Slow and Fast Proteins—A Myth? Part 1

Dear BLOG readers, Dear PEAK customers,

There are several ways to classify proteins. First, there are animal and plant proteins. They are distinguished by their source.

The second distinction separates proteins with a high insulin index from those that have only a minor effect on insulin secretion.

A third distinction is biological value, which refers to the ability of dietary protein to be converted into the body’s own protein. A final distinction categorizes proteins as “fast” and “slow,” referring to their bioavailability—that is, the time it takes for the amino acids broken down from a protein to become available in the bloodstream.

Numerous dietary approaches are based on this theory of fast and slow proteins and sometimes use this framework to make recommendations regarding the timing of consumption for specific types of protein.

Since I’m one of those people who likes to trace unwritten rules back to their origins, today I’ll address the question of whether the theory of fast and slow proteins holds true and what conclusions can be drawn from it to help optimize one’s diet plan in terms of protein intake.

 

Gastric Dwell Time and Absorption Rate

Gastric dwell time

When it comes to absorption—that is, the uptake of amino acids from the intestine into the bloodstream—we actually need to start our investigation a bit earlier, specifically where a significant portion of protein digestion takes place: in the stomach. Different foods are “held” there for varying lengths of time until the stomach releases them in measured doses into the intestine. In technical terms, this is referred to as “gastric retention time,” which denotes the amount of time food spends in the stomach. It depends on several factors and typically ranges from one to six hours.

Solid or poorly chewed food remains in the stomach longer than liquids. In addition to consistency, osmolarity—that is, particle concentration—also plays a role in gastric retention time. This is particularly important for liquids, and especially when monosaccharides are ingested, since they stimulate the osmoreceptors of the duodenum in different ways. Nutrient composition also influences gastric retention time. In particular, fats and carbohydrates lead to a longer retention time due to their effect on the osmoreceptors and chemoreceptors of the digestive tract. Ultimately, a high energy density also results in a prolonged gastric retention time. However, it is important to note that, per unit of time, high-calorie meals still release more energy into the small intestine than meals with a low energy density.

This brings us to the crux of the issue regarding gastric retention time in relation to protein intake—a concept that is repeatedly misinterpreted.

Conclusion

Depending on a food’s properties, it remains in the stomach for a longer or shorter period before the nutrients are released into the small intestine.

 

Absorption Rate

While a long gastric retention time means that a meal takes a long time to completely leave the stomach, it does not mean that this is the time it takes for the first pre-digested nutrients to leave the stomach. Since the stomach releases fully processed nutrients in measured doses to the small intestine, this is not the case.

Absorption of Whey Protein

When whey protein is consumed, the first amino acids can be detected in the blood’s amino acid pool as early as 30 minutes after ingestion. The peak concentration is reached after about 45 to 120 minutes. After about three hours, the concentration has returned to normal.

Not All Whey Is the Same

Since whey protein is available in several forms, it stands to reason that there are also differences in absorption. Virtually pre-digested proteins, such as whey hydrolysate, cause blood amino acid levels to rise more quickly than whey concentrates.

Kalmann et al. estimate that the first amino acids from protein hydrolysates enter the bloodstream as early as 15 minutes after ingestion, but that this elevated concentration returns to baseline levels after just 90 minutes.

The absorption rate here is comparable to that of free amino acids. High-quality whey isolates also feature an absorption rate that can almost rival that of free amino acids.

The absorption of whey concentrates takes slightly longer, as this form lacks short-chain peptides, resulting in a slightly greater digestive burden.

Interesting
fact: Soy protein isolate is known to have an absorption rate that is nearly identical to that of whey protein.

 

Casein takes its time

Casein is known to remain in the stomach for up to 7 hours. However, as early as one hour after ingestion, a moderate amount of amino acids can be detected in the blood, which then persists for several hours.

Incidentally, this characteristic does not apply to casein hydrolysate, which is why the typical time-release effect associated with casein cannot be expected here.

Medium-Fast

Soy protein and egg protein are often described in the literature as “medium-fast” proteins, and that is exactly what they are. The gastric retention time for both is about 2–3 hours, meaning it takes a little longer for a significant amount of amino acids from them to be detected in the blood.

 

Conclusion

In general, it doesn’t take very long for the first amino acids from any protein source to appear in the blood. The major difference lies in the amount per unit of time and the time-release effect—that is, the duration during which an elevated amino acid concentration is maintained.

 

Summary

The theory of fast- and slow-acting proteins holds true in practice as well. While hydrolysates generally lead the way in absorption thanks to their short amino acid chains, it’s safe to assume that a larger amount of amino acids from whey protein—and also from soy protein in its isolate form—will reach the bloodstream significantly faster than those from egg protein, soy protein concentrate, and casein. Unfortunately, anyone who bases their assessment solely on the time spent in the stomach is on the wrong track.

 

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 M, 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.