Slow vs. Fast Digesting Protein
You’re likely familiar with the concept of slow vs. fast digesting carbs. Slow carbs are complex and often minimally processed, while fast carbs are simple and tend to be highly processed. Classically, slow digesting carbs are best eaten at meal times, while fast carbs play their role when we need fuel ... fast (ha). This is why you’ll see weightlifters eat gummies or grapes during training sessions, and runners consuming glucose gels or applesauce mid-race. This variation affects the rate at which our bodies absorb sugar into the bloodstream, and how long after eating it takes us to get hungry again.
Parallel to this concept is slow vs. fast digesting proteins. If you’ve consumed a whey protein shake in a small window before or after working out, then ingested casein several hours later before bed, you’ve taken advantage of your body’s variable response to different protein types.
Source: Examine.com
Whey (fast digesting) is a liquid milk by-product formed in the making of cheese that accounts for 20% of the protein in milk. Casein (slow digesting) is a solid milk by-product, composing the bulk of milk protein at 80%. Both are complete proteins (containing all essential amino acids), though casein boasts a 117 DIAAS (digestible indispensable amino acid score) vs. 85 for whey. DIAAS accounts for amino acid digestibility at the end of the small intestine (providing a more detailed indicator of the protein’s contribution to the body’s amino acid and nitrogen contributions). Casein results in slower gastric emptying - staying with you longer, releasing amino acids into the bloodstream several hours after consumption and potentially enhancing nitrogen retention and use relative to whey. Casein does tend to be more expensive and a bit harder to find than its whey counterpart. Whey protein absorbs quickly, spiking blood leucine (an indicator of protein deposition) after consumption before rapidly fizzling out.
So how does speed of protein digestion affect my gains?
For one form of protein to be more effective than the other at supporting strength or hypertrophy, the ratio between Muscle Protein Breakdown (MPB) and Muscle Protein Synthesis (MPS) would have to be significantly skewed in favor of building muscle. While resistance training gets the ball rolling on muscle protein synthesis (and keeps it going for up to 48 hours after exercise), MPB is simultaneously prevented by inducing a state of hyperinsulinemia. Protein consumption enhances both of these effects (increased MPS and decreased MPB).
Theoretically, fast-digesting protein sources high in leucine (a key amino acid player for stimulating MPS) are hypothesized to be the most beneficial for MPB due to rapidly maximizing amino acid bioavailability. Meanwhile, casein’s strength lies in preventing MPB. The reality is that, while it is possible that whey and casein have differing effects on muscle adaptations after exercise, there has been little evidence translating what we know about rate of digestion to an increase in actual performance factors. Note this 2013 study on female athletes by Wilborn et. al., in which whey and casein both demonstrated positive effects on body composition and performance, with no significant difference between the two. See also this 2017 study currently under review by Fabre et.al. demonstrating similar results. If anything, this supports the hypothesis that a combination of the two is most effective due to their complementary effects on the various processes of muscle rebuilding.
All in all, if you’re worried about protein considerations, I wouldn’t worry too much about fast vs. slow digesting protein types. Your body is smart, adaptable, and will continue to build muscle as you feed it a sufficient quantity of quality, complete amino acids sources surrounding your resistance training. If you enjoy shakes and like the taste of casein, go for it (make sure to check for third party testing). If not, it’s never a bad idea to come back to the basics: doing your best to divy up the totality of your daily protein requirements with some level of frequency throughout the day (3 meals with 30g + 2 snacks with 15g for a 130 lb female athlete, as an example) so that amino acids are distributed for metabolizing in regular intervals to support strength, performance and health.
Until next week,
Laura
