Field-deployable measurements of free-living individuals to determine energy balance: fuel substrate usage through δ13C in breath CO2 and diet through hair δ13C and δ15N values

Carbon isotopes of breath CO2 vary depending on diet and fuel substrate used. This study examined if exercise-induced δ13C-CO2 changes in substrate utilization were distinguishable from baseline δ13C-CO2 variations in a population with uncontrolled diet, and compared hair isotope values and food logs to develop an isotope model of diet. Study participants included nine women with diverse Body Mass Index (BMI), age, ancestry, exercise history, and diet. Breath samples were collected prior to and up to 12 h after a 5- or 10 K walk/run. Indirect calorimetry was measured with a smartphone-enabled mobile colorimetric device, and a field-deployable isotope analyzer measured breath δ13C-CO2 values. Diet was assessed by food logs and δ13C, δ15N of hair samples. Post-exercise δ13C-CO2 values increased by 0.54 ± 1.09‰ (1 sd, n = 9), implying enhanced carbohydrate burning, while early morning δ13C-CO2 values were lower than daily averages (p = 0.0043), indicating lipid burning during overnight fasting. Although diurnal δ13C-CO2 variation (1.90 ± 0.77‰) and participant baseline range (3.06‰) exceeded exercise-induced variation, temporal patterns distinguished exercise from dietary isotope effects. Hair δ13C and δ15N values were consistent with a new dietary isotope model. Notwithstanding the small number of participants, this study introduces a novel combination of techniques to directly monitor energy balance in free-living individuals.