Is ‘Zone 2’ the magic effort level for exercise?

What is Zone 2 training?

The buzz around Zone 2 is based on the work of Inigo San Millan, a professor at the University of Colorado School of Medicine. A former professional cyclist, San Millan has long helped train cyclists – including three-time Tour de France winner Tadej Pogacar – in addition to his academic research.

His work with cyclists led him to classify exercise into six different training zones, based in part on what type of fuel your muscles are burning, to explain how the body responds to different workout intensities. In the easiest zone, which for an average person might be a brisk walk, you’re burning mostly fat.

As you push harder, you burn more fat – but only up to a point. Beyond a certain level of effort, your body starts relying more on carbohydrates and ramps down fat-burning. Lactate levels in your blood also begin to creep up, a sign that your muscles are working harder. From a metabolic perspective, San Millan said, “something funky happens” when you cross this threshold.

Over the years, he has observed that top cyclists tend to spend a lot of their training time just below this point. That’s not the only type of training they do, he emphasised – a point that sometimes gets lost in the social media hype about Zone 2. But the more time they spend in this zone, he said, the better they get.

Why might Zone 2 be good for you?

There’s little debate that racking up lots of relatively easy aerobic exercise is good for your health, but the question is whether Zone 2 is uniquely beneficial compared to slightly easier or harder workouts.

Training at this intensity, in San Millan’s view, teaches your muscles to make the most efficient use possible of both fat and carbohydrates. More specifically, he believes Zone 2 is the optimal level to trigger improvements in your mitochondria, the so-called “powerhouses” in our cells that use oxygen to turn food into energy.

If you stay in Zone 1, his thinking goes, your mitochondria won’t be working hard enough to spur significant improvements. If you ramp up to Zone 3, the point at which you start struggling to speak in full sentences, your muscles will switch over to burning more carbohydrates and less fat. Zone 2, then, is the sweet spot.

It’s well established that having more mitochondria is associated with positive outcomes like better insulin sensitivity and athletic performance. But San Millan, along with a growing cohort of other scientists, believes they also function as a sensitive predictor of future metabolic problems: the first place that signs of trouble appear on the long road to chronic conditions like type 2 diabetes, heart disease and even Alzheimer’s disease.

In their theory, the prescription for warding off such ailments is to exercise at Zone 2, which forces the mitochondria to adapt and grow.

How do you know when you’re in Zone 2?

Figuring out exactly how hard you need to push yourself or what heart rate corresponds to Zone 2 requires a visit to an exercise physiology lab, which isn’t practical for most people. You can’t trust the heart-rate zones on treadmills or other exercise equipment, since San Millan’s six-zone system is one of several different classification systems, each with different zone boundaries.

Another method that has gained popularity among tech-forward self-optimisers is to use a pinprick of blood to test your lactate levels intermittently throughout a workout.

Alternatively, you can skip the data and focus on how hard the exercise feels.

In his book, Attia describes the appropriate effort as between easy and moderate: “slow enough that one can maintain a conversation, but fast enough that the conversation might be a little strained”. He suggests starting with two half-hour sessions a week and progressing until you’re doing at least three hours a week in total.

What does the science say?

The case for Zone 2 relies mostly on observational data, but scientists haven’t yet put these claims to the test in rigorous training studies.

San Millan views the pace you can sustain in Zone 2 as an indirect measure of how many mitochondria you have and how well it’s working. Over the years, he has noted that those who do the most Zone 2 training – pro cyclists being his primary example – see the biggest improvements in this pace.

But when researchers perform muscle biopsies to directly measure how many mitochondria are present, Zone 2 doesn’t fare as well, according to Kristi Storoschuk, a doctoral candidate in muscle physiology at Queen’s University who is one of the few researchers in the world currently studying the topic. Instead, intense exercise well above Zone 2 produces the biggest effects on mitochondria. That’s also the conclusion of a newly published systematic review by scientists in Norway and Denmark, and it’s a perspective shared by many scientists in the field.

If the interest in Zone 2 leads people to assume that easier is always better, Storoschuk warned, they might end up exercising at such a low intensity that their mitochondria don’t benefit at all.

It’s difficult to make apples-to-apples comparisons between different types of workouts, which might explain these seemingly conflicting findings. An hour of Zone 4 may have greater fitness benefits than an hour of Zone 2, for example, but it’s also much harder.

The real magic of Zone 2, said Martin MacInnis, a professor at the University of Calgary who studies exercise and mitochondria, may turn out to be that it allows you to recover quickly and do it again the next day – and perhaps even enjoy it. The best workout for boosting mitochondrial health, he said, is probably whichever one you’re willing to do regularly.

Indeed, Stephen Seiler, a Norwegian sports scientist, has observed that top athletes across a variety of endurance sports spend about 80% of their time doing relatively easy training and 20% going hard. This 80/20 pattern seems to produce the ideal combination of how much and how hard you can train – and it’s probably a good recipe for optimising your health as well.

That’s a perspective shared even by San Millan, who never intended to suggest that there’s only one type of workout worth doing. “Zone 2 is part of it,” he said, “but you need to do other intensities as well.”

This article originally appeared in The New York Times.

Written by: Alex Hutchinson

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