Urinary Stable Nitrogen Isotopes Ratios Are Responsive to Controlled Short-Term Calorie Restriction

Abstract Purpose Increased nitrogen losses observed during periods of energy deficiency are attributed to increased breakdown of endogenous proteins. Stable nitrogen isotope ratios (δ15N) measured in human hair have previously been shown to reflect prolonged periods of increased protein breakdown. The goal of this study was to determine whether stable nitrogen isotopes measured from urinary urea (δ15Nurea) represent a potential biomarker of more immediate changes in endogenous protein utilization. Methods We analyzed samples from recreationally active males (n = 6, 25.2 ± 1.0 years, BMI: 23.5 ± 0.6 kg/m2) who took part in a repeated measures cross-over study during which they underwent 4 days of calorie restriction (~ 15 kcal/kg FFM) without and with exercise (CR-EX, CR + EX) and two control conditions in energy balance (CON-EX, CON + EX). δ15Nurea was analyzed from urine samples. In addition, the trophic shift (Δ15N) was calculated as δ15Nurea - δ15Ndiet, with δ15Ndiet obtained from food logs. Results δ15Nurea was significantly elevated in CR-EX (4.35 ± 0.16‰) when compared to CR + EX (3.84 ± 0.14‰; p = 0.050, d = 1.56), CON-EX (3.65 ± 0.09‰; p = 0.026, d = 1.63) and CON + EX (3.34 ± 0.13‰, p = 0.001, d = 3.52). δ15Nurea was also significantly higher in CR + EX when compared to CON + EX (p = 0.050, d = 1.5). Δ15N was positive in CR-EX (0.24 ± 0.15‰) and negative in all other conditions (CR + EX: -0.55 ± 0.46‰; CON-EX: -0.96 ± 0.19; CON + EX: -1.1 ± 0.15). CR-EX differed significantly from CON-EX (p = 0.005, d = 2.55) and CON + EX (p = 0.006, d = 2.3). Conclusion Results from this analysis suggest that δ15Nurea as well as Δ15N can serve as alternative tools to monitor changes in endogenous protein turnover during phases of calorie restriction.