Whey protein is a biologically active protein that contains all the amino acids in an optimal balance, including the essential amino acids, with rapid digestibility for faster delivery into the bloodstream. Whey also features the branched-chain amino acids (BCAAs), including a high proportion of leucine. The amino acids in whey benefit body composition by supporting lean body mass and maintenance of muscle tissue.

Combining whey protein with resistance training has been shown to contribute to improvements in lean mass, fat mass, and muscular strength. One meta-analysis found it may benefit younger, healthy individuals more than those with pathological conditions or an older population. There may also be sex differences in the effects, with women experiencing increases in lean mass.

A meta-analysis found that while whey supplementation did not increase muscle mass in an older population, it did increase the concentration of leucine and mixed muscle protein fractional synthesis rate, which may still improve muscle function. Studies have found that whey protein supplementation supports increases in muscle protein synthesis, skeletal muscle mass, muscular strength, and functional capacity in preconditioned older women. The muscle benefits may also result in enhanced performance, with studies finding improvements to aspects of function and performance, including push-ups, grip strength, and aerobic exercise. Additionally, whey protein may support net protein balance after exercise and help with muscle recovery after resistance training.

One reason for the muscle-supporting effects of whey is its high leucine content. Consumption of whey protein significantly increases muscle leucine concentration and activates mechanistic target of rapamycin complex 1 (mTORC1) to stimulate muscle protein synthesis. Although leucine may play an important role in muscle building, one study found that whey protein led to significantly greater increases in strength, as shown through bench press performance compared to a placebo and taking leucine alone.

Another potential action behind the benefits of whey protein to body composition is increasing nesfatin concentrations, which impact glucose homeostasis, can reduce glucose levels, and may act as an anorectic hormone to increase satiety and impact appetite. It may also promote an increase in resting metabolic rate. Additionally, whey protein consumption may increase satiety hormones, including glucagon-like peptide-1 (GLP-1), dipeptidyl peptidase, and cholecystokinin (CCK), as well as imparting positive effects on leptin and ghrelin. A meta-analysis found that consumption of whey protein led to a significant reduction in long-term appetite and prospective food consumption.

Whey protein has also been found to be beneficial for cardiovascular health and metabolic health. Systematic reviews and meta-analyses found that whey supplementation significantly reduced both systolic and diastolic blood pressure, fasting blood sugar levels, HbA1c, insulin, homeostasis assessment-estimated insulin resistance (HOMA-IR), triglycerides, and total cholesterol, in addition to improving body composition and waist circumference. Whey protein supplementation has also been shown to reduce aortic stiffness and aortic hemodynamic load. Some of the cardiovascular benefits may be due to the isoleucine-proline-alanine tripeptide found in whey protein, which has been shown to exhibit angiotensin-converting enzyme (ACE) inhibitory properties and to augment nitric-oxide mediated vasodilation to benefit blood pressure.

Whey is also rich in the amino acid cysteine, which helps increase glutathione levels to support the body’s natural antioxidant defense system. Cysteine is one of the building blocks of glutathione and the rate-limiting amino acid for its synthesis.