Zone 2 is everywhere—podcasts, socials, longevity threads. But a 2025 narrative review took a hard look at the claims and asked a simple question: Is Zone 2 training really the best way to improve mitochondrial capacity and cardiorespiratory fitness? Read on to discover the findings translated into plain, actionable science.
What exactly is Zone 2 training?
Zone 2 training is easy, steady aerobic work just below your first lactate threshold (LT1)—you can hold a conversation, breathing is controlled, and you could keep going for a while. On a bike or treadmill, it feels comfortable-moderate. Rough guides:
- Talk test: You can speak in full sentences.
- Rate of Perceived Exertion (RPE): ~3–4/10 effort.
- Heart rate: Often ~60–70% of max (varies widely).
- Blood lactate: ~1.7-2.0 mmol/L.
- Power/pace: Below LT1; sustainable for 45–90+ minutes.
Note: Zone boundaries are individual. Fitness level shifts where “Zone 2” lands—what’s easy for an elite athlete may be challenging for a beginner.
The hype vs. the science
The hype
Zone 2 training is having “a moment”. It is all over the media, promoted as the optimal intensity for mitochondrial health, fat oxidation, and longevity—sometimes with the suggestion that you should avoid going harder to “protect” those benefits.
What the new review found
These recommendations largely stem from observational data of elite endurance athletes who engage in large volumes of Zone 2 training and possess high mitochondrial and fatty acid oxidative capacity in the first place.
Zone 2 does have benefits, but the evidence does not support it being superior to higher intensities for most key adaptations—especially when your training time is limited. In many cases, intervals and harder efforts drive bigger, faster changes in mitochondrial signaling, cardiorespiratory fitness (VO2max), and overall cardiometabolic health.
Mechanisms & adaptations—what the data shows
1) Mitochondria & gene signaling
- Acute studies show small/inconsistent changes in energetic stress markers (AMP/ADP, PCr depletion), with mixed activation of AMPK → PGC-1α signaling.
- The majority of the available evidence argues against the ability of Zone 2 training to increase mitochondrial capacity, refuting the current popular media narrative that Zone 2 training is optimal for mitochondrial adaptations.
- Some protocols (typically long durations) do upregulate mitochondrial gene expression.
- >Zone 2 (heavy/severe domains): Repeatedly shows greater energetic stress (PCr↓, pH↓, lactate↑), stronger AMPK/p38/CaMKII activation, and larger or faster mitochondrial signaling responses.
Implication: Yes, Zone 2 triggers adaptations. But higher intensity drives stronger signals. When researchers compare like-for-like volumes, intervals and hard efforts generally produce larger mitochondrial responses than Zone 2 training alone.
2) Fat oxidation (FAO)
- Researchers were surprised to find only 1 study that measured rates of FAO following confirmed Zone 2 training, suggesting further research in this area is needed.
- It does not appear that Zone 2 training elicits unique benefits for FAO capacity that cannot be achieved by higher intensities.
- Beginners / sedentary / metabolic impairment: Zone 2 can increase the intensity at which fat oxidation peaks (FATmax) and the maximal rate of fat oxidation (MFO), as well as shift substrate use toward fat, although increases are likely limited to sedentary/untrained populations.
- Trained individuals: Evidence for further FAO gains from additional Zone 2 is limited; adaptations often require higher intensity, volume, or novel stimuli.
Implication: Zone 2 can improve fat oxidation in beginners, but sustained progress usually requires more than Zone 2 alone.
3) Cardiorespiratory fitness (CRF; VO2max)
- Across studies/meta-analyses, protocols with higher intensity outperform equal-volume Zone 2 for VO2max—particularly when time is constrained.
Implication: For health risk reduction and performance, intensity is a powerful—and time-efficient—driver. If you’ve only got a few hours each week, higher intensity work tends to outperform Zone 2 for improving VO2max.
Why time efficiency matters
Most of us aren’t training 10–20 hours per week like elite endurance athletes. At realistic volumes (e.g., 1–4 hours), work above LT1 tends to yield larger improvements per minute in VO2max and mitochondrial signaling than Zone 2 alone. That’s not an anti–Zone 2 message, it’s a pro-efficiency message. Use Zone 2 strategically for recovery, low stress volume, and base—but make sure intensity is part of the plan.
Where CAROL Bike and REHIT fit
REHIT (Reduced Exertion HIIT) on CAROL Bike gives you the shortest, most efficient workout: 2×20-second sprints inside a 5-minute ride. These brief, supra-maximal efforts are proven to trigger robust AMPK/p38/CaMKII signaling, providing a potent VO2max and cardiorespiratory fitness stimulus. With CAROL Bike’s workouts, you can reset your VO2max by 12% in just 8 weeks.
The bike’s Instant Resistance and AI-personalization set the exact intensity for you, so you safely and accurately hit the stimulus that research highlights—without spending hours in Zone 2. This makes REHIT ideal for maximum gains per minute and for time-crunched schedules, while still being easy to layer with Zone 2 sessions for additional low-stress volume.
REHIT on CAROL Bike delivers superior health and fitness benefits compared to zone 2 training in 90% less time.
Practical training templates
If you’re aiming for well-rounded fitness, these example schedules show how to balance Zone 2 training with exercise at higher intensities.
If you’re a beginner
- 2–3x/week Zone 2 (20–40 min) to build endurance and confidence.
- + 1x/week short REHIT/HIIT to spark VO2max and mitochondrial signals.
- Optional light strength 1–2×/week.
If you’re time-crunched
- 2–3x/week REHIT/HIIT (5–30 min) for efficiency.
- + 1x/week Zone 2 45–90+ minutes for recovery and fat metabolism.
- Keep total weekly time realistic so you actually stick with it.
If you’re an endurance athlete
- Keep Zone 2 for base and recovery.
- Layer higher intensity sessions (like REHIT) to drive continued adaptation.
- Taper intensity in race weeks.
A concise comparison
FAQs
Is Zone 2 training necessary?
Not strictly, but it’s useful. Zone 2 training is great for building a base, supporting recovery, and adding low stress volume. It’s not a replacement for intensity if you want the biggest gains in fitness with limited time.
Can I skip Zone 2 if I’m very busy?
If time is limited, you’ll generally get faster health and fitness returns from higher exercise intensities.
Won’t high intensity be too hard to sustain?
That’s why very short, well-controlled protocols like REHIT on CAROL Bike work—short bursts, big stimulus, minimal time cost.
How many high intensity sessions per week?
Most people do well with 2–3 high-intensity sessions (like REHIT/HIIT), plus easy movement on other days.
TL;DR
- Zone 2 training has its benefits, but it is not superior to higher intensities for mitochondrial adaptations or VO2max. Popular claims that Zone 2 is ‘optimal’ aren’t strongly backed by direct experimental evidence; they mostly stem from observing elite endurance athletes who also train substantial amounts at higher intensities.
- FAO improvements with Zone 2 training are most evident in beginners; trained athletes often need harder work for continued gains.
- For the general public, relying only on Zone 2 training could mean missing out on larger benefits that come from higher intensity exercise.
- If you’re short on time, prioritize REHIT/HIIT; use Zone 2 training strategically for base and recovery.
- Public-health “moderate intensity” targets may not always be met by what some individuals call “Zone 2”; calibrate using LT1/HR/RPE, not labels alone.
If your aim is maximum improvement per minute, evidence favors higher-intensity stimuli. CAROL’s REHIT makes that stimulus precise, safe, and consistent—so you can build a sustainable routine that respects both biology and your calendar. Use REHIT for the heavy lifting; keep Zone 2 training for the base you enjoy and recover on.
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