Changes in Urinary Stable Nitrogen Isotope Ratios During Controlled Short-Term Energy Deficit: a Proof-of-principle Analysis.

Purpose Stable isotope ratios of nitrogen (delta N-15) have previously been shown to increase in human hair during periods of catabolism. The goal of this study was to assess changes in delta N-15 in urinary urea (delta N-15(urea)) and Delta N-15 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, similar to 15 kcal/kg FFM) without and with exercise (ED-EX, ED + EX) and control conditions in energy balance (CON-EX, CON + EX). delta N-15(urea) was analyzed from urine samples, and Delta N-15 was calculated as delta N-15(urea)-delta N-15(diet), with delta N-15(diet) obtained from diet prescriptions. Results delta N-15(urea) was significantly elevated in ED-EX (4.4 +/- 0.2 parts per thousand) when compared to CON-EX (3.7 +/- 0.1 parts per thousand; p = 0.026) and CON + EX (3.34 +/- 0.13 parts per thousand, p = 0.001). As a consequence, Delta N-15 was positive in ED-EX (0.2 +/- 0.2 parts per thousand) and remained negative in ED + EX (- 0.6 +/- 0.5 parts per thousand), CON-EX (- 1.0 +/- 0.2) and CON + EX (- 1.1 +/- 0.2). Differences in Delta N-15 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 delta N-15(urea) and subsequently Delta N-15 are responsive to a short-term energy deficit, likely due to increased amino acid oxidation to meet energy demands and preferable elimination of N-14.