Changes in urinary stable nitrogen isotope ratios during controlled short-term energy deficit: a proof-of-principle analysis.

PURPOSE

Stable isotope ratios of nitrogen (δN) have previously been shown to increase in human hair during periods of catabolism. The goal of this study was to assess changes in δN in urinary urea (δN) and ΔN 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) 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). δN was analyzed from urine samples, and ΔN was calculated as δN-δN, with δN obtained from diet prescriptions.

RESULTS

δN 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, ΔN 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 ΔN 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 δN and subsequently ΔN are responsive to a short-term energy deficit, likely due to increased amino acid oxidation to meet energy demands and preferable elimination of N.