Milk Protein is significantly better than soy at building muscle mass, according to research from McMaster University in Canada. The study found that drinking two glasses of skim milk after a work-out created twice as much as the same amount of protein from soy drinks. This is an interesting finding , since soy and milk proteins are considered to be complete proteins that are basically equivalent from a nutritional standpoint. The effectiveness of milk in stimulating muscle gain much better than soy. If muscle mass gain is your goal with weightlifting, then drink milk.