Abstract
PURPOSE: Stable isotope ratios of nitrogen (δ(15)N) have previously been shown to increase in human hair during periods of catabolism. The goal of this study was to assess changes in δ(15)N in urinary urea (δ(15)N(urea)) and Δ(15)N during a short-term controlled energy deficit. METHODS: We analyzed samples from 6 recreationally active men (25 ± 1 years, BMI: 23.5 ± 0.6 kg/m(2)) who participated in a repeated measures cross-over study involving 4 days of energy deficit (ED, ~ 15 kcal/kg FFM) without and with exercise (ED-EX, ED + EX) and control conditions in energy balance (CON-EX, CON + EX). δ(15)N(urea) was analyzed from urine samples, and Δ(15)N was calculated as δ(15)N(urea)-δ(15)N(diet), with δ(15)N(diet) obtained from diet prescriptions. RESULTS: δ(15)N(urea) was significantly elevated in ED-EX (4.4 ± 0.2‰) when compared to CON-EX (3.7 ± 0.1‰; p = 0.026) and CON + EX (3.34 ± 0.13‰, p = 0.001). As a consequence, Δ(15)N was positive in ED-EX (0.2 ± 0.2‰) and remained negative in ED + EX (- 0.6 ± 0.5‰), CON-EX (- 1.0 ± 0.2) and CON + EX (- 1.1 ± 0.2). Differences in Δ(15)N were significant between ED-EX and CON-EX (p = 0.005) and ED-EX and CON + EX (p = 0.006). CONCLUSION: Our results suggest that δ(15)N(urea) and subsequently Δ(15)N are responsive to a short-term energy deficit, likely due to increased amino acid oxidation to meet energy demands and preferable elimination of (14)N.