Slow and Fast Proteins—A Myth? Part 2

Dear BLOG readers, Dear PEAK customers,

In Part 1 of my two-part series, I discussed the distinction between fast and slow proteins, as well as the gastric retention time and absorption rate of various proteins. The conclusion is that there is indeed a distinction between slow- and fast-acting proteins. However, these terms are often misinterpreted.

Slow and Fast Proteins—A Myth? Part 1

Today, I’d like to build on the research from Part 1 to discuss whether a faster or slower absorption rate is likely to have advantages—or perhaps even disadvantages.

 

Differences Based on Timing of Intake?

In principle, one can assume that the amino acid concentration after ingesting the corresponding protein is equally high at any time of day. What changes significantly, however—especially when consumed after a workout—is the blood flow to the muscle cells. Studies show that this flow is 30–100% higher than at rest, thereby positively influencing protein synthesis through this mechanism.

Conclusion

Increased blood flow to the muscles is particularly evident after a workout.
We’ll need to refer back to this finding later...

 

Absorption Rate and Protein Turnover

We know from Part 1 that there are fast and slow proteins—a fact that is certainly reassuring to many. What we do not yet know, however, is the extent to which these differences affect the body’s own protein turnover—that is, the processes of protein synthesis and breakdown in our bodies.

Study

To investigate this, Boirie, Y., Dangin, M., Gachon, P., et al. examined the influence of a 30-gram serving of casein and the same amount of whey protein on blood amino acid concentrations over time. To distinguish whether the measured amino acids originated from the respective meal or from endogenous protein turnover, radioactive tracers were attached to the amino acid leucine in the administered protein. The study participants were 16 young, healthy men with an average age of 24 years and an average BMI of 21.

As shown in Figure 1, the results reveal the overall increase in leucine, how endogenous protein turnover behaves in parallel, and how the intake of the two proteins ultimately affected leucine levels.

As expected, whey protein triggers a rapid and more pronounced increase in leucine, thereby strongly inhibiting endogenous protein breakdown in the short term and ultimately resulting in a high, rapid rise in leucine that peaks approximately 60 minutes after ingestion. After only a short time, a downward trend in blood amino acid levels can be observed.

Although casein also produced an elevated leucine level after about 60 minutes, it did not reach the same magnitude as whey protein. In contrast to whey protein, endogenous protein breakdown was inhibited over a much longer period, which ultimately resulted in 34% less protein breakdown over the course of the measurement period.

Effects on protein breakdown similar to those observed with casein were not observed with whey protein; instead, protein synthesis increased more rapidly in the short term.

Conclusion

Whey protein causes a rapid increase in absolute amino acid levels, but these levels drop quickly again. Unlike casein, it rapidly increases protein synthesis but has no significant effect on protein breakdown; therefore, one can speculate that a combination of both protein types can generally be considered the most potent choice.

 

Fast or Slow Proteins—Which Is the Better Choice?

A general question that now arises regarding our topic is whether digestion rate should actually be a deciding factor when purchasing a protein supplement, or whether the amino acid profile of the respective protein alone determines its positive effects.

To find this out, Yves Boirie, Bernard Beaufrere, and their colleagues have uncovered some extremely interesting study results. In the course of their research, they made the following observations, some of which we are already familiar with:

• Whey protein is absorbed more quickly than micellar casein
• The amino acids in whey protein (primarily BCAAs, and leucine in particular) contribute to a significantly greater extent to energy supply
• Micellar casein can generally—but especially after a workout—prevent the breakdown of muscle protein
• Micellar casein is superior to faster-digesting whey protein in terms of utilization

Regarding the effects of digestion rate, the researchers found that this factor appears to be even more important than the amino acid profile of the protein in question. For further investigations, they used different proteins but standardized the amino acid compositions and nitrogen content to determine the pure effect of absorption rate.

In a comparison of the effects of 30 g of casein and a mixture of 30 g of free amino acids, the amino acid mixture resulted in faster absorption and an increase in protein synthesis. However, neither of these contributed to an improved leucine balance nor to increased protein synthesis; instead, they resulted in enhanced amino acid oxidation, similar to the rate observed with standardized whey protein. Protein breakdown was only slightly inhibited by the amino acid mixture, whereas the intake of slow-digesting casein significantly inhibited breakdown for over seven hours.

In a further study, a single dose of protein was compared to multiple doses of 2.5 g each administered every 20 minutes. As discussed in the chapter on gastric retention time in Part 1, it was expected that the protein drink containing the full 30 g would be digested and utilized more quickly. As in the previous studies, some of its amino acids were utilized for energy production, while protein breakdown was not prevented. Spreading the intake over several hours resulted in a steadily elevated amino acid concentration for four to five hours; however, this concentration remained at a much lower level than that observed with the single dose. The small doses reduced protein breakdown but did not significantly affect protein synthesis. The researchers concluded that the magnitude of the change in amino acid levels and the duration of the increase are decisive factors. Rapidly available proteins (such as whey protein) are better suited for consumption immediately after exercise, since the muscles absorb protein more effectively at that time (as mentioned above). Slow-release protein ensures improved absorption at all other times of the day and better protects body tissues from breakdown.

Conclusion

With the exception of the post-workout window, slower-digesting proteins appear to be the better alternative when it comes to building, protecting, and maintaining muscle mass.

 

Is Net Yield a Disadvantage of Fast-Digesting Proteins?

Now that a fast digestion rate has been shown to be advantageous—at least at certain times of the day—it’s important to provide a brief summary of the overall picture. In 2008, Julius Oben et al. published an interesting study in the Journal of the International Society of Sports Nutrition, in which they raised what appear to be valid doubts about the net bioavailability of rapidly available proteins. They investigated the effects of a single dose of 50 g of whey protein concentrate on actual absorption yield—once with and once without the additional intake of a digestive enzyme complex—in young, healthy, and non-overweight participants. The CG (control group) received a placebo; TG-A-5 received 5 g of a protease enzyme complex; and TG-A-2.5 received 2.5 g of the same enzyme complex.

The results showed significant differences in the levels of amino acids in the blood. Of secondary importance here are the differing effects of the 2.5-g and 5-g supplements on net absorption. Of crucial importance, however, is the observed reduced absorption of amino acids, caused by an excessively short transit time and what appears to be an overload on the body’s own proteases. Of the 50 g of protein consumed, only 15 g were actually absorbed into the bloodstream—and this was with the use of a whey protein concentrate, which we know is far from matching the digestion rate of hydrolysates or isolates.

Conclusion

It is possible that fast-digesting proteins are a bit too fast and overwhelm our proteases—the body’s protein-breaking enzymes—which may result in a reduced net yield of amino acids in the blood. From this perspective, the addition of exogenous enzymes to fast-digesting proteins should definitely be reconsidered.

 

Summary

After reviewing all the data, studies, and facts, one can conclude that differences in protein absorption rates are not a myth and are indeed significant when it comes to using the right protein at the right time. Overall, slow-release proteins have the edge for both muscle building and muscle maintenance, regardless of their amino acid composition. Only after a workout does one benefit from consuming a fast-acting protein, as this ensures a rapid increase in amino acid concentration and drives protein synthesis. The question of the actual net yield and the efficiency of protein-digesting enzymes in relation to the consumption of fast-acting proteins remains open.

With athletic regards
, Holger Gugg

 

 

Sources

Boirie, Y., Dangin, M., Gachon, P., et al. “Slow and fast dietary proteins differently modulate postprandial protein accretion.” Proc Natl Acad Sci U S A. Dec. 23, 1997; 94(26): 14930–14935

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC25140/

https://www.spektrum.de/lexikon/ernaehrung/magenverweildauer/5520

D. Kalman et al. Journal of the International Society of Sports Nutrition, July 2007

NBJ’s Sports Nutrition and Weight Loss Report 2007–2008. Nutrition Business Journal. Boulder, CO. New Hope Natural Media, January 2008.

Paul GL. The rationale for consuming protein blends in sports nutrition. J Am Coll Nutr. Aug. 2009;28 Suppl:464S-472S. Review.

Boirie Y, Dangin M, Gachon P, Vasson MP, Maubois JL, Beaufrère B. Slow and fast dietary proteins differently modulate postprandial protein accretion. Proc Natl Acad Sci U S A. Dec 23, 1997;94(26):14930-5.

Dangin ^M1, Boirie Y, Garcia-Rodenas C, Gachon P, Fauquant J, Callier P, Ballèvre O, Beaufrère B (2001). The digestion rate of protein is an independent regulating factor of postprandial protein retention. Am J Physiol Endocrinol Metab. Feb 2001;280(2):E340-8.