Whey is one of the two major classes of protein in milk. (The other is casein, the “curds” of “curds and whey.”) Proteins are made of amino acids, and whey contains high levels of the amino acid cysteine. This is the basis for many of its proposed uses. It also contains branched chain amino acids ( BCAAs). However, while there is no question that whey is a highly digestible and rich protein source, there is no meaningful supporting evidence that it provides any specific health benefits.

When milk is converted into cheese, whey is the liquid that is left behind. There is no specific dietary requirement for whey, as the amino acids it contains are present in a wide variety of other foods, as well.