Field-deployable measurements of free-living individuals to determine energy balance: fuel substrate usage through δ ¹³ C in breath CO ₂ and diet through hair δ ¹³ C and δ ¹⁵ N values

Carbon isotopes of breath CO ₂ vary depending on diet and fuel substrate used. This study examined if exercise-induced δ ¹³ C-CO ₂ changes in substrate utilization were distinguishable from baseline δ ¹³ C-CO ₂ 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 δ ¹³ C-CO ₂ values. Diet was assessed by food logs and δ ¹³ C, δ ¹⁵ N of hair samples. Post-exercise δ ¹³ C-CO ₂ values increased by 0.54 ± 1.09‰ (1 sd, n = 9), implying enhanced carbohydrate burning, while early morning δ ¹³ C-CO ₂ values were lower than daily averages (p = 0.0043), indicating lipid burning during overnight fasting. Although diurnal δ ¹³ C-CO ₂ variation (1.90 ± 0.77‰) and participant baseline range (3.06‰) exceeded exercise-induced variation, temporal patterns distinguished exercise from dietary isotope effects. Hair δ ¹³ C and δ ¹⁵ N 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.