if a sufficient amount of protein is ingested during recovery (post-workout). So what does this mean? The positive net protein balance provides the basis for muscle hypertrophy with repetitive sessions of resistance training and protein consumption post-workout when consistent over the course of weeks and months. There have been multiple studies conducted that support the claim/importance of consuming protein (i.e. protein shake and/or meal) post-workout while your body is recovering. The ideal window to consume post-workout protein is believed to be within one hour of completing your training session.

What about aerobic exercise? Or high intensity interval training (HIIT)? These types of exercises are repetitive, low force muscle contractions/movements performed at sustained period of 10-30min or longer. While these exercises have been shown to enhance cardiovascular fitness (cardio will be discussed in a later post) do they increase MPS? The answer is, yes. Multiple studies using a variety of these types of cardiovascular exercises and approaches to MPS have found that there is an increase in MPS and muscle development. However, there were no significant changes in MPS when post workout protein was ingested.

VII.     References

Bagheri, R., Robinson, I., Moradi, S. (2022). Muscle Protein Synthesis Responses Following Aerobic-Based Exercise or High-Intensity Interval Training with or Without Protein Ingestion: A Systematic Review. Sports Med 52, 2713–2732.

Bierau, J., Crombag, J., Gorissen, S., Senden, J., van Loon, L., Verdijk, L., & Waterval., W. (2018). Protein content and amino acid composition of commercially available plant-based protein isolates. Springer Link 50, 1685-1695.

Burd, N., van Loon, L., & van Vilet, S. (2015). The Skeletal Muscle Anabolic Response to Plant- versus Animal-Based Protein Consumption. The Journal of Nutrition, 145 (9), 1981–1991.

Halton, T. & Hu, F. (2004). The effects of high protein diets on thermogenesis, satiety and weightloss: A critical review. Journal of the American College of Nutrition 50 (5), 373-385.

Lamming D.W., Richardson, N.E., & Trautman, M.E. (2017). Protein restriction and branched-chain amino acid restriction promote geroprotective shifts in metabolism.

Pesta, D.H., Samuel, V.T. (2014). A high-protein diet for reducing body fat: mechanisms and possible caveats. Nutr Metab (Lond), 11, 53.

Protein’s Functions in the Body. (2017). Medicine LibreTexts. Retrieved Nov 24, 2022, from

Tang, J. E., Moore, D. R., Kujbida, G. W., Tarnopolsky, M. A., & Phillips, S. M. (2009). Ingestion of whey hydrolysate, casein, or soy protein isolate: effects on mixed muscle protein synthesis at rest and following resistance exercise in young men. Journal of Applied Physiology, 107(3), 987-992. 10.1152/japplphysiol.00076.2009

Trautman, M. E., Richardson, N. E., & Lamming, D. W. (2022). Protein restriction and branched-chain amino acid restriction promote geroprotective shifts in metabolism. Aging Cell, 21, e13626. acel.13626

Williams AC & Hill LJ. Meat and Nicotinamide: A Causal Role in Human Evolution, History, and Demographics. Int J Tryptophan Res. 2017 May 2;10:1178646917704661. doi: 10.1177/1178646917704661. PMID: 28579800; PMCID: PMC5417583.




Note from the author: I refrained from in-text citation in this blog entry since blogs are informal and do not necessarily have to adhere a particular style of formatting. I took direct quotes from some of the references which is not something I normally ever do, but I wanted this post to be a consolidation of research and felt it best to include direct quotes. I’ve included a reference section so you can read where I pulled all of this information from. This is not just off the top of my head nor did I use an advanced degree and certifications to make this law. I supported my point using scientific data – what all good personal trainers and nutritionists should do. If your trainer can’t do that or doesn’t believe in or know the science – find a new one. Lastly, I am not a medical provider and am not providing any medical advice. Speak with your medical provider prior and during any engagement with physical activity or dietary and nutrition changes.

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