Whey protein is one of the main components of milk, the other being casein. Whey protein has become popular as a sports supplement in its powdered form because it has a number of interesting properties. In particular, whey protein is rapidly absorbed in comparison to casein, and for this reason many sports nutritionist advocate whey protein as a source of nitrogen following a workout. This is based on the premise that post-workout protein is able to stimulate protein synthesis through nitrogen delivery to the circulation, and thus may increase skeletal muscle formation. However, the rapid absorption of whey is dependent on the absence of other proteins from the stomach and small intestine. Concomitant consumption of whey with other foods can slow this absorption and negate whey proteins primary advantage over other proteins like casein and egg protein. Considering that whole protein can remain in the stomach for some hours, consuming whey protein even a long time after consumption of meat may inhibit its absorption rate considerably.
During research into whey protein, the authors are careful to compare digestion rates in subjects with empty stomachs so as not to affect the results. Assuming that such results transfer into the real world is a mistake many advocates of whey protein make. Of course often such advocates are also in the business of selling whey protein, and for this reason their judgement is either biased deliberately, or more usually through ignorance. Studies comparing casein and whey protein do show faster rates of absorption for whey protein. Casein is absorbed more slowly and results in lower peak plasma nitrogen levels when compared to whey protein. However, the slow passage of casein through the stomach ensures that its delivery to the plasma is more prolonged. When whey protein nitrogen delivery to the circulation has already diminished, casein continues to provide a source of nitrogen. Of course this is a moot point if any residual food remains in the stomach, in which case whey protein likely behaves in a similar way to casein.