Zoned Out: So Which Damn Cardio Zone(s) Should You Aim For?

There are many reasons that Cranky Fitness stopped pretending to be a blog about fitness, and became Just Cranky instead. One of them is: health and fitness research keeps contradicting itself. Who can trust it anymore?

And yet, I can't help it: I still keep clicking on headlines and reading studies and listening to experts and watching little how-to videos. I'm frequently asking Chatty, my AI health research assistant, for her cheerfully confident yet often inaccurate advice.

But every time I'm convinced I finally know what the "best" answer is to a health question? Some new take from an equally credible source comes along and says, nope, that was all wrong! How to sort out these scientific cat fights? Who do I listen to? 

Is it best to just "zone out" and ignore it all and go with our guts? 

So I'm still struggling a bit with cardio due to hypoparathyroidism, a rare and completely bonkers endocrine disorder I managed to acquire after cancer surgery. So it feels super important to try to optimize the cardio I can get away with.

But it turns out Expert Advice has changed a bit since back in the Cranky Fitness days! So what's the surprising new twist?  

But First: Remember Cardio Zones?

This is the boring, background research part of health blogging that I hate doing. So since (a) I'm not a health blogger anymore and (b) no one is reading this, let alone paying me anything, I'll do the Bezos and Musk thing and just fire the human (me) and let Chatty have a go at it.

If you already know about zones, just skip on right through! 

Cardio (heart-rate) zones = intensity bands based on % of your max heart rate or metabolic thresholds.

Most models use 5 zones, from very easy to maximal effort.

Typical 5-zone model 

• Zone 1 →  ~50–60% max HR — very easy / recovery

• Zone 2 →  ~60–70%  — aerobic endurance base

• Zone 3 → ~70–80% — moderate-hard “tempo”

• Zone 4 → ~80–90% —  threshold / hard

• Zone 5 → ~90–100% — near-max / VO₂max

These zones roughly track metabolic changes. For example, crossing from Zone 2 to Zone 3 corresponds to passing a lactate threshold where acid production rises faster than the body clears it. Um, and what does that mean, exactly? No clue. It just sounds impressively sciencey. 

Note: if you want to double check her sources, Chatty cites Cleveland Clinic and Mass General Brigham.

So What's New in Zone Training Advice?

So back when I was fitness blogging, "Cardio" meant Zone 3 and Zone 4.  And that's what we mostly did. Zone 2 was for warm up and cool down. 

And the new cool kid on the block was Zone 5, which is where you end up when doing HIIT, or High Intensity Interval Training.

I wrote quite a few posts back in the day about the benefits of High Intensity Interval Training, because it's awesome for you in countless ways. But because the protocols were so brutal, I invented my own alternative: Somewhat High Intensity Interval Training, or SHIIT for short. 

And guess what? Cranky Fitness was ahead of the curve. Turns out that turning up the intensity to nearly barf-inducing levels is still encouraged.

Enter... Polarization!

No, this does not refer to training with cool sunglasses. 

 

It means: focusing on low and high intensity zones and blowing off most of the stuff in the middle.

So why are some experts recommending that we ditch most of our old mainstay Zone 3 and 4, and instead, spend the majority of our time in boring lower intensity Zone 2 training, with a sprinkling of agonizing Zone 5?

Because research studies and elite training programs started seeing better results that way. And it turns out there are unique longevity and metabolic benefits that humble Zone 2 and horrific Zone 5 provide that you don't get from the middle zones.

Cardio Zone 2 Benefits 

So what's so great about lower intensity cardio? 

Hit it, Chatty!

Mitochondrial function & density

• Zone 2 stimulates mitochondrial efficiency, oxidative capacity, and fat metabolism.

Insulin sensitivity & glucose control

• Improves insulin sensitivity and glucose handling, helping prevent metabolic disease.

Capillary density and oxygen use

• Improves oxygen delivery and cellular oxygen utilization via mitochondrial and capillary adaptations.

Fat oxidation & metabolic flexibility

• Encourages fat as fuel → important for long-duration energy and metabolic health.

Why this matters for health:

• Mitochondria = central to aging, energy, and chronic disease risk

• Insulin sensitivity = central to cardiometabolic risk

• Aerobic base = allows sustainable lifetime activity

Cardio Zone 5 (HIIT) Benefits

So in case you were hoping that the benefits of God-awful HIIT training turned out to be over-hyped and were minimal enough to ignore, um... sorry. Can we have a recap of the benefits, Chatty?

• Higher VO₂max is strongly associated with lower cardiovascular disease risk and lower all-cause mortality. 
• HIIT improves insulin sensitivity and helps muscle cells take up and use glucose more efficiently. This supports metabolic flexibility and reduces risk for metabolic syndrome and type 2 diabetes.
• HIIT is associated with reductions in total body fat and body fat percentage.
• Some studies also show reductions in visceral fat, the fat stored around organs that is strongly linked to cardiometabolic disease risk.
• High-intensity exercise can increase brain-derived neurotrophic factor (BDNF), which supports neuron growth, brain plasticity, and cognitive function.
• HIIT can also increase vascular growth factors that support blood vessel development in the brain. These effects are often strongest immediately after intense exercise sessions.
• HIIT improves endothelial function, which is how well blood vessels dilate and regulate blood flow. This supports cardiovascular health and may help reduce long-term vascular disease risk.
• HIIT activates signaling pathways which supports mitochondrial biogenesis and improves muscle oxidative capacity, even with relatively low total training time.

Why this all matters for health:

• VO₂max is one of the strongest measurable predictors of long-term health and survival. High-intensity exercise also provides signals that are difficult to replicate with lower-intensity training alone, including strong metabolic, vascular, and neurological adaptation signals. 

Also from personal experience: Zone 5 makes you feel like a badass. It's painful but quick, and incredibly rewarding. 

So what are you missing out on if you concentrate on Zone 2 and 5 and start blowing off Zone 3 and Zone 4? 

According to Polarization People: maybe not as much as you thought!

Chatty? 

Mid-range training is especially good at improving lactate threshold — the ability to sustain a relatively hard pace for longer periods — which matters for events like distance running, cycling, or hiking at a strong pace. It can also improve exercise efficiency at moderate-hard intensities, helping activities at the “comfortably hard” level feel easier. Some people also find Zone 3 training psychologically and practically useful because it’s challenging but sustainable and easier to structure than very hard intervals. For people training for performance, racing, or time-limited fitness gains, this zone can be very valuable. For general health and longevity, however, many of the core metabolic and cardiovascular benefits can be achieved through a combination of lower-intensity aerobic work and smaller amounts of high-intensity work, which is why some modern training approaches minimize — but don’t necessarily eliminate — time spent in the middle intensity range.

On the Other Hand...

Polarization is still not mainstream.

In fact, one of the biggest advocates for this sort of training style is the recently disgraced Peter Attia. Although I don't know that his disgustingly close relationship with a wealthy pedophile necessarily nullifies benefits people have discovered from his training advice. After all, wealthy pedophiles pay good money for expert advice to maximize longevity. (Although it didn't seem to work out too well for Mr. Epstein). 

And it's not just pedophiles who think emphasizing Zone 2 and Zone 5 is smart. Zone 5 has been trendy for quite a while now, and Zone 2 is gaining traction in the exercise world, especially among endurance coaches. And for some reason, every podcaster and longevity expert I've encountered lately seems to be advocating for this, or at least in my weird little fitness-nerd corner of the world.

Yet mainstream advice still emphasizes doing pretty much everything, with the hope that you will at least do something. It's mostly the more cutting edge longevity folks who are telling us to pile up the Zone 2, season with Zone 5, and not worry so much about 3 and 4.

Biggest Drawbacks of Zone 2: How the Hell to Know When You're in it?

Experts disagree on exactly where Zone 2 starts and stops, and it's really hard for some of us to tell when we're there: Especially when our heart rates and the "talk test" and the perceived rate of exertion all point us in different directions.  It could be the subject of a whole other extremely Cranky post! Here's a starter article from Cleveland Clinic with cardio zone advice, but if you are considering Zone 2 training it's probably worth doing a lot more research, so you can get a definitive answer personalized for your situation, and then later find out it was totally wrong.

For example, I spent a year or two trying to build up Zone 2 mileage only to discover that my Garmin watch most likely had my zones wrong, and that I was probably in Zone 1 for much of that time.  No wonder it felt like I wasn't making any progress. Much crankiness ensued, and now I'm experimenting with different heart rate targets.

Second Biggest Drawback: You Have to Go Longer and it Gets Really Freaking Boring

As I recall, the recommended amount is about 3 hours a week of Zone 2 as a minimum. Which is a lot, especially if you also want to do HIIT, strength training, stretching, meditation etc, etc, etc. Zone 2 is endurance training, and it's interesting to some of us to discover that our brain's endurance craps out way more quickly than our body's endurance! At least I have my secret weapon, my Holofit Virtual Reality world to escape to. (Note: I get no money recommending it, I just think it's cool).

Biggest Drawback of Zone 5, High Intensity

It sucks. 

So What Kind of Cardio Should You Be Doing? 

Short answer: whatever the hell kind seems to work for you. You can find research to justify pretty much any kind of cardio.

Motivation and consistency are SO much more important than picking the absolute best protocol.

For me, a polarized plan is pretty ideal. I love the feeling of accomplishment I get doing HIIT, even if only once a week. It's so damn good for you.  And I try to also work in at least 3 days of Zone 2, because it's slightly easier to recover from with my fucked up spazzy calf muscles than Zones 3 and 4 are, and I like the idea of grabbing some of those metabolic and longevity benefits unique to Zone 2. But we're all special little snowflakes with our own beautifully complex constellations of goals, vulnerabilities and preferences. 

How about y'all, what are you  doing for exercise? Is cardio even still a thing?