Eggs are slow proteins, while some components of milk are fast proteins.

Fast and Slow Proteins: Separating Fact from Fiction

Fast protein and slow protein

In 1997, a French scientist named Yves Boirie published a paper entitled, “Slow and fast dietary proteins differently modulate postprandial protein accretion”. His paper is published on the web here.

In it, he described research which led him to discover that the two main proteins found in milk – whey and casein – are digested at different rates.

Protein digestion increases plasma amino acid concentration. The rate at which this increase occurs determines whether a protein source is considered “slow” or “fast”. Whey is digested more quickly than casein. It is a “fast” protein and casein is a “slow” protein. You can digest 18 grams of whey in two hours; you require three hours to fully digest the same amount of casein.

Different food sources are digested at varying rates

Different food sources are digested at varying rates

Whey protein causes the concentration of amino acids in the blood plasma to peak at a higher level than casein.

While a meal of casein is digested slowly and plasma amino acid concentrations remain relatively constant throughout the digestion process, whey digests in half the time and peaks the amino acid concentration before falling off rapidly.

How does fast and slow protein tie in with weight lifting?

This research took the fitness world by storm.

Immediately, supplement manufacturers claimed Boirie’s research indicates that whey protein supplements are essential to weight lifters because the fast whey protein is metabolized immediately after exercise when an athlete is in an especially anabolic state.

Although whey does peak plasma amino acid levels, it does not begin to digest any faster than other sources of protein.

They ran advertisements explaining that whey’s fast digestion characteristics, coupled with the fact that the research also implied that whey was capable of boosting the rate of protein synthesis more than casein, made it perfect for use as a post-workout snack. Thus, even trainees who got more than enough protein in their diet were advised to supplement with whey after a workout.

The high price of whey protein supplements, which is almost entirely profit, gives manufacturers and marketers an incentive to make unfounded claims about what was once a by-product of cheese making and fit only for processed food or animal feed.

These days, people are beginning to realize that whey protein is not a panacea for anxious weight lifters who want to bulk up. Although whey does peak plasma amino acid levels, it does not begin to digest any faster than other sources of protein. And since it is upwards of an hour before whey boosts the plasma amino acid levels, the strategy of using whey as a post-workout snack is called into question.

Boiries protein digestion rate chart comparing casein to whey

Boirie's protein digestion rate chart comparing casein to whey

As Boirie’s graph (above) shows, although whey protein causes plasma amino acid levels (represented by the blood’s leucine concentration) to rise higher, both whey and casein don’t enter the bloodstream in significant concentrations until around an hour after ingestion.

Whey boosts protein synthesis, but casein reduces protein degradation. Both serve bodybuilders well.

Furthermore, although the supplement manufacturers were quick to tout whey’s propensity for boosting protein synthesis, they failed to mention that casein causes a reduction in protein degradation. The effects on overall protein accretion of whey versus casein are a wash: either one will give your body what it needs.

Current research indicates that post-workout carbohydrates are more important than proteins. A snack consisting of fast carbs (simple sugars) prevents the protein breakdown that often follows exercise.

What does this mean for my bulking diet

There is no question that skeletal muscle uses free amino acids for protein synthesis at an increased rate immediately after exercise.

However, it’s not clear that a post-workout hit of whey protein is of much help. Since dietary protein takes hours to digest – casein can take up to eight hours to digest and enter the bloodstream as amino acids – the amino acids used in your post-workout anabolic window are probably left over from a previous meal.

You will benefit from carbohydrates immediately after working out. There is no definitive proof that you benefit from a post-workout whey supplement.

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