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Milk contains two types of protein: casein and whey. Whey protein, which contains fractions known as beta-lactoglobulin or lactalbumin, was for years considered a "junk" product of the dairy industry. More recently, however, the excellent nutritional properties of whey have been recognized. These features include ease of digestion, a superior amino-acid balance - including a rich supply of branched-chain aminos - and immune-stimulating factors that survive the digestion process intact. A recent study even found that a specially modified form of whey protein inhibits replication of the AIDS virus.
Casein, on the other hand, has been around for years; it was what people traditionally referred to when they talked about milk protein. Although whey is thought to be biologically superior to casein, you will still see casein in the form of sodium caseinate and similar versions in various protein supplements. Casein isn't likely to fade out lust yet: It's not only a good protein, it's much less expensive than whey.
But if you separate the hype from reality, such as the stratospheric biological values presented in ads for certain commercial whey supplements, you may wonder just how superior whey is to good old casein.
A new study, reported in the American Journal of Clinical Nutrition (63:546, 1996), looked at the digestive effects of whey compared with casein protein. The study noted that in the process of digestion, the uptake of casein and whey proteins appears to be radically different. For one, whey protein is much more soluble than casein.
This is evident during the first stage of protein digestion, which occurs in the stomach. Stimulated to flow by the presence of protein, the stomach's naturally occurring hydrochloric acid begins to degrade casein into polypeptides, or amino-acid chains. This has a clotting effect on casein, slowing its entry into the intestine.
In contrast, whey takes the express route. Because it has a more-soluble structure than casein, whey bypasses initial breakdown in the stomach, arriving intact as a whole protein at the next stop along the protein-digestion route: the small intestine. Subsequently, the breakdown of whey occurs farther down in the small intestine.
The study revealed that casein stimulates secretion of pancreatic digestive factors and slows down digestive transit time through the gut more efficiently than whey does. Thus, the two milk proteins are digested at different rates. This may increase the overall efficiency of milk-protein digestion and partially explain why milk protein has such a high biological value (aside from its having a rich essential amino-acid content).
The researchers in this study also suggested that proteins that are more rapidly absorbed, such as whey, should be consumed with a rapidly absorbed form of carbohydrate (i.e., carbs with a high glycemic-index rating, such as sugars, except fructose). Slower-absorption proteins, such as casein, should be linked with slower-acting carb sources (i.e., low glycemicindex carbs). Combining proteins with carbs in this manner aids protein absorption and fosters an increased positive nitrogen balance.
Casein, on the other hand, has been around for years; it was what people traditionally referred to when they talked about milk protein. Although whey is thought to be biologically superior to casein, you will still see casein in the form of sodium caseinate and similar versions in various protein supplements. Casein isn't likely to fade out lust yet: It's not only a good protein, it's much less expensive than whey.
But if you separate the hype from reality, such as the stratospheric biological values presented in ads for certain commercial whey supplements, you may wonder just how superior whey is to good old casein.
A new study, reported in the American Journal of Clinical Nutrition (63:546, 1996), looked at the digestive effects of whey compared with casein protein. The study noted that in the process of digestion, the uptake of casein and whey proteins appears to be radically different. For one, whey protein is much more soluble than casein.
This is evident during the first stage of protein digestion, which occurs in the stomach. Stimulated to flow by the presence of protein, the stomach's naturally occurring hydrochloric acid begins to degrade casein into polypeptides, or amino-acid chains. This has a clotting effect on casein, slowing its entry into the intestine.
In contrast, whey takes the express route. Because it has a more-soluble structure than casein, whey bypasses initial breakdown in the stomach, arriving intact as a whole protein at the next stop along the protein-digestion route: the small intestine. Subsequently, the breakdown of whey occurs farther down in the small intestine.
The study revealed that casein stimulates secretion of pancreatic digestive factors and slows down digestive transit time through the gut more efficiently than whey does. Thus, the two milk proteins are digested at different rates. This may increase the overall efficiency of milk-protein digestion and partially explain why milk protein has such a high biological value (aside from its having a rich essential amino-acid content).
The researchers in this study also suggested that proteins that are more rapidly absorbed, such as whey, should be consumed with a rapidly absorbed form of carbohydrate (i.e., carbs with a high glycemic-index rating, such as sugars, except fructose). Slower-absorption proteins, such as casein, should be linked with slower-acting carb sources (i.e., low glycemicindex carbs). Combining proteins with carbs in this manner aids protein absorption and fosters an increased positive nitrogen balance.