Breath carbon stable isotope ratios identify changes in energy balance and substrate utilization in humans

Rapid detection of shifts in substrate utilization and energy balance would provide a compelling biofeedback tool for individuals attempting weight loss. As a proof of concept, we tested whether the natural abundance of exhaled carbon stable isotope ratios (breath delta C-13) reflects shifts between negative and positive energy balance. Volunteers (n = 5) consumed a 40% energy-restricted diet for 6 days followed by 50% excess on day 7. Breath was sampled immediately before and 1 h and 2 h after breakfast, lunch and dinner. Exhaled breath delta C-13 values were measured by cavity ring-down spectroscopy. Using repeated measures analysis of variance (ANOVA) followed by Dunnett's contrasts, pre-breakfast breath values on days 2-6 were compared with day 1, and postprandial day 7 time points were compared with pre-breakfast day 7. Energy restriction diminished pre-breakfast breath delta C-13 by day 3 (P<0.05). On day 7, increased energy intake was first detected immediately before dinner (-23.8 +/- 0.6 vs -21.9 +/- 0.7 parts per thousand, P = 0.002 (means +/- s.d.)), and breath delta C-13 remained elevated at least 2 h post dinner. In conclusion, when shifting between negative and positive energy balance, breath delta C-13 showed anticipated isotopic changes. Although additional research is needed to determine specificity and repeatability, this method may provide a biomarker for marked increases in caloric intake.