REF-167 · Mendes et al., 2022 · Scientific Reports, · Paper · PubMed

Superior Cardiometabolic and Cellular Adaptive Responses to Multiple Versus Single Daily Sessions of HIIT in Wistar Rats

Cweak evidence: opinion or lab dataMeasured on a mixed groupWritten before October 2026, being re-checked

Participants

Model: male Wistar rats, n=30 in three groups of 10: untrained (n=10), 1×HIIT (n=10), 3×HIIT (n=10)
Duration: 8 weeks, treadmill running five times per week
⚠️ ANIMAL MODEL — direct extrapolation to humans is not valid

Protocol

1×HIIT: one daily session of 15 min treadmill running
3×HIIT: three daily sessions of 5 min, 4 h apart
Total volume: identical (15 min/day in both trained groups)
Measurements: resting VO2 and VO2max (metabolic chamber), blood pressure and heart rate (plethysmography), body composition (DEXA), glucose and insulin tolerance tests; after euthanasia: cardiac function, histology and morphology of heart, gastrocnemius and visceral fat; mitochondrial density of gastrocnemius and left ventricle (electron microscopy)

Key Results

Parameter1×HIIT3×HIITDifference
Resting VO2↑↑ (similar)no difference
VO2max↑↑ (similar)no difference
Cardiac function↑↑ (similar)no difference
Mitochondrial density↑↑ (similar)no difference
Visceral fat weight↓↓↓3×HIIT superior
Adipocyte size↓↓↓3×HIIT superior
Insulin tolerance↑↑↑3×HIIT superior

Authors' conclusion: multiple short daily bouts of HIIT may be superior to single daily sessions in improving cardiometabolic and cellular adaptations in rats.

Mechanisms

Inference, not tested in this paper (the abstract reports outcomes only): repeated daily bouts mean repeated post-exercise metabolic activation and repeated signalling episodes, which could favour fat-tissue and insulin outcomes without changing the aerobic-capacity response; the equal mitochondrial and VO2max response suggests the 15-min dose, however split, was the operative stimulus for those.

Practical Application

Rat study (n=30, 8 weeks, 15 min/day of treadmill HIIT): splitting the daily dose into three 5-min sessions 4 h apart produced the same gains in resting VO2, VO2max, cardiac function and mitochondrial density as one 15-min session, but larger reductions in visceral fat and adipocyte size and a larger improvement in insulin tolerance. Animal data: a hypothesis for humans, not a prescription.
For aerobic capacity the split showed no advantage even in rats — no reason from this paper to break up VO2max sessions.
Inference, not tested in humans: if the metabolic-health advantage of splitting held, it would matter for a fat-loss phase with flexible scheduling, not for performance training.
Not from this paper; the base's own practice: keep standard VO2max formats (REF-045, REF-046) for capacity work; for the human comparison of one long vs two short sessions see REF-159 (acute physiological responses, no adaptation measured).

⚠️ Critical Context

This is a rat study. The human comparison in REF-159 (one 6×10 min session vs two 3×10 min sessions) measured acute responses only: the single long session produced a within-session drift in HR, lactate and RPE and a higher session RPE, which the authors read as a larger stimulus — no adaptation outcome was measured there either. Neither paper shows a chronic human advantage for either format. In capacity-focused programs, prioritize human evidence.

Limitations

Rats, male only; treadmill running; 8 weeks.
Outcomes include visceral fat and insulin tolerance — metabolic health, not endurance performance.
No human replication in this entry.
How this entry was checked and changed
Content correction 2026-09-27
n=30 (10 + 10 + 10) and the measurement methods added from the abstract; "5 days/week" -> five times per week (abstract).
AMPK / EPOC×3 / PGC-1α mechanisms -> not in the abstract; relabelled inference without named pathways.
"Maintain standard 4×8min, 5×6min, 30/15 protocols", "Morning Zone 2 + evening short HIT" -> house advice; relabelled base practice with pointers.
Cross-reference "REF-160 Talsnes 2024" -> the one-long-vs-two-short human study is REF-159; REF-160 is Reynolds (long runs and durability). "Single long session SUPERIOR for aerobic drift (human data)" -> REF-159 measured acute responses only; reworded.
Cross-reference REF-162 (duplicate) -> REF-306.