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

Stable isotope ratios of nitrogen (δ¹⁵N) have previously been shown to increase in human hair during periods of catabolism. The goal of this study was to assess changes in δ¹⁵N in urinary urea (δ¹⁵N_urea) and Δ¹⁵N during a short-term controlled energy deficit.

We analyzed samples from 6 recreationally active men (25 ± 1 years, BMI: 23.5 ± 0.6 kg/m²) 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). δ¹⁵N_urea was analyzed from urine samples, and Δ¹⁵N was calculated as δ¹⁵N_urea–δ¹⁵N_diet, with δ¹⁵N_diet obtained from diet prescriptions.

δ¹⁵N_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, Δ¹⁵N 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 Δ¹⁵N were significant between ED-EX and CON-EX (p = 0.005) and ED-EX and CON + EX (p = 0.006).

Our results suggest that δ¹⁵N_urea and subsequently Δ¹⁵N are responsive to a short-term energy deficit, likely due to increased amino acid oxidation to meet energy demands and preferable elimination of ¹⁴N.