Absorption of Protein: How Much is Too Much? 

Protein has always been an important part of a balanced diet, but these days it seems like high protein diets are all the rage.  Athletic trainers and weight loss professionals alike tout the benefits of more protein.   

Nutrition fads tend to target one nutrient as either “bad” or “good”, and then you have a slew of extreme diets based on that theory.  Recently, I have even had clients asking about the carnivore diet, which promotes eating a 100% animal product diet.  Literally, nothing but protein.  And fat.  Not a good idea for anyone. 

Extreme diets aside, we do need protein in our diets, and as athletes you need more than your average person.  That said, there are plenty of sources of protein in the average American diet, so most of us don’t have to struggle to get enough.   

The question I will tackle in this article is whether there is an upper limit to how much protein we should consume, either on a per-meal basis or on a total daily intake basis.   

What is protein? 

Protein is often referred to as the building blocks of life.  Considering proteins form all of the tissues and cells in your body and play integral roles in all bodily functions, I’d say that’s an understatement.   

A protein is a structure including one or more chains of individual molecules called amino acids. Your body can produce some amino acids.  Some, however, must be obtained from food and are therefore called essential amino acids (EAA). 

How is protein absorbed? 

Before protein can be absorbed, it must first be digested.  This means that whole proteins are broken down into individual amino acids or groups of amino acids, called peptides, that are small enough to pass through the intestinal wall. 

Protein absorption refers to amino acids and peptides passing from the digestive tract into the blood stream for use by cells to either form new proteins or use for energy.   

How much protein can we absorb at one time? 

This is a topic of ongoing research and, while we don’t currently have any absolute answers, I will attempt to explain what we do know so you can make educated decisions that are right for you. 

There isn’t necessarily a limit on absorption for entire proteins, however evidence suggests that there may be competition between individual amino acids.  When you have a large quantity of one amino acid and less of another, the more abundant amino acid is likely to be absorbed at a higher rate than the less abundant amino acid. (1

That said, most of us don’t do a complete analysis of individual amino acids before we eat.  What we really need to know is how much total protein should we eat at one time?  As athletes, development and repair of muscles is what we’re really interested in.  Scientist types call this muscle protein synthesis (MPS).     

Some studies suggest that maximum MPS is achieved at 20-25 grams of protein in one sitting. (1)   What this means is that any additional protein consumed will not increase MPS and will instead be used by the body as fuel.  This isn’t the whole story, though.   

“Fast-acting” vs. slowly absorbed proteins 

Whey protein, which has been used in many of the studies that support a maximum total protein intake of 20-25 g at a time, is what we consider a “fast-acting” protein.  In other words, it is absorbed quickly at a rate of 10 g/hour. (1)   

On the other hand, cooked egg protein is considered a slowly absorbed protein, with an absorption rate of 3 g/hour.  It has been suggested that the slower absorption rate may increase how much of the protein your body can use for MPS. (1)   

Just like when trying to pour liquid through a funnel, if you try to pour an entire gallon of water through a kitchen funnel at one time, it will overflow.  But, if you pour a little at a time, you can get that entire gallon into whatever vessel you want.  So, the idea is that the longer it takes for a protein to fully absorb, the higher the percentage that is used for MPS versus being used for energy. 

It makes sense but we need more studies comparing fast vs. slow proteins to confirm this theory. 

Impact of other nutrients on protein absorption 

Another factor that may play a role in how much of the protein you eat gets used for MPS is what you eat with it.  When you consume isolated protein, such as in a whey protein shake, the absorption rate of the protein is what it is. 

However, when you have carbohydrates and/or fat with your protein, overall digestion and absorption slows. (1) As with slowly absorbed protein, the extended release of amino acids into the blood likely leads to the body being able to use more of those amino acids for muscle synthesis. 

Where the wild proteins are 

So, when not all proteins or amino acids get absorbed in the small intestine, where do they go?  They make their way down to your large intestine, where your gut bacteria can ferment them.  Note, any protein or partial protein that makes it all the way to your gut is protein that your body hasn’t fully used. (2

There are several factors that influence how much protein survives to the large intestine.  First, the more protein you eat, the more will make it to your gut.  Second, the digestibility of a protein plays a role in whether we can absorb it.  Lastly, some people have decreased ability to digest proteins, leading to increased whole or partial proteins arriving in the gut. (2,3)  

As with carbohydrates, your body will excrete some of the proteins that survive to the gut.  Others will be broken down, or fermented, by your gut microbiota (the collection of microbes residing in your large intestine).  Bacterial fermentation produces by-products, some of which may be beneficial, but others may have negative effects on health. (2

Protein digestibility 

Some proteins are more easily digested than others.  In general, we are more easily able to digest animal proteins than plant proteins. (2,3) Note, I am not arguing against a plant-based diet, as plant proteins can provide plenty of protein and offer a variety of other health benefits.  I am simply stating the facts. 

For many plant proteins, such as beans, cooking at moderate temperatures help break down some of the proteins, making them easier to digest. (2)  Fermentation can also improve digestibility. (2

Consequences of excessive protein 

Here’s where there is some controversy in the health and wellness world.  Some argue that a healthy person can handle eating a very high protein diet without issue.  Others cite burden on the kidneys as a reason to consume moderate amounts of protein.  Looking at how protein intake impacts gut health offers yet another perspective to consider. 

Fermentation of amino acids by gut bacteria has been linked to production of potentially harmful byproducts. (2,3)  The byproducts of amino acid fermentation likely are not all harmful, however many of them are not well understood at this time.   

What we do know is that some of the bacteria that ferment amino acids can also ferment fiber.  When both fiber and amino acids are available, these bacteria will choose fiber. (2,3) In other words, eating more fiber from fruits, veggies, whole grains, and resistant starch may offset potential negative effects from high protein diets! 

While there are no long-term studies on the effects of a high protein diet on recreational athletes, there is evidence that athletic training helps to maintain a healthy microbial composition in the gut of elite athletes, also protecting against some of the potential negative effects of high protein intake. (3)   

Finding balance 

Unlike carbohydrates and fats, we don’t store proteins for later, so it’s important to consume enough daily. (3) For most adults, protein needs are estimated at between 0.8 to 1 gram of protein per kilogram of body weight (0.36-0.45 g/lb).  Athletes, however, require more.   

The  Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine jointly recommend 1.2-2 grams per kilogram of body weight (0.55-0.9 g/lb) for athletes. (4)  Studies indicate that more than 2.2 grams per kilogram of protein provides no benefit to athletes regardless of type, intensity, or duration of exercise. (1

It should be noted that timing of protein intake matters.  MPS is at its highest immediately following exercise, however studies show that muscle seems to be more sensitive to protein intake up to 24 hours post-exercise. (3)  

Athletes should consume 15-25 grams of protein (0.25-0.3 g/kg) as soon as possible following exercise. (4)  Where you fall within that range depends on not only your weight, but also the intensity and duration of exercise.  Additional protein to meet total daily needs should be spread throughout the remainder of the day. 

Summary 

While there is no limit on how much protein your body can absorb, there may be limitations on how much of that protein can be used efficiently both on a per-dose basis and on a total daily basis. 

Any proteins that are not fully utilized for MPS are either used for energy or they continue to the gut where they can be fermented by gut bacteria.  The fermentation process produces by-products that interact with the rest of the body, some of which are beneficial and some of which are harmful. 

The key to overall health and performance is finding the balance between consuming enough protein to build, maintain and repair tissues and bones while limiting excessive intake that may have harmful effects on the body and gut. 

Evidence suggests that consuming between 20-25 grams of protein at one time maximizes muscle synthesis and repair. Total daily protein of greater than 2.2 grams/kg of body weight provides no additional benefit to muscles or performance. 

In the future, we will likely be able to make more personalized recommendations by assessing an athlete’s gut bacterial composition.  In the meantime, keep protein intake within the recommended ranges, continue to exercise regularly, and consume plenty of fiber daily to maintain a healthy and happy body and gut! 

Need help finding the right balance for you? If you’re interested in working with me, contact me to schedule a free 20 minute call to see if we are a good fit to work together!

4 thoughts on “Absorption of Protein: How Much is Too Much? ”

  1. Hello Kathy! Great article! Question for you: I take between 1000 – 1500 mg of lysine tablets per day to ward off cold sores ( so awful! ) and I believe this is an amino acid, yes? After reading your article, that discusses imbalance in amino acids, do you think there is any concern in taking these supplements, which do seem to be effective in reducing cold sores for me? Just want to be cautious! Thanks for your advice!

    1. As long as you haven’t had any GI issues and aren’t taking any meds that have adverse interactions with lysine, you should be ok. Lysine is an essential amino acid, so just get a variety of protein types from whole foods throughout your day to ensure plenty of the other EAAs!

  2. lrhvermontgmailcom

    Hi Kathy! I work with a lot of RDs and I find this article of yours incredibly helpful in understanding proteins and how they are metabolized. Appreciate the links provided to supporting documents! Keep the info and insights coming!

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