Cold-water immersion is popular among athletes, based on the belief it enhances post-exercise recovery. But growing evidence suggests it might actually impair the recovery process.

In fact, we previously showed that cold-water immersion after exercise reduces muscle protein synthesis rates (the process driving muscle recovery and adaptation). In that study, we hypothesized that this might be due to reduced muscle blood flow and nutrient delivery.

So, in our new study (in young active adults), we investigated the effect of post-exercise cold-water immersion on:

• Muscle microvascular blood flow
• Amino acid incorporation into muscle tissue

We used a within-subject, unilateral design: one leg was submerged in cold water (8°C), and the other in thermoneutral water (30°C).

Key findings:

• Microvascular blood flow was 70% lower in the cold leg when compared to the thermoneutral leg.
• The reduction in microvascular blood flow correlated with the lower amino acid incorporation into muscle tissue

What does this mean?

Our findings provide further mechanistic evidence that cooling strategies after exercise may hinder muscle recovery and adaptation.

Link to our study:
Betz et al, Post-exercise cooling lowers skeletal muscle microvascular perfusion and blunt amino acid incorporation into muscle tissue in active young adults, Med Sci Sports Exerc, 2025

Go to the next uncatogorized infographic:
Hot water baths do not increase muscle growth?

Source: Does cooling affect muscle blood flow?

Protein distribution: beneficial, detrimental, or inconsequential?

The concept of protein distribution suggests that how you distribute your meals throughout a 24-hour period strongly impacts your overall anabolism.

There are contrasting lines of thought of what meal frequency is preferred.

In intermittent fasting or time-restricted feeding, food is consumed in a relatively short feeding window. By not eating for most of the day, the goal is to be in a fasted/catabolic state, which is believed to be healthy. For example, it is thought to speed up the removal of damaged proteins (autophagy).

However, not eating does not equal being in a fasted state. If you have a lot of protein in your feeding window, that protein can still be digested for a long time, depending on the amount and type of protein in your last meal (> 12 h). In other words, not eating for 12 hours does not necessarily mean you have spent any time in a fasted state.

In sports nutrition, the opposite strategy is applied: eat multiple meals spread out over the day to optimize anabolism. The main thought here is that the anabolic response to a meal is only short-lived (although our recent work STRONGLY challenges this).

In practice, most protein is consumed as whole-food mixed meals that are slowly digested and will sustain high levels of plasma amino acids and anabolism throughout the day, regardless of meal frequency. Therefore, the amount of time spent with elevated amino acid levels and in an anabolic state likely has a lot more to do with the amount and type of protein consumed than meal frequency.

While there is some evidence that protein distribution may impact anabolism, the majority does not support it. In fact, even with intermittent fasting, there is no clear detrimental impact on muscle loss. Intermittent fasting represents an extreme model of supposedly poor protein distribution. Thus if you don’t observe an effect with such an extreme model, it seems silly to argue about 4 vs 5 meals a day for example.  Therefore, protein distribution appears to have little to no practical impact on muscle mass in practice.

Our paper:
Trommelen et al. Protein Intake Distribution: Beneficial, Detrimental, or Inconsequential? IJSNEM, 2024

Source: Protein intake distribution: Beneficial, Detrimental, or Inconsequential for Muscle Anabolism?

Protein is essential to build muscle.

But does the type of protein matter?

Typically, bodybuilders eat a lot of meat. But more recently, plant-based diets have been getting a lot of attention.

A plant-based diet may have benefits with respect to ethical, sustainability, and general health issues.

But now you even start to hear claims that plant protein is just as good, or even better, than animal protein for athletes.

So in this article, we’re going to discuss:

• why you need protein for muscle growth
• why plant protein CAN be less effective
• why you can still maximize gains on a plant-based diet

1. Why we need protein

We recently published a scientific review article that discussed the potential of plant protein to stimulate muscle growth (Pinckaers, 2021).

So let’s first quickly cover the basics.

Why do we need protein in the first place? After you ingest protein, it is digested in smaller building blocks called amino acids.

The amino acids then get absorbed from the gut into the bloodstream.

From the circulation, these amino acids are taken up by tissues such as muscle.

And finally, the amino acids are then incorporated into protein.

In muscle, this process is called muscle protein synthesis.

So, if you don’t eat protein, your tissues have no building blocks for growth.

You can’t build muscle out of thin air.  

A lot of research has shown that the ingestion of protein stimulates muscle protein synthesis (Trommelen, 2019) and in the long-term improves muscle mass gains (Cermak, 2011).

Therefore, most athletes consume a high-protein diet (Gillen, 2017).

So clearly, the amount of protein is important.

But what about protein quality?

Are all types of protein equally effective?

For example, how does plant protein compare to animal protein?

2. Plant protein has a lower quality?

Several studies suggest that plant protein is less potent at stimulating muscle protein synthesis when compared to animal protein (e.g. 1, 2, 3).

In line, some studies suggest that plant protein supplementation results in lower muscle mass gains when compared to animal protein (4, 5)

However, it’s important to note that most studies observed no significant differences between plant and animal protein.

But we’ll get back to that later on…

For now, it’s important to realize there are:

• some studies suggest that plant-based protein has a lower quality
• more studies suggest no difference between plant and animal protein
• no studies suggest plant-protein has a higher quality

So together, this is a somewhat unfavorable picture for plant protein.

However, this does not mean that a plant-based diet has to limit your gains.

Because if we understand why plant protein sometimes appears to have a lower quality…then maybe we can fix them.

2.1 – Lower essential amino acid content

There are various factors that can explain the lower quality of plant protein.

The first is a lower essential amino acid content.

Dietary protein is made up of about 20 common amino acids.

Several of these are classified as essential amino acids.

Essential amino acids cannot be made in sufficient quantity by the body, and therefore it is essential that you consume them in your diet.

Per gram of protein, animal proteins usually have more essential amino acids when compared to plant proteins.

2.2 – Limiting amino acids

But it’s not just the total amount of essential amino acids that matters.

It’s also the composition of essential amino acids.  

Imagine building a house.

You can’t build a functional house if you have tons of bricks and windows but NO doors.

It’s the same with building muscle protein.

You need all the individual essential amino acids.

So if a protein source lacks 1 of the EAA, you miss an ingredient for protein synthesis.

And many plant proteins are deficient in one or more essential amino acids.

For example, many plant proteins are deficient in lysine or methionine.

So both the total amount of essential amino acids and the essential amino acid profile are important factors for protein quality.

But there is another issue.

2.3 – lower digestibility

If protein is not properly digested, the amino acids will never reach the muscle.

And plant protein typically has a lower digestibility than animal-based protein.

This is because plant products often have anti-nutrients.

So, plant protein has multiple characteristics that make its quality inferior to animal protein.

So let’s discuss some potential solutions.

3 – Potential solutions to compensate

3.1 – Eat more

The first solution is straightforward: eat more plant protein.

Ultimately, the goal of eating protein is to provide your tissues with enough of all the amino acids.

So, you can compensate for the lower digestibility and the suboptimal amino acid profile by simply eating more.

However, eating more may not always be practical.

On average, protein density is much lower in plant protein compared to animal protein.

Getting 20 g of protein from a steak is easy. But good luck getting 20 g of protein from potatoes.

And it would also bring along a lot of additional calories.

And indeed, vegetarians and especially vegans, have a lower protein intake than meat eaters (Davey, 2003).

Of course, there are plant products with high protein density.

But simply switching to a plant-based diet will likely lower protein intake if you don’t make a conscious effort to have a high protein intake.

So simply eating more food does not seem an efficient solution.

3.2 – Protein extraction

However, protein extraction may be helpful to get more protein in.

Protein supplements are an easy way to get in extra protein, without additional calories from carbohydrates, or without filling you up as much as whole-foods.

In addition, protein supplements are highly digestible, solving another issue.  

So adding a protein supplement to your diet can be a huge help.

3.3 – Protein fortification

Another strategy is to fortify foods that are low in one or two essential amino acids.

For example, there are a lot of plant-based meat replacements.

Some of these products are now getting enriched with lysine and/or methionine, because those are often lacking.

Just that small addition can help improve the overall protein quality.

3.4 – Protein blends

And another option to solve the issue of an essential amino acid lacking is by strategically mixing two or more plant proteins.

If one protein is low in lysine and high in methionine, it’s the perfect partner for a protein that is high in lysine and low in methione.   

4. Why do many studies find no difference?

Ok, now we have a good understanding of potential issues of plant protein, and how to solve those issues.

Now, lets get back to what I said earlier.

I’ve mentioned there are many studies that show no difference in muscle mass gains when animal and plant protein is supplemented.

In these studies, subjects typically perform exercise approximately 3 times a week for a couple of weeks.

After each training session, they receive either a plant or animal protein shake.

But is it really surprising that many of these studies observe no effect?

Not really.

Imagine that of a 20 g whey protein shake, all 20 g gets built into muscle.

It definitely would not, but we’ll just use it as an example.

Of the 20 g soy protein shake, only 10 g gets built into muscle.

That would mean animal protein is twice as effective. That’s a huge difference!!!

With 3 training session a week, the whey protein would result in 60 g of muscle mass, and the soy in 30 g.

During a 10 week study, that would equal 600 vs 300 g muscle mass gain.

I would say that 300 g is a meaningful difference.

But in research, you have to be conservative with potential conclusions.

The first issue is the measurement equipment.

Even the best measurement equipment is not perfect and introduces variation.

So then you don’t know if the difference is real or simply measurement variation.

And then you also have to stick to specific statistical rules.

So often, the data observes a slightly higher gain in one group when compared to the other.

But then the statistics are not significant, which basically means you are not sure enough that the difference is real and not a coincidence (simplified explanation, for more details see 6).

And that was based on 100% of the whey protein going to muscle mass.

But in reality, our research has shown you should be very happy if 20% of the ingested protein ends up in your muscle.

So the difference would be more like 120 vs 60 g.

So even if animal protein is twice as effective….

…it would still be extremely difficult to show it in relatively short studies comparing a few animal vs protein shakes per week.

5. Full vegan diet

To really figure out a difference between plant vs animal protein, you can compare an entire vegan diet to an omnivorous or even a carnivore diet where the difference occurs at every meal.

And fortunately, this has recently been investigated (Hevia-Larrain, 2021).

One group of subjects were omnivorous and the other group were vegans.

They performed a resistance training program for 12 weeks.

And both groups gained the same amount of muscle.

So even when the whole diet was plant-based, it did not reduce muscle mass gains.

But there are a few things to take into consideration.

The protein intake was high: 1.6 g/kg/d.

As we discussed, as long as you consume enough protein, protein quality is not that big of a concern.

But habitual intake was only 0.9 and 1.2 g/kg/d for the vegan and omnivorous group.

So a habitual diet may not be enough.

In this study the subjects had to use highly-digestible supplement to reach their target intake.

So this study shows that even an entirely plant-based diet can gain the same as an omnivorous diet.

But it does not mean this is always the case.

For example, be careful with lower protein intakes or when the entire diet is from lower digestible whole-foods.

6. Conclusions

I often see US vs THEM mentality in these type discussions.

For example, some omnivores seem to want the plant-based protein to be bad. 

But remember that omnivores also eat quite a lot of plant protein – around 40% (Gillen, 2017).

So it doesn’t make much sense to want plant protein to be of lower quality, that means you’re worse off yourself.

So to summarize:

1. plant-based protein has various characteristics that lower its quality
   • lower EAA content
   • unbalanced EAA profile
   • lower digestibility
2. some studies suggest that plant protein seems to result in lower muscle mass gains
3. there are various solutions to solve these issues, such as eating more protein to compensate and using highly digestible protein supplements
4. with a bit of effort, it is possible to maximize the gains with a plant-based diet

Our scientific review publication:
Pinckaers et al. The Anabolic Response to Plant-Based Protein Ingestion. Sports Med, 2021

Source: Plant or animal-based protein for muscle growth?

Studies are typically relatively short (weeks to months). However, we often want to know what happens with longer-term adherence to interventions such as a training protocol or a diet.

Therefore, data from studies needs to be extrapolated; make assumptions what would happen if the interventions would be continued. 

Extrapolation requires assumptions. Essentially, we’re trying to make an educated guess what would happen. This guess should be based on the available data and logic.

For example, if a study observes 5 kg fat loss after 12 weeks on a diet, that does not necessarily mean you can expect an additional 5 kg fat loss in the next 12 weeks. 

It is important to realize that extrapolation is heavily influenced by personal bias.

For example, if you are a proponent of a certain training style or diet, it is more likely that your extrapolation is (too) optimistic.

It has been shown that a ketogenic diet decreases exercise performance, while a high carbohydrate diet increases exercise performance during a 3-week training program.

A common critique is that this study gives a misleading picture, because the ketogenic diet is claimed to require a longer adaptation phase. Once the athlete is fully keto-adapted, the superiority of the ketogenic diet would become apparent (scenario 4).

But scenario 4 is a drastic change compared to the observed data, requires several assumptions, and therefore the burden of proof would be on those would claim this scenario is most likely. 

An assumption of this scenario is that keto-adaptation requires >3 weeks, but there is no convincing supporting evidence.

The second assumption of this scenario is that further keto-adaptation would drastically improve exercise performance. However, it could also be argued that further keto-adaptation would only further decreases exercise efficiency and performance (scenario 1).

Source: Extrapolation or wishful thinking

Protein provides the building blocks for tissues such as muscle mass.

Ingested protein is digested into amino acids. Subsequently, these amino acids are:

• Absorbed in the gut
• Released into the circulation
• Taken up by tissues
• Incorporated into tissues (protein synthesis)

Protein synthesis allows tissues such as muscle to recover, adapt, and grow. Therefore, many athletes consume a high protein diet. Protein and (free) amino acids supplementation are also common strategies to increase protein intake.

But is there a difference between protein and amino acids supplements? If protein is digested into amino acids, is it simply the same? Or perhaps amino acid supplements are more effective because they don’t require digestion?

While protein powders have a high absorption (>90%), amino acids are thought to have essentially 100% absorption. In addition, amino acids will be absorbed more rapidly and result in a large spike of amino acids into the circulation. Such a spike is thought to be an anabolic signal that triggers muscle protein synthesis.

So theoretically, a free amino acids supplement could potentially be more efficient than a protein powder that contains exactly the same amount and composition of amino acids.

Therefore, we designed a study to compare a protein versus amino acid supplement.

Study design

We investigated the impact of a:

1. 30 g milk protein supplement
2. 30 g free amino acid supplement (matched for amino acid content with the milk protein)

We measured how much of the ingested amino acids were released into the circulation, muscle protein synthesis, and whole-body protein net balance (more outcomes in the paper).

Results

As expected, the amino acid supplement resulted in a faster release of amino acids into the circulation (indicating a faster absorption rate).

In addition, the total amount of ingested amino acids released into the circulation was higher in the amino acid group. However, it appeared that the milk protein group was still in the process of catching up at the end of the 6-h study period, so the difference may have become smaller.

While the protein supplement resulted in a positive whole-body protein net balance, the increase was lower when compared to the free amino acid supplement.

Whole-body protein net balance strongly correlated with the ingested amount of amino acids released into the circulation. Since the total release of ingested amino acids was still increasing in the milk group, it is likely that the difference between the treatment in whole-body protein net balance would also decrease beyond the 6-h mark. In addition, we believe that people should be careful in drawing practical conclusions based on whole-body protein metabolism but should rather focus on muscle protein synthesis (for a more in depth explanation, see XYZ).

Therefore, we also measured protein synthesis specifically in muscle tissue. Both supplements were effective at stimulating muscle protein synthesis, but there was no difference between the treatment.

Discussion

These data suggest that when matched for amino acid content, the form of supplementation (protein or amino acids) does not impact muscle protein synthesis. However, it is possible that the results would be different in other scenarios.

For example, the subjects received 30 g of protein/amino acids. This may have been enough to maximally stimulate muscle protein synthesis in both treatments. A difference in the efficiency of the supplements may only become clear when smaller (suboptimal) amounts are ingested (<20 g).

In addition, potential benefits from amino acids may become more pronounced when they are compared to less digestible protein sources or in subjects with suboptimal digestion.

Our study:
Weijzen et al. Ingestion of Free Amino Acids Compared with an Equivalent Amount of Intact Protein Results in More Rapid Amino Acid Absorption and Greater Postprandial Plasma Amino Acid Availability Without Affecting Muscle Protein Synthesis Rates in Young Adults in a Double-Blind Randomized Trial. J Nutr, 2021

Source: Free amino acids or intact protein for anabolism?

Everyone who lifts weights knows the very tight feeling in your muscles during and after a heavy set, which is often called “the pump”. One of the factors that gives this tight feeling is an increase in muscle blood flow.

In this article, we’ll cover everything you need to know on blood flow and its importance for muscle recovery and growth.

What is muscle blood flow and why is it important?

The main function of blood is transport. On one hand, blood delivers useful substances to the muscle, such as oxygen, nutrients, hormones, and even more. On the other hand, blood removes waste products from the muscle, such as CO2.

The heart pumps blood around the vascular system, which goes from large arteries into smaller arteries and eventually to the microcirculation of the muscle. The muscle microcirculation consists of arterioles and the smallest blood vessels: the capillaries.

The capillaries are located in between the muscle fibers and this is where the delivery of oxygen, nutrients, and hormones, but also the removal of waste products takes place. However, it is good to note that blood flow through the capillaries is regulated by the arterioles being open or closed. After the blood passes through the capillaries, it goes back to the heart via the small veins (i.e. venules) and the larger veins (see figure below for overview).

It is clear that muscle blood flow plays a very important role in normal muscle functioning. However, the importance of blood flow becomes is even more pronounced during exercise. During exercise, the metabolic demand (need for energy) of muscle increases. To match these energy needs, muscle blood flow can increase up to 100-fold compared to resting levels.

This incredible increase in muscle blood flow is a product of several factors, such as increases in:

• Cardiac output (how much blood the heart is pumping into the arteries)
• Arterial blood flow (blood supply towards the muscle)
• Capillary recruitment (small blood vessels that open up)

Acute effects

Muscle protein synthesis is the process driving muscle adaptations and growth.

Increasing blood flow with a pharmaceutical agent has been shown to increase muscle protein synthesis (Timmerman, 2010). However, when an additional pharmaceutical agent was added to prevent an increase in blood flow, it also prevented the increase in muscle protein synthesis (see figure below)1In this study, insulin increased blood flow and as a result MPS. However, in most situations this does not happen. See our infographic (1), blog post (2) or paper (3) here (in increasing level of complexity) .

This suggests that an increase in blood flow can stimulate muscle protein synthesis.

However, it is important to realize that pharmaceutical agents typically have a much stronger effect than what normally occurs in our bodies. For example, some food substances, such as cacao, can also increase blood flow. However, this effect is much less compared to pharmaceutical agents and not strong enough to stimulate muscle protein synthesis (Phillips, 2016).

These acute studies show that an increase in blood flow can increase muscle protein synthesis rates, but only when the increase in blood flow is very large.

Chronic effects

However, blood flow is an ongoing process, it never stops. So can long-term changes in blood flow impact muscle growth?

Unfortunately, it is practically impossible to study long-term changes in blood flow. Instead, we can look at anatomical structures related to blood flow, such as the number of capillaries in muscle tissue (i.e. capillarization).

We have performed a study to investigate the impact of capillarization on muscle mass gains (Snijders, 2017). Older subjects were divided in a low or a high capillarization group. The low capillarization group did not gain muscle mass during a 12-week resistance training program. In contrast, the high capillarization group did gain muscle mass. Similar results were found in another study in older men and women (Moro, 2019). Therefore, it appears that low muscle capillarization could limit muscle mass gains during resistance training in older adults.

It should be noted that older adults have lower capillarization when compared to young.  Therefore, it is possible that capillarization is not a limiting factor in younger adults. However, there are some indications that capillarization can also be an important factor in muscle regulation in young adults. For example, it has been shown that capillarization is related to the activation of satellite cells, which are the stem cells of muscle tissue that are important for muscle adaptation (Nederveen, 2018).

So can you increase capillarization?

Yes, it is possible by being very active and exercising a lot. So if you’re already training to maximize muscle growth, there may not be much more that you can do.

However, many researchers (including myself) are looking into new strategies to increase capillarization and blood flow, such as different types of exercise, heating and cooling, blood flow restriction, and massage.

Source: Blood Flow – Key to muscle growth?

The main purpose of a journal club is to learn how to (critically) read a scientific paper and discuss it with others. We will go through the paper, and discuss it very systematically. This way you can see our thought process and learn how to do it yourself. At the end, we’ll also briefly cover practical takeaways of the study and of related research on the topic. Finally, there will be Q&A about the paper or any other topic chat wants to ask about.

Paper that is discussed:
Nyakayiru et al. Beetroot Juice Supplementation Improves High-Intensity Intermittent Type Exercise Performance in Trained Soccer Players. Nutrients, 2017

See the upcoming live stream schedule

Source: Beetroot juice to improve exercise performance – Journal Club #01

What is the commonality between the justice system and statistics?

And why is this important to know for evidence-based nutrition and fitness?

In the justice system, you are considered innocent until proven guilty. This means that you don’t have to prove that you’re ”not guilty” of a crime. “Not guilty” is the default option. So unless convincing evidence shows that you are guilty of something, you will be considered not guilty.

However, just because a judge or jury finds you “not guilty”, this does not mean that you actually are not guilty.

It’s not hard to imagine that you can commit a crime and get away with it, because there’s no evidence. Clearly, the verdict “not guilty” does not necessarily prove that you are not guilty.

In order words: “the lack of evidence for being guilty is not proof of evidence of being guilty”.

Traditional statistics are kind of similar. The default option is that a statistical comparison (e.g. between the effect of two diets on fat loss) is considered “not significantly different”.

However, many people incorrectly interpreted a lack of evidence for a difference as proof of evidence of equality.

In order words: they believe that a study which observed no significant difference is proof that the two diets produce identical results.

When no significant difference is observed, a more accurate conclusion is that things remain unclear.

Maybe there is actually a difference, but the data of this study simply did not provide enough evidence. For example, if the study has a small number of subjects, it will have relatively little data set. It’s very difficult for a small data set to be be considered strong proof of something (it’s easy to wonder what would happen if you included more subjects). So a study with a small number of subjects often observes no significant difference between diets, even if in reality one diet is superior.

Maybe the diets do produce identical results. That’s possible, but you simply cannot conclude that based on standard statistics.

If you find the video interesting, please help us share it. If you want way more detail and practical examples, see our article:
No significant difference? Oh really? How to recognize weaknesses in muscle hypertrophy studies.

Source: Research statistics: guilty or not guilty?

Our new review:
Gut amino acid absorption in humans: concepts and relevance for postprandial metabolism

*Note: this is a methodological review. Therefore, it is not focused on practical takeaways. But feel free to ask your protein questions in the comments*

Not all dietary protein has the same nutritional quality. An important factor that impacts protein quality is its absorption in the gut.  

For example, protein powders have a very high absorption (~95%), while whole food plant-based proteins are typically lower (~70%).

We found that much of the literature discussing protein absorption is confusing, inconsistent, and/or incorrect.

For example, digestibility, absorption, release in the circulation, and bioavailability seem to be used interchangeably, while we consider them all different. See the table below for the definition we propose:

Hence, the common term “amino acid digestibility” is a misnomer. Amino acids are not digested, they are absorbed. “Amino acid absorbability” is clearer and more accurate.

The same is true for the various essays used to assess gut absorption.

Apparent, standardized, true, and real ileal digestibility are used inconsistently. As a consequence, this inconsistency and inaccuracy spills over to protein quality scores.

Another limitation of protein quality scoring systems (e.g. DIAAS) is that they account for total absorption, but not absorption rate.

However, a more rapid absorption may result in a greater anabolic response.

Therefore, a kinetic approach is preferred to take into account all experimental and individual factors.

Postprandial whole-body protein net balance can only be assessed using intrinsically labeled protein, but the method is expensive and complex (Trommelen, 2021).

In summary,

• There is a need for more consistency and accuracy in the terminology of protein digestion and amino acid absorption.
• Protein quality scoring systems have limitations.
• Intrinsically labeled protein can accurately assess the anabolic response.

Open access reference:
Trommelen et al, Clinical Science Open Science, 2021

Source: Gut amino acid absorption in humans: concepts and relevance for postprandial metabolism

There are many advantages of this method.

Building amino acid tracers into dietary protein allows assessment of protein digestion and absorption into the circulation. This is preferred over plasma amino acid concentrations that are just a proxy.

Assessing protein digestion and absorption is also an essential step in accurately assessing whole-body protein net balance (Trommelen, 2021; or the blog version here).

Protein synthesis can also be specifically measured in muscle tissue. This measurement requires that the enrichment of amino acid tracer used to calculate it is in a steady-state (simplified: the percentage of tracer vs the normal amino acid). However, normal protein only has unlabeled protein. Therefore, the ingestion of protein results in a drop of the amino acid tracer enrichments (called dilution; see panel A below).

Intrinsically labeled protein solves the issue. Because there are also amino acid tracers in the protein, the plasma amino acid enrichments is not disturbed (panel C). This will allow better assessment muscle protein synthesis rates.

In addition, a unique tracer in the dietary protein (so not in the infusion), allows the assessment of the amount of dietary protein that is build into muscle tissue (de novo muscle protein synthesis; panels E and F).

Instead of using intrinsically labeled protein, amino acid tracers can also be added to protein in an attempt to stabilize plasma amino acid enrichments (panel A below). However, because free amino acids are much more rapidly digested than dietary protein, the speed at which these amino acids appear into the circulation does not match the speed of the protein. While adding free amino acid is generally an improvement compared to doing nothing, it is suboptimal compared to the application of intrinsically labeled protein.

When there is no steady-state, it is also important to have a high sampling frequency to assess the disturbance. A low sample frequency can give the false impression there is a steady-state simply because you miss the peaks and nadirs.

In addition to an explanation of the methodology, the review also describes findings established with the intrinsically labeled protein method.

For example, it is clear that physical activity strongly impacts how much of ingested protein is built into muscle tissue. This is a spectrum, where doing nothing is the worst (e.g. prolonged bed rest) and intense resistance training is the best.

Much more details here in our OPEN-ACCESS full article:
Trommelen et al. Comprehensive assessment of postprandial protein handling by the application of intrinsically-labelled protein in vivo in human subjects. Proc Nutr Society, 2021

Source: Comprehensive assessment of postprandial protein handling by the application of intrinsically labelled protein [OUR REVIEW]