# Rethinking the role of fat oxidation: substrate utilisation during high-intensity interval training in well-trained and recreationally trained runners

PMID: 27900134
Journal: BMJ Open Sport & Exercise Medicine
Published: 2015
Authors: Hetlelid KJ, Plews DJ, Herold E, Laursen PB, Seiler S

## Question

How do well-trained and recreationally trained runners differ in substrate use during a self-paced high-intensity interval running session?

## Summary

During one self-paced treadmill HIIT session, well-trained male runners oxidized much more fat than recreationally trained runners while reporting similar effort and lactate responses.

## Population

- Male recreationally trained and well-trained runners.
- Sample size: 18
- Age: RT 28 +/- 6; WT 30 +/- 4 years
- Sex: Male
- Fitness level: Recreationally trained or well-trained
- Health status: Apparently healthy

## Methodology

- Acute comparative exercise physiology study
- Single HIIT session after preliminary testing
- Laboratory motorized treadmill at 5% gradient
- Controlled study design.

## Protocol

- Self-paced 6 x 4 min treadmill HIT.
- Modality: Treadmill running.
- Work intervals: 4 min self-paced highest sustainable speed.
- Recovery: 2 min recovery.
- Sets or repetitions: 6.
- Intensity: About 89-94% VO2max and 93-95% HRmax peak responses.
- Session duration: 34 min.
- Frequency: Single session.
- Program length: Acute testing.
- Progression: None.
- Same HIT in recreationally trained runners.
- Modality: Treadmill running.
- Work intervals: 4 min self-paced.
- Recovery: 2 min.
- Sets or repetitions: 6.
- Intensity: Same instructions, lower absolute speed.
- Session duration: 34 min.
- Frequency: Single session.
- Program length: Acute testing.
- Progression: None.

## Outcomes

### Fat oxidation
Status: improved
Well-trained runners had much higher fat oxidation during HIT.

0.57 +/- 0.18 vs 0.20 +/- 0.11 g/min; ES 1.74 +/- 0.93.

### CHO oxidation
Status: no clear change
Carbohydrate oxidation was similar between groups.

3.61 +/- 0.94 vs 3.79 +/- 1.01 g/min; unclear ES.

### RPE/lactate
Status: no clear change
Groups reported similar effort and lactate despite different work rates.

RPE about 16.2 in both groups; lactate similar.

## Practical Insights

- The same perceived effort can correspond to very different absolute workloads depending on training status.
- Fuel-use claims from trained athletes should not be generalized to beginners.

## Limitations

- Acute session only
- Small male sample
- Indirect calorimetry equations used above recommended intensity range
- Diet not controlled beyond instructions

## Safety And Adherence

- No adverse events were reported.
- All participants completed the acute laboratory HIIT session; no dropout was reported.
- Safety context included trained participants familiar with treadmill exercise/HIT, consent and ethics approval, continuous HR/gas monitoring, lactate/RPE checks, and pretest restrictions on hard training, alcohol, tobacco, and caffeine.

## Original Sources

- [PubMed](https://pubmed.ncbi.nlm.nih.gov/27900134/) (pubmed)
- [DOI](https://doi.org/10.1136/bmjsem-2015-000047) (doi)
- [PMC full text](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5117036/) (full text)

## Agent Guidance

Preserve the paper-level scope of this note. Do not generalize beyond the population, protocol, measured outcomes, and limitations above.