Whey protein has been proposed to be an ideal source of protein during weight loss due to its nutrient partitioning effects that would enhance fat loss and preserve lean tissue. A distinguishing feature of whey is its complex protein source with a high prevalence of essential amino acids, needed for stimulating skeletal muscle protein synthesis. Whey also contains a higher proportion of branched chain amino acids (BCAA), especially Leucine, which triggers protein synthesis and helps build lean body mass. In addition to promoting muscle anabolism and strength, there is evidence that whey protein may also promote body weight and fat loss. In a preliminary study, overweight men and women assigned to a whey protein group lost significantly more weight and body fat than the carbohydrate supplement group with no other changes in habitual diet and exercise habits. Greater weight and fat loss could be attributed to an increased thermogenesis because higher quality animal proteins induce greater 24 hr thermic effects compared to soy and isocaloric amounts of carbohydrate. Since whey is nutrient dense but relatively low in energy (~4 kcal/g), supplementation is an efficient method to promote skeletal muscle anabolism while promoting catabolism in fat cells and therefore is hypothesized to improve both muscle growth and fat loss. Researchers confirmed these effects in a recent 12 week trial that involved overweight subjects consuming a specialized whey fraction containing 10 g of protein 20 min before breakfast and another 10 g 20 min before dinner. About 25% of the amino acids in the whey fraction were BCAA half of which was leucine. A control group consumed a beverage matched for energy. At the end of the 12 week weight loss trial, fat loss was nearly two-fold greater in the whey group (2.81 kg vs 1.62 kg) and lean body mass loss was two-fold reduced in the whey group (-0.75 vs -1.55 kg). Thus daily intake of a small of whey (10 g) before breakfast and dinner was able to partition nutrients and result in greater fat loss and preservation of lean body mass during weight loss.

Frestedt JL, Zenk JL, Kuskowski MA, Ward LS, Bastian ED. A whey-protein supplement increases fat loss and spares lean muscle in obese subjects: a randomized human clinical study. Nutr Metab (Lond). 2008 Mar 27;5:8.