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Energy Deficiency, but not Aerobic Exercise, is Associated with Increased Stable Nitrogen Isotope Ratio of Urinary Urea
Author(s) -
Koehler Karsten,
Flenker Ulrich,
Schaenzer Wilhelm,
Huelsemann Frank
Publication year - 2016
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.30.1_supplement.430.1
Subject(s) - lean body mass , chemistry , urea , isotopes of nitrogen , nitrogen balance , medicine , endocrinology , resting energy expenditure , aerobic exercise , zoology , basal metabolic rate , urine , protein turnover , urinary system , isotope ratio mass spectrometry , blood urea nitrogen , nitrogen , creatinine , biochemistry , energy expenditure , body weight , mass spectrometry , chromatography , biology , protein biosynthesis , organic chemistry
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 (δ 15 N) 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. PURPOSE To determine the effects of ED with and without exercise on the stable nitrogen ratio of urinary urea (δ 15 N urea ). METHODS Healthy, 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). δ 15 N urea 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 (δ 15 N diet ), which was computed as previously described [1]. RESULTS Weight 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). δ 15 N urea adjusted for δ 15 N diet (δ 15 N urea ‐δ 15 N diet ) 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 δ 15 N urea ‐δ 15 N diet 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 δ 15 N urea ‐δ 15 N diet.CONCLUSION The increase in δ 15 N urea 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 δ 15 N urea are reflective of increased body protein breakdown during ED. Support or Funding Information The study was supported by a University seed grant.