Much Ado About Zone 2

The awesomely titled ‘Much Ado About Zone 2: A Narrative Review Assessing the Efficacy of Zone 2 Training for Improving Mitochondrial Capacity and Cardiorespiratory Fitness in the General Population’ was published last June in Sports Medicine. You can read the abstract here, but there’s no free link to the full paper I’m afraid.

This one has had folks in all corners of Instagram talking. The premise of the paper is taking aim at podcasters and influencers who often claim that zone 2 training is the optimal intensity for mitochondrial adaption and fat oxidation, which then gets linked to metabolic health and chronic disease prevention. The aim of the authors was to evaluate this claim.

A brief intro, if you’re not familiar with the concepts of zone 2, mitochondria or fat oxidation. There are many ways of describing different intensities of exercise, but zone 2 has become popular as part of a five zone model as a shorthand for an ‘easy’ intensity, one at which your breathing is relaxed and you can speak in full sentences. Your mitochondria are your cells’ ‘engines’ - they generate energy from the food you eat via aerobic respiration. Fat oxidation is the process of ‘burning’ fat in the mitochondria to create energy. Improving fat oxidation is useful for endurance runners because we store large reserves of fat in the body, but have only limited resources of carbohydrate, our other main fuel.

The paper argues that the claims made for the benefits of zone 2 are based on the practices of elite athletes, who do indeed do very high volumes of zone 2 training, but also do regular seriously hard training. Elite athletes also train for many hours per week - does it follow that someone doing two hours of training should aim for the same balance of training types as someone doing twenty hours? They’re spot on that it’s clearly nonsensical to base claims for the value of only doing zone 2 training on what the elites do - it’s akin to saying ‘Jenny is healthy, and she eats lots of oranges and Oreos, therefore Oreos have significant health benefits’.

The authors carried out a literature review to see if they could find evidence that prescribing zone 2 training for the general population (i.e. relatively untrained, sedentary people) is more beneficial than prescribing higher intensity training. Full disclosure; there’s a lot of exercise physiology in this paper, and some of it is beyond my level of expertise. But the short answer is - no. The evidence doesn’t support the idea that the general public will experience the most health and fitness benefit by solely doing zone 2 training. In fact, as far as I can tell, someone who is doing the government-recommended 150 minutes per week of ‘moderate-intensity’ exercise may well be better off training mainly above zone 2.

This had some internet commentators going crazy - their advocacy of high-intensity exercise programmes had finally paid off. Proponents of zone 2 training had finally got their comeuppance. But…not so fast.

We aren’t the general public - although we’re not (mostly) elite athletes either. Where does this leave us? I have to admit that there were a couple of bits in this paper that challenged my thinking. I generally talk to athletes about why lower intensity training is beneficial (alongside, of course, their harder sessions), and one of the things I mention is the mitochondrial adaption that zone 2 generates. But, this review says that they found limited evidence for this, while they found much stronger data for mitochondrial response to higher intensity stimulus. Similarly, the paper found mixed evidence for the idea that zone 2 training is the best stimulus for fat oxidation - some studies said yes, others no.

Does it all mean that we are doing it wrong? Zone 2 is out, HIIT classes are in? I still don’t think so. The authors themselves note that there is a reason why the ‘mainly easy, some hard’ training method is so popular - it’s because it works for the best in the world. Elite athletes and coaches are constantly pushing at the boundaries of training theory, and if a full-throttle high-intensity diet was the optimum, you can guarantee Kilian, Courtney and Eliud would be doing it.

Of course, just because elites do it doesn’t mean it’s right for us. So how do we figure out what is? Something this paper doesn’t mention at all is the physical impact of higher intensity training. The faster you get, the more load you have to absorb from your running (by which I mean, the literal forces as your feet hit the ground get higher, and this creates more damage to your muscles for your body to repair). This means more recovery, and begins to limit how much high intensity you can do. It’s one reason why a number of coaches and athletes are working with uphill treadmill sessions for their harder runs - they can do much higher volumes of hard work without hammering their bodies. I’d argue this is an important determinant of the balance of your training load. If you can train for say, three hours per week, and your pace, even at high intensity, is relatively slow, there’s a good chance you’ll be able to recover well and adapt to that training. If you are more advanced, training for ten hours per week, and your pace is faster, a big additional argument for balancing high intensity with easy effort develops - even if you’re not certain of the energy system benefits, you will break yourself without easy running.

The paper also doesn’t mention the psychological and emotional aspects of training. Even if you only have three hours per week to train, do you want to do all your training at high intensity? This might be fun for some, while others might really enjoy those chilled zone 2 runs.

My takeaway? Maybe I’ll worry less about my lower-volume runners working harder. This paper suggests that they’ll get the benefits of zone 2 training even if they’re doing a much bigger proportion of their running above zone 2 than is traditionally recommended, and at lower overall volumes they’re unlikely to hurt themselves. But for my higher volume runners, I’m sticking firmly to ‘mainly easy, some hard’.

If you’ve read the paper I’d love to chat about it - really interested to hear what others’ key messages are!