"vitamin A is all important to the human diet because it allows the body to use proteins and minerals, enhances the immune system , fights infection, and ensures proper vision and reproduction. the true form of vitamin A, or retinol, is found only in animal fats and organs such as liver. the body is able to convert beta carotene from plants into vitamin A if bile salts are available, but bile secretion in the body is stimulated by the consumption of fat. even then, the conversion isn't very efficient. butter and full fat dairy foods from pastured cows are rich sources of vitamin A, as is cod liver oil." (taken from, The Makers Diet, by Jordan S. Rubin) in light of this information i now use whole milk for mixing my protein shakes, since they are normally consumed at an acceptable time for fat intake anyway, but the bulk of my milk drinking is still from skim milk because i drink it constantly and don't want that much fat.