Exhaled 13CO2/12CO2 Delta Value as a Metabolic Marker for Metabolism in Performance Training
Authors: Bütz Daniel E , Schoeller Dale A , Breunig Emily , Brassil John , Chu Johnny , Weidmann Damien 
Published: November 2023

Naturally occurring stable carbon isotope ratios (13C/12C) in body proteins, fats and carbohydrates reflect the isotope ratios of the diet, however, the 13C isotopes are not equally distributed in these tissue types. As energy is stored during the conversion of dietary glucose to lipids, enzymatic processes discriminate against 13C, resulting in stored lipids being about 3.5 parts per mil lighter than carbohydrates. Energy metabolism is fueled by a mixture of carbohydrate, protein and lipid as they are converted to CO2 and expired. Changes in the ratio of macronutrient oxidation, either due to exercise or starvation, influence the 13CO2/12CO2 delta value of expired breath (BDV). Previous work has shown that the BDV is correlated to respiratory quotient (RQ) during exercise. Therefore, the BDV may be useful for simple and non‐invasive tracking changes in macronutrient oxidation during exercise. Tracking macronutrient metabolism via the BDV in performance trained athletes during conditioning exercise is a previously unexplored application of this concept. By measuring and understanding shifts in macronutrient metabolism during exercise, athletes and trainers can re‐program nutrition and exercise regimens to optimize energy availability, recovery, injury prevention, and weight management.

We hypothesized that the BDV correlates to workload only during energy balance and can serve as a biomarker for metabolic energy need in performance‐trained athletes. We recruited subjects from the University of Wisconsin football program, and categorized them into 3 groups based on body type (i.e. ectomorphic, mesomorphic and endomorphic body types). Breath samples were taken immediately before, 30 and 60 minutes into, immediately after, and 20 after conditioning training.

Breath samples were analyzed using two breath 13CO2/12CO2 spectrometers, the Picarro G2101i (Picarro, Santa Clara, CA, USA) and the LIR100 (MIRICO LTD, Harwell Oxford, Didicot, UK). The change in the BDV from the pre‐workout baseline was calculated for each timepoint and reported as delta over baseline (DOB). Breath samples were compared against the reported relative perceived exertion (RPE) and total workload measured by the Catapult monitoring system (Catapult Sports, Chicago, IL).

Both the G2101i and the LIR100 breath spectrometers yielded similar results. During one per day conditioning trainings there was a linear increase in the BDV during the workout period that persisted up to 20 minutes after exercise in each of the body type groups, indicating an increased reliance on glucose metabolism for energy relative to baseline. The increase in the BDV was correlated to the intensity of the workout, and the total work load of the individual players. During the second practice on a two a day schedule, the BDV decreased linearly during the workout period, particularly in the endomorphic and mesomorphic body type groups. According to our hypothesis, a decrease in the BDV is likely due to an increased reliance on lipid oxidation to perform work when muscle glycogen is depleted due to negative energy balance.

These data suggest that athletes and trainers can use the BDV during performance workout trainings to understand and optimize the metabolic fuel sources being used to perform work, and may provide immediate feedback on rest, recovery programs and nutritional strategies during intense training.

Support or Funding Information

Research supported in part by NIGMS SBIR Grant 1R44GM110844‐ 01, and the Wisconsin Alumni Research Foundation. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the view of the funding agencies. Data presented here were part of a non‐provisional patent application P140010US01 entitled “Methods of Determining Energy Balance Using Breath Carbon Isotope Ratios” filed in January, 2014. Daniel E. Bütz has an ownership interest in Isomark, LLC, which has licensed the technology discussed in this publication. Damien Weidmann is also Chief Scientific Officer of MIRICO Ltd.