Zone 2 Training and Mitochondria: What Low-Intensity Cardio Does for Cellular Energy

Zone 2 training is steady cardio just below your first lactate threshold, an effort where you can still talk in full sentences. It supports your mitochondria mainly by letting you build plenty of easy, repeatable training volume. But research shows it isn't uniquely effective: harder interval work also improves mitochondrial function, often in less time.

That second point is where most Zone 2 content goes quiet. This guide explains what Zone 2 is, what it does inside your muscles, where the evidence is strong and where it's been oversold, and how to combine it with other training if you're over 40 and short on time. For the biology of energy production first, start with our guide to how your cells make energy and what changes after 40.

What is Zone 2 training?

Zone 2 is low-to-moderate intensity exercise sitting just below the first lactate or ventilatory threshold, the point where your body starts relying noticeably more on carbohydrate and lactate begins to rise. A 2025 expert viewpoint in the International Journal of Sports Physiology and Performance by Sitko, Seiler and colleagues recommended doing it immediately below that threshold.

The same paper admits there's no clear consensus on the definition. Some systems use three zones, others five, and heart rate percentages vary. In practice, Zone 2 feels like this:

• you can speak in full sentences, but you wouldn't want to sing

• breathing is deeper than at rest but controlled

• the effort feels sustainable for a long time, depending on your fitness

• common forms are brisk uphill walking, easy cycling, rowing or slow jogging

Heart rate formulas such as 220 minus age are rough guides, so the talk test is often more reliable than a watch, especially if you're new to training.

What does Zone 2 do to your mitochondria?

Regular endurance exercise increases how many mitochondria your muscle cells have and how well they work, a process called mitochondrial biogenesis. Zone 2 contributes mainly through volume: it's easy enough to do often and for long sessions, and total volume is a key driver of mitochondrial content.

The clearest finding comes from a 2018 review in Sports Medicine by Granata, Jamnick and Bishop. It concluded that training volume may be a critical factor for changes in mitochondrial content, while relative intensity is an important determinant of changes in mitochondrial respiratory function. Put simply, volume tends to build more mitochondria and intensity tends to make them work better, although the split isn't absolute.

A 2017 review by MacInnis and Gibala adds that the cellular signals for biogenesis, including PGC-1α, depend largely on exercise intensity, and that sprint interval training can raise mitochondrial content about as much as moderate continuous training in far less total time. For the cell biology of how mitochondria are built, split and recycled, see our guide on how cells maintain their mitochondria.

Is Zone 2 overhyped?

Partly. Zone 2 is a good tool, but the claim that it's the best or only intensity for mitochondrial health isn't supported by current evidence.

A 2025 narrative review in Sports Medicine by Storoschuk, Moran-MacDonald, Gibala and Gurd, titled "Much Ado About Zone 2", concluded that current evidence doesn't support Zone 2 as the optimal intensity for improving mitochondrial or fat-burning capacity in the general population, and that higher intensities may deserve priority when training time is limited. The 2025 expert viewpoint makes a similar point: the adaptations expected from Zone 2 might not be unique to it.

Our reading is this: Zone 2 is a foundation, not a complete programme. Its value is that you can do a lot of it without being wrecked, which makes consistency easy. It becomes a problem when it crowds out the harder sessions and strength work that also matter, particularly as you age.

Why does this matter more after 40?

Muscle mitochondrial capacity tends to fall with age and inactivity, but it responds well to training in older adults, and harder sessions appear to matter.

In a 2005 PNAS study of 146 healthy adults aged 18 to 89, Short and colleagues found muscle mitochondrial ATP production declined by roughly 8% per decade and tracked closely with aerobic capacity. In a 2017 Cell Metabolism study, Robinson and colleagues compared 12 weeks of high-intensity interval training, resistance training and combined training in younger and older adults. Only interval and combined training improved aerobic capacity and mitochondrial respiration, and interval training reversed many age-related differences in mitochondrial proteins.

That's a strong case for including some harder sessions once you're past 40, not just steady cardio. Our guide to what human research supports for cellular energy after 40 covers the wider picture, including sleep and nutrition.

Where does NAD+ fit in?

NAD+ is a coenzyme your mitochondria need to turn food into ATP, and exercise increases the enzyme that recycles it. That makes training the first and best-evidenced way to support NAD+ metabolism in muscle.

In a 2019 study in Physiological Reports, de Guia and colleagues found that NAMPT, the key enzyme in NAD+ recycling, was lower in older muscle. Aerobic training raised NAMPT by 12% in younger and 28% in older adults, and resistance training raised it by 25% and 30% respectively. Both cardio and strength work support the NAD+ recycling machinery, and older adults saw the larger relative gains.

NAD+ precursor supplements are where many people go next. The main options sold in the UK differ in formulation more than in mechanism. Tru Niagen is built around nicotinamide riboside (NR) on its own. Elysium Health's Basis combines 250 mg of NR with 50 mg of pterostilbene per two-capsule serving and carries NSF Certified for Sport. Longevity+, NovusDNA's NAD+ precursor formula, combines nicotinamide riboside chloride with pterostilbene and adds TMG (trimethylglycine), and is made by a UK brand that tests its products through Campden BRI, a UKAS-accredited UK laboratory.

To be clear about the limits: findings in muscle tissue don't show that any NAD+ precursor improves exercise performance or mitochondrial function in people, and no NR product carries an authorised UK health claim. If you're exploring this research, treat a precursor as an addition to training and sleep, never a replacement.

How to use Zone 2 in a busy week

Combine two or three Zone 2 sessions with one interval session and two strength sessions. That fits within the UK Chief Medical Officers' guidelines of at least 150 minutes of moderate or 75 minutes of vigorous activity a week, plus muscle strengthening on at least two days.

Here's an example for a time-poor adult over 40. It's a starting template based on the evidence above, not a prescription, so adjust it to your fitness and any health conditions.

A few practical rules we'd add:

1. If you're new to exercise, do Zone 2 and strength only for the first four to six weeks before adding intervals.

2. Keep Zone 2 genuinely easy. Most people drift too hard, which turns it into tiring "grey zone" training.

3. If time is short, keep the strength sessions and one interval session, and shorten Zone 2 rather than dropping the others.

4. If you have a heart condition, high blood pressure or another health concern, speak to your GP before starting interval training.

FAQ

How long does it take for Zone 2 training to increase mitochondria?

Studies have measured changes in mitochondrial content and function within weeks of starting regular endurance training, and many trials run for six to twelve weeks. Robinson and colleagues, for example, saw improvements in mitochondrial respiration after 12 weeks of interval and combined training. The exact timeline depends on your starting fitness, age, weekly volume and consistency. Untrained people usually see the fastest early gains, which is a good reason to start gently and build steadily.

Is Zone 2 better than HIIT for mitochondria?

Not according to current evidence. A 2018 review found volume mainly drives mitochondrial content and intensity mainly drives mitochondrial function, and a 2017 review found sprint intervals can raise mitochondrial content as much as moderate training in far less time. A 2025 review in Sports Medicine concluded Zone 2 isn't the optimal intensity for mitochondrial capacity in the general population. For most people, the best approach combines plenty of easy volume with some harder sessions.

How do I know if I'm in Zone 2 without a lab test?

Use the talk test: you should be able to speak in full sentences but not sing, and feel you could keep going for an hour or more. Heart rate guides often put Zone 2 at around 70% to 80% of maximum, but age-based formulas can be off by a wide margin, so if your watch and your breathing disagree, trust your breathing. Lab testing of lactate or ventilatory thresholds is more precise but not necessary for most people.

Can walking count as Zone 2?

Yes, for many people, especially brisk walking uphill or on an incline treadmill. Whether walking reaches Zone 2 depends on your fitness: for a less active person a brisk walk may sit squarely in the zone, while a fit runner may need to jog or cycle. Use the talk test rather than speed. Walking is also one of the easiest ways to build the weekly volume behind mitochondrial adaptations, and it counts towards the UK guideline of 150 moderate minutes.

What else supports mitochondrial energy besides exercise?

Sleep, overall nutrition and maintaining muscle mass all matter, and fatigue can have medical causes such as low iron, low B12 or thyroid problems, which the NHS advises discussing with a GP. Exercise remains the best-evidenced lever: both aerobic and resistance training raise the enzyme that recycles NAD+ in muscle. NAD+ precursor supplements are an area of active research and are best seen as an addition to training and sleep, not a replacement.

Zone 2 training supports your mitochondria mainly by letting you build easy, repeatable volume. It works best as the base of a plan that also includes intervals and twice-weekly strength training, especially after 40, when mitochondrial capacity and NAD+ recycling tend to decline and both respond well to training.

For more on the science behind ATP production and mitochondria, explore NovusDNA's cellular-energy guides. If you're interested in NAD+ precursor research, you can also learn about Longevity+, while keeping exercise, sleep and nutrition as the foundations.

This article is for general information and isn't medical advice. Speak to your GP before starting a new exercise programme if you have a health condition. Food supplements should not replace a varied, balanced diet and healthy lifestyle.