Australian researchers addressed this question in a recent study that compared the use of whey isolate to casein protein by bodybuilders. Two groups supplemented their diets with 1.5 grams per kilogram body weight per day of either a whey or casein supplement while performing their normal resistance training for 10 weeks. That equals about 120 grams of protein for a person weighing 175 pounds.
As expected, the whey group had a greater increase in lean body mass (11 lbs.) compared to the casein group (1.8 lbs.). In addition, the whey group lost more fat (-3.3 lbs.) compared to the casein group (-0.4 lbs.). The greater increases in lean body mass translated into greater strength gains over the 10 weeks in the whey group.
Many studies have showed the benefits of casein for building muscle, but at a slightly slower rate. Casein has the added advantage of being slower digesting than whey. When combined with whey, casein can add a timed-release effect to your protein supplement.