Energy deficiency (ED) is typically associated with the loss of lean mass, which occurs secondary to increased body protein breakdown. Exercise, which stimulates both protein synthesis and breakdown, has the capacity to attenuate the loss of lean mass during ED, but the contribution of protein sources, i.e. body protein vs. dietary protein, to protein turnover during ED and/or exercise remains only poorly understood. Analysis of the stable nitrogen isotope ratio (δ15N) of urea, the primary end‐product of protein metabolism, could provide useful information about the source of protein breakdown and, subsequently, for dietary protein requirements to preserve lean mass during ED.PURPOSETo determine the effects of ED with and without exercise on the stable nitrogen ratio of urinary urea (δ15Nurea).METHODSHealthy, trained, young men (n=6, 25.2 ± 2.4 y, 79.7 ± 7.7 kg; 9.6 ± 1.5% body fat) were included in this 4‐way crossover study. Participants underwent two 4‐day ED conditions during which energy availability (EA) was reduced to 15 kcal/kg fat free mass (FFM) per day, once while conducting exercise to expend 15 kcal/kg FFM (ED+EX) and once while not exercising (ED‐EX). Participants also underwent two 4‐day control conditions in energy balance (EA of 40 kcal/kg FFM), once with exercise (C+EX) and once without exercise (C‐EX). δ15Nurea was assessed from urine samples using elemental analysis‐isotope ratio mass spectrometry (EA‐IRMS, Eurovektor EA 3000) and was adjusted for dietary nitrogen isotope ratio (δ15Ndiet), which was computed as previously described [1].RESULTSWeight loss was similar in both ED conditions (ED‐EX: −2.4±0.7 kg, p<0.001; ED+EX: −1.8±1.0 kg, p=0.004). δ15Nurea adjusted for δ15Ndiet (δ15Nurea ‐δ15Ndiet) was significantly elevated in ED‐EX when compared with C‐EX (0.3±0.4‰ vs. −1.1±0.4‰, p=0.01) and in ED+EX when compared with C+EX (−0.6±1.0‰ vs. −1.1±0.4‰, p=0.01); whereas, there was no significant difference in δ15Nurea ‐δ15Ndiet between ED‐EX and ED+EX and between C‐EX and C+EX. Multiple linear regression identified EA (p=0.015), dietary protein intake (p=0.001), and the interaction between EA and protein intake (p=0.03) as significant predictors of δ15Nurea ‐δ15Ndiet.CONCLUSIONThe increase in δ15Nurea during both ED conditions confirms that endogenous protein breakdown is increased in the energy‐deficient state. However, our results do not indicate that exercise per se is associated with changes in endogenous protein breakdown. Future studies with controlled dietary protein intake are needed to verify that changes in δ15Nurea are reflective of increased body protein breakdown during ED.Support or Funding InformationThe study was supported by a University seed grant.