Running 8000 m fast or slow: Are there differences in energy cost and fat metabolism?

PURPOSE: To compare energy requirements and substrate use for running a given distance fast versus slow under field conditions.

METHODS: Ten males and four females who were moderately endurance trained (32 +/- 6 yr; 71 +/- 11 kg; body mass index: 22.7 +/- 2.3 kg x m(-2); VO2max: 62.0 +/- 6.0 mL x min(-1) x kg(-1); individual anaerobic threshold [IAT]: 13.7 +/- 1.4 km x h(-1)) performed an incremental running protocol to determine IAT. Subsequently, two 8000-m runs at 70 and 95% IAT were performed on separate days in randomized order on an indoor track. Energy expenditure (EE) was measured by means of a portable metabolic device (indirect calorimetry). A meaningful difference in EE was defined as >10%.

RESULTS: EE was significantly greater during the 95% IAT run than during the 70% IAT run (2650 +/- 276 and 2554 +/- 348 kJ, respectively). However, this difference was only 3.8 +/- 4.8%. Including measurements up to 10 min postexercise, the difference rose to 5.1 +/- 4.7% (95% IAT: 2830 +/- 301 kJ; 70% IAT: 2692 +/- 368 kJ). There was no significant difference between the absolute amounts of fat oxidized during the runs (70% IAT: 26 +/- 5 g; 95% IAT: 20 +/- 5 g). During the 95% IAT run, significantly more carbohydrates were metabolized than during the 70% IAT run (108 +/- 14 and 90 +/- 5 g, respectively). The difference in EE between the two runs increased from the first to the third part of the running distance (first: no significance; second: P < 0.01; third: P < 0.001).

CONCLUSION: Energy requirements for a commonly run distance in recreational endurance training differ significantly but not relevantly between slow and fast speeds. However, increasing total running distance might lead to larger differences.