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Just How Much Protein Can The Body Really Absorb?

Published: 13th May 2018. Last updated: 21th July 2019.

Shaun Ward MSc ANutr

Staff Writer


Introduction

Many nutrition articles claim that the body has a limit to the amount of protein that can be absorbed in a single meal or throughout the day.

This has led to the idea that the body can only utilize a certain amount of protein at one time to help with things such as muscle repair and adaptation.

If true, it would seem a better idea to distribute protein intake across the day in multiple meals.

But is this really true? What do people mean when they are talking about “absorption”? How does this effect your diet?

Let’s take a look at what the evidence says:

Protein Digestion and Protein Absorption

It is very common for people to confuse the words “digestion” and “absorption”, which can make the conversation about protein absorption extremely confusing.

Without this fundamental understanding it is hard for anyone to try and wrap their heads around this topic.

Protein Digestion

Protein digestion refers to the physical and chemical breakdown of protein into its individual amino acid components within the stomach and the small intestine.  

This digestion process occurs by using enzymes (proteases and peptidases) to break down the amino acid bonds that are linked together when a protein is in its whole form.  

Digestion is necessary before starting to absorb these small amino acid components through the intestines and into the bloodstream.

Protein Absorption

This is where much of the confusion stems from.

The definition of protein absorption can have 2 meanings depending on who you ask, neither of which are necessary wrong.

The 2 definitions are:

  • Protein absorption is the transfer of amino acids through from the gut into systemic circulation (blood).
  • Protein absorption is the transfer of amino acids through the gut, into the systemic circulation, and then delivered to cells (such as muscle) to be used.

As you may notice, the definitions follow the same path, but one also accounts for the amount of amino acids that are taken up by cells from the bloodstream.

Such a small difference can make a huge impact when analyzing this topic. Just because amino acids are in the bloodstream does not mean they will be utilized by muscle cells for protein synthesis. In fact a recent study showed that only ~10% of the amino acids provided to healthy men in a 20 gram dose of casein protein were used for muscle protein synthesis despite ~50% availability in the systemic circulation within hours of consumption [1].

The remaining amino acids may be stored for later use, converted to glucose for energy production, or to produce urea and neurotransmitters.

Based on the first definition which only accounts for amino acids getting into the bloodstream, protein absorption is virtually unlimited.

The more protein you eat, the longer it will take to completely digest, but most of the protein will end up in the bloodstream at some point.

According to the International Journal of Sport Nutrition and Exercise Metabolism, amino acids are absorbed into the blood at a rate of 1.3-10 grams per hour [2]. The exact rate is dependent on the context of the whole meal, as dietary fat and fiber will significantly slow down digestion speeds, and more liquidized sources of protein will have faster digestion speeds.

People Confuse “Absorption” with Muscle Protein Synthesis

Going a step further than just delivery of amino acids into the blood or cells is the principle of muscle protein synthesis.

After all, the main reason why most people consume protein is to induce an anabolic state and begin to synthesize new muscle proteins – not to just have amino acids floating around their blood!

Most people think they are talking about absorption when they are really talking about muscle protein synthesis. The latter not only requires amino acids to be delivered to the blood and muscle cells, but then for those muscle cells to actually use these amino acids for muscle repair and remodeling purposes.

Following on from this, several publications indicate that there is a limit to the maximum stimulation of muscle protein synthesis from a single meal, said to occur with intakes of 20-30 grams of protein [3] [4].

Newer studies report this may be actually be higher than first thought, with a 40 gram protein dose achieving a 20% higher muscle protein synthesis compared to a 20 gram dose [5].

This line of evidence led the scientific community to believe that the maximum utilization of protein from a single meal for muscular purposes was ~40 grams.

Excess protein beyond this at a single meal was not thought to incur additional benefits to muscle protein synthesis, despite sustained amino acid availability and anabolic signaling within the muscle cell [6].

Perhaps unsurprisingly, this anabolic response was also shown to be transient, meaning muscle protein synthesis could only be elevated for ~3 hours before returning to baseline levels [7].

Therefore, recommendations followed to consume ~20-40 grams of protein every 3 hours to keep muscle protein synthesis levels raised throughout the day.

So despite protein absorption not being limited, the usefulness of a protein dose within the muscle cell was thought to be limited.

Does This Miss The Bigger Picture?

the most recent evidence again goes one step further (sorry, we said it was a complex topic!).

Focusing only on the muscle protein synthesis response from dietary protein may not be enough, as this fails to account for effects on muscle protein breakdown, or for the “storage pools” of amino acids in the body that can be used in future.

Studies have found that the body has large pools of digested amino acids within the gut and muscle cells that may not be used immediately, but may be used later in the day or even following days. This retention is basically a temporarily storage pool of essential amino acids [8].

So although the immediate benefit of protein in a single meal may be limited at ~40 grams, these temporary storage pools may work to raise muscle protein synthesis again once the effects have worn off.

In addition, only measuring the muscle protein synthesis response to a meal does not account for muscle protein breakdown responses. Logically, even if muscle protein synthesis remains the same, a reduction in muscle protein breakdown from a meal would increase the net muscle protein balance.

There is no point synthesizing 40 grams of protein if you are simultaneously breaking down 40 grams of protein!

Interestingly, when studies have measured rates of muscle protein synthesis and breakdown at the same time, the provision of 70 grams of protein has a superior effect on net muscle protein balance than 40 grams of protein [9].

This was primarily attributed to a greater attenuation of protein breakdown due to the increased amount of insulin secreted from the larger protein dose.

Insulin is an anabolic hormone that is released when protein or carbohydrates are consumed. One of its roles is to inhibit muscle protein breakdown, and thus impacts the total anabolic response to a meal.

When insulin and muscle protein breakdown are considered, this theory therefore predicts that there is no practical limit to the anabolic response from protein intake. With greater and greater protein intake, a progressively greater insulin response will result, with subsequent suppression of protein breakdown, and thus a greater anabolic response.

Studies in which muscle protein synthesis and breakdown have been measured confirm this, with a proposed linear relationship between protein intake at a meal and net muscle protein balance [10] [11].

These findings have driven experts to now accurately predict changes in lean body mass over time in tightly controlled conditions by assessing muscle protein synthesis and breakdown responses to a meal [12].

This could help explain why some recent systematic review conclude that intermittent fasting has similar effects on fat-free mass compared with continuous and distributed eating patterns [13]

Does Exercise Also Have an Influence on Protein Responses?

Exercise sessions consisting of resistance exercises at intensities greater than 60% of your 1 repetition max (1RM) are able to increase muscle protein synthesis by 200-300% [14].

These improvements in muscle protein synthesis may be sustained for up to 4 hours without protein intake, but can go beyond 24 hours when there is adequate protein intake throughout the day [15].

Therefore, adequate protein intake is necessary after exercise to enhance the magnitude and duration of the increase in muscle protein synthesis [16].

More importantly, this type of exercise has shown to delay any potential “muscle-full” effect for up at least 24 hours after a single bout of exercise. This may help to explain why chronic adaptations of muscle growth over time are generally independent of feeding patterns [17].

So even if there is a limit to muscle protein synthesis under normal circumstances, in the context of a resistance-training athlete it is protein sufficiency, rather than protein timing, that is the most important aspect of successful muscle adaptation [18].

What About Daily Protein Limits?

I know what you’re thinking – if there is no apparent limit to the benefits of protein at a single meal, does this mean that there is no limit to protein intake across an entire day? Is more always better?

As much as that sounds great, unfortunately that is not the case.

In the context of a 24 hour period, there does appear to be a law of diminishing returns in terms of the adaptive responses that are seen with daily protein intakes.

When studies analyze the benefits of various daily protein intakes, more is better but only up until a certain point.

The most recent American College of Sports Medicine position stand on dietary practices for athletes recommends a protein intake of 1.2-1.7 grams per kilogram of bodyweight per day to maximize training responses and muscle recovery.

Generally, going beyond 2 grams if protein per kg of bodyweight per day can be viewed as excessive with no additional benefits – outside of extreme cases such as very restrictive diets.

So how comes there is no limit to protein in a meal but there is throughout the day?

Great question, but the precise answer is not yet known, and hopefully the next stage of research can clarify this.

The best research at the moment hypothesizes that it may be due to a regulatory protein called myostatin, which is directly produced by skeletal muscle to limit muscle fiber growth. It appears tissue-specific growth inhibitors such as myostatin may be an evolutionary adaptation to stop continual growth that may ultimately lead to serious health problems in future [19].

Conclusion

I know what you’re thinking – if there is no apparent limit to the benefits of protein at a single meal, does this mean that there is no limit to protein intake across an entire day? Is more always better?

As much as that sounds great, unfortunately that is not the case.

In the context of a 24 hour period, there does appear to be a law of diminishing returns in terms of the adaptive responses that are seen with daily protein intakes.

When studies analyze the benefits of various daily protein intakes, more is better but only up until a certain point.

The most recent American College of Sports Medicine position stand on dietary practices for athletes recommends a protein intake of 1.2-1.7 grams per kilogram of bodyweight per day to maximize training responses and muscle recovery.

Generally, going beyond 2 grams if protein per kg of bodyweight per day can be viewed as excessive with no additional benefits – outside of extreme cases such as very restrictive diets.

So how comes there is no limit to protein in a meal but there is throughout the day?

Great question, but the precise answer is not yet known, and hopefully the next stage of research can clarify this.

The best research at the moment hypothesizes that it may be due to a regulatory protein called myostatin, which is directly produced by skeletal muscle to limit muscle fiber growth. It appears tissue-specific growth inhibitors such as myostatin may be an evolutionary adaptation to stop continual growth that may ultimately lead to serious health problems in future [19].

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