Substrate Utilization During Walking Trials Among Normal Weight Through Severely Obese Adults

Abstract

PURPOSE: To examine the relationship between body mass index (BMI) and substrate utilization during walking trials at absolute and relative workloads. METHODS: Adult (42 ± 7 yr) males (n = 17) and females (n = 42) with a of BMI 32.9 ± 8.3 kg/m2 (range 19.4 - 48.5 kg/m2) completed assessments. Regular PA level was determined with the International Physical Activity Questionnaire. Indirect calorimetry measured resting oxygen consumption (VO2) to determine each participant's 1 metabolic equivalent (MET) value. Indirect calorimetry was also used to measure VO2 and carbon dioxide production (VCO2) during 3 walking trials of at least 5 min durations. Substrate utilization was quantified as the respiratory exchange ratio (RER), defined as VCO2/VO2. Walking speeds of trials included a standard speed of 2.5 mph, and the speeds at which an individual achieved moderate physical activity (MPA, defined as 3 METS) and vigorous physical activity (VPA, defined as 6 METS). Multiple variable linear regression was used to examine the relationship between BMI and RER for each walking trial. RESULTS: Controlling for age and gender, BMI was positively associated with RER at an absolute workload (2.5 mph), (r2 = 0.23; p = 0.003); thus, indicating a lower utilization of fat for energy with higher BMI. However, BMI did not predict RER during relative workloads equivalent to MPA (p = 0.15) and VPA (p = 0.07). Subjects' self-reported regular PA level was not significantly related to RER in any of the trials. CONCLUSIONS: These findings suggest that while fat oxidation is lower in subjects with a higher BMI at an absolute workload, when walking at equivalent relative workloads, BMI does not account for variation in substrate utilization. Further research needs to confirm these finds and elucidate how other variables may affect oxidation of fuel sources.