Written by Richard B. Kreider, Ph.D., FACSM, FISSN, FACN, FNAK
01 December 2021

 

 

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Maximum Muscle Growth: Fast vs. Slow Protein

 

ByRichard B. Kreider, Ph.D., FACSM, FISSN, FACN, FNAK

 

It’s well known that athletes engaged in intense training need to eat enough protein in their diets to repair tissue and promote growth.1 Research has also indicated that ingesting protein with carbohydrate prior to exercise can lessen the degree of muscle catabolism observed during exercise, while ingesting protein with carbohydrate following exercise can enhance glycogen and protein synthesis.2 However, not all protein is the same. Dietary proteins vary in amino acid content as well as the amount of fat and other nutrients they contain.3 These differences can affect the digestion rate of a protein and the manner in which amino acids are released into the blood. Research has shown that protein synthesis may be regulated in part by the manner in which amino acids are released into the blood. Theoretically, ingesting the right type of protein following exercise may help stimulate protein synthesis to a greater degree. This discussion addresses the way you can structure the timing and delivery of proteins in your diet in order to optimize protein synthesis during training.

 

Protein Basics      

Proteins vary depending on the amino acid, fat and micronutrient content. For example, proteins are comprised of various essential, conditionally essential, and non-essential amino acids (see Table 1). Proteins that contain all the essential amino acids are considered complete proteins, while those that don’t are considered incomplete.4,5 The quality of a protein can be classified using the Protein Efficiency Ratio (PER) and the Protein Digestibility Corrected Amino Acid Score (PDCAAS). The PER is determined by assessing the weight gain of growing rats fed a particular protein in comparison to a standard protein (egg whites). The higher the PER value, the greater the quality of the protein. The PDCAAS method compares the amino acid profile of a protein to the essential amino acid requirements in humans established by the Food and Agriculture Organization.

           

A protein with a PDCAAS of 1.0 indicates that it exceeds the essential amino acid requirements of the body and therefore is an excellent source of protein.6 Table 2 describes the protein and fat content of common dietary proteins7 while Table 3 lists the PER and PDCAAS for various types of protein.4 As you can see, low-fat dairy products, skinless light chicken, and fish are generally considered among the best sources of low-fat dietary protein. Soy represents the highest quality plant protein, while egg, milk proteins (casein and whey) and bovine colostrum represent the highest quality animal proteins.  

 

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Fast and Slow Protein Concept

           

Research has indicated that several factors affect protein synthesis after consuming a meal, including the amount of calories consumed, the quantity and quality of protein ingested, the insulin response to the meal, and the digestibility of the food.3 The digestibility of food is affected by the fat, starch and fiber content of the food, as well as the acidity. Generally, the higher the fat, starch and fiber content, the slower a food is digested. The digestion rate influences the time course of amino acid release in the blood. In this regard, foods containing protein that are digested faster generally result in a greater release of amino acids in the blood over a shorter period of time. Conversely, foods that contain protein that are digested at a slower rate typically promote a smaller, but more prolonged, increase in amino acids.5

           

Different types of protein also have different time courses of amino acid release. For example, since casein clumps when exposed to acid in the stomach, it’s digested at a relatively slow rate, which results in a modest but prolonged increase in amino acids in the blood.3 On the other hand, whey protein is comprised of a mixture of soluble proteins that are digested rather rapidly, resulting in a more pronounced but shorter increase in amino acids in the blood.3

                       

The time course of amino acid release following ingestion of a protein has been shown to affect protein metabolism and synthesis. For example, Boirie and colleagues8 compared the effects of ingesting 30 grams of whey (fast protein) and 43 grams of casein (slow protein) on protein utilization and synthesis. The amount of whey and casein evaluated was chosen to match the proteins for leucine content, since the researchers used labeled leucine methodology to assess protein use and synthesis rates.

           

In the aforementioned study, the researchers found that whey protein ingestion promoted a greater increase in amino acid levels in the blood and a greater rate of protein storage during the first two hours after feeding, compared to casein. However, the rate of protein storage returned to baseline within three to four hours after feeding. In addition, whey protein had no effect on the rate of protein breakdown. Casein ingestion promoted a more modest but prolonged increase in amino acid concentrations that were greater than the whey group during three to six hours after feeding. This resulted in a greater amount of protein storage, as well as less protein breakdown. The investigators observed similar results when comparing 30 grams of whey to 30 grams of casein, even though the casein had less leucine.

           

In a follow-up study, Dangin and colleagues9 evaluated the effects of ingesting a single serving of casein to a single serving of whey protein, as well as several smaller whey protein meals. All protein meals were matched to nitrogen and amino acid content. In this way, they could determine whether fast (whey) and slow (casein) proteins influenced protein synthesis. The researchers found that even though the two types of protein had the same nitrogen content and amino acid profile, ingestion of the “slower” protein promoted a greater amount of protein synthesis during a seven-hour post-feeding period than ingestion of whey protein or smaller amounts of whey protein several times. The researchers suggested that the time course of amino acid absorption after protein ingestion has a major influence on protein synthesis, presumably because proteins that are digested and absorbed at a faster rate may be used to a greater degree for energy metabolism. 

 

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The Concept Analyzed

         

The studies described above have served as the theoretical basis for various nutritional supplement companies to develop “time releasing” or “slow protein” formulations. Some have even suggested that casein may serve as a more effective source of protein than whey.10 Although these findings are interesting, there are several points to consider when interpreting results of these studies.

           

First, these studies only evaluated the effects on protein synthesis of consuming different types of protein. As the researchers point out, it’s unclear how co-ingesting fast or slow proteins with other nutrients (e.g., carbohydrate, fat, etc.) may affect protein synthesis rates.3 For example, insulin is one of the primary regulators of protein synthesis. It’s possible that in the presence of carbohydrate, it may be more advantageous to ingest a “fast” rather than a “slow” protein in order to enhance amino acid availability and promote an insulin-mediated protein synthesis. Moreover, carbohydrate availability directly affects the use of amino acids for energy metabolism.  Consequently, the greater use of amino acids as fuel observed following “fast” protein ingestion might be negated in the presence of carbohydrate. To support these contentions, studies have reported that ingesting protein with carbohydrate following exercise increases insulin levels and enhances protein synthesis in comparison to carbohydrate or protein alone.11-14

           

Second, the primary differences between the fast and slow proteins on protein synthesis were observed between three and seven hours after ingesting the slow protein “meal.” Athletes engaged in intense training typically ingest four to six meals per day, as well as snack between meals in order to maintain adequate caloric intake.2 Consequently, athletes who are attempting to maintain or increase bodyweight seldom go four to seven hours between meals or snacks. This means there may be less practical value in ingesting slow proteins in athletes who eat frequently throughout the day. It’s also possible, however, that ingestion of slow proteins may help athletes maintain muscle mass to a better degree when dieting or when observing prolonged fasting periods. In addition, since most people sleep six to eight hours a day, it’s also possible that ingesting a slow protein prior to going to bed may enhance protein synthesis while sleeping. More research is needed to examine the potential value of consuming slow-digesting proteins on protein synthesis and training adaptations.

           

Finally, although there is some theoretical basis for consuming slow proteins, there is no evidence that ingesting casein promotes greater gains in strength and muscle mass during training than whey protein. One study15 often cited to support contentions that casein supplementation during training is more effective than whey protein actually compared ingesting a vitamin/mineral fortified carbohydrate/protein meal replacement powder containing a “unique blend of milk protein isolates (caseinate, glutamine, whey protein concentrate, egg white)” to a whey protein supplement. Consequently, the greater gains in muscle mass and strength observed could not be attributed to ingesting a slow protein (i.e., casein) alone.

           

Conversely, several studies indicate whey protein stimulates protein synthesis8,16 and may have greater immunoenhancing effects17,18 and may have some anticarcinogenic properties19,20 in comparison to other forms of protein (including casein). For example, Lands and colleagues21 reported that ingesting a supplement containing whey protein (20 grams/day) during 12 weeks of training promoted better gains in immune function, performance and body composition alterations than ingesting casein. These findings suggest whey protein may provide greater health and/or performance benefits than casein.   

 

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Summary      
Dietary proteins vary depending on the amino acid, fat, and micronutrient content. The digestion rate of food and proteins can affect the time course of amino acid release in the blood. There is some evidence that the digestion rate of specific proteins may affect amino acid release, protein synthesis and protein degradation. Fast proteins (e.g., whey protein) appear to rapidly increase amino acid levels and protein synthesis during the first two hours after feeding. Slow proteins (e.g., casein) promote a more modest but sustained increase in amino acid levels and protein synthesis particularly following four to seven hours after feeding. Theoretically, ingesting slow-digesting proteins may promote a greater protein synthesis during prolonged fasts and/or during sleep. However, no study has evaluated these hypotheses and there’s no evidence that ingesting slow proteins promotes greater training adaptations than fast proteins. More research is needed before conclusions can be drawn. Until then, I suggest meeting your protein needs with a balance of fast and slow proteins, and that you consume protein with carbohydrate after exercise to optimize protein synthesis and recovery.
Richard B. Kreider, Ph.D., FACSM, FISSN, FACN, FNAK is Professor and Executive Director of the Human Clinical Research Facility and Director of the Exercise & Sport Nutrition Lab at Texas A&M University. Dr. Kreider conducts research related to the role of exercise and nutrition on health, disease, rehabilitation, and performance.

 

Table 1. Essential, Conditionally Essential and Nonessential Amino Acids

 


Essential Amino Acids

*Isoleucine                               

*Leucine                                   

Phenylalanine

Threonine

Tryptophan

*Valine

 

Conditionally Essential Amino Acids

Cysteine (Cystine)                                                        

Histidine

Proline

Tyrosine

 

Nonessential Amino Acids

Aspartic Acid                           

Citruline

Glutamic Acid

Glycine

Serine

* Branched-Chain Amino Acid

Adapted from Di Pasquale5 

 

 

table

 

 

 

table 3 e1

 

References:

 

               

 

1. Kreider RB. Dietary supplements and the promotion of muscle growth with resistance exercise. Sports Med 27:97-110, 1999

 

               

 

2. Leutholtz B, Kreider R. Exercise and Sport Nutrition. In: Wilson T, Temple N (eds): Nutritional Health Humana Press. Totowa, NJ, 2001, pp 207-239

 

               

 

3. Beaufrere B, Dangin M, Boirie Y. The "fast" and "slow" protein concept. In: Furst P, Young V (eds): Proteins, Peptides and Amino Acids in Enteral Nutrition, vol 3 Karger. Basel, Germany, 2000, pp 121-133

 

               

 

4. Bucci L, Unlu L. Proteins and amino acid supplements in exercise and sport. In: Driskell J, Wolinsky I (eds): Energy-Yielding Macronutrients and Energy Metabolism in Sports Nutrition CRC Press. Boca Raton, FL, 2000, pp 191-212

 

               

 

5. Di Pasquale M. Proteins and amino acids in exercise and sport. In: Driskell J, Wolinsky I (eds): Energy-Yielding Macronutrients and Energy Metabolism in Sports Nutrition CRC Press. Boca Raton, FL, 2000, pp 119-162

 

               

 

6. Sarwar G, McDonough FE. Evaluation of protein digestibility-corrected amino acid score method for assessing protein quality of foods. J Assoc Off Anal Chem 73:347-356, 1990

 

               

 

7. Pennington J, Bowes A, Church H. Bows and Churches Food Values of Portions Commonly Used (ed 15) Harper and Row, New York, 1989

 

               

 

8. Boirie Y, Dangin M, et al. Slow and fast dietary proteins differently modulate postprandial protein accretion. Proc Natl Acad Sci U S A 94:14930-14935, 1997

 

               

 

9. Dangin M, Boirie Y, et al. The digestion rate of protein is an independent regulating factor of postprandial protein retention. Am J Physiol Endocrinol Metab 280:E340-348., 2001

 

               

 

10. Wright J. The case for casein: does this protein outwhey the competition?, Flex Magazine, vol 11, 2000, pp 237-240

 

               

 

11. Biolo G, Williams BD, et al. Insulin action on muscle protein kinetics and amino acid transport during recovery after resistance exercise. Diabetes 48:949-957, 1999

 

               

 

12. Levenhagen DK, Gresham JD, et al. Post-exercise nutrient intake timing in humans is critical to recovery of leg glucose and protein homeostasis. Am J Physiol Endocrinol Metab 280:E982-993, 2001

 

               

 

13. Tipton KD, Rasmussen BB, et al. Timing of amino acid-carbohydrate ingestion alters anabolic response of muscle to resistance exercise. Am J Physiol Endocrinol Metab 281:E197-206, 2001

 

               

 

14. Rasmussen BB, Tipton KD, et al. An oral essential amino acid-carbohydrate supplement enhances muscle protein anabolism after resistance exercise. J Appl Physiol 88:386-392, 2000

 

               

 

15. Demling RH, DeSanti L. Effect of a hypocaloric diet, increased protein intake and resistance training on lean mass gains and fat mass loss in overweight police officers. Ann Nutr Metab 44:21-29, 2000

 

               

 

16. Fruhbeck G. Slow and fast dietary proteins. Nature 391:543-534, 1998

 

 

 

17. Bounous G, Batist G, Gold P. Immunoenhancing property of dietary whey protein in mice: role of glutathione. Clin Invest Med 12:154-161, 1989

                          

 

18. Bounous G: Whey protein concentrate (WPC) and glutathione modulation in cancer treatment. Anticancer Res 20:4785-4792, 2000

 

               

 

19. Hakkak R, Korourian S, et al. Diets containing whey proteins or soy protein isolate protect against 7,12-dimethylbenz(a)anthracene-induced mammary tumors in female rats. Cancer Epidemiol Biomarkers Prev 9:113-117, 2000

 

               

 

20. Kennedy RS, Konok GP, et al. The use of a whey protein concentrate in the treatment of patients with metastatic carcinoma: a phase I-II clinical study. Anticancer Res 15:2643-2649, 1995

 

               

 

21. Lands LC, Grey VL, Smountas AA: Effect of supplementation with a cysteine donor on muscular performance. J Appl Physiol 87:1381-1385, 1999

 

               

 

22. Murray R, Horswill C. Nutrient requirements for competitive sports. In: Wolinsky I (ed): Nutrition in Exercise and Sport CRC Press. Boca Raton, FL, 1997, pp 521-558

 

 

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