An FSR of 0. This translates to a completely new muscle every 3 months. However, protein ingestion would disturb this steady state, as a lot of normal amino acids will enter the blood, thus throwing off the tracer amino acid to normal amino acid ratio.

However, our lab has gotten fancy in this area. We have been able to produce highly enriched intrinsically labeled protein. This means that the amino acid tracers have been build into our protein supplements.

So as our intrinsically protein supplements are absorbed, both amino acid tracers and normal amino acids enter the blood. Therefore the steady state is not disrupted and FSR can be calculated more accurately. Example paper FSR with and without intrinsically labeled protein : Holwerda, You can trace the amino acids from the protein: first, as they are digested, then as they appear in the blood, subsequently they are taken up by the muscle, and ultimately some of them are built into actual muscle tissue.

So we can measure how much of the protein you eat, actually ends up into muscle tissue. This is called de novo muscle protein synthesis. Example paper de novo muscle protein synthesis: Trommelen, When measuring muscle protein synthesis, we can measure mixed muscle protein synthesis all types of muscle protein together.

But muscle protein can be further specified into fractions. The main muscle protein fraction is myofibrillar protein. Myofibrillar proteins contract and represent the majority of the muscle mass. This fraction is highly relevant for building muscle mass and strength. Mitochondrial proteins only represent a small part of the muscle.

Mitochondria are the powerhouses of the muscle, they burn carbohydrate and fat for fuel. Therefore mitochondrial protein synthesis is more informative about energy production capacity in the muscle and more relevant for endurance athletes and metabolic health. Sarcoplasmic protein contains various organelles such as the endoplasmic reticulum and ribosomes.

Intramuscular connective tissue protein represents collagen protein in the muscle. This collagen helps transfer the muscle force generated by myofibrillar protein.

Example paper myofibrillar vs mitochondrial protein synthesis: Wilkinson, , or intramuscular connective tissue protein synthesis Trommelen, More recently, deuterium oxide D2O, also called heavy water is getting popular to measure muscle protein synthesis.

Example paper deuterium oxide: Brooks, There is evidence that a variety of signaling molecules are involved in the regulation of muscle protein synthesis.

Most notably, the protein from the mTOR pathway. Research of these molecular markers is very important to better understand how physiological processes are regulated and ultimately can be influenced by exercise, nutrition or even drugs. Therefore, you should be very skeptical to draw conclusions based on studies that only measure molecular markers of muscle protein synthesis and muscle protein breakdown.

Methods are not necessarily good or bad. But the interpretation of the data based on these methods can be wrong. We recently showed that resistance exercise does not increase whole-body protein synthesis Holwerda, So should we conclude that resistance exercise is not effective to build muscle?

Whole-body protein metabolism measures the synthesis of all proteins in the body. Other tissues in the body have much higher synthesis rates, and therefore, the muscle only has a relatively small contribution to the total whole-body protein synthesis rates.

In the same study, we also measured muscle protein synthesis using the FSR method both with and without intrinsically labeled protein and de novo muscle protein synthesis. All 3 methods showed that resistance exercise was anabolic for the muscle.

Imagine we would have only measured whole-body protein synthesis. Our study would give the wrong impression that resistance exercise is not anabolic, as we saw no increase in whole-body protein synthesis rates.

Shortly, it says that very large protein meals are beneficial because they reduce protein breakdown. Again, this study gave the wrong impression, because whole-body protein breakdown was mistaken for muscle protein breakdown the latter was not measured in this study.

Both the 40 gram and the 70 gram dose were equally effective at stimulating muscle protein synthesis. One of the purposes of measuring muscle protein synthesis is to study if an intervention helps to build muscle or maintain muscle mass. Let me first get something out of the way: we have no bias for either acute or long-term studies.

We run both at our lab, and do some of the most expensive studies in the field of either type. A lot of people seem to think that based on this study, muscle protein synthesis measurements do not translate to actual muscle mass gains in the long term. But that conclusion is way beyond the context of the study.

This study measured muscle protein synthesis in the 6 hours after a single exercise bout. However, resistance exercise can increase muscle protein synthesis for several days.

So a 6-hour measurement does not capture the entire exercise response. This study showed that measuring muscle protein synthesis for 6 hours does not predict muscle mass gains. That is totally different from the conclusion that muscle protein synthesis regardless of measurement time does not predict muscle mass gains.

This was followed up by a study which used the deuterium oxide method to measure muscle protein synthesis rates during all the weeks of training not just a few hours after one session , and found that muscle protein synthesis did correlate with muscle mass gains during the training program Brooks, More recently, a study found that muscle protein synthesis measured over 48 hours after an exercise bout did not correlate with muscle mass gains in untrained subjects at the beginning of an exercise training program, but it did at three weeks of training and onwards Damas, While untrained subjects have a large increase in muscle protein synthesis after their initial exercise sessions, they also have a lot of muscle damage.

So muscle protein synthesis is mainly used to repair damaged muscle protein, not to grow. After just 3 weeks of training, muscle damage is diminished, and the increase in muscle protein synthesis is actually used to hypertrophy muscles.

So do these studies show that muscle protein synthesis predicts muscle mass gains, but only in the right context. A huge benefit of muscle protein synthesis studies is that they are more sensitive than studies that measure actual muscle mass gains. This means that muscle protein synthesis studies can detect an anabolic effect easier than long term studies which simply miss it long term studies might draw the wrong conclusion that something does not benefit muscle growth when it actually does.

For example, it has been shown time and time again that protein ingestion increases muscle protein synthesis. Muscle mass gain is simply a very slow process. You need to do a huge study, with a huge amount of subjects, who consume additional protein for many months, before you will actually see a measurable effect of protein supplementation.

We performed a meta-analysis combining the results of individual studies on the effect of protein supplementation on muscle mass gains. We demonstrated that only 5 studies concluded that protein supplementation had a benefit, while 17 did not! However, most of the studies that showed no significant benefit, did show a small non-significant benefit.

When you combine all those results, you increase the statistical power and you can conclude that protein supplementation actually does improve muscle mass. So in this case, most long-term studies gave the wrong impression, and muscle protein synthesis studies are actually preferred.

There are a lot of long-term studies that have a relative small number of subjects and a small study duration and conclude that an intervention did not work for example, protein supplementation, or X versus Y set of exercise for example.

However, the studies were doomed to begin with. They needed to be 3 times as big and 2 times as long to have a chance to find a positive effect. Now if the effect of giving additional protein is already extremely hard to detect in long-term studies, how realistic is it to find smaller effects?

For example, optimizing protein intake distribution throughout the day has been shown to optimize muscle protein synthesis rates Mamerow, Areta, However, this effect is smaller than adding another protein meal. So the effect of protein distribution is almost impossible to find in a long-term study.

For such a research question, acute muscle protein synthesis studies are simply much better suited. The second big benefit of muscle protein synthesis studies is that they give a lot more mechanistic insight. They help you understand WHY a certain protein is good or not that good at stimulating muscle protein synthesis for example, its digestion properties, amino acid composition etc.

These kinds of insights help to better understand what triggers muscle growth and come up with new research questions. These kind of insights are very hard to obtain in long-term studies, which typically only show the end result of the mechanisms.

The benefits of measuring muscle protein synthesis include the sensitivity, controlled environment, and they allow you to investigate questions that are almost impossible to answer in long-term studies.

Again, we do both and each has its purpose and build on each other. Usually, muscle protein synthesis studies are performed to see if something work as they are very sensitive and why it works.

Only when you have both, you have pretty convincing evidence that your intervention does what you claim it to do.

Multiple sets increase muscle protein synthesis more than a single set Burd, A higher weekly training volume number of sets to muscle results in a greater muscle mass gains Schoenfeld, It is often recommended that a rep range of reps per set is optimal for muscle growth.

The American College of Sport Medicine position stand states ACSM, :. For novice untrained individuals with no RT experience or who have not trained for several years training, it is recommended that loads correspond to a repetition range of an repetition maximum RM.

For intermediate individuals with approximately 6 months of consistent RT experience to advanced individuals with years of RT experience training, it is recommended that individuals use a wider loading range from 1 to 12 RM in a periodized fashion with eventual emphasis on heavy loading RM using 3- to 5-min rest periods between sets.

However, these recommendations lack evidence. The main takeaway here is that there are no magic rep ranges that are superior for muscle growth. It is unclear whether each set should be taken to failure. Muscular failure decreases performance on subsequent sets, thereby reducing training volume.

Perhaps performing a set with reps left in the tank will still give a near-maximal stimulus to the muscle, without much of the associated fatigue. If sets are not taken close to failure, the muscle protein synthetic response will be small Burd, But at least in untrained subjects, training close to failure appears to produce similar muscle mass gains as training to complete failure Nóbrega, A longer rest period between sets increases the larger post-exercise muscle protein synthetic response compared to a short rest period 5 vs 1 min McKendry, In agreement, a longer interset rest period improves muscle mass gains compared to a shorter rest period 3 vs 1 min Schoenfeld, A single bout of resistance exercise can stimulate muscle protein synthesis for longer than 72 hour, but peaks at 24 h Miller, Indeed, training each muscle group at least twice a week results in larger muscle mass gains Schoenfeld, The total muscle protein synthetic MPS response determined by the increase in MPS rates and the duration of these increased rates is decreased in trained subjects compared to untrained subjects Damas, However, the pattern of this decreased response is differs between mixed muscle protein synthesis the synthesis of all types of muscle proteins and myofibrillar protein synthesis the synthesis of contractile proteins: the relevant measurement for muscle mass.

The increase in mixed muscle protein synthesis is shorter lived in trained subjects. In contrast, myofibrillar protein synthesis rates do not increase as much in trained subjects, but the duration of the increase does not appear impacted.

The larger increase in the total muscle protein synthetic response seems like a logical explanation why untrained people can make faster much gains than experienced lifters.

However, this is not necessarily true. In untrained subjects, there is not only a large increase myofibrillar protein synthesis, but also in muscle damage following resistance exercise. A large portion of the myofibrillar protein synthesis is used to simply repair damaged muscle proteins, rather than growing muscle proteins.

In more trained subjects, here is a smaller increase in myofibrillar protein synthesis, but there is also much less or even minimal muscle damage following resistance exercise just weeks of training is enough to see these effects.

This means that in a trained state, the increase in myofibrillar protein synthesis can actually be used to actually increase muscle mass. When you correct for muscle damage, myofibrillar protein synthesis rates measured over 48 hour post-exercise recovery are similair in untrained subjects and after 10 weeks of training Damas, Of course, most athletes would hardly consider someone trained after just 10 weeks.

Unfortunately, little is know about how years of serious training impacts the muscle protein synthetic response to resistance exercise. Twenty gram of protein gives a near-maximal increase in MPS after lower body resistance.

When data of several studies was combined and the amount of protein was expressed per bodyweight, it was found that on average 0. However, the authors suggest a safety margin of 2 standard deviations to account for inter-intervidual variability, resulting in a dose of protein that would optimally stimulate MPS at an intake of 0.

More recently, it has been shown that the amount of lean body mass does not impact the response to protein ingestion Macnaughton, The authors speculated that this was related to the fact that this was following a session of whole-body resistance exercise compared to the lower-body exercise used in previous studies.

Protein sources differ in their capacity to stimulate MPS. This is best illustrated by study which compared the muscle protein synthetic response to casein, casein hydrolysate and whey protein. Casein is a slowly digesting protein. When intact casein is hydrolyzed chemically cut into smaller pieces , it resembles the digestion of a fast-digesting protein.

Consequently, hydrolyzed casein results in higher MPS rates than intact casein. However, the muscle protein synthetic response to hydrolyzed is lower than that of whey protein.

While both proteins are fast digesting, whey protein has a higher essential amino acid content including leucine Pennings, Animal based protein sources are typically have a high essential amino acid content and appears more potent than plant protein to stimulate MPS Van Vliet, However, there this can potentially compensated by ingesting a greater amount of plant protein Gorissen, Leucine is the amino acid that is thought to be most potent at stimulating MPS.

Peak plasma leucine concentrations following protein ingestion typically correlate with muscle protein synthesis rates Pennings, This supports the notion that protein digestion rate and protein leucine content are important predictors for anabolic effect of a protein source.

This is best illustrated by study Churchward-Venne, which compared the muscle protein synthetic response to five different supplemental protocol:. All five conditions increased muscle protein synthesis rates compared to fasting conditions. As expected from our earlier discussion on the optimal amount of protein, 25 gram of protein increased MPS rates more than just 6.

Interestingly, the addition of 2. The addition of a larger amount of leucine 4. This indicates that the addition of a relatively small amount of leucine to a low dose of protein can be as effective as a much larger total amount of protein.

Isoleucine and valine use the same transporter for uptake in the gut as leucine. Therefore, it is speculated that isoleucine and valine compete for uptake with leucine, resulting in a less rapid leucine peak which is thought to be an important determinant of MPS rates.

Carbohydrates slows down protein digestion, but have no effect on MPS Gorissen, In agreement, adding large amounts of carbohydrates to protein does not improve post-exercise MPS rates Koopman, However, the addition of carbohydrates to post-exercise protein has no effect on muscle protein synthesis or breakdown rates.

The effects of insulin on muscle protein breakdown rates are described in more detail in section 2, and the effects of insulin on muscle protein synthesis are further described in section 7.

Adding oil to protein does not slow down protein digestion or MPS Gorissen, It possible that oil simply floats on top of a protein shake in the stomach, and that a solid fat would delay digestion. One study has reported a greater increase in net muscle balance following full-fat milk compared to fat-free milk although this study used the 2 pool arterio-venous model which is not the most reliable measurement.

Most research has looked at isolated protein supplements in liquid form such as whey and casein shakes. This supports the protein dose-response relationship observed with protein supplements where 20 g of protein gives a near maximal increase in MPS.

Minced beef is more rapidly digested than beef steak, indicating that food texture impacts protein digestion.

However, there was no difference in MPS between these protein sources. Beef protein is more rapidly digested than milk protein. However, milk protein stimulated MPS more than beef in the 2 hours Burd, Between 2 and 5 hours, there was no significant difference between the sources.

This indicates that digestion speed does not always predict the muscle protein synthetic response of a protein source. As discussed in the previous section, the addition of carbohydrate powder or oil to a liquid protein shake does not impact muscle protein synthesis.

However, it is unknown how the components of large mixed meals interact. For example, the addition whole-foods carbohydrates such rice, potatoes, or bread to whole-food protein sources such as chicken. It can be speculated that the protein in mixed meals is less rapidly digested, which is typically but not certainly not always associated with a lower increase in MPS.

As described in my systematic review, insulin does not stimulate MPS Trommelen, Regardless whether insulin levels were kept low similar to fasted levels or very high, MPS rates were the same in all conditions.

In my systematic review, I describe the effect of insulin in other conditions including in the absence of amino acid infusion, but the conclusion remains that insulin does not stimulate MPS under normal conditions Trommelen, However, it should be noted that insulin stimulates MPS at at supraphysiological above natural levels doses Hillier, In the bodybuilding world, insulin is sometimes injected at supraphysiological doses to stimulate muscle growth.

Insulin inhibits muscle protein breakdown a bit, but only a little is needed for the maximal effect this is discussed in dept in section 2. Exercise improves the muscle protein synthetic response to protein ingestion.

Therefore, it has been suggested that protein intake immediately post-exercise is more anabolic than protein ingestion at different time points.

Probably the best evidence to support the concept of protein timing is a study which showed that protein ingestion immediately after exercise was more effective than protein ingestion 3 h post-exercise though this study used the 2 pool arterio-venous method which is not a great measurement of muscle protein synthesis Levenhagen, In contrast, a different study observed no difference in MPS was found when essential amino acid were ingested 1 h or 3 h post-exercise Rasmussen, In addition, resistance exercise enhances the muscle protein synthetic response to protein ingestion for at least 24 hour Burd, It is certainly possible that the synergy between exercise and protein ingestion is the largest immediately post-exercise and then slowly declines in the next 24 h hour.

However, these data suggest that there is not a limited window of opportunity during which protein is massively beneficial immediately post-exercise, that suddenly closes within a couple of hours.

Overal, no clear benefit to protein timing has been found in studies measuring muscle protein synthesis studies. As such studies are much more sensitive to detect potential anabolic effects compared to long-term studies measuring changes in muscle mass, it unlikely that long-term studies will observe benefits of protein timing.

However, this effect was largely explained by the fact that the protein supplementation increased total protein intake, rather than the specific timing of protein intake.

We performed a study to assess protein intake in well-trained Dutch athletes. Even some Olympic athletes were included.

We observed that athletes consumed ~1. The majority of the protein was consumed in the three main meals: breakfast, lunch and dinner. While this intake pattern has a reasonable distribution throughout the waking hours, amino acid availability is potentially low during the night.

This begs the question: does protein distribution throughout the day matter for muscle protein synthesis? Several studies suggest that protein should be reasonably distributed for optimal anabolism.

For example, an even balance of protein intake at breakfast, lunch and dinner stimulates MPS more effective than eating the majority of daily protein during the evening meal Marerow, But a too high distribution resulting in many mini snacks may also be suboptimal.

Providing 20 g of protein every 3 hours stimulates MPS more than providing the same amount of protein in less regular doses 40 g every 6 hours , or more regular doses 10 g every 1. While there are more studies that support the concept of protein distribution, there are even more studies that suggest it has no clear benefit.

If your goal is to absolutely maximize gains, it theoretically makes sense to try to aim for at least a reasonable protein distribution protein rich meals divided throughout the day. The muscle full effect is the observation that amino acids stimulate MPS for a short period, after which there is a refractory period where the muscle does not respond to amino acids.

More specifically, after protein intake, there is an lag period of approximately min before MPS goes up and peaks between minutes, after which MPS returns rapidly to baseline even if amino acid levels are still elevated Bohe, Atherthon, The muscle full effect has given birth to a theory on how to optimize protein intake throughout the day in the online fitness community.

It suggests that after amino acids levels have been elevated, you should let them drop down back to fasting levels to sensitize the muscle to amino acids again. Subsequently, protein intake will stimulate MPS again.

The suggested mechanism seems unlikely as many food patterns result in elevated amino acid levels throughout the whole day.

The traditional bodybuilding diet consists of very frequent, very high protein meals e. chicken, rice, broccoli 6 times a day. In fact, it was specifically designed with the goal of keeping amino acids elevated throughout the whole day so there would always be enough building blocks for form new muscle tissue.

Or intermittent fasting where all daily protein is eaten in a short time period usually 8 hours. These diets would only allow for a single ~90 min increase in MPS during a whole day. This is best illustrated in a study where the effect of protein was assessed in both rested and post-exercise conditions Churchward-Venne, Protein intake alone stimulates MPS in the h period after ingestion.

Subsequently, MPS rates fall back to basal rates. However, in post-exercise condition, protein stimulated MPS rates in both the h and the h period. It appears that the muscle full effect is not present in acute post-exercise conditions. As discussed above, an effective protein distribution optimizes MPS.

Protein supplementation Only three days of dieting already reduce basal MPS Areta, This shows that an energy deficit is suboptimal for MPS, however you can grow muscle mass while losing fat Longland, It is unclear if eating above maintenance is needed to optimize MPS.

Second, I will continue to further elaborate sections based on your feedback and add additional sections in the future. Lastly, please reference specific sections from this article when you see a discussion on muscle protein synthesis. People mistaking whole-body protein synthesis for muscle protein synthesis: see section 4.

Someone skeptical about a conclusion from a paper because muscle protein breakdown was not measured? Section 2 buddy. Someone claiming that protein supplementation is not effective based on a long-term study he read that found no improvement in muscle mass: section 5 got you covered.

Feel free to ask me questions about the methods, or interpretation on protein metabolism studies in comments or on Facebook. If I work out 3 days a week e. Tuesday, Thursday, Sunday and am looking to do a lean bulk, would you suggest ensuring there is more of a surplus the day of exercises and the following day during heightened protein synthesis than for example on Saturday when I would have had 2 days rest from the gym?

To clarify, I am assuming that on Saturday, there would not be much protein synthesis occurring from the Thursday workout, so if I were to have a surplus, would it be better to eat more on the Thursday and Friday and possibly to a maintenance calorie day on Saturday to limit fat when muscle gain is not likely to happen?

Energy balance on the short term does not seem to impact muscle protein synthesis. Your body sort of keeps track of the last couple of days and longer , rather than just the moment. You could play around with your strategy. We comprehensively discuss a range of physiological and methodological variables that, in our view and others Mitchell et al.

Physiological variables relate to inherent variability in the response of MPS to exercise and nutrition, the modulation of muscle protein metabolism with changing training status, the influence of a multitude of training paradigms, and genetics.

Methodological variables relate to subtle, yet important, technical considerations with regard to measurements of MPS and muscle hypertrophy. Muscle hypertrophy represents the primary phenotypic adaptation to RET Goldberg et al. The precise definition of skeletal muscle hypertrophy is a topic of current debate among the scientific community Damas et al.

Traditionally, muscle hypertrophy is defined as an increase in skeletal muscle mass and cross-sectional area CSA at the whole tissue and cellular levels Haun et al.

This definition is underpinned by the notion that an accretion of contractile i. The plasticity of skeletal muscle is mediated, at least in part, by the constant turnover or remodeling of muscle proteins. In this regard, two metabolic processes, MPS and muscle protein breakdown, act concurrently in response to various stimuli to repair, replace, and generate new muscle proteins leading to phenotypic adaptations.

Accordingly, it has been proposed that muscle hypertrophy following RET stems from a cumulative accretion of muscle proteins resulting from the repeated increase in response of myofibrillar—MPS to successive bouts of REx Hawley et al. Hence, according to this traditional definition of muscle hypertrophy, it may seem intuitively satisfying that assessment of the acute response of MPS to REx provides an informative tool when devising RET and nutritional interventions to maximize muscle hypertrophy in athletes and other exercisers.

We acknowledge that an alternative definition of muscle hypertrophy relates to an increase in skeletal muscle size accompanied by an increase in mineral, protein, or substrate abundance e.

This contemporary, and arguably more comprehensive, model of muscle hypertrophy also accounts for the growth of nonmyofibril components.

Accordingly, three different types of muscle hypertrophy have been proposed, namely myofibrillar hypertrophy, sarcoplasmic hypertrophy, and connective tissue hypertrophy, each with their own biological definition Haun et al.

Finally, connective tissue hypertrophy is defined as an increase in volume of the extracellular matrix of skeletal muscle accompanied by an increase in mineral or protein content.

A critical evaluation of skeletal muscle hypertrophy as a biological construct is beyond the scope of this text, and the reader is referred to several recent reviews on this topic Damas et al.

Nonetheless, we suggest that all three types of hypertrophy likely contribute to measured changes in muscle mass with RET, almost certainly to varying degrees depending on the type of training, as well as the type and timing of the measurement. Moreover, these factors potentially add to variability in the measurement of muscle hypertrophy with RET, leading to potential confusion for informing practice.

A seminal study by Chesley et al. Subsequently, it was shown that this response persisted for up to 48 hr postexercise Phillips et al. Biolo et al. Next, the first studies were published that demonstrated ingestion of essential amino acids immediately following REx increased the postexercise stimulation of MPS, resulting in a net accretion of muscle protein Rasmussen et al.

Several methods have been used to measure the acute response of MPS to exercise and nutrition in humans. The most common approach is the precursor—product method that allows for the determination of muscle protein fractional synthesis rate FSR. In practice, this method utilizes stable isotope labeled amino acids i.

Traditionally, FSR was calculated for mixed muscle proteins, that is, all muscle protein fractions combined. Methodological advances during the s allowed for the separation of muscle protein fractions Hasten et al. Another recent advancement in the field is centered around the re-emergence of the orally administered deuterium oxide D 2 O tracer method to measure free-living integrative rates of MPS.

Today, separation techniques have evolved further to measure FSR at the individual muscle protein level using D 2 O.

The focus of the review is on studies that directly determined MPS using the measurement of FSR. The controversy surrounding the value of acute i. More recently, an elegant study by Mitchell et al. This study was novel in examining the within-participant i. No measurement of muscle protein breakdown was conducted in this study.

As such, this study design offered insight into whether any heterogeneity in the muscle hypertrophic response to RET could be explained by differences in the acute response of MPS to REx between the 23 participants that conducted the study. The muscle hypertrophic response was determined by measurement of pre—post RET changes in quadriceps volume and lean body mass using magnetic resonance imaging and dual-energy X-ray absorptiometry, respectively.

The acute response of MPS was measured over a 6-hr recovery period following the first of 64 bout of REx. Perhaps surprisingly to many at the time, and refuting their original hypothesis, the study by Mitchell et al. Moreover, no correlation of the change in MPS from rest with the change in muscle volume was reported Mitchell et al.

Indeed, this observation is consistent with the results of a comparable 16 weeks RET study by Mayhew et al. In this study, no relationship was observed between the acute response of mixed MPS measured in the fasted state 24 hr after the initial bout of REx and muscle hypertrophy as determined by measurement of muscle fiber cross-sectional area.

Taken together, these data suggest that acute measurements of MPS offer limited quantitative value for predicting individualized chronic changes in muscle mass following progressive RET, at least when the acute response of MPS is measured following the initial exercise session of the RET period.

These studies have contributed to some confusion—particularly for practitioners, students, and others without specialist knowledge of the strengths and limitation of stable isotope methodology—and controversy over the interpretation of data from the measurement of MPS in response to exercise and nutrition Mitchell et al.

In contrast, multiple lines of evidence support the notion that the acute response of MPS to REx, with or without nutritional intervention, is predictive of chronic changes in muscle mass with RET when repeatedly exposed to a comparable exercise or nutritional intervention, at least when studied on an averaged, group basis.

First, ingesting an 18 g bolus of milk protein immediately after REx stimulated a greater acute response of MPS than a dose-matched soy protein beverage in young men Wilkinson et al.

This finding was consistent with a longitudinal training study that reported a greater change in muscle hypertrophy when a milk protein beverage was consumed immediately after each REx session of a week RET program versus a soy protein beverage in young men Hartman et al.

Similarly, the greater acute response of MPS to ingesting 20 g of whey protein versus casein immediately post REx Tang et al. Second, the acute response of MPS to REx when manipulating exercise workload low vs. high Burd, West, et al.

Finally, REx-induced increases in putative anabolic hormones were not shown to increase the acute response of MPS West et al. When combined with data generated by Mitchell et al.

In our view, and that of others Damas et al. Several physiological factors, related to both the acute response of MPS to REx and the muscle hypertrophic response to RET, appear to contribute to the observed discrepancy between measured rates of MPS and muscle hypertrophy.

Muscle hypertrophy is a complex physiological process that is altered as training progresses. For the initial response of MPS to predict subsequent muscle hypertrophy during RET, it must be assumed that the measured response of MPS to REx is uniform throughout the training period.

However, it is clear that the response of MPS is modified from the initiation of RET and as training progresses Kim et al. This modification takes place on a number of levels that include the timecourse amplitude and duration and nature directed to anabolic or nonanabolic processes of the MPS response, as discussed below.

There is considerable evidence from both cross-sectional Phillips et al. In the untrained state, the acute response of MPS has been shown to peak later, but remain elevated for longer, after REx compared with the trained state Phillips et al.

Conversely, in the trained state, the acute response of MPS to REx is more rapid but shorter lived than the untrained state Phillips et al. As a result, the overall acute stimulation of MPS after REx is generally considered to be greater in untrained versus trained individuals, at least when the absolute workload of REx is matched between training states Damas et al.

Given that training status clearly modulates the acute response of MPS to REx, it follows that the relationship between the acute MPS response to REx and chronic muscle growth response to RET may be altered over the time course of the training process.

To date, the most comprehensive study to examine the influence of training status on the relationship between the acute MPS response to REx and the muscle growth response to RET was conducted by Damas et al. The RET program was divided into three phases, namely the initial i.

Measurements of the acute MPS response to REx and muscle mass were obtained at each phase of RET. This elegant study design offered unique insight into the temporal relationship between acute measurements of MPS in response to REx, assessed in both the trained and untrained state, and the subsequent muscle growth response during RET.

The study by Damas, Phillips, Libardi, et al. In this regard, no relationship was observed between the acute response of myofibrillar—MPS to the initial REx bout of the RET period and the change in muscle mass following 10 weeks of RET. As detailed above, this observation is consistent with previous studies that reported no association between the acute response of MPS to the initial REx bout and the change in muscle volume Mitchell et al.

In contrast, the acute response of MPS to REx measured at Weeks 3 and 10 were associated with chronic changes in muscle mass over the week RET period Damas, Phillips, Libardi, et al.

These data are consistent with recent studies that reported associations between acute measurements of MPS and muscle hypertrophy over 3 Brook et al. Taken together, these data indicate the relationship between acute measurements of MPS and chronic changes in the muscle growth response becomes apparent as the training status of the individual progresses Table 1.

The predictive value of the acute response of MPS to nutrition and exercise interventions seems to be greater in trained than untrained individuals, who are not accustomed to muscle loading during REx Damas et al.

Thus, the researcher or practitioner may wish to consider the relative value of acute measurements of MPS for predicting chronic changes in muscle growth when formulating training and nutrition recommendations, at least for trained individuals. Relationship Between Acute Measurements of MPS and Chronic Changes in Muscle Mass in Response to RET.

One physiological mechanism proposed to explain the temporal relationship between acute measurements of MPS in response to REx and chronic changes in the muscle growth response to RET relates to the nature of the response of MPS to REx Damas et al. Damas et al.

This trend aligned with the acute 48 hr muscle damage response to REx that was highest after the initial unaccustomed REx bout, but was attenuated by the early Week 3 phase of RET. The authors reasoned that during the early phase of a training program, the increased response of MPS to REx and protein ingestion is related more to the repair and remodeling of existing older, perhaps damaged, proteins Damas, Phillips, Lixandrao, et al.

Consistent with this notion, the greater muscle damage response to unaccustomed eccentric-based exercise versus a work-matched bout of concentric exercise has been shown to correspond with a greater acute response of MPS to eccentric REx Moore et al.

As RET progresses, the responses of MPS to REx and nutrition become more refined toward muscle hypertrophy. This notion is supported by data showing that both mitochondrial and myofibrillar—MPS are increased following a REx in the untrained state Wilkinson et al.

However, following 10 weeks of RET, only myofibrillar FSR is increased. Taken together, these data suggest that with the progression of RET, and as the degree of exercise-induced muscle damage starts to diminish, the acute stimulation of MPS is directed almost exclusively to the accretion of new muscle proteins, thus explaining the correlation between acute rates of MPS and the muscle growth response during the later phase of RET Trommelen et al.

The inherent variability in the response of MPS to REx and nutrition, as well as the response of muscle hypertrophy to RET, also contributes to our inability to utilize acute metabolic data to predict an individual response to RET Figure 1.

This variability in response to exercise and nutrition is reported consistently Jackman et al. While the source of this individual variability is not fully understood at this time, genetic variability must be a contributing factor Clarkson et al.

Attempts to control prestudy activity and diet are common in these studies, yet the variability is evident. Moreover, in many studies, the population from which participants are selected is kept fairly tight. Yet, even when the range of muscle mass is restricted, there is considerable variation in the response of MPS Macnaughton et al.

The methodological conditions under which MPS is determined that may influence the measured response will be discussed below. However, in the examples illustrated in Figure 1 , the method used to determine MPS, as well as the conditions under which it was measured, in each individual were identical within studies.

Hence, methodological issues alone do not account for all the observed variability. Inherent variability in the metabolic response to REx and nutrition contributes to uncertainty in predicting muscle growth based on measured rates of MPS in individuals.

a Individual fasted FSR at rest REST and with ingestion of 30 g protein following resistance exercise FEDEX in two groups of trained young weightlifters and b individual FSR in response to ingestion of 20 and 40 g whey protein following REx in trained young weightlifters.

a Adapted from McGlory et al. Citation: International Journal of Sport Nutrition and Exercise Metabolism 32, 1; One potential contributing factor to the variability of the response of MPS to identical REx and protein feeding conditions Figure 1 might be differences in translational capacity, that is, the total number of ribosomes capable of producing peptide chains Wen et al.

The MPS is the metabolic process from which functional proteins are produced from polypeptide chains created by ribosomes. The measurement of FSR essentially represents translational efficiency, that is, the rate of translation for a given number of ribosomes.

It is clear that ribosome number, that is, translational capacity, does not change acutely following REx Brook, Wilkinson, Mitchell, et al. Thus, translational capacity may help explain the individual variability in response of MPS to anabolic stimuli. The lack of ability to predict long-term muscle hypertrophic responses to RET with the acute measurement of MPS does not necessarily reflect the overall worth, or lack thereof, of information obtained from acute metabolic studies.

Contributing factors to the uncertain relationship between the acute MPS response to REx and nutrition, and the muscle hypertrophic response to RET, include a lack of consistency in methods utilized, as well as inherent variability resulting from the methods used Mitchell et al.

There also is heterogeneity in the response of muscle mass to RET that contributes to this disconnect. Accordingly, there are numerous reasons to suggest that the study design and methods chosen to determine hypertrophy in RET studies contributes to this quite heterogeneous response.

A full evaluation of these methods is beyond the scope of this review, so interested readers are referred to an excellent presentation of the methodology by Haun et al.

Several factors related to study design and methods used to assess MPS must be considered when interpreting the relationship between the acute response of MPS- and RET-induced changes in muscle mass.

Over the past 25—30 years, the vast majority of studies investigating the response of MPS have utilized the precursor—product method with direct incorporation of the stable isotopically labeled amino acids into muscle protein to determine FSR. Accurate prediction of muscle hypertrophy during RET by determining FSR in response to REx and nutrition requires certain assumptions to be made and met.

First, we must assume that the initial measurement of FSR is representative of every subsequent stimulation of MPS for the remainder of the RET period, that is, the responses remain unchanged throughout RET see discussion above.

Next, the measured FSR captures the true response of MPS to REx and protein ingestion. Thus, methodological choices will be critical for determining the true response of MPS. Methodological considerations influence the ability to capture the true response of MPS with measurement of the FSR in response to exercise and nutrition.

Until recently, the majority of studies measuring FSR included an infusion of a labeled amino acid and multiple muscle biopsy samples. The FSR is reported as an hourly rate of synthesis in the time between the muscle samples. An important issue for any infusion study to determine FSR is the limited time period for incorporation of the labeled amino acid.

One critical assumption is that the time between biopsies captures the true period of stimulation of MPS. Thus, regardless of the maximal magnitude of the response, if the second muscle sample is taken before the response of MPS returns to baseline, a portion of the true response of MPS may be missed and the determined FSR would be an underestimation Figure 2.

Of course, the converse would be true if the biopsy is taken too late to capture the true response. a Infusion of [ 13 C 6 ] phenylalanine and muscle samples taken at timepoints that capture the entire true response of MPS and b infusion ends and muscle samples are taken at 0 and 4 hr, but the true response of MPS remains elevated above baseline for 6 hr, so the response is underestimated.

Another factor that contributes to a mismatch between the true response of MPS to REx is the prolonged enhancement of the utilization of amino acids from protein ingestion for MPS following a REx bout Figure 3. The REx sensitizes the muscle to the anabolic stimulation of elevated amino acid levels from protein feeding Biolo et al.

It is clear that the sensitivity of muscle to amino acids remains enhanced for at least 24 hr following the exercise Burd et al. Thus, any protein containing meal consumed within this hr time period will result in a MPS response that is greater than that in response to a meal not preceded by REx.

An acute measurement of MPS based on an infusion of labeled amino acids and biopsies for only a few hours after exercise would not be capable of capturing the contribution to muscle hypertrophy resulting from all of these enhanced postprandial elevations of MPS Figure 3a.

Thus, an acute measurement limited to only a few hours after REx would not reflect the entire influence of the exercise on MPS and subsequent muscle hypertrophy further contributing to the observed mismatch between measurement of MPS and changes in muscle mass with training.

The response of MPS is enhanced following REx and this is captured by D 2 O measurement of MPS. Over the past 15 years, another method has been revisited to determine an integrated FSR in free-living participants over a time period that is not limited by an infusion, that is, the D 2 O method Figure 3.

Thus, MPS in various situations and in response to various exercise and nutrition interventions can be determined over the time course of days to weeks. The determined rate of MPS integrates the response to all physical activity and nutrient consumption during that time, including the prolonged response of MPS to subsequent meals following REx Figure 3b.

Thus, the D 2 O method could be argued to provide a more holistic assessment of MPS without the limitations inherent with the requirement for infusion of stable isotopes for measurement of MPS.

It is perhaps not particularly surprising that integrated rates of MPS over longer time periods than are possible with isotope infusion studies, as well as inclusion of habitual physical activity and enhanced periods of postprandial MPS in response to exercise hours to days earlier, are better correlated with subsequent muscle hypertrophy.

Several studies utilizing the D 2 O measurement of FSR have reported correlations of MPS with subsequent muscle hypertrophy Brook et al. Therefore, this method for assessing MPS seems to be more suitable for predicting muscle hypertrophy with RET.

The disconnect between the initial measurement of MPS and subsequent muscle hypertrophy during RET may be due to methodological choices made for measurement of changes in muscle mass in addition to MPS. Differences in study design and methods chosen to determine changes in muscle mass, in addition to inherent individual variability in the response of muscle to training Mobley et al.

Factors including training duration, sleep quality, nontraining physical activity, nutrition, and other lifestyle variables may impact the training response Haun et al.

Proper control of many of these factors is virtually impossible in most RET study situations. This variability is further complicated by the various permutations possible with various combinations of these factors Haun et al.

Perhaps a more prosaic factor contributing to the disconnect between the acute response of MPS and subsequent muscle hypertrophy with RET relates to the inherent limitations of methods used to measure changes in muscle mass in humans. Reported changes in muscle mass with RET are heavily dependent on the method chosen to assess those changes.

Hence, the critical reader should consider the limitations of these methods when evaluating any particular training study.

Changes in muscle mass may be measured on one or more of several levels, that is, biochemical, ultrastructural, histological, and gross anatomical levels.

When multiple methods from these levels of hypertrophy are used, the agreement between methods is often poor Haun et al. Moreover, as detailed above, there are different types of hypertrophy that must be considered in combination with the method chosen to assess changes in muscle mass.

Three types of hypertrophy have been proposed: connective tissue, sarcoplasmic, and myofibrillar. For example, there is evidence that hypertrophy measured at the early stage of a RET program may result from edema-induced, that is, muscle swelling and sarcoplasmic hypertrophy Damas, Phillips, Libardi, et al.

This means that if muscle hypertrophy is based on dual-energy X-ray absorptiometry or other methods without consideration of changes in intramuscular fluid, overestimations of true hypertrophy will be made.

Clearly, changes in muscle mass with fluid infiltration are not related to MPS. These methodological factors should be considered when assessing the relationship between the acute response of MPS to changes in muscle mass with RET.

Based on our critical evaluation of existing evidence, we can make three practical implications. In this review, we have attempted to provide an evidence-based critical evaluation for the use of results from acute metabolic studies to predict changes in muscle mass with RET.

This lack of predictive power is especially true if the individual is beginning an unaccustomed exercise program. Nevertheless, this discrepancy should not be used to determine the value of studies measuring MPS in response to REx and protein nutrition.

There are multiple examples of studies in which the acute response of MPS does predict the average hypertrophy on a group level Hartman et al. Moreover, measurement of the acute response of MPS to REx and nutrition interventions can provide valuable information.

Regardless of training status, the acute response of MPS is indicative of protein turnover and muscle remodeling critical for recovery from exercise and adaptation to training. The measurement of integrated MPS that includes the enhanced postprandial response of MPS to protein ingestion in free-living individuals certainly may provide predictive information about subsequent muscle growth, albeit not in individuals undergoing unaccustomed exercise.

Moreover, the acute measurement of MPS also provides more sensitivity than chronic training studies over a much shorter time frame and can thus be viewed as a good starting point for determining nutritional recommendations. Given the nature of measurement of FSR, if a difference is detected in an acute study, for example, between different protein sources, then we can conclude with high confidence that the measured difference is physiologically relevant, at least qualitatively.

In this regard, the protein source that engenders the greater FSR may be considered the higher quality protein source irrespective of whether chronic studies are able to detect differences in muscle hypertrophy under comparable conditions of protein source manipulation.

Thus, we can use that information to inform subsequent RET studies. Finally, the acute measurement of MPS in response to exercise and nutrition offers valuable mechanistic information. In fact, delineation of mechanisms of muscle protein metabolism was the aim of many of the seminal studies that are now used to contribute to the development of recommendations Biolo et al.

Thus, whereas practitioners should be aware of the potential pitfalls with reliance on acute metabolic studies for making nutritional recommendations for athletes and exercisers, with proper interpretation a great deal of valuable information may be gleaned from these studies.

Acute measurement of MPS in response to various nutrition and exercise interventions should be viewed as yet another tool in the toolbox for use by practitioners and others. Balagopal , P. Skeletal muscle myosin heavy-chain synthesis rate in healthy humans.

American Journal of Physiology, 1 , Biolo , G. Increased rates of muscle protein turnover and amino acid transport after resistance exercise in humans.

American Journal of Physiology, , E — E An abundant supply of amino acids enhances the metabolic effect of exercise on muscle protein. Brook , M. Contemporary stable isotope tracer approaches: Insights into skeletal muscle metabolism in health and disease.

Experimental Physiology, 7 , — Synchronous deficits in cumulative muscle protein synthesis and ribosomal biogenesis underlie age-related anabolic resistance to exercise in humans.

The Journal of Physiology, 24 , — Skeletal muscle hypertrophy adaptations predominate in the early stages of resistance exercise training, matching deuterium oxide-derived measures of muscle protein synthesis and mechanistic target of rapamycin complex 1 signaling.

The FASEB Journal, 29 11 , — The metabolic and temporal basis of muscle hypertrophy in response to resistance exercise. European Journal of Sport Science, 16 6 , — Burd , N.

Skeletal muscle remodeling: Interconnections between stem cells and protein turnover. Exercise and Sport Sciences Reviews, 45 3 , — Resistance exercise volume affects myofibrillar protein synthesis and anabolic signalling molecule phosphorylation in young men.

The Journal of Physiology, 16 , — Enhanced amino acid sensitivity of myofibrillar protein synthesis persists for up to 24 h after resistance exercise in young men.

The Journal of Nutrition, 4 , — Low-load high volume resistance exercise stimulates muscle protein synthesis more than high-load low volume resistance exercise in young men. PLoS One, 5 8 , Article e Chesley , A. Changes in human muscle protein synthesis after resistance exercise.

Journal of Applied Physiology, 73 4 , — Clarkson , P. ACTN3 genotype is associated with increases in muscle strength in response to resistance training in women. Journal of Applied Physiology, 99 1 , — Damas , F.

The development of skeletal muscle hypertrophy through resistance training: The role of muscle damage and muscle protein synthesis.

European Journal of Applied Physiology, 3 , — A review of resistance training-induced changes in skeletal muscle protein synthesis and their contribution to hypertrophy. Sports Medicine, 45 6 , — Resistance training-induced changes in integrated myofibrillar protein synthesis are related to hypertrophy only after attenuation of muscle damage.

The Journal of Physiology, 18 , — Early resistance training-induced increases in muscle cross-sectional area are concomitant with edema-induced muscle swelling.

European Journal of Applied Physiology, 1 , 49 — Figueiredo , V. Revisiting the roles of protein synthesis during skeletal muscle hypertrophy induced by exercise. American Journal of Physiology—Regulatory, Integrative and Comparative Physiology, 5 , R — R Franchi , M.

Early structural remodeling and deuterium oxide-derived protein metabolic responses to eccentric and concentric loading in human skeletal muscle.

Physiological Reports, 3 11 , Article e Goldberg , A. Mechanism of work-induced hypertrophy of skeletal muscle. Hartman , J. Consumption of fat-free fluid milk after resistance exercise promotes greater lean mass accretion than does consumption of soy or carbohydrate in young, novice, male weightlifters.

The American Journal of Clinical Nutrition, 86 2 , — Hasten , D. Isolation of human skeletal muscle myosin heavy chain and actin for measurement of fractional synthesis rates. American Journal of Physiology, 6 , Article E Haun , C. A critical evaluation of the biological construct skeletal muscle hypertrophy: Size matters but so does the measurement.

Frontiers in Physiology, 10, Hawley , J. Promoting training adaptations through nutritional interventions. Journal of Sports Science, 24 7 , —