Benefits
Muscle growth and repair with resistance training
A meta-analysis of 49 randomized trials (1863 people) found protein supplements during at least 6 weeks of resistance training added about 0.3 kg of fat-free mass and 2.5 kg to one-rep max strength. Benefits were smaller with age and stopped rising above about 1.6 g/kg/day of total protein. These trials tested protein supplements in general, not whey concentrate alone.
Strength gains with resistance training
A meta-analysis of 22 randomized trials (680 younger and older adults) found protein supplements during more than 6 weeks of resistance training increased leg press strength by about 13.5 kg and fat-free mass by about 0.7 kg versus placebo. Endurance was not studied, and the trials used various protein supplements rather than whey concentrate alone.
Fast rise in muscle protein synthesis after exercise
In young men, whey (as hydrolysate) raised blood leucine more and lifted muscle protein synthesis about 122 percent more than casein and 31 percent more than soy after leg exercise, with each drink matched for 10 g of essential amino acids. A small short-term study (6 men per group) of a related whey form.
Short-term fullness and GLP-1 response to a whey breakfast
In a small study of 25 healthy young adults, a breakfast with 10 percent of energy from whey protein reduced hunger more than casein or soy. At 25 percent protein, appetite ratings did not differ, though whey raised GLP-1 and insulin the most. In neither case did people eat less at lunch.
Lactose and fat content compared with whey isolate
Whey concentrate keeps more lactose and fat than whey isolate, which is filtered further. People who are sensitive to lactose may tolerate isolate or a lactase enzyme better; anyone with a milk allergy should avoid all whey.
Mechanism of action
Muscle Protein Synthesis (MPS)
Whey supplies all the essential amino acids and is rich in leucine, which activates the mTOR pathway that starts muscle protein synthesis. Because whey is digested quickly, blood amino acid and leucine levels rise fast after a dose, which in short-term studies lifts muscle protein synthesis more than slowly digested casein.
Satiety signals
In a breakfast study, whey raised blood levels of several amino acids and of GLP-1 and insulin more than casein or soy. GLP-1 is a gut hormone that signals fullness. In that study the hormone rise did not translate into eating less at the next meal.
Clinical trials
Systematic review, meta-analysis and meta-regression of randomized controlled trials lasting at least 6 weeks that combined resistance training with dietary protein supplements. (Morton et al. 2018, Br J Sports Med)
1863 healthy adults across 49 trials.
Protein supplements increased one-rep max strength by 2.49 kg, fat-free mass by 0.30 kg and muscle fiber size versus control. The effect on fat-free mass shrank with age and was larger in people already resistance trained, and there was no further gain above about 1.62 g/kg/day of total protein. Protein supplements in general, not whey concentrate alone. The senior author has received support from the US National Dairy Council.
Systematic review and random-effects meta-analysis of randomized controlled trials of protein supplements during more than 6 weeks of resistance training. (Cermak et al. 2012, Am J Clin Nutr)
680 younger and older adults across 22 trials.
Protein supplements increased fat-free mass by 0.69 kg (95% CI 0.47 to 0.91) and leg press one-rep max by 13.5 kg (95% CI 6.4 to 20.7) versus placebo, in both younger and older people. Protein supplements in general, not whey concentrate alone.
Acute study of three groups given a drink with 10 g of essential amino acids as whey hydrolysate, micellar casein or soy protein isolate, at rest and after one-leg resistance exercise. (Tang et al. 2009, J Appl Physiol)
18 healthy young men (6 per group).
Whey raised blood leucine and essential amino acids more than casein or soy. Muscle protein synthesis after whey was about 93 percent higher than casein at rest and about 122 percent higher than casein and 31 percent higher than soy after exercise. Whey hydrolysate rather than concentrate, small groups, and a few hours of measurement.
Randomized single-blind study of custard breakfasts with casein, soy or whey at a normal (10 percent of energy) or high (25 percent) protein level, measuring appetite, hormones and lunch intake. (Veldhorst et al. 2009, Physiol Behav)
25 healthy young adults (average age 22).
At 10 percent protein, whey reduced hunger more than casein or soy. At 25 percent, appetite ratings did not differ. Whey gave the largest rises in GLP-1 and insulin. Energy intake at lunch did not differ between breakfasts, so lower hunger did not lead to eating less in this study.