Marathon Training Increases Fat Metabolism in Women During Moderate-Intensity Steady State Running

Abstract

Marathon training (MT) may be associated with metabolic and cardiovascular adaptions, including increased fat utilization, improved running economy, and decreased heart rate during submaximal running. Improvements in these measures are important given the high metabolic cost of running a 26.2 mile marathon. PURPOSE: To assess steady state running variables pre- and post-MT. METHODS: Sixty recreational female runners (ages 21.2 ± 1.6) completed a 2 mile time trial (2MI) to establish baseline fitness. A submaximal (SUBMAX) 6 minute treadmill run at 75% of 2 mile velocity was done 1-2 weeks later. An 18 week, progressive MT program was followed by all subjects, followed by a 3 week taper, prior to the completion of a marathon road race. The 2MI and SUBMAX were repeated during the taper period, with the same pace used for SUBMAX in the pre- and post-test. The final 5 minutes of the SUBMAX were averaged for analysis. Oxygen use (O2) and respiratory exchange ratio (RER) were assessed continuously during SUBMAX via metabolic cart. A subset of 15 subjects also wore heart rate (HR) monitors to assess HR during SUBMAX. Paired samples t-tests were used to assess changes in 2MI, O2, RER, and HR from pre- to post-training. Change in O2, HR, RER and 2MI were also calculated as percentages, and correlations between the changes were assessed using Pearson’s r. RESULTS: During SUBMAX, RER decreased from pre- to post-testing (0.90 ± 0.05 vs. 0.86 ± 0.05; p ≤ 0.001), 2MI improved (16.9 ± 1.6 min vs. 15.8 ± 1.6 min; p ≤ 0.001), HR decreased (160.2 ± 7.2 vs. 152.4 ± 8.6; p = 0.001), but O2 did not change (30.3 ± 4.3 vs. 30.1 ± 4.1; p = 0.590). Change in RER was correlated with change in HR (r = .687; p = 0.014), and inversely correlated with change in O2 (r = -0.0259; p = 0.046). CONCLUSIONS: Consistent with previous studies, this study found that endurance training decreases RER during moderate exercise. This change is indicative of increased fat metabolism at the same absolute work rate. In this population, training-related changes can be expected in substrate utilization but not oxygen cost of running, possibly due to the higher oxygen cost of fat metabolism. This suggests that it may be more appropriate to look at energy cost, rather than oxygen cost of running, when assessing running economy.