Changes In Running Economy And Substrate Utilization During Acute Low Energy Availability

A compensatory downregulation of metabolic processes in athletes has been observed during periods of low energy availability (LEA) to conserve energy expenditure. However, it is unclear how LEA affects submaximal running economy and substrate utilization. PURPOSE: This study investigated running economy and substrate utilization in response to 5 days of LEA in female runners. METHODS: Nine recreational runners (age, 26 ± 4 yr; body mass, 66.5 ± 8.6 kg; fat-free mass (FFM), 47.8 ± 4.6 kg; VO2max, 44.9 ± 7.5 ml/kg/min; mean ± SD) underwent 5 days of a controlled LEA condition (15 kcal/kgFFM/day) consisting of dietary restriction and treadmill running. Running economy was assessed at three submaximal stages of 4 min each on Days 1 and 5. Participants chose a starting speed of 5.0 or 5.5 mph based on their current aerobic fitness, followed by a 0.5 mph increase in speed during the next two stages. Oxygen uptake (VO2) and carbon dioxide production (VCO2) were measured using indirect calorimetry (TrueOne 2400, Parvo Medics, Murray, UT). Running economy was assessed by relative VO2 (ml/kg/min) and substrate utilization was determined using the respiratory exchange ratio (RER). Paired t-tests were used to compare relative VO2 and RER at each submaximal stage on Day 5 vs Day 1. RESULTS: Five days of LEA significantly reduced RER at all three stages coinciding with decreased carbohydrate (CHO) and increased fat oxidation rates (Table 1). Fat oxidation increased by 40%, 73%, and 110% during stages 1, 2, and 3, respectively (Day 1 vs. 5). Despite the shift in substrate utilization to a less efficient fuel source, no changes in relative VO2 were detected at any stage. CONCLUSION: Substrate utilization during submaximal running adapted in favor of fat oxidation after 5 days of LEA. However, no changes in VO2 were observed, suggesting moderate intensity running performance may not be impaired during acute periods of LEA. - Stage Variable Day 1 Day 5 P 63 ± 8% VO2max RER 0.87 ± 0.04 0.81 ± 0.02 0.001 CHO (g/min) 1.20 ± 0.35 0.72 ± 0.18 0.003 Fat (g/min) 0.43 ± 0.17 0.60 ± 0.12 0.003 VO2 (ml/kg/min) 27.9 ± 2.6 27.4 ± 2.07 0.406 70 ± 10% VO2max RER 0.93 ± 0.02 0.87 ± 0.02 <0.001 CHO (g/min) 1.85 ± 0.36 1.34 ± 0.29 0.001 Fat (g/min) 0.26 ± 0.10 0.45 ± 0.10 <0.001 VO2 (ml/kg/min) 30.7 ± 2.3 30.5 ± 1.9 0.751 74 ± 11% VO2max RER 0.95 ± 0.03 0.89 ± 0.04 0.001 CHO (g/min) 2.20 ± 0.52 1.60 ± 0.47 0.002 Fat (g/min) 0.20 ± 0.12 0.42 ± 0.14 0.002 VO2 (ml/kg/min) 32.6 ± 1.6 32.8 ± 1.7 0.243