‘Zone In’ for Cardio that Works

Whenever I start talking to clients about ‘cardio,’ responses typically fall into one of three categories:

1. Cardio is life (and they tend to do too much; often too high-intensity)
2. Cardio sucks (and they tend to do too little or none)
3. I do cardio because I sort of think I have to (but they aren’t quite sure if they’re doing enough, too much, or even what kind of cardio is ‘worth it’ to do)

With the Type 1 clients (aka the “cardio bunnies,” the HIIT addicts, the Spin-classers), I find myself trying to explain the importance of moderate-intensity cardio and not blowing past their anaerobic threshold every single session.

With the Type 2 clients (aka the “heavy lifters,” the ones who skip all cardio for fear of losing their gainz), I find myself trying to explain why some cardiovascular exercise matters for heart health, and why a bit of it is genuinely useful for body recomposition if that’s a goal too.

And with the Type 3 clients (aka “everyone else,” the people who are a little confused about the role of cardio in a regular exercise programme), I find myself advising around everything from F45 to marathon running to hot Pilates to answer the question, “what’s the ‘best’ kind of cardio to do?”

For a while now, there’s been a tidy, science-flavoured answer for all three types: Zone 2 cardio. I gave you that answer myself, in this very post, back in 2024. And I still think it’s a good one, for a lot of people.

But the research has moved since I first wrote this, and if I’m going to keep sending you here, I owe you the update — including the parts that complicate the story.

What Zone 2 actually is

“Zone 2” became the cardio format du jour thanks to people like Peter Attia and exercise physiologist Iñigo San Millán. The idea, roughly: at a specific intensity — one comfortably below your lactate threshold — your mitochondria get the strongest training stimulus, and your body leans more heavily on fat as fuel.

Most recreational exercisers, the theory goes, blow straight past this zone into Zone 3, 4, or 5, which is why so many of us feel like we’re working hard and still not seeing the metabolic payoff we expect.

The reason it caught on, and the reason I still think it’s worth your time, is that it isn’t painful and doesn’t demand any heroics. Elite endurance athletes spend somewhere around 80 percent of their training time at this easy, sustainable intensity — which tells us something counterintuitive: what separates the pros from the rest of us often isn’t how hard they go, it’s how disciplined they are about going easy most of the time.

Here’s the bit I need to be straight with you about

A narrative review published in Sports Medicine in July 2025, out of Queen’s University and McMaster, went looking for the evidence behind that “elite athletes do it, so you should too” logic — and didn’t find it holds up quite as cleanly as the podcasts suggest.

Their point, and it’s a fair one: the case for Zone 2 as the optimal intensity for mitochondrial and fat-oxidative adaptation comes almost entirely from watching elite endurance athletes, not from testing the theory on people like you and me. For the general population, they argue, higher-intensity work may produce comparable — sometimes larger — mitochondrial and metabolic gains, especially at the training volumes most of us can realistically fit into a week.

Here’s the context that made this click for me. An elite athlete’s “80 percent easy” is 80 percent of a forty-hour training week. Their capacity for hard effort is essentially maxed out already, so directing the rest toward Zone 2 makes sense — there’s nowhere else for that time to usefully go.

You, on the other hand, are nowhere near that ceiling (and I say that with love; nor am I). If you’re doing two or three sessions of cardio a week, you have enormous untapped capacity for higher-intensity work, and telling you to spend all of it going slow may not be the leverage move it sounds like.

So — do I think you should abandon Zone 2? No. I think it’s still a legitimate, well-supported way to build an aerobic base, protect your heart, and train without wrecking your joints or your nervous system. What I’m no longer willing to tell you is that it’s the single best answer, or that your Peloton habit is metabolically wasted effort.

It isn’t. All different kinds of cardio can belong in your week. More on the ratios below.

The numbers, with an honest caveat

If you want your precise, personalised threshold, book a VO2 max test at LIFT Clinic — it’s the only way to know, rather than estimate, exactly where your fat-burning zone starts and stops.

If you’re working it out yourself: a 2025 consensus paper from fourteen sport scientists and coaches, published in the International Journal of Sports Physiology and Performance, settled on roughly 70–80 percent of your maximum heart rate as the practical target — the bottom a smidge higher than the 60–82 percent I’ve defined in the past.

Your watch will estimate this using the 220-minus-your-age formula, which is a genuinely rough guide — a separate 2025 study looking at individual variability found that none of the field methods (heart rate percentage, talk test, perceived effort) agree particularly well with each other, or with a lab-measured lactate threshold. None of that means the number is useless. It means treat it as a sensible estimate, not a diagnosis, and don’t panic if your “Zone 2” looks different from your same-aged friend.

Which is exactly why I still love the talk test as your most honest tool: if you’re on a phone call while exercising, the other person should be able to tell you’re moving, but you should be able to speak in full sentences without gasping. It’s imprecise in the same way the heart rate number is imprecise (sorry not sorry) — but it’s imprecise in a way you can feel, understand, and actually do.

On duration: the research still points to 45–60 minutes per bout, several times a week, as the dose that produces the mitochondrial adaptations people are chasing — San Millán’s own recommendation runs three to four sessions a week at that length.

I won’t pretend that’s not a big ask on time (as a working mother of three, time is my own most precious resource – and when I spend it, I need to know there’s ROI).  So if 45-minute blocks of cardio genuinely aren’t happening this week, a shorter, harder session is still doing far more for your mitochondria than nothing at all.

Why this matters more in midlife

Here’s what didn’t make it into the original version of this post, and it should have: your VO2 max — the single best marker we have of cardiovascular fitness — doesn’t just decline with age. It declines faster once oestrogen starts falling.

Postmenopausal women have measurably lower VO2 max than perimenopausal women of the same age, independent of body composition. That’s not a reason to panic. It’s a reason this particular kind of training earns a place in your week that it perhaps didn’t at thirty.

And the stakes are higher than “fitter” or “leaner.” Cardiorespiratory fitness is now one of the most consistently replicated predictors of how long you live, full stop — a 2024 umbrella review in the British Journal of Sports Medicine, drawing on 20.9 million observations across nearly 200 cohort studies, found it holds up better than almost any other measurable health marker.

One widely cited study of over 122,000 people found the least-fit group carried roughly five times the mortality risk of the fittest group, with no ceiling on the benefit of improving further. And, as a reminder I always feel obliged to include in discussions about overall fitness and mortality, a 2025 study of nearly 400,000 people found that fit people who were classified as overweight showed no significant increase in mortality risk compared with fit people at a “normal” weight.

Read that again. Fitness, not the number on the scale, is doing the heavy lifting for your health.

Two things I’d add specifically for you, that a generic cardio post won’t tell you:

1. Zone 2 alone is unlikely to be enough to counter the insulin resistance and visceral fat shifts that come with falling oestrogen — it needs to sit alongside strength training and some genuine high-intensity effort, not replace them.
2. If your Zone 2 modality of choice is swimming or cycling, it’s doing nothing for your bone density, and that’s a live concern at this stage of life. Make sure something weight-bearing — walking, hiking, the stair machine, running if your joints tolerate it — is also in the mix.

One final correction, while I’m being honest with you

I want to gently retire the phrase “burns the most fat,” which I used rather loosely in the previous draft of this post.

It’s true that at lower intensities, a higher proportion of the calories you burn come from fat rather than carbohydrate. It is not true that this means you’re burning more fat overall, or that it’s the superior choice for fat loss per se.

You will burn more total calories from harder efforts, even though the ratio shifts toward carbohydrate. What drives fat loss is total energy expenditure over time, not the fuel mix in any single session. But that said – harder efforts are also harder to recover from, and amid the persistent daily stressors common in midlife (teenagers, aging parents, career pressure, hormonal disruptions), that’s not a minor concern.

Zone 2’s real value to you isn’t as a fat-loss shortcut. It’s metabolic flexibility, mitochondrial health, and a stronger heart – and dare I say, those are even more critical.

So, coming back to my three types of clients

There is a best-practice answer here, and it’s still a blend of all three approaches.

You need some cardio, not none.

You need a meaningful chunk of it to be easy and sustainable (that’s the Zone 2 bit).

And you need some of it to feel heart-poundingly hard, not as an indulgence for the athletic types, but because the newest evidence says that’s where a real share of your fitness-related adaptation is actually happening — especially for those of us watching our VO2 max start to slide as the years go by.

Now take your marching orders, in whichever zone you can actually get to this week, with my (evidence-supported) blessing.