# Effects of sprint interval training on substrate oxidation in adults living with and without obesity: The i-FLEX study

PMID: 34110721
Journal: Physiological Reports
Published: 2021-06
Authors: Colpitts BH, Seaman K, Eadie AL, Brunt KR, Bouchard DR, Sénéchal M

## Question

Does 4 weeks of sprint interval training alter resting or exercise substrate oxidation differently in adults living with and without obesity?

## Summary

After 4 weeks of supervised Wingate-style sprint interval training, adults with obesity showed lower submaximal exercise RER and higher submaximal fat oxidation, while adults without obesity did not show the same substrate shift. The study was small, single-arm, and lacked a non-exercise control group.

## Population

- Adults with and without obesity completing 4 weeks of supervised SIT.
- Sample size: 34
- Age: No-obesity 41.7 +/- 13.3; obesity 39.3 +/- 11.8 years
- Sex: 19 female, 15 male
- Fitness level: Obesity group physically inactive; no-obesity group not required to be inactive
- Health status: With or without obesity; diabetes and impaired glucose tolerance excluded

## Methodology

- Single-arm quasi-experimental intervention with obesity-status comparison
- 4 weeks
- Supervised laboratory cycling intervention

## Protocol

- Sprint interval training.
- Modality: Stationary cycling.
- Work intervals: 30 s.
- Recovery: 4 min active recovery at 59 W.
- Sets or repetitions: 2-4 sprints.
- Intensity: All-out Wingate at 7.5% bodyweight load.
- Session duration: 9-18 min plus cooldown.
- Frequency: 3 sessions/week.
- Program length: 4 weeks.
- Progression: Intervals progressed from 2 to 4.

## Outcomes

### Submax RER
Status: improved
Adults with obesity reduced submaximal RER after SIT.

Obesity 0.95 +/- 0.08 to 0.92 +/- 0.06; time x group p=0.001.

### Fat oxidation
Status: improved
Submaximal fat oxidation increased in adults with obesity.

105.5 to 208.0 g/min; p=0.009.

### Insulin sensitivity
Status: mixed
Insulin sensitivity decreased in the no-obesity group and increased nonsignificantly in the obesity group.

Group x time p=0.033.

### VO2peak
Status: mixed
VO2peak improved in the no-obesity group but not significantly in the obesity group.

No-obesity p=0.004; obesity p=0.352.

## Practical Insights

- SIT may change fuel use during exercise in adults with obesity, but this is mechanistic evidence.
- Long recoveries and screening are central to the studied sprint protocol.
- Do not translate Wingate SIT directly to broad unsupervised bodyweight workouts.

## Limitations

- No non-exercise control group
- Small sample
- Short intervention
- Different activity eligibility by obesity status
- VO2peak rather than VO2max
- Prior training experience not recorded

## Safety And Adherence

- Three participants dropped out for injury/illness unrelated to the intervention or personal reasons.
- HR and BP were checked after cooldown before participants left sessions.

## Original Sources

- [PubMed](https://pubmed.ncbi.nlm.nih.gov/34110721/) (pubmed)
- [DOI](https://doi.org/10.14814/phy2.14916) (doi)
- [PMC full text](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8191399/) (full text)

## Agent Guidance

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