Whey Protein: Why Is It Superior to Other Proteins to
Stimulate MPS?
Whey protein has been shown to stimulate MPS to a greater
extent than casein and soy protein at rest and following exercise
in young and older individuals (Tang and others 2009; Pennings
and others 2011; Burd and others 2012; Yang and others
2012b). The differential MPS response to protein feeding is a
function of the quality of the ingested protein. Proteins differ in
their quality based on their amino acid (AA) content, digestibility,
and bioavailability (FAO/WHO Expert Consultation 1991).
Upon ingestion whey protein is rapidly digested and, since it is
acid-soluble, it exits the stomach rapidly resulting in a pronounced
rise in blood AA, which is thought to be critical to the stimulation
of MPS (Katsanos and others 2006; Dreyer and others 2008;
Tang and others 2009; West and others 2011). In comparison
to other supplemental proteins, whey and soy proteins are considered
to be “fast”-digested proteins, while casein is considered
to be a “slow”-digested protein as it clots due to the acid pH
of the stomach and exits slowly into the small intestine (Boirie
and others 1997; Bos and others 2003). Of importance is that
whey protein is a complete protein, meaning it contains all of the
essential amino acids (EAAs), and has a high proportion of the
branched-chain AA (BCAA) leucine, a key AA in the stimulation
of the MPS (Anthony and others 2001). In fact, while both soy
and casein are also complete proteins, whey protein has greater
EAA and leucine contents than casein, soy, and collagen proteins.
In addition to having a lower leucine content, the bioavailability
of amino acids from soy protein to support MPS is inferior to