HIIT vs Zone 2: Which Type of Cardio Should You Prioritise?

Defining the Two Approaches

Zone 2 training means sustained aerobic exercise at low intensity — roughly 60–70% of maximum heart rate, where you can hold a full conversation. At this intensity, the body predominantly burns fat and produces minimal lactate. The primary adaptation is mitochondrial: Zone 2 increases mitochondrial density, improves fat oxidation efficiency, and builds the aerobic base.

HIIT (High-Intensity Interval Training) involves repeated short bursts at high intensity (typically 85–95% of max heart rate) separated by rest or low-intensity recovery periods. The primary adaptations include increased VO2 max, improved cardiac output, and better lactate threshold.

What Each Is Best For

The 80/20 Model

Elite endurance athletes and researchers like Stephen Seiler have popularised the "polarised training" model: approximately 80% of training volume at low intensity (Zone 2 or below) and 20% at high intensity. This ratio has been observed in Olympic cyclists, rowers, runners, and cross-country skiers.

The evidence base for this model in recreational athletes is growing but less established. However, the principle that chronic high-intensity training leads to overreaching, impaired recovery, and reduced adaptation is well-supported.

The Mistake Most People Make

Most recreational exercisers train in the "grey zone" — moderate intensity that is too hard to be Zone 2 (and capture its full adaptations) but not hard enough to be genuinely high-intensity (and capture HIIT adaptations). Heart rate monitors are useful here: if you cannot sustain a conversation but are not truly working hard, you are likely in this suboptimal middle ground.

Peter Attia's formulation: Zone 2 should feel "uncomfortably easy." If you feel like you could go harder but just don't, you're probably in the right zone.

Practical Prescription

For most adults with general health goals (cardiovascular health, longevity, body composition), a weekly structure might include:

• 3–4 sessions of Zone 2 cardio, 45–60 minutes each
• 1–2 HIIT sessions (4×4 intervals or similar), with at least 48 hours of recovery between sessions
• 2 strength training sessions (which have different but complementary adaptations)

This is not a rigid prescription — individual recovery capacity, available time, and enjoyment all matter. But the key insight is that both types serve distinct and complementary roles, and doing only HIIT (which is the default for time-efficient exercisers) leaves significant adaptations on the table.