Is rice protein a “fast” or “slow” protein? Does it matter?

Scientists at the University of Tampa recently (2014) investigated the absorption kinetics of Growing Naturals rice protein and compared it to a whey protein isolate in a double-blind crossover study.  They wanted to see how long it would take the amino acids in each protein to reach the bloodstream.  This was a follow up study to the 2013 study which found that rice protein produced significant gains in muscle much like whey protein.

The “speed” of a protein may offer insight into the type of benefit it can have on your body.  For example, in sport nutrition some studies show that the digestion rate of proteins differently influence how tissue (muscles, skin, nails) is broken down and/or rebuilt.  After resistance exercise, there is no conclusive data as to which (“fast” or “slow”) is better for muscle gains–what’s most important is to eat protein (rather than not to).  What about blending proteins? Recent evidence has shown that a blend of proteins (fast + intermediate) might be more beneficial than choosing one over the other after a workout because the muscles are exposed to amino acids for a prolonged period of time.  For satiety (the state of being full), a slow protein (like casein) has shown a stronger satiating effect than a fast protein like whey.

For this study, 10 trained male subjects were randomly assigned to consume GN rice protein- or a whey protein isolate beverage after a 12 hour overnight fast.  Both drinks contained the same amount of protein, calories, carbs, and volume.  Blood samples were taken right before the protein drink was consumed and then 1, 2, 3, and 4 hours after to analyze changes in concentration from hour to hour.  After seven days, the procedure was repeated with subjects consuming the other protein drink.

Results showed that rice protein peaked in the blood at 93 minutes compared to 69 minutes for the whey protein, classifying it as an “intermediate-absorbing” protein—in between slow and fast.  Uniquely enough, when scientists looked at the absorption of the amino acids individually, they found that the leucine from rice protein was absorbed faster than leucine from whey. Why is this interesting? Leucine is the amino acid known to solely (on its own) activate muscle building processes in the body.  No other amino acid is known to do this.  And despite levels being slightly lower in the rice protein, it was absorbed faster than that of whey’s.  (Scientists don’t know for sure) but this might offer some additional explanation as to why rice was as good as whey in the 2013 study.

In terms of amounts absorbed, the results showed that rice protein had a 6.8% lower concentration in the blood than that of whey protein—but the difference was not statistically different.  This was great, albeit surprising considering rice protein has been documented as having a 13% lower digestibility than that of whey.

Ultimately, more research is needed to truly define the benefit of rice protein as an “intermediate-absorbing” protein.


figure 3 - absorption


Reference: Purpura et al. Comparison of blood amino acid concentrations following ingestion of rice and whey protein isolate: A double-blind crossover study. J Nutr Health Sci. 2014.

Print Friendly