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Originally Posted by Lyle McDonald
]A recent paper (6) has kicked off an entirely new category/fad of protein nutrition and marketing, that of fast versus slow dietary proteins. This idea is conceptually similar to the glycemic index (GI) concept which is applied to carbohydrates, and represents the speed at which they digest and affect blood glucose and insulin levels.
In this study, healthy subjects with a normal protein intake (16% of total calories) were fasted for 10 hours and then given either 30 grams of whey protein or 30 grams of casein (milk) protein. The primary finding of the study was that whey protein caused blood leucine levels (which are used as a marker of a variety of metabolic processes in the body) to increase rapidly, hitting a peak in 1 hour. However leucine levels decreased equally rapidly returning to normal by 4 hours. In contrast, casein caused a much slower rise in blood leucine levels, reaching a lower peak around 1 hour, but maintained that level for almost 7 hours (see figure 1).
Additionally, the researchers found that whey protein stimulated protein synthesis (which refers to the building of larger proteins from individual AAs) with no effect on protein breakdown (which refers to the breakdown of larger proteins to individual AAs), while the casein inhibited protein breakdown without affecting protein synthesis. Another observation was that whey protein increased leucine oxidation (burning) slightly more than the casein (31% vs. 24%), probably due to the faster rate of entry. Finally, leucine balance (determined as the amount ingested versus the amount stored) was higher for casein than whey. These observations lend themselves to multiple interpretations.
On the one hand, the effects on protein synthesis and breakdown are interesting and it appears that whey is an 'anabolic' protein while casein is 'anti-catabolic' protein, at least over a span of 7 hours. However, body leucine stores are also important for a variety of reasons and it could be argued that casein is superior in that it led to net leucine retention by the body. I'm sure that how the study is interpreted will depend on whether the person who is interpreting it wants to sell whey, casein or a blend of the two.
This one study has already generated an article in the prestigious journal Nature (7) as well as in several bodybuilding magazines, with authors suggesting that whey and casein can be used to elicit differential physiological effects and spur growth. Protein supplements containing mixes of the so-called fast and slow proteins have also appeared on the market, with the idea being that one can get increased protein synthesis AND decreased protein breakdown, as well as keeping blood AA levels more stable.
To say that too much has been read into this single study would be putting it mildly. There are a number of issues which have been completely ignored by those reporting on this article which are discussed here. The first and perhaps most important point is that the subjects were fasted for 10 hours prior to being given the protein supplements. Protein synthesis and breakdown rates are very different after a 10 hour fast compared to the rates in the middle of the day after food has been eaten. After an overnight fast, muscle protein synthesis rates may be 50% lower than after food has been consumed (8). This means that any effects from a protein meal would be expected to be much higher first thing in the morning, versus a similar comparison done at a different time of the day.
Additionally, it is well known that the mixing of nutrients (i.e. carbohydrates and protein or carbohydrates, protein and fat) changes the absorption rate of nutrients into the bloodstream (1). Similarly, the presence of undigested food from a previous meal also affects digestion rate. All this study tells us is what will happen when whey or casein is taken by itself, after a 10 hour fast, on an empty stomach. To draw meaningful conclusions from this study to what might happen with the consumption of whey or casein with dietary fat (i.e. flax oil in the blender drink) or carbohydrates, or to other times of the day is impossible.
A final unanswered question is where the protein synthesized during the whey trial was stored (7). That is, the methodology of the study only told the researchers that protein was being synthesized and stored, not where it was going. This is, in fact, a major problem with most human protein research: it is generally difficult to know where stored protein is going unless a biopsy is taken. Since the goal of bodybuilders is to influence muscle protein synthesis, and not just increase whole body protein synthesis, it is important to know where the ingested protein is going.
Sufficed to say that it is just as reasonable to assume that it was being stored as liver protein as it is to assume it was being stored as muscle protein. Of course, saying that whey will increase liver protein synthesis won't sell a lot of supplements.
As a final comment, most serious bodybuilders eat a protein containing meal every 2-3 hours as it is. Since blood leucine didn't drop until the 4 hour mark in the whey trial, is it going to make a huge difference whether a bodybuilder consumes whey or milk protein if they are eating every 3 hours? And if casein keeps blood leucine levels up for 7 hours, and whole proteins take even longer to fully digest, is it truly necessary to ingest protein every 3 hours during the day?
Some have suggested consuming a mix of whey and casein right before bedtime to get a sustained release of AA's into the bloodstream and there may be some validity to this. Of course, any whole protein, combined with some carbs, fat and fiber would accomplish just as much.
A final question that this study raises has to do with the post-workout protein feeding. Consider that even the whey protein took 1 hour to raise blood leucine levels to it's peak. If the idea is to provide AAs to a recovering muscle immediately after training, it might make more sense to consume protein an hour or two before the workout, so that AAs are hitting the bloodstream as the workout is ending.
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