I tend to mix my protien powder with lactose free skim milk and water here is the reason why it was from a supplement companies website
Why mixing whey protein with milk does not provide optimal results
At first, it may seem strange to learn that whey protein should not be mixed in milk or milk products like yogurt and ice cream. First of all, most whey protein supplements taste better in milk than in water; and secondly, whey protein is obtained by collecting the clear fluid (called whey) produced during the processing of cheese.
Let’s consider some of the following facts:
The enzymes necessary to break down and digest milk are renin and lactase. They are all but gone by the age of three in most humans.
There is an element in all milk known as casein and there is three hundred times more casein in cow’s milk than in human’s milk. Casein coagulates in the stomach and forms large, tough, dense, difficult-to-digest curds that are adapted to the four-stomach digestive apparatus of a cow. Once inside the human system, this thick mass of goo puts a tremendous burden on the body to somehow get rid of it.
Unfortunately some of this gooey substance hardens and adheres to the lining of the intestines and prevents the absorption of nutrients into the body.
The most serious difficulty with milk consumption is the formation of mucus in the system. This mucus coats the mucous membranes and also seriously affects absorption.
To fully understand why whey protein supplements should not be mixed with milk, this knowledge should be combined with a review of the technology that goes into high quality whey protein supplements. Whey protein isolate, a highly separated fraction found only in top quality products, is very expensive and has an extremely high bioavailability (meaning it is very easily utilized by the body) because its particle sizes are so small. Whey protein concentrate has also gone through a huge amount of separation from the original starting material and the major waste product in this procedure is casein. This casein is then sold to other companies which produce low grade products. If you were to check out the ingredients on your favourite pudding desert, for example, you will likely see sodium caseinate because it is a cheap product that provides substance.
The idea behind a high quality whey protein is particle size. Digestion is most effective when particles are small enough to be taken up by the cells of the intestine. If food is not broken down into small enough pieces the body will simply NOT be able to use it. All research on the absorption of food reaches the same conclusion: the bigger the molecule, the more difficulty it has penetrating the mucus barrier lining the intestines.
So, why not mix milk and whey protein? Milk contains a lot of casein which adheres to the intestinal wall and blocks the absorption of the smaller whey protein molecules. Also, it generates a barrier of mucus internally which not only drives down the absorption of protein, but also the absorption of other vital nutrients.