Design: We used a single-blind, randomized, parallel-group prospective
trial. During a 4-wk period, we provided hypoenergetic (w40%
reduction compared with requirements) diets providing 33 6 1 kcal/
kg LBM to young men who were randomly assigned (n = 20/group)
to consume either a lower-protein (1.2 g $ kg21 $ d21) control diet
(CON) or a higher-protein (2.4 g $ kg21 $ d21) diet (PRO). All
subjects performed resistance exercise training combined with highintensity
interval training for 6 d/wk. A 4-compartment model assessment
of body composition was made pre- and postintervention.
Results: As a result of the intervention, LBM increased (P , 0.05) in
the PRO group (1.2 6 1.0 kg) and to a greater extent (P , 0.05)
compared with the CON group (0.1 6 1.0 kg). The PRO group had
a greater loss of fat mass than did the CON group (PRO: 24.8 6 1.6 kg;
CON: 23.5 6 1.4kg; P , 0.05). All measures of exercise performance
improved similarly in the PRO and CON groups as a result
of the intervention with no effect of protein supplementation. Changes
in serum cortisol during the intervention were associated with changes
in body fat (r = 0.39, P = 0.01) and LBM (r = 20.34, P = 0.03).
Conclusions: Our results showed that, during a marked energy deficit,
consumption of a diet containing 2.4 g protein $ kg21 $ d21 was
more effective than consumption of a diet containing 1.2 g protein $
kg21 $ d21 in promoting increases in LBM and losses of fat mass
when combined with a high volume of resistance and anaerobic
exercise. Changes in serum cortisol were associated with changes
in body fat and LBM, but did not explain much variance in either
measure. This trial was registered at clinicaltrials.gov as
NCT01776359. Am J Clin Nutr 2016;103:738–46.