# Regulators of human white adipose browning: evidence for sympathetic control and sexual dimorphic responses to sprint interval training

PMID: 24603718
Journal: PloS one
Published: 2014
Authors: Scalzo RL, Peltonen GL, Giordano GR, Binns SE, Klochak AL, Paris HL, Schweder MM, Szallar SE, Wood LM, Larson DG, Luckasen GJ, Hickey MS, Bell C

## Question

Do sympathetic stimulation and sprint interval training alter proposed browning regulators such as FGF21, FNDC5, and irisin in humans?

## Summary

In a small study of young healthy adults, 3 weeks of all-out cycling sprint interval training improved time to exhaustion but did not change skeletal muscle FNDC5 or overall circulating irisin. Plasma irisin responses differed by sex in exploratory analyses.

## Population

- Young healthy non-obese adults in the sprint interval training study.
- Sample size: 19
- Age: 24 +/- 1 years
- Sex: 7 men and 12 women
- Fitness level: Healthy adults with VO2max 35.4 +/- 1.5 mL/kg/min.
- Health status: Healthy non-obese adults, although mean BMI was 26.8 +/- 1.1 kg/m2.

## Methodology

- Two human studies; Study 2 was a pre-post sprint interval training intervention
- 3 weeks for sprint interval training
- University laboratory

## Protocol

- Sprint interval training.
- Modality: Cycling.
- Work intervals: 30 s all-out maximal sprint.
- Recovery: 4 min recovery.
- Sets or repetitions: 4-8 repetitions.
- Intensity: All-out.
- Session duration: Not fully reported; sprint block varied by repetitions plus 4 min recoveries.
- Frequency: 9 sessions over 3 weeks.
- Program length: 3 weeks.
- Progression: Increased from 4 to 8 sprints.

## Outcomes

### Time to exhaustion
Status: improved
Time to exhaustion improved after SIT.

38.7 +/- 4.4 to 49.3 +/- 7.4 min, p=0.048.

### FGF21
Status: worse/safety concern
FGF21 decreased after SIT.

338 +/- 78 to 251 +/- 36 pg/mL in abstract; p=0.046.

### Irisin/FNDC5
Status: mixed
FNDC5 and overall irisin did not change; irisin showed a sex interaction.

FNDC5 p=0.79; irisin overall p>0.787; sex x training p=0.012.

## Practical Insights

- Long recoveries with 30 s all-out cycling sprints can improve time-to-exhaustion in a short program.
- Biomarker findings should be treated as exploratory and not translated into consumer claims.

## Limitations

- Small sample
- No control group
- Exploratory sex interaction
- Young healthy participants
- No adherence or adverse-event reporting
- Surrogate biomarker outcomes

## Safety And Adherence

- Adverse events were not reported.
- The article says participants were informed of risks and consented; biopsies were performed under local anesthesia.
- Session attendance, dropout, and tolerability values were not reported for the SIT intervention.

## Original Sources

- [PubMed](https://pubmed.ncbi.nlm.nih.gov/24603718/) (pubmed)
- [DOI](https://doi.org/10.1371/journal.pone.0090696) (doi)
- [PMC full text](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3946216/) (full text)

## Agent Guidance

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