Whey is essential in the bodybuilding world today because of its ability to be digested very rapidly. This allows the protein to become available for muscle building very quickly. Most commonly it is used after workouts to help increase levels of amino acids in the blood, which are taken up by the muscles to ultimately increase mass. In addition, during exercise, whey helps open up blood flow by inhibiting an angiotensin-converting enzyme which originally constricts blood vessels; this allows better flow of nutrients to necessary areas to help repair and rebuild muscle tissues.

The use of whey protein as a source of amino acids and its effect on reducing the risks of diseases such as heart disease and cancer is the focus of ongoing research. Whey is an abundant source of branched-chain amino acids (BCAAs) which are used to fuel working muscles and stimulate protein synthesis. In particular, leucine plays a key role in initiating the transcription pathway that fires up protein synthesis. When leucine is ingested in high amounts, such as with whey protein supplementation, there is greater stimulation of protein synthesis, which may speed recovery and adaptation to stress (exercise).

Whey protein contains the amino acid cysteine which can be used to make glutathione. However, this amino acid is not essential for the synthesis of glutathione and some studies have suggested that the amount of cysteine in the diet may have little effect on glutathione synthesis. However, another study suggested that large amounts of whey protein can increase cellular glutathione levels. Glutathione is an antioxidant that defends the body against free radical damage and some toxins, and studies in animals have suggested that milk proteins might reduce the risk of cancer.

Whey protein memiliki asam amino cysteine yang dapat digunakan untuk membuat glutathione. Namun demikian asam amino ini bukan hal penting untuk sintesis dari glutathione dan beberapa penelitian menganjurkan bahwa sejumlah cysteine pada makanan memiliki efek yang kecil dalam sintesis glutathione. Namun demikian penelitian lain menyarankan sejumlah besar dari whey protein dapat meningkatkan level seluler glutathione. Glutathione adalah antioxidant yang mempertahankan tubuh melawan kerusakan sel akibat radikal bebas dan beberapa racun, penelitian pada binatang telah di sarankan protein susu mungkin mengurangi resiko dari penyakit kanker.