Zone 2 Cardio Isn't Enough on Its Own: What the 2025 Research Review Really Found

Performance Lab • Fitness and Longevity • 7 min read

By Jason T., Founder, Black Ridge Performance

Zone 2 cardio has become the default advice for anyone chasing a longer, healthier life. It is not wrong. But a 2025 research review suggests it is only part of the story, especially for men over 40 who do not have unlimited hours to train. Here is what the science actually says, and what it means for your weekly routine.

What Zone 2 Actually Does

Zone 2 means working at a pace where you could still hold a conversation, roughly 60 to 70 percent of your max heart rate. It feels almost too easy, which is exactly why some men skip it in favor of harder efforts. That is a mistake, because consistent Zone 2 work increases mitochondrial content in your muscle cells.

Mitochondria are the small structures inside cells that turn fuel into usable energy. More of them, working well, means your muscles get better at burning fat for fuel instead of relying only on sugar. This matters more as you age, since mitochondrial density and function naturally decline over time. That decline is linked to lower exercise capacity and higher risk for metabolic disease.

So the appeal of Zone 2 is grounded in real physiology, not just a trend. The problem is what happened next: it got promoted so heavily that some men started treating it as the only training that matters.

The 2025 Review That Changed the Conversation

A 2025 narrative review in the International Journal of Sports Physiology and Performance took a closer look at who actually benefits most from Zone 2 as a primary strategy. The finding was straightforward. For people training fewer than six hours a week, which covers most men juggling jobs, kids, and everything else, higher intensity sessions like intervals or hill repeats produced comparable or better improvements in VO2max, in less total time.

VO2max is a measure of how efficiently your body uses oxygen during exercise, and it is one of the stronger predictors of long term health researchers track. If your training time is limited, the review suggests spending all of it on slow, steady cardio is not the most efficient path to improving that number.

This does not mean Zone 2 is useless. It means it works best as one piece of a bigger structure rather than the whole plan. Researchers converged on a blended weekly approach: two to three Zone 2 sessions of 30 to 60 minutes, one to two higher intensity sessions, and two to three resistance training sessions for muscle mass and bone density. Separate research in adults over 40 backs this up, finding that combining aerobic and resistance training produced larger gains in functional capacity and cardiometabolic markers than either one alone. The long slow session still counts. It just should not be the only thing on your calendar.

Why Time Efficiency Matters After 40

At 25, spending an extra hour on a treadmill was annoying. At 45, it might not exist. Between work, kids, and the general chaos of adult life, training windows shrink. This is exactly why the 2025 review matters. It found that for anyone training less than six hours a week, which covers most men with a job and a family, higher intensity intervals produced VO2max gains equal to or better than pure Zone 2 work, in a fraction of the time.

This does not mean Zone 2 is useless. It builds real mitochondrial density and improves fat burning capacity, both of which matter more as you age. Mitochondrial function tends to decline over the decades, and that decline is linked to lower exercise capacity and higher metabolic disease risk. Zone 2 training is a legitimate tool against that trend.

The problem is treating it as the only tool. If your weekly training time is capped at three or four hours, spending all of it on easy paced cardio leaves real fitness gains on the table. Efficiency is not about cutting corners. It is about matching the training method to the time you actually have.

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Building a Blended Weekly Plan

The researchers did not land on an either-or answer. They landed on a blend. A practical week looks like two to three Zone 2 sessions of 30 to 60 minutes, one to two higher intensity sessions such as intervals or hill repeats, and two to three resistance training sessions. That is not a radical schedule. It is closer to what a well-rounded training week has always looked like, just with better justification for each piece.

The resistance training piece is not optional filler. Separate research on adults over 40 found that combining aerobic and resistance training produced larger improvements in functional capacity and cardiometabolic markers than either type of training alone. Muscle mass and bone density both need direct mechanical loading to maintain themselves. Cardio alone will not do that job, no matter how many zones you train in.

None of this requires a complicated system. A basic week might include two easy paced rides or runs, one session of hard intervals or hill repeats, and two or three lifting sessions built around compound movements. The specific split can flex around your schedule. What matters is that no single method is asked to do all the work. Zone 2 supports your aerobic base and metabolic health. Intensity supports VO2max in less time. Resistance training supports the muscle and bone density that keep you functional. Skipping any one of them just shifts the burden onto the other two, and eventually that catches up with you.

What This Means for Your Heart and Metabolism

Zone 2 training earns its reputation honestly. Riding a bike or walking uphill at a pace where you can still talk gives your muscle cells a reason to build more mitochondria. Think of mitochondria as the small engines inside your cells that turn fuel into energy. More of them, working better, means your body gets more efficient at burning fat and less reliant on quick sugar hits. That matters more after 40, since mitochondrial function tends to decline with age and that decline is linked to lower exercise capacity and higher risk of metabolic disease.

The 2025 review does not throw this out. It just adds a correction most longevity content skipped. If you only have a handful of hours to train each week, higher intensity sessions like intervals or hill repeats can match or beat Zone 2 for improving VO2max, which is a solid marker of heart and lung fitness. Sitting in fat-burning mode for an hour feels productive, but it is not automatically the best use of limited time.

The heart and metabolism benefit from both approaches, just in different ways. Zone 2 builds the aerobic base and improves fuel efficiency at low effort. Higher intensity work pushes your heart's pumping capacity and improves how well your body uses oxygen under stress. Neither one replaces the other. They stack.

Getting Started Without Overhauling Your Routine

You do not need to rebuild your week from scratch. The research points to a simple blend: two to three Zone 2 sessions of 30 to 60 minutes, one or two higher intensity sessions, and two or three resistance training sessions. That is a full week, but it is not an extreme one, and most of these sessions can be shorter than an hour.

If you already walk, bike, or jog at an easy pace, keep doing that two or three times a week. Add one session where you push harder for short bursts, like six to eight rounds of one minute hard followed by two minutes easy. Hill repeats work the same way if you prefer outdoor terrain over intervals on a track. This does not need to be a separate workout bolted onto an already packed schedule. It can replace one of your existing cardio sessions.

Resistance training rounds out the picture, and the research is clear that skipping it leaves results on the table. Combining aerobic work with resistance training produced better improvements in functional capacity and cardiometabolic markers than either one alone in adults over 40. Two sessions a week hitting the major muscle groups is enough to start seeing that benefit. You do not need a complicated program, just consistent effort on compound movements like squats, presses, and rows.

None of this requires perfect execution. Missing a session here and there will not undo the pattern. The goal is a weekly rhythm that includes some easy aerobic work, some harder effort, and some resistance training, repeated over months. That combination is what the research actually supports, not a single magic zone on a heart rate monitor.

This article is for general education and is not medical advice. Talk with your doctor before making changes to your health, wellness, or supplement routine.