SLOW AND FAST DIETARY PROTEINS

We all know that there are differences in carbohydrates—high glycemic, low glycemic, simple
sugars, starches, etc. And we know that different carbohydrates are absorbed in the gut and appear
in the blood at different rates depending on various factors—for example, simple sugars are
absorbed quickly and more complex ones, depending on how quickly they can be broken down,
are absorbed more slowly. This makes up the basis for the glycemic index of not only foods but also
whole meals since the presence of protein and fat with the carbohydrates usually slows down the
absorption over the whole digestive process. Fast and slow carbohydrates have different metabolic
effects on the hormones and on various metabolic processes.

Now we also have slow (e.g., whey and soy) and fast (e.g., casein) dietary proteins. The speed
of absorption of dietary amino acids by the gut varies according to the type of ingested dietary
protein and the presence of other macronutrients. The speed of absorption can affect postprandial
(after meals) protein synthesis, breakdown, and deposition.44,45

It has been shown that the postprandial amino acid levels differ a lot depending on the mode of
administration of a dietary protein; a single protein meal results in an acute but transient peak of
amino acids whereas the same amount of the same protein given in a continuous manner, which
mimics a slow absorption, induces a smaller but prolonged increase.

Since amino acids are potent modulators of protein synthesis, breakdown, and oxidation,
different patterns of postprandial amino acidemia (the level of amino acids in the blood) might
well result in different postprandial protein kinetics and gain. Therefore, the speed of absorption by
the gut of amino acids derived from dietary proteins will have different effects on whole-body
protein synthesis, breakdown, and oxidation, which in turn control protein deposition.

For example, one study looked at both casein- and whey-protein absorption and the subsequent
metabolic effects.46 In this study two labeled milk proteins, casein and whey protein, of different
physicochemical properties were ingested as one single meal by healthy adults, and postprandial
whole-body leucine kinetics was assessed. Whey protein induced a dramatic but short increase of
plasma amino acids. Casein induced a prolonged plateau of moderate hyperaminoacidemia, probably
because of a slow gastric emptying. Whole-body protein breakdown was inhibited by 34%
after casein ingestion but not after whey-protein ingestion. Postprandial protein synthesis was
stimulated by 68% with the whey-protein meal and to a lesser extent (þ31%) with the casein meal.

Under the conditions of this study, that is a single protein meal with no energy added, two
dietary proteins were shown to have different metabolic fates and uses. After whey-protein
ingestion, the plasma appearance of dietary amino acids is fast, high, and transient. This amino
acid pattern is associated with an increased protein synthesis and oxidation and no change in protein
breakdown. By contrast, the plasma appearance of dietary amino acids after a casein meal is slower,lower, and prolonged with a different whole-body metabolic response: protein synthesis slightly
increases, oxidation is moderately stimulated, but protein breakdown is markedly inhibited.

This study demonstrates that dietary amino acid absorption is faster with whey protein than with
casein. It is very likely that a slower gastric emptying was mostly responsible for the slower
appearance of amino acids into the plasma. Indeed, casein clots in the stomach whereas whey
protein is rapidly emptied from the stomach into the duodenum. The results of the study demonstrate
that amino acids derived from casein are indeed slowly released from the gut and that slow and fast
proteins differently modulate postprandial changes of whole-body protein synthesis, breakdown,
oxidation, and deposition.

After whey-protein ingestion, large amounts of dietary amino acids flood the small body pool in
a short time, resulting in a dramatic increase in amino acid concentrations. This is probably
responsible for the stimulation of protein synthesis. This dramatic stimulation of protein synthesis
and absence of protein breakdown inhibition is quite different from the pattern observed with classic
feeding studies and with the use of only one protein source.

In conclusion, the study demonstrated that the speed of amino acid absorption after protein
ingestion has a major impact on the postprandial metabolic response to a single protein meal. The
slowly absorbed casein promotes postprandial protein deposition by an inhibition of protein
breakdown without excessive increase in amino acid concentration. By contrast, a fast dietary
protein stimulates protein synthesis but also oxidation. This impact of amino acid absorption speed
on protein metabolism is true when proteins are given alone, but as for carbohydrate, this might be
blunted in more complex meals that could affect gastric emptying (lipids) and/or insulin response
(carbohydrate).