Does zone 2 training increase mitochondria? Not more than harder work

Zone 2 training does increase mitochondria, but it is not the best intensity for building them. A 2025 narrative review in Sports Medicine took the popular claim on directly and concluded that current evidence does not support Zone 2 as the best intensity for improving either mitochondrial capacity or the muscle's ability to burn fat [1]. Zone 2 means low-intensity exercise below the lactate threshold, the effort level at which lactate, a by-product muscles make when working hard, starts collecting in the blood faster than the body clears it [1].

The recommendation traces back to observational data on elite endurance athletes, who do large volumes of easy training and have high mitochondrial and fat-burning capacity [1]. Copying their easy pace leaves out the volume it sits inside. A description of how world-class middle-distance and long-distance runners actually train puts weekly mileage at 150 to 180 kilometres, with three or four threshold interval sessions and one session hard enough to push VO2 max, the most oxygen your body can use while exercising hard, folded into that total every week [2].

On intensity the review does not stop at a neutral verdict. It argues that prioritising intensities above Zone 2 is what matters most for cardiometabolic health when training volume is low [1]. That is the reverse of how the easy pace is usually sold to someone with three or four hours a week.

One small trial shows how little easy exercise on its own can move the machinery. Thirteen recreationally trained adults walked two 45-minute bouts per session across ten days, and their VO2 max rose while every mitochondrial marker the researchers measured stayed flat: PGC-1-alpha, the cellular switch that tells muscle to build new mitochondria, the protein that reads the mitochondria's own DNA, a calcium-sensing enzyme, and all four complexes of the electron transport chain, the assembly line inside each mitochondrion that makes the energy [10]. The walking was done in roughly 42 degrees Celsius heat, because the study was testing heat acclimation rather than training intensity [10].

What does change mitochondria has less to do with a heart-rate zone than with the size of the whole stimulus. The functional result of training is set by volume, intensity and frequency together, plus how long the new proteins last before the body breaks them down [15]. Sustained endurance training is what produces mitochondrial biogenesis, the growth of new mitochondria, which means more mitochondrial volume and more capacity, and with it a better ability to use fat for fuel at easy and moderate efforts [12][15]. Exercise is still the most effective behaviour we know of for improving mitochondrial health [14].

The signals that trigger mitochondrial growth answer to how hard a muscle fibre is working, not to a pace on a watch. Two of them are calcium released inside the fibre and AMPK, a fuel gauge that senses when a cell is running low on energy [2][7]. In mouse muscle, AMPK raises levels of NAD+, a molecule cells use to move energy around, which activates an enzyme called SIRT1, which in turn switches on PGC-1-alpha and the genes under its control [7]. Interval sessions seem to reach the same pathways by another road: short fast repetitions recruit more motor units, meaning bundles of muscle fibres and the nerves driving them, than steady easy running does, even when the overall metabolic demand stays moderate [2].

Mitochondrial health is not only a question of how many there are. Inside muscle, mitochondria form a connected network held in shape by two opposing jobs: building and merging on one side, splitting and clearing out the damaged parts on the other [14]. A 2026 review of Drp1, the protein that does the splitting, finds that its activity has to stay inside a narrow band, because both too little and too much splitting make muscle waste and malfunction [6]. In ageing muscle Drp1 activity runs low; in metabolic disease it runs high. Moderate exercise appears to push it back toward the middle from either direction, through the reactive oxygen species it generates, which are the reactive by-products of burning fuel with oxygen [6].

In mice, that repair work is visible. Old mice had less running endurance and lower mitochondrial respiratory capacity, meaning how much energy their mitochondria could produce, along with fragmented mitochondrial networks and more reactive oxygen leaking out [11]. Training restored maximum respiratory capacity whether or not the animals had PGC-1-alpha, but rebuilding the fragmented network only worked when PGC-1-alpha was present [11].

Which stimulus you apply decides which machinery you get. Endurance work drives mitochondrial growth and a shift toward fatigue-resistant muscle fibres, while heavy resistance work drives the building of contractile proteins and force, by largely separate molecular routes [15].

The central source here is a narrative review, an argued reading of the published literature rather than a new experiment, and its target is the advice being given to the general public rather than how athletes should train [1].

No study in this set ran the experiment a reader would most want: assign ordinary adults to a Zone 2 programme or a harder programme, then measure what happened inside their muscle.

The ten-day walking study is small and peculiar. Thirteen recreationally trained adults, two 45-minute bouts per session, in roughly 42 degrees Celsius heat, run as a test of heat acclimation [10]. It cannot settle what months of easy training do to mitochondria.

Much of the mechanism is animal and laboratory work. The ageing experiments were done in mice [11], the AMPK pathway was mapped in mouse muscle [7], and the Drp1 review draws largely on laboratory models [6]. None of it tells you what is happening in a particular person's muscle.

The older-adult programme was a feasibility pilot in 19 people with no comparison group, and it measured whether people could follow the instructions, not whether their mitochondria changed [5].

The evidence does not apply to:

• elite endurance athletes, whose training volume is several times larger and on whom the Zone 2 case was built in the first place [1]
• anyone for whom exercise intensity is a medical question, including people with heart or metabolic disease, where how hard to train is decided with a doctor
• frail or much older adults, for whom reviews call for an individually prescribed mix of strength, aerobic, balance and flexibility work rather than one intensity [13]

None of these studies measured lifespan, disease or death. The outcomes are muscle proteins, mitochondrial function and fitness.

Take someone with four hours a week who has read that easy Zone 2 riding is how you build mitochondria. The review's conclusion lands on exactly that case: at a low weekly volume, the intensity of those few hours is what the evidence puts its weight on, and the athlete template being copied assumes 150 to 180 kilometres a week with harder sessions already inside it [1][2].

The easy pace does have one practical advantage, which is that people can find it and keep doing it. In a pilot of a video course on mitochondrial fitness for adults aged 50 and over, 18 of the 19 who carried on for a month could take their own pulse and work out their Zone 2 heart rate within two attempts, 13 of 19 kept a daily walking log, and the share doing regular moderate aerobic activity rose from 6 of 17 participants to 10 of 17 [5]. Walking a mile took them 17.4 to 27.1 minutes [5]. That pilot tested whether the habit sticks, not whether anyone's mitochondria changed.