Here’s a quick quiz for you. Milk contains two primary types of protein, whey and casein. Studies show that they’re absorbed at different rates, with whey being absorbed rapidly and casein slowly
releasing amino acids into the blood for more than seven hours. Which would suppress appetite more? A new study focused on just that question.1 Past studies have shown that protein has a greater satiation effect than fat or carbs. On the other hand, the appetite-suppressing effect of protein is blunted in people who habitually eat a high-protein diet—typical of bodybuilders. That’s most noticeable in those who increase their protein intake from a lower level. With a regular higher-protein intake, amino acids are more rapidly cleared and oxidized from the blood, mitigating its effect on appetite.
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That’s also the reason a high-protein diet isn’t likely to lead to increased fat deposition, since all nutrients, including protein, fat and carbs, can be converted into fat. Active people, such as those engaged in regular exercise, burn up excess protein in the liver, thus preventing its conversion into glucose or fat.
One notable difference between whey and casein is that the amino acids in whey are rapidly processed and absorbed into the blood. Casein, on the other hand, curdles in the stomach, leading to a much slower breakdown and release of amino acids. Because a high concentration of amino acids is vital for promoting muscle protein synthesis, some studies suggest that whey’s rapid release of aminos makes it superior to casein for that purpose. On the other hand, casein’s slow release of amino acids makes it more conducive to providing a pronounced anticatabolic effect by maintaining a positive nitrogen balance in the blood over the course of several hours.
The amino acids in the blood dictate the appetite-suppressing properties of protein. Whey is the superior appetite-suppressing protein. In a new study, subjects drank a beverage containing 48 grams of a commercial whey supplement or one containing the same amount of casein and attended an all-you-can-eat buffet 90 minutes later. Those who drank the whey ate 19 percent less food than those who drank the casein.
In the second part of the study researchers wanted to confirm the notion that the quick entry of amino acids made whey more satiating and to figure out if any gut hormones known to affect appetite were affected by whey or casein. That part of the study showed, as expected, that whey increased blood amino acid levels 28 percent more than casein over a three-hour period.
The whey drink increased the levels of several gut hormones that are known to suppress appetite. It increased the blood levels of cholecystokinin (CCK) by 60 percent, glucagonlike peptide-1 by 65 percent and glucose-dependent insulinotropic polypeptide by 36 percent. That last hormone isn’t normally increased by protein alone, but both the whey and casein drinks also contained cream and maltodextrin, a quick-acting carbohydrate.
Thus, whey appears to decrease appetite through its rapid release of amino acids, which in turn promotes the release of several gut hormones known to depress appetite and increase feelings of satiety.
1 Hall, W.L., et al. (2003). Casein and whey exert different effects on plasma amino acid profiles, gastrointestinal hormone secretion and appetite. Brit J Nutrition. 89:239-48.
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