For years, milk protein has consistently been rated the number one source of protein for anyone engaged in exercise or bodybuilding. The essential amino acid of content of milk protein is what's responsible for its high biological value. It is a rich source of branched-chain amino acids, often called "muscle amino acids" since they are metabolized in muscle, while other amino acids are metabolized in the liver. Of the two primary milk proteins, whey and casein, the whey portion (comprising 20% of native milk protein) is slightly richer in essential amino acids compared to the other protein in milk, casein (80% of milk protein). The two proteins also differ in their digestibility, with whey being absorbed rapidly, reaching a peak blood level of amino acids within an hour of ingestion. Casein provides a more sustained effect, since it tends to curdle in the stomach due to the presence of certain protein fractions. As such, the amino acids in casein are more gradually released, often for as long as seven hours. In terms of muscle protein synthesis, which is the underlying cornerstone of muscle hypertrophy or growth, whey tends to produce faster results because of its more rapid release of essential amino acids, which drive the muscle protein synthesis process. One amino acid in particular, leucine, appears to be the primary promoter of muscle protein synthesis through its stimulation effects on a protein called mTOR-1. When this protein is turned on by whey, a cascade ensues that results in upgraded muscle protein synthesis.
Besides its effects on promoting muscle growth through increased muscle protein synthesis, milk protein also provides a number of valuable health benefits. This is due to the presence in milk protein of various smaller proteins called peptides. Research shows that these peptides may be active in the body, producing such beneficial effects as lowering elevated blood pressure, providing an immune boost, lowering inflammation, decreasing appetite.Milk protein also is great for maintaining lean mass under dieting conditions.
But according to a new opinion paper, milk is not for every body, contrary to what those ubiquitous ads from the American Dairy Association have long touted. The authors of the paper assert that people who are obese have increased amounts of branched-chain amino acids, as well as glutamine in their blood. The increased presence of these amino acids promotes insulin release, exacerbating the already elevated insulin levels that are common with excess body fat. This tends to perpetuate obesity, since insulin promotes lipogenesis, or the synthesis of body fat, especially in the presence of excess calories or carbohydrates. In addition, the choronic elevation of insulin induced by the excess BCAA in the blood of the obese can cause destruction of the pancreatic cells that produce insulin, thus making a bad situation even worse. As evidence, the authors cite the common observation that people who've undergone a gastric bypass procedure (used to treat morbid obesity) show a significant drop in their blood levels of BCAA, along with weight-loss and improved insulin sensitivity. On the other hand, since this procedure results in a drastic drop in food intake, along with a concomitant decline in total caloric intake, you would expect to see a boost in insulin sensitivity and a loss of fat. The drop in BCAA may just be a bystander effect that is not crucial.
The authors also note that in studies that have compared milk protein to meat, the milk protein has produced a greater release of insulin. Again, this effect is more related to the faster release of amino acids from milk protein, especially whey. In addition, the greater fat content in meat delays the digestion of its protein content, which results in a slower release of amino acids, and consequent blunted release of amino acids in comparison to milk protein.
The authors of the paper also implicate leucine as a cause of obesity. They base this on studies in which rats on high fat diets were supplemented with leucine. They note that mTOR-1 is a promoter of fat accretion. This effect, however, is seen more frequently in animal studies, and is never seen when the studies involve human subjects. In addition, studies with humans show an opposite effect of leucine: a tendency to promote fat oxidation. Despite this, the authors suggest that over-stimulation of mTOR is a major cause of type-2 diabetes. Again, there is little or no human proof of this assertion. If it were true, those who chronically boost mTOR levels through consuming a high protein diet should all have type-2 diabetes, and obviously this isn't the case.
Other studies show that in those who exercise, BCAA actually promotes fat oxidation. In active people, BCAA promotes the synthesis of glycerol, a component of fat. This glycerol is stored in intramuscular fat as triglycerides. This intramuscular fat is a primary source of energy during exercise, and does not cause metabolic problems in those who are fit and active. In those who are sedentary, however, the intramuscular fat is not tapped into as much, and an increase is linked to muscular insulin resistance.
The authors of the anti-milk paper also cite an in vitro or isolated cell study, in which exposing prostate cells to milk protein increased the risk of prostate cancer by 32%. However,. this level of cellular exposure to milk protein never happens in the human body, a fact not mentioned by the authors. There is no evidence that consuming milk protein will cause cancer. They then note that restricting BCAA in fruit flies extends their lifespan. They suggest that it isn't calorie restriction, but instead BCAA restriction that is responsible for the extended lifespans seen in caloric-restricted animals. This is so nonsensical that it's laughable. In fact, animal studies have shown that BCAA may be involved in life extension. There is, however, some truth to the fact that a lifelong promotion of mTOR stimulation may shorten lifespan for reasons too complex to discuss here.
Melnik, B, et al.Metabolic effects of milk protein intake strongly depend on pre-existing metabolic exercise status. Nut Metabol 2013;10:60.
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