How Much Protein Can Your Body Absorb?

There is quite a bit of controversy surrounding the topic of protein such as how much can be utilized and how much is needed for muscle growth. 

For those following a regimented resistance training program, there is debate about the total amount of protein one should have each day and also the amount of protein that can be utilized for muscle tissue-building purposes in a single meal.

The Recommended Daily Allowance (RDA)

The RDA of 0.8 g/kg/day represents the relative quantity of high-quality protein needed to maintain nitrogen balance in 97.5% of apparently healthy males and females aged 19 years and older (1).

However, this RDA is not an appropriate guideline to follow when considering the “optimal” protein intake to promote muscle growth.

That is because dietary reference intakes don’t address energy status (i.e., energy restriction – which promotes lean mass catabolism) as a potential modifier of protein needs.

How much protein can you have in a day?

One of the key points to clarify straight away is that unlike carbohydrates and fats, our bodies don’t have a way to store extra protein. 

Therefore, a dose of 20-30 grams of protein per meal maxes out the protein synthesis response for most people. There is data showing that a higher amount is needed in some individuals (2,3) which can be due to the source of protein.

For example, plant-based protein requires higher amounts compared to animal-based protein for the same effect. 

A dose of about 20 grams of whey protein can max out protein synthesis because it is very high in the branched-chain amino acid leucine. Keep in mind that building muscle tissue is a balance between protein synthesis and degradation.

To build muscle, we either need to increase protein synthesis or decrease protein degradation or a combination of both.

As mentioned previously, under stressful conditions such as energy restriction and physical activity, the RDA for protein is no longer appropriate.

Under catabolic conditions, higher protein intakes are proposed to diminish the loss of lean body mass and under anabolic conditions, higher protein intakes are proposed to increase the gain of body lean mass.  

A recent meta-analysis assessed protein intakes greater than the RDA compared with the RDA on changes in whole-body lean mass.

The major finding was that protein intakes greater than the RDA beneficially influenced changes in lean mass when individuals were purposefully stressed by the catabolic stressor of dietary energy restriction with and without the anabolic stressor of resistance training. 

Consuming approximately 1.3 g/kg/day of protein induces beneficial changes in lean mass in adults (4).

Another recent summary of multiple research studies evaluated the dose-response relationship between protein intake and the increase in lean body mass while assessing this relationship within the context of the absence or presence of resistance training.

This was the first meta-analysis to examine the dose response-relationship between a wide range of protein intakes and an increase in muscle mass in the presence or absence of resistance training (3).

The main findings were as follows:

• Protein supplementation was significantly effective for increasing lean body mass with or without resistance training. This meta-analysis demonstrates for the first time that protein supplementation is significantly effective without resistance training in a diverse population without specific serious health conditions.
• Dose-response analyses indicated that total protein intake over a wide range of doses (0.5 to 3.5 g/kg bodyweight/day) was positively correlated with an increase in lean body mass. These results suggest that slightly increasing protein intake for several months, even by as little as 0.1 g/kg BW/day may increase or help maintain muscle mass.
• The rate of increase in the effect of protein supplementation rapidly diminished after 1.3 g/kg/day was exceeded. Resistance training distinctly suppressed this decline suggesting that it may contribute to maintaining or improving the efficiency of protein anabolism. Therefore, increasing protein intake combined with a resistance training program is recommended to optimize lean body mass.

How much protein can you have in one sitting?

It’s been proposed that muscle protein synthesis is maximized in young adults with an intake of approximately 20-25 g of high quality protein per meal, and that anything above this amount is thought to be oxidized for energy or transaminated to form alternative bodily compounds (5).

It’s important to note that a number of variables influence dietary protein metabolism including the composition of the given protein source, the composition of the meal, the amount of protein ingested, and the specifics of the exercise routine (6).

In addition, individual variables such as age, training status, and the amount of lean body mass also impact muscle-building outcomes. 

Given that muscular development is a function of the dynamic balance between muscle protein synthesis and muscle protein breakdown, both of these variables must be considered in any discussion on dietary protein dosage (7).

In summary, quantifying a maximum amount of protein per meal that can be utilized for muscle anabolism has been a challenging pursuit due to the multitude of variables open for investigation. One of the most comprehensive synthesis of findings concluded that 0.4 g/kg/meal would optimally stimulate muscle protein synthesis (5).

Although for some older men, an intake as high as ~0.6 g/kg/meal is required.

How much protein can your body absorb?

A long-standing misconception in the general public is that there is a limit to how much protein can be absorbed by the body. From a nutritional standpoint, the term “absorption” describes the passage of nutrients from the gut into systemic circulation. Based on this definition, the amount of protein that can be absorbed is virtually unlimited.

Essentially, unless protein has some kind of inherent characteristic that limits its bioavailability (e.g., plant-based proteins that are bound up in the fibrous plant material are less bioavailable) you will absorb all of the protein you eat. 

The important question is how much will be used for muscle building.  

There is also a school of thought proposing that you shouldn’t eat too much protein because the excess is oxidized for energy.  Although this is true, this is good thing when it comes to fat loss because protein is thermogenic.

In fact, protein is the most thermogenic of all the macronutrients.

If body composition and fat loss are important to you, then a higher protein intake is good.

Conclusion

The collective body of evidence indicates that a total daily protein intake of approximately 1.6 g/kg/day is ideal if your goal is to maximize resistance training-induced gains in muscle mass and strength.  This amount holds true in non-dieting (excess calories or number of calories consumed is about same as expended) conditions.

However, 1.6 g/kg/day should not be viewed as a universal limit beyond which protein intake will be wasted or used for physiological demands aside from muscle growth. There is recent evidence involving resistance trainees and young male bodybuilders that demonstrate that a dose of 2.2 g/kg/day is ideal for muscle building purposes.

This reinforces the practical need to individualize dietary programming, and remain open to exceeding estimated averages. If the primary goal is to build muscle, a relatively simple and elegant solution to reach a minimum of 1.6 g/kg/day of protein is to have a target intake of approximately 0.4 g/kg/meal across a minimum of four meals per day.

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References:
1.    Trumbo, P., Schlicker, S., Yates, A. A., Poos, M., Food, and Nutrition Board of the Institute of Medicine, T. N. A. (2002) Dietary reference intakes for energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein and amino acids. J Am Diet Assoc 102, 1621-1630
2.    Morton, R. W., Murphy, K. T., McKellar, S. R., Schoenfeld, B. J., Henselmans, M., Helms, E., Aragon, A. A., Devries, M. C., Banfield, L., Krieger, J. W., and Phillips, S. M. (2018) A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Br J Sports Med 52, 376-384
3.    Tagawa, R., Watanabe, D., Ito, K., Ueda, K., Nakayama, K., Sanbongi, C., and Miyachi, M. (2020) Dose-response relationship between protein intake and muscle mass increase: a systematic review and meta-analysis of randomized controlled trials. Nutr Rev
4.    Hudson, J. L., Wang, Y., Bergia Iii, R. E., and Campbell, W. W. (2020) Protein Intake Greater than the RDA Differentially Influences Whole-Body Lean Mass Responses to Purposeful Catabolic and Anabolic Stressors: A Systematic Review and Meta-analysis. Adv Nutr 11, 548-558
5.    Morton, R. W., McGlory, C., and Phillips, S. M. (2015) Nutritional interventions to augment resistance training-induced skeletal muscle hypertrophy. Front Physiol 6, 245
6.    Bilsborough, S., and Mann, N. (2006) A review of issues of dietary protein intake in humans. Int J Sport Nutr Exerc Metab 16, 129-152
7.    Schoenfeld, B. J., and Aragon, A. A. (2018) How much protein can the body use in a single meal for muscle-building? Implications for daily protein distribution. J Int Soc Sports Nutr 15, 10