Expired breath δ(13)CO2 measured in real time serves as a useful biomarker of altered macronutrient metabolism in response to changes in energy balance. Altered breath δ(13)CO2 is believed to be a result of changes in macronutrient oxidation and the kinetic isotope effect where enzymatic processes discriminate against metabolites naturally enriched with (13)C. Use of breath δ(13)CO2 as a rapid biofeedback of energy balance status will enhance an individual's ability to modify behavior during weight loss efforts. Herein we describe a novel approach for immediate biofeedback for energy deficit using a moderate exercise challenge. Our new mid-infrared isotope ratio-meter for δ(13)CO2 is a step toward miniaturization of a personal device for instant biofeedback for people attempting to lose weight.