Introduction
For most of my medical career, I did what millions of Americans do every morning: I lined up my bottles and swallowed a handful of pills. Multivitamin. Vitamin D. Fish oil. CoQ10. A probiotic. Sometimes more, depending on what I'd read that month.
I practiced what I preached. Long bike rides on weekends. Hours in the saddle chasing some notion of cardiovascular excellence. I mentioned in an earlier article that I once did a 100-mile ride in Savannah. Brutal doesn't begin to describe it. But I thought I was investing in longevity.
Looking back, I had the equation half right and half wrong. The half-right part: cardiovascular fitness genuinely predicts how long you'll live. The data on that is overwhelming. The half-wrong part: how I was building that fitness was inefficient at best, potentially counterproductive at worst.
In 2025, a major controversy erupted in exercise science that forced me to reconsider everything I thought I knew about aerobic training. Researchers challenged the very foundation of what's been called "Zone 2" training: that sweet spot of moderate effort everyone from elite coaches to longevity podcasters had been promoting as the secret to mitochondrial health and extended lifespan.
I went deep into the research. Here's what I learned about what actually extends life, what might be wasting your time, and what I'm doing differently now at 75.
The One Number That Predicts Everything
If I could measure only one thing to predict how long you'll live, it wouldn't be your cholesterol. It wouldn't be your blood pressure or your fasting glucose. It would be your VO2 max: the maximum amount of oxygen your body can use during intense exercise.
A 2018 study from the Cleveland Clinic followed 122,007 patients over eight years. The researchers divided people into fitness categories based on their VO2 max and tracked who died. The results were striking. People with low cardiovascular fitness had five times the mortality risk of those with elite fitness. That's a larger risk differential than smoking, diabetes, or coronary artery disease.
Here's what struck me most: moving from low fitness to merely below-average fitness cut mortality risk roughly in half. You don't have to become an elite athlete. You just have to get out of the basement.
The study found no upper limit of benefit. Even the most fit individuals (top 2%) showed lower mortality than those who were merely "high" fitness. More cardiovascular capacity, within any reasonable range, appears to equal longer life.
Other large studies confirm this pattern. A 2022 analysis of 750,000 US veterans found that each 1-MET increase in fitness (roughly 3.5 ml/kg/min of VO2 max) correlated with a 13 to 15 percent reduction in mortality. The relationship held across age groups, sexes, and health conditions.
So the question isn't whether cardiovascular fitness matters. It clearly does, more than almost any other modifiable factor. The question is how to build it efficiently. And that's where the controversy begins.
The Zone 2 Reckoning
For the past few years, "Zone 2" training has been the darling of the longevity world. The idea is appealing: exercise at a moderate intensity (roughly 60-70% of your maximum heart rate) where you can still hold a conversation. At this effort level, your mitochondria supposedly get the optimal stimulus for biogenesis: the creation of new cellular power plants.
Coaches like Dr. Iñigo San Millán, who trains Tour de France champion Tadej Pogačar, popularized this approach. Peter Attia discussed it extensively on his podcast. The message filtered down to recreational exercisers: spend 3 to 4 hours per week in Zone 2 and your mitochondria will flourish.
Then came June 2025. A narrative review in Sports Medicine by researchers Storoschuk, Gibala, and Gurd analyzed 167 sources and concluded that current evidence does not support Zone 2 as the optimal intensity for improving mitochondrial function. In fact, intensities below roughly 60% of maximum aerobic power appear to fail at inducing significant mitochondrial gains in untrained individuals.
The core problem, according to this analysis: Zone 2 doesn't generate enough metabolic disturbance. The cellular signaling molecules that trigger mitochondrial adaptation (things like AMP and ADP) don't rise enough to flip the switches. As Brendon Gurd, one of the paper's authors, put it: "Everything I understood was that Zone 2 is probably the worst intensity, not the best intensity for building mitochondria."
That's a strong statement. Is it right?
The honest answer: partially. The review has real limitations. It relied heavily on acute, single-session studies in sedentary populations. It didn't examine all the pathways through which Zone 2 might work, including oxidative stress signaling. And there's a disclosure worth noting: one co-author holds equity in a company whose services relate to exercise.
But the critique isn't wrong to point out that the Zone 2 enthusiasm outran the evidence. Much of the support for Zone 2 came from observing what elite endurance athletes do. They spend enormous volumes at low intensity: 15 to 25 hours per week. For them, Zone 2 serves a specific purpose. It allows recovery between hard sessions while still accumulating aerobic stimulus. It builds the aerobic foundation that supports their high-intensity work.
The problem is extrapolating that to someone exercising 3 to 5 hours per week. At that volume, the adaptation dynamics are completely different. You're not recovering from crushing interval sessions. You're trying to get maximum return on limited time.
This is where I went wrong for years. I was grinding away at moderate intensities, thinking I was optimizing my mitochondria. What I was actually doing was spending a lot of time in the least efficient zone for someone with my schedule.
What Actually Works: The Polarized Approach
Here's the synthesis I've arrived at after reviewing the evidence. It's called polarized training, and it flips the conventional wisdom on its head.
The idea: instead of spending most of your time in the moderate middle zone, you split your training between very easy and very hard. Roughly 80% of your exercise at low intensity (Zone 1, easy enough to chat freely) and 20% at high intensity (Zone 4-5, where conversation becomes impossible). Almost nothing in the middle.
This pattern emerged from observing what actually produces results in elite endurance athletes. But unlike Zone 2 advocacy, it's been tested directly in recreational exercisers. A University of Salzburg study found that athletes adhering closest to the 80/20 distribution achieved the greatest improvements in VO2 max and time to exhaustion. A study co-authored by Stephen Seiler, who developed much of this framework, showed recreational 10K runners improved 5.0% with polarized training versus 3.6% with threshold training.
The high-intensity component appears to be the key driver. A protocol called the Norwegian 4×4 has the strongest evidence: four intervals of four minutes at 90-95% of maximum heart rate, with three minutes of active recovery between. Helgerud and colleagues demonstrated a 7.2% VO2 max improvement in just eight weeks, training only three times weekly. Some studies show gains of 10-13%.
Here's the critical insight for people like me who don't have unlimited time: approximately 40% of people doing only moderate-intensity continuous exercise for six months show no measurable VO2 max improvement. They're "non-responders." High-intensity interval training (HIIT) appears to eliminate this problem. Almost everyone responds to intensity.
What about the low-intensity portion? Walking. Lots of walking. This might sound anticlimactic, but there's an evolutionary logic to it. Our hunter-gatherer ancestors moved constantly at low intensity (foraging, traveling) with occasional bursts of maximum effort (hunting, escaping). They didn't jog for an hour at moderate pace. That combination: high-volume very-easy movement plus occasional all-out effort, matches our physiology better than chronic moderate grinding.
The Cardiac Question: What's Real, What's Overblown
I need to address something you may have heard: that too much cardio damages the heart. This claim has circulated for years, sometimes attached to scary images of marathon runners dropping dead.
Dr. James O'Keefe, a preventive cardiologist, proposed what's called the "reverse J-curve": the idea that extreme exercise increases mortality risk after a certain threshold. He pointed to troponin elevations after marathons (a marker of heart stress), right ventricular dysfunction in endurance athletes, and structural changes like myocardial fibrosis.
The mechanisms he describes are real. Hearts do remodel under sustained endurance load. But here's what the large-scale mortality data actually shows: no upper limit of harm for cardiovascular fitness. The Cleveland Clinic study I mentioned found that even elite-level fitness was associated with lower mortality than high fitness. Elite endurance athletes, as a population, live longer than average, not shorter.
The one risk that does appear genuine: atrial fibrillation. A meta-analysis of six case-control studies found a five-fold increased AFib risk in endurance athletes. The threshold seems to be around 1,500 to 2,000 cumulative lifetime hours of intense endurance training. That's a lot of hours: roughly 10 years of training 3-4 hours per week at high intensity.
Some data also shows higher coronary artery calcium scores in marathon runners. But critically, these plaques are predominantly calcified (stable) rather than the soft, rupture-prone plaques that cause heart attacks. The Cooper Center Longitudinal Study followed over 21,000 men for a decade and found that highly active individuals with elevated calcium scores showed no increased mortality. Fitness appears to mitigate the risk.
My read: the cardiac toxicity hypothesis is real at extreme volumes but has been overstated as a general concern. If you're exercising 3 to 6 hours per week, the primary risk is doing too little, not too much. The AFib concern matters mainly to people accumulating thousands of hours of intense endurance training over decades: competitive marathoners, ultra-endurance athletes, serious amateur triathletes. For most of us training for health rather than competition, it's not the relevant constraint.
What I'm Doing Now
At 75, I've restructured my approach based on this evidence. I'm not claiming I've perfected it. I'm experimenting, tracking, adjusting.
High-intensity intervals once or twice per week. I do a modified version of the Norwegian 4×4 on my rowing machine. Four minutes hard (heart rate around 85-90% of max), three minutes easy. Four rounds. Total time including warm-up and cool-down is about 35-40 minutes. My Garmin tells me when I'm in the right zone. I don't do more than two of these sessions per week because recovery matters.
Walking. Lots of walking. I aim to exceed 10,000 steps daily. Nothing fancy. Just moving at a pace where I could easily hold a conversation. Sometimes I listen to podcasts. Sometimes I just think. This is my Zone 1 volume: the easy stuff that doesn't stress the system but keeps me active.
Strength training three times per week. This isn't cardio, but it matters for longevity. I've written about movement complexity elsewhere. The point here is that I'm not spending all my exercise time on aerobic work anymore.
Heart rate monitoring during intervals. For the high-intensity work, I use a chest strap monitor. Wrist-based monitors aren't accurate enough at high heart rates. I want to know I'm actually hitting 85-90% of max during the hard efforts, not just guessing. For someone my age, that means getting my heart rate into the 130-140 range during the intervals. Your numbers will be different depending on your age and fitness.
A rough calculation for Zone 2 heart rate if you want to target it: take 180 minus your age as an upper limit. For me at 75, that's 105 bpm. The Karvonen method is more precise: subtract your resting heart rate from your max, take 60-70% of that, then add your resting heart rate back. Either way, true Zone 2 should feel genuinely easy. If you can't speak in complete sentences, you've drifted higher.
The Bottom Line
Cardiovascular fitness is the single strongest predictor of longevity we can actually modify. The data is overwhelming. Getting from low fitness to average fitness cuts mortality risk roughly in half.
How you build that fitness matters more than I understood for most of my life. The 2025 Zone 2 controversy is legitimate: for time-limited exercisers (most of us), grinding away at moderate intensity may be the least efficient path. The polarized approach, splitting effort between very easy and genuinely hard, appears to work better.
The cardiac risk concerns are real at extreme volumes but overblown as general guidance. Atrial fibrillation risk rises with thousands of cumulative hours of intense endurance training. For recreational exercisers, the danger is almost always too little activity, not too much.
I'm not claiming certainty about any of this. Exercise science is messy. Individual responses vary enormously. What works for a 75-year-old former physician may not work for you.
But here's what I'm confident about: the version of cardio I did for decades, those endless moderate-effort miles, wasn't optimal. It wasn't harmful. It was better than nothing. But it wasn't the best use of my time.