Milk contains two protein fractions. These fractions are whey and casein. Consuming whole milk necessitates the consumption of both fractions because they are present as a mixture along with the other macronutrients. However, it is possible to separate these two fractions and freeze dry them creating isolated supplements of whey and casein. Evidence suggests that whey and casein have different effects on postprandial metabolism in humans, and there have been a number of studies that have investigated the differing physiological effects of whey and casein. Many such studies have focussed on the immediate gastrointestinal effects of whey and casein. In this regard, whey protein may stimulate a slightly different profile of gut hormones during digestion and absorption, and this may explain the differing postprandial effects seen between these two proteins. However, other factors such as physical interactions in the gut, as well as differences in postprandial metabolism may also contribute to differing physiological effects.
In a recent study, researchers compared the effects of isolated whey protein or isolated casein protein on the postprandial triglyceride response to a high fat meal. A third treatment included isolated whey protein, but the protein was supplemented with additional α-lactalbumin, one of the main proteins found in whey that may have particular effects on the immune system. The overweight male subjects incorporated the protein as part of a high fat meal. The results of the study showed that the casein treatment significantly reduced the postprandial triglyceride concentrations following the high fat meal, suggesting that a direct interaction between casein and dietary fat had occurred. This reduction in postprandial triglycerides was equivalent to a 22 % in the area under the curve for postprandial triglycerides over 6 hours. Similar trends were shown to plasma chylomicrons, the main transport lipoproteins of postprandial triglycerides, suggesting the effect was not caused by a reduction in endogenous fat synthesis.
When a mixed meal is consumed containing casein protein, the fat forms globules within the chyme. However, the presence of casein protein in the acid pH of the stomach may cause the dispersal of fat droplets leading to a phase separation between protein and fat. While this does not delay the absorption of the protein, it may significantly delay the absorption of the fat.
The reason for the reduction in postprandial triglycerides is not clear, but the authors of the study did investigate possible causes. One line of reasoning they scientifically tested was the ability of casein to physically interact with dietary fat in the gut. In this regard what the authors found was that in the mixed fat and protein meals, the fat globules were dispersed within the chyme producing fat globules in the range of 10 to 20 μm in size. However when the pH decreased to that in the stomach, the presence of casein, but not whey, caused a phase separation of the fat and protein, such that these globules dispersed. Therefore at the pH of the stomach, the fat globules that were present with casein lost their solubility within the chyme and this caused the presence of two distinct phases, that of fat and that of the protein. While this did not affect the absorption rate of the casein, it did slow the absorption of the fat, and this resulted in a significant reduction in the rate of appearance of postprandial fat in the blood.
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