FEATURED ARTICLE: Protein and Muscle Resizing
Milk contains two primary proteins: whey and casein. Several studies have examined their uptake. Whey is rapidly absorbed,
reaching peak values of amino acid entry into the blood within 90 minutes, after which amino acid blood levels return to
normal. In contrast, casein curdles in the stomach, leading to a slow release of amino acids that can last up to seven
hours.
The rapid release of amino acids promotes protein synthesis after exercise. It also promotes the breakdown of those amino
acids in the liver, explaining why they disappear after 90 minutes. While rapid amino acid appearance favors increased muscle
protein synthesis following exercise, that's only pad of the muscle-building equation. The other aspect involves an
anti-catabolic effect, also controlled by the presence of amino acids in the blood.
Since muscle protein synthesis occurs for 36 hours following a weight workout, an optimal nutritional environment will
constantly supply amino acids during that time. Casein fills the bill here, since its amino acids are released over seven
hours. As a result, studies show that casein is superior to whey for anti-catabolic effects.
A number of studies both dispute and support those findings. One study found that both proteins produced an equal amount of
protein synthesis after exercise. Another found that providing more frequent whey feedings had an anticatabolic effect
similar to that of casein. Still another found that with a constant infusion of amino acids into the blood, all muscle protein
synthesis ceases after four hours.
Adding to the confusion are two recently published studies that examined the anabolic effects of milk protein metabolism. The
first study looked at precisely how different milk proteins affect the body's anabolic processes. It compared the metabolic
fates of three types of protein formulations: 1) micellar casein (MC); 2) milk-soluble protein isolate (MSPI); and 3) total
milk protein (TMP), which contains both casein and whey. The proteins were labeled with radioactive tracers to map their
metabolism in 23 healthy subjects divided into three groups. Each group got one of the three protein formulations. Prior to
the study, they were all on a standardized diet containing the same amount of protein in each meal for one week.
The fate of the proteins was measured over an eight-hour period. The MSPI, which had the most rapid digestion rate, also
produced the highest rate of amino acid breakdown into urea, the liver's major metabolic protein waste product. The rapid
release of the amino acids from MSPI promoted the liver breakdown of its amino acids, despite the high amino acid content of
MSPI, including leucine, a branched- chain amino acid largely responsible for including muscle protein synthesis. The breakdown
occurred within two hours and produced levels of urea twice as high as occurred with the other two protein forms. After eight
hours MSPI showed an amino acid breakdown 7 percent higher than that of casein.
While the high branched-chain-amino-acid content of MSPI, which is similar to that of whey, should have led to a sustained
muscle protein synthesis effect, the I3CMs in the blood returned to baseline after four hours, in contrast to the sustained
BCM levels produced by the other two protein sources. The rapid breakdown of amino acids from MSPI makes it unlikely to have
anabolic effects in muscle, according to the authors.
The total milk protein produced the highest nitrogen retention of the three, which would point to a greater anabolic effect
in muscle. That was thought to be related to an early metabolic and hormonal effect of the whey protein fraction, followed by
a sustained effect from the casein portion.
Another study came to a different conclusion. It compared hydrolyzed whey isolate-similar to the MSPI used in the previous
study-to casein on strength, body composition and plasma glutamine levels during a supervised 10-week bodybuilding program.
Thirteen noncompetitive male bodybuilders took either whey isolate (Wl) or casein (C) in amounts of 1.5 grams per kilogram of
bodyweight per day throughout the study. Thus, a 200-pound bodybuilder would get 135 grams of protein supplement daily. The
study used a double-blind design, so neither group knew who got the whey or the casein.
While the primary focus of the study compared the effects of the two major milk protein supplements, a secondary focus was on
their effect on plasma glutamine levels. Some studies show that higher-protein diets have an inverse effect on glutamine levels.
Other studies show that weightlifters have lower blood levels of glutamine than other athletes. High-intensity anaerobic training,
such as bodybuilding, is also known to lower plasma glutamine. Since whey is rich in SCAns, which are the precursors of glutamine
synthesis, the authors wanted to see how a whey or casein supplement would affect glutamine in the trained bodybuilders.
After 10 weeks neither group showed any changes in glutamine levels. The whey group, however, had significantly greater lean
mass gain, greater fat loss and greater strength gain. On the other hand, the whey group got an average of 250 more calories
daily than the casein group. That's significant because past studies show a direct relationship between calorie intake and protein
requirements, in that a higher protein intake is supported by the intake of additional calories for purposes of increasing muscle.
Interestingly, the authors suggest that whey's higher content of the amino acid cysteine led to a reduced production of urea.
That's in direct opposition to the findings of the study that showed greater urea production from a whey like protein source
because of its rapid amino acid uptake. The cysteine content of whey isolate is also thought to account for the fat loss. The
casein group experienced no change in fat loss during the study.
Those in the whey group also tended to reduce their intake of food protein in favor of the whey. That could relate to the satiety
effect of concentrated protein, which can have a marked effect on appetite. On the other hand, as mentioned above, the whey group
consumed slightly more calories than the casein group.
So which protein is better for muscle-building purposes, whey or casein? It's probably best to play it safe by getting both forms,
such as with a total-milk-protein supplement. That provides the best of both worlds, with the rapid uptake and increased muscle
protein synthesis provided by whey, along with the sustained, anticatabolic effect produced by casein. That equation equals more
muscle growth.
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