We all know that there are differences in carbohydrates – high glycemic, low glycemic, simple sugars, starches, etc. And we know that different carbohydrates are absorbed in the gut and appear in the blood at different rates depending on various factors. For example simple sugars are absorbed more quickly than more complex ones, the rate of absorption of the latter depending on how quickly the complex sugars especially in the form of starches, can be broken down and subsequently absorbed.

The rate of absorption, and its subsequent effects on insulin levels, makes up the basis for the glycemic index of not only foods but whole meals since the presence of protein and fat with the carbohydrates usually slows down the absorption over the whole digestive process. Fast and slow carbohydrates have different metabolic effects on the hormones and on various metabolic processes.

Now we also have slow (for example casein) and fast (for example whey and soy) dietary proteins. The speed of absorption of dietary amino acids by the gut varies according to the type of ingested dietary protein and the presence of other macronutrients. The speed of absorption can affect postprandial (after meals) protein synthesis, breakdown, and deposition.1,2

Its been shown that the postprandial amino acid levels differ depending on the mode of administration of a dietary protein; a single protein meal results in an acute but transient peak of amino acids whereas the same amount of the same protein given in a continuous manner, which mimics a slow absorption, induces a smaller but prolonged increase.

Since amino acids are potent modulators of protein synthesis, breakdown, and oxidation, different patterns of postprandial aminoacidemia (the level of amino acids in the blood) might well result in different postprandial protein kinetics and gain. Therefore, the speed of absorption by the gut of amino acids derived from dietary proteins will have different effects on whole body protein synthesis, breakdown, and oxidation, which in turn control protein deposition.

For example, one study looked at both casein and whey protein absorption and the subsequent metabolic effects.3In this study two labeled milk proteins, casein (CAS) and whey protein (WP), of different physicochemical properties were ingested as one single meal by healthy adults and postprandial whole body leucine kinetics were assessed. WP induced a dramatic but short increase of plasma amino acids. CAS induced a prolonged plateau of moderate hyperaminoacidemia, probably because of a slow gastric emptying. Whole body protein breakdown was inhibited by 34% after CAS ingestion but not after WP ingestion. Postprandial protein synthesis was stimulated by 68% with the WP meal and to a lesser extent (+31%) with the CAS meal.

Under the conditions of this study, i.e., a single protein meal with no energy added, two dietary proteins were shown to have different metabolic fates and uses. After WP ingestion, the plasma appearance of dietary amino acids is fast, high, and transient. This amino acid pattern is associated with an increased protein synthesis and oxidation and no change in protein breakdown. By contrast, the plasma appearance of dietary amino acids after a CAS meal is slower, lower, and prolonged with a different whole body metabolic response: protein synthesis slightly increases, oxidation is moderately stimulated, but protein breakdown is markedly inhibited.

This study demonstrates that dietary amino acid absorption is faster with WP than with CAS. It is very likely that a slower gastric emptying was mostly responsible for the slower appearance of amino acids into the plasma. Indeed, CAS clots into the stomach whereas WP is rapidly emptied from the stomach into the duodenum. The results of the study demonstrate that amino acids derived from casein are indeed slowly released from the gut and that slow and fast proteins differently modulate postprandial changes of whole body protein synthesis, breakdown, oxidation, and deposition.

After WP ingestion, large amounts of dietary amino acids flood the small body pool in a short time, resulting in a dramatic increase in amino acid concentrations. This is probably responsible for the stimulation of protein synthesis. This dramatic stimulation of protein synthesis and absence of protein breakdown inhibition is quite different from the pattern observed with classic feeding studies and with the use of only one protein source.

In conclusion, the study demonstrated that the speed of amino acid absorption after protein ingestion has a major impact on the postprandial metabolic response to a single protein meal. The slowly absorbed CAS promotes postprandial protein deposition by an inhibition of protein breakdown without excessive increase in amino acid concentration. By contrast, a fast dietary protein stimulates protein synthesis but also oxidation. This impact of amino acid absorption speed on protein metabolism is true when proteins are given alone, but as for carbohydrate, this might be blunted in more complex meals that could affect gastric emptying (lipids) and/or insulin response (carbohydrate).

In light of the fact that both hyperaminoacidemia 4,5,6 and resistance exercise7,8,9,10,11 independently stimulate muscle protein synthesis, a recent study (by Wilkinson et al. 2007) looked at how different proteins differ in their ability to support muscle protein accretion.12

The study investigated the effect of oral ingestion of either fluid nonfat milk or an isonitrogenous and isoenergetic macronutrient-matched soy-protein beverage on whole-body and muscle protein turnover after an acute bout of resistance exercise in trained men. The authors hypothesized that the ingestion of milk protein would stimulate muscle anabolism to a greater degree than would the ingestion of soy protein, because of the differences in postprandial aminoacidemia. compared whey against casein.

In this study arterial-venous amino acid balance and muscle fractional synthesis rates were measured in young men who consumed fluid milk or a soy-protein beverage in a crossover design after a bout of resistance exercise.

The primary finding of the current study was that intact dietary proteins, as against say portions of intact proteins such as concentrates or isolates of whey, soy or casein, can support an anabolic environment for muscle protein accretion.

Two other studies done to date found that the ingestion of whole proteins after resistance exercise can support positive muscle protein balance.13,14 However this study was the first to show that the source of intact dietary protein (i.e., milk compared with soy) is important for determining the degree of postexercise anabolism.

The study (by Wilkinson et al. 2007) also found a significantly greater uptake of amino acids across the leg and a greater rate of muscle protein synthesis in the 3 h after exercise with the milk-protein consumption as compared to soy-protein ingestion. Thus milk protein promoted a more sustained net positive protein balance after resistance exercise than did soy protein.

The authors concluded that since the milk and soy proteins provided equal amounts of essential amino acids, and that the level of EAAs drive protein synthesis,15 it’s likely that differences in the delivery of and patterns of change in amino acids are responsible for the observed differences in net amino acid balance and rates of muscle protein synthesis. Because of differences in digestion rates, milk proteins may provide a slower pattern of amino acid delivery to the muscle than soy protein.

Ingestion of soy protein results in a rapid rise and fall in blood amino acid concentrations, whereas milk protein ingestion produces a more moderate rise and a sustained elevation in blood amino acid concentrations.16Interestingly, these increases in anabolic p
rocesses were seen without any concurrent increases in whole-body protein oxidation. Part of the explanation for this lack of increase is that the test meals consumed by participants in this study had 30% of total energy from fat, which would likely have slowed digestion and, thus, the rate appearance of amino acids into general circulation. As well, the dose of protein used (7.5 g indispensable amino acids) did not stimulate amino acid oxidation.

Previous studies that examined the effect of ingestion of similar quantities of crystalline amino acids on muscle protein turnover have shown that increases in net protein balance with the ingestion of 40 g crystalline indispensable amino acids (8.3 g leucine)17 were similar in magnitude to that seen with the ingestion of only 6 g crystalline amino acids (2.2 g leucine)18. These data suggest that, when large quantities of amino acids are ingested, amino acids are likely being directed to deamination and oxidation.

The authors of this study (by Wilkinson et al. 2007) proposed that the digestion rate and, therefore, the ensuing hyperaminoacidemia that differed between the milk and soy groups after exercise is what affected the net uptake of amino acids in the exercised leg.

However, regardless of their conclusions, because there are variations between the proteins, it’s still possible that the differences in amino acid composition between the two proteins had some effect on protein accretion. For example, the analysis of the proteins in this study found that the content of methionine in the soy protein (1.4%) was lower than that in milk protein (2.6%).


Because of different absorption kinetics, proteins from different sources are used differently in various tissues, including locally by the gut, by the liver, and by skeletal muscle. As well, the kinetics change not only with the source of protein, but also when protein intake is increased.19

Recent studies have alluded that whey protein may be the best protein to use after training. However, this is not the case when one looks at the immediate beneficial effects of whey protein on protein synthesis, the counter productive effects on insulin, and the lack of long term effects on protein synthesis.

A recent study looked at the effects of protein supplentation on body composition, muscular strength, muscular endurance, and anaerobic capacity during 10 weeks of resistance training.20

Thirty-six resistance-trained males were split into three groups and followed a 4 days-per-week split body part resistance training program for 10 weeks. Protein supplements were randomly assigned, prior to the beginning of the exercise program. Group one received carbohydrate placebo, group two whey protein + casein, and group three whey protein plus branched-chain amino acids and glutamine.

In this study, the combination of whey and casein protein promoted the greatest increases in fat-free mass after 10 weeks of heavy resistance training.



  • Dangin M, Boirie Y, Garcia-Rodenas C, Gachon P, Fauquant J, Callier P, Ballevre O, Beaufrere B. The digestion rate of protein is an independent regulating factor of postprandial protein retention. Am. J. Physiol. Endcrinol. Metab 2001:280: E340-E348.
  • Dangin M, Boirie Y, Guillet C, Beaufrere B. Influence of the protein digestion rate on protein turnover in young and elderly subjects. J Nutr 2002;132(10):3228S-33S.
  • Boirie Y, Dangin M, Gachon P, Vasson MP, Maubois JL, Beaufr B. Slow and fast dietary proteins differently modulate postprandial protein accretion. Proc. Natl. Acad. Sci. USA 1997;94: 14930-14935.
  • Bohe J, Low A, Wolfe RR, Rennie MJ. Human muscle protein synthesis is modulated by extracellular, not intramuscular amino acid availability: a dose-response study. J Physiol 2003;552:315–24.Biolo G, Tipton KD, Klein S, Wolfe RR. An abundant supply of amino acids enhances the metabolic effect of exercise on muscle protein. Am J Physiol 1997;273:E122–9.
  • Bohe J, Low JF, Wolfe RR, Rennie MJ. Latency and duration of stimulation of human muscle protein synthesis during continuous infusion of amino acids. J Physiol 2001;532:575–9.
  • Biolo G, Tipton KD, Klein S, Wolfe RR. An abundant supply of amino acids enhances the metabolic effect of exercise on muscle protein. Am J Physiol 1997;273:E122–9.
  • Chesley A, MacDougall JD, Tarnopolsky MA, Atkinson SA, Smith K. Changes in human muscle protein synthesis after resistance exercise. J Appl Physiol 1992;73:1383–8. Biolo G, Maggi SP, Williams BD, Tipton KD, Wolfe RR. Increased rates of muscle protein turnover and amino acid transport after resistance exercise in humans. Am J Physiol 1995;268:E514–20.
  • Phillips SM, Tipton KD, Aarsland A, Wolf SE, Wolfe RR. Mixed muscle protein synthesis and breakdown after resistance exercise in humans. Am J Physiol 1997;273:E99–107.
  • Phillips SM, Tipton KD, Ferrando AA, Wolfe RR. Resistance training reduces the acute exercise-induced increase in muscle protein turnover. Am J Physiol 1999;276:E118–24.
  • Yarasheski KE, Zachwieja JJ, Bier DM. Acute effects of resistance exercise on muscle protein synthesis rate in young and elderly men and women. Am J Physiol 1993;265:E210–4.
  • Biolo G, Maggi SP, Williams BD, Tipton KD, Wolfe RR. Increased rates of muscle protein turnover and amino acid transport after resistance exercise in humans. Am J Physiol 1995;268:E514–20.
  • Wilkinson SB, Tarnopolsky MA, Macdonald MJ, Macdonald JR, Armstrong D, Phillips SM. Consumption of fluid skim milk promotes greater muscle protein accretion after resistance exercise than does consumption of an isonitrogenous and isoenergetic soy-protein beverage. Am J Clin Nutr. 2007;85(4):1031-40.
  • Elliott TA, Cree MG, Sanford AP, Wolfe RR, Tipton KD. Milk ingestion stimulates net muscle protein synthesis following resistance exercise. Med Sci Sports Exerc 2006;38:1–8.
  • Tipton KD, Elliott TA, Cree MG, Wolf SE, Sanford AP, Wolfe RR. Ingestion of casein and whey proteins result in muscle anabolism after resistance exercise. Med Sci Sports Exerc 2004;36:2073–81.
  • Volpi E, Kobayashi H, Sheffield-Moore M, Mittendorfer B, Wolfe RR. Essential amino acids are primarily responsible for the amino acid stimulation of muscle protein anabolism in healthy elderly adults. Am J Clin Nutr 2003;78:250–8.
  • Bos C, Metges CC, Gaudichon C, et al. Postprandial kinetics of dietary amino acids are the main determinant of their metabolism after soy or milk protein ingestion in humans. J Nutr 2003;133:1308–15.
  • Tipton KD, Ferrando AA, Phillips SM, Doyle D Jr, Wolfe RR. Postexercise net protein synthesis in human muscle from orally administered amino acids. Am J Physiol 1999;276:E628–34.
  • Borsheim E, Tipton KD, Wolf SE, Wolfe RR. Essential amino acids and muscle protein recovery from resistance exercise. Am J Physiol Endocrinol Metab 2002;283:E648–57.
  • Fouillet H, Juillet B, Gaudichon C, Mariotti F, Tomé D, Bos C. Absorption kinetics are a key factor regulating postprandial protein metabolism in response to qualitative and quantitative variations in protein intake. Am J Physiol Regul Integr Comp Physiol. 2009 Dec;297(6):R1691-705.
  • Kerksick CM, Rasmussen CJ, Lancaster SL, Magu B, Smith P, Melton C, Greenwood M, Almada AL, Earnest CP, Kreider RB. The effects of protein and amino acid supplementation on performance and training adaptations during ten weeks of resistance training. J Strength Cond Res. 2006 Aug;20(3):643-53.