and others 2012). On the other hand, rates of MPS are lower
and net negative protein balance ensues during periods of energy
deficit (Pasiakos and others 2010; Areta and others 2014) and with
increasing age (Figure 1D) (Volpi and others 2000; Cuthbertson
and others 2005; Phillips 2009), resulting in muscle mass loss.
However, it has now been established that this loss of muscle mass
during energy deficit and with aging can be attenuated by the
consumption of a higher-protein diet (Houston and others 2008;
Josse and others 2011;Wycherley and others 2012; Geirsdottir and
others 2013).
Protein-containing foods include animal sources such as meat,
fish, dairy products, and eggs and vegetarian sources such as tofu,
legumes, and quinoa. Of note, the proteins in dairy products such
as milk, yogurt, and cheese are of the highest quality and their
consumption following a bout of resistance exercise (Wilkinson
and others 2007), during a period of resistance training (Hartman
and others 2007) and during caloric restriction (Josse and others
2011) result in a greater positive protein balance than that seen
with other protein sources. The 2 proteins in milk are casein
(about 80% of protein content) and whey (about 20% of protein
content). Whey is a “by-product” of cheese production (or can
be specifically isolated through a complicated filtration of milk)
and its protein is one of the most commonly used supplements
by athletes and sports nutrition product consumers to support
muscle hypertrophy and improve body composition. The purpose
of this review is to examine the existing data supporting a role for
protein consumption, in particular whey protein, in the regulation
of muscle mass and body composition across the adult lifespan.