The Study That's Changing the Conversation

For decades, the endurance world has operated on a comforting assumption: push hard enough, long enough, and your cardiovascular system becomes virtually bulletproof. High VO2 max, low resting heart rate, lean body composition — these markers have long been treated as ironclad protection against heart disease.

The new international research, highlighted by Radboud University and covered by NOS — the Netherlands' largest public news outlet — challenges that assumption directly. The findings indicate that long-term, high-intensity exercise spanning a decade or more can actually increase the risk of specific heart conditions in athletes aged 35 and older.

The two conditions identified are particularly noteworthy:

• Cardiac arrhythmias — irregular heartbeat patterns that can range from mildly disruptive to seriously dangerous
• Coronary artery calcification — a buildup of calcium deposits in the artery walls that can restrict blood flow and elevate the risk of heart attack

These aren't conditions typically associated with active, healthy people. That's precisely what makes this research so significant. It suggests that for a specific subset of athletes — older, highly trained, long-tenured — the cardiovascular system may experience wear and structural change that standard fitness metrics simply don't capture.

Defining "High-Intensity": Where the Line Is Drawn

Before alarm bells start ringing, it's important to understand exactly what kind of training this research is describing. Not all exercise carries the same risk profile — and that distinction matters enormously.

Researcher Thijs Eijsvogels was direct about the threshold in his interview with NOS:

"We are talking about exercise where your heart rate and breathing rise to the point that you can no longer hold a normal conversation."

This is the classic talk-test definition of high-intensity effort — the zone where you're working hard enough that stringing together a full sentence becomes genuinely difficult. For triathletes, this maps roughly to threshold and above-threshold efforts: hard bike intervals, race-pace running, sustained high-output swimming.

What doesn't qualify

• A comfortable morning jog where you're chatting with a training partner
• A moderate-paced cycling session at conversational effort
• Recovery swims and easy aerobic base work

What does qualify

• Sustained tempo and threshold sessions
• High-intensity interval training (HIIT)
• Race-effort training across all three disciplines
• Long-course racing at competitive intensity

Equally important is the timeline factor. This study isn't describing what happens after a hard training block or a competitive season. The elevated risks are associated with athletes who have maintained this level of intensity and volume for five to ten years or more. A few tough months before an A-race doesn't create the same cumulative stress on the heart as a decade of relentless high-output training.

The U-Shaped Curve: Finding Your Training Sweet Spot

Here's the genuinely good news buried inside this research: exercise is still the most powerful tool available for reducing the risk of premature death and chronic illness. The study's goal is not to push athletes toward the couch. It's to help them find the zone where training delivers maximum benefit without tipping into diminishing returns.

The data points toward what researchers describe as a U-shaped benefit curve — a framework every triathlete should understand:

Think of it like almost any good thing in life: the first dose delivers the most benefit. Each additional dose delivers a little less. And at some point — particularly when that dose is very large and very sustained — the curve bends the other way.

For most recreational triathletes training for sprint and Olympic distance events, this curve is largely good news. You're likely operating within a range that delivers strong health benefits. For athletes pursuing Ironman-distance racing, multi-race seasons, or ultra-endurance events with years of high-volume training behind them, the picture becomes more nuanced.

It's worth noting that scientists have not yet identified the exact tipping point where sport becomes "too much." Individual variation plays a significant role — genetics, recovery habits, stress levels, sleep, and nutrition all influence how the body responds to training load. The U-shaped curve is a population-level framework, not a personal verdict.

Red Flags and Monitoring Strategies for Veteran Athletes

For triathletes and endurance athletes over 35, this research delivers a clear message: regular monitoring is no longer optional — it's essential.

The most reassuring aspect of what this research team recommends is that early detection is both possible and effective. The key is knowing what to look for and building a proactive approach to health screening into your training life.

Warning Signs to Take Seriously

• A sudden, unexplained drop in performance — If your pace, power output, or perceived effort at a given heart rate shifts significantly without an obvious training explanation, that's worth investigating.
• Shortness of breath during previously manageable efforts — If an effort level that used to feel routine now leaves you gasping, don't chalk it up to a bad day. Pay attention.

These signals won't always indicate a serious problem — fatigue, illness, overtraining, and life stress can all produce similar symptoms. But they represent your body's way of flagging that something has changed, and they deserve a clinical response rather than a training override.

The Medical Monitoring Schedule

The research team's recommendation for athletes in this age bracket is straightforward: a comprehensive medical check-up every five years to monitor key cardiovascular risk markers. Those markers include:

• Cholesterol levels (both LDL and HDL)
• Blood sugar (including HbA1c for longer-term glucose tracking)
• Blood pressure (resting and, ideally, under exercise conditions)

Here's the part that catches many veteran athletes off guard: being able to finish a Long Distance triathlon does not make you immune to these risk factors. As the research makes clear, the internal markers — the ones you can't feel or see in your race results — remain the most accurate predictors of heart health.

An athlete can be lean, fast, and highly trained while simultaneously carrying elevated cholesterol or early signs of arterial calcification. The performance data on your GPS watch won't reveal that. A blood panel will.

Practical Implementation: Training Smart After 35

Understanding the research is one thing. Applying it to your actual training life is another. The goal isn't to dismantle what you've built — it's to make smarter decisions about how you build going forward.

Assess Your Current Position on the Curve

Start by honestly evaluating your training volume and intensity distribution:

• How many hours per week are you training on average, across the full year?
• What percentage of that training sits in the high-intensity zone (the "can't hold a conversation" range)?
• How many consecutive years have you been training at this level?

If you're consistently above five hours per week with a significant chunk at high intensity, and you've been doing this for a decade or more, this research is speaking directly to you.

Strategies for Finding Your Personal Sweet Spot

• Polarize your training — The most evidence-supported model for long-term endurance training involves keeping the bulk of your work (roughly 80%) at genuinely easy, aerobic intensities, reserving high-intensity efforts for specific, purposeful sessions. This approach builds fitness without the same cumulative cardiac stress as year-round threshold training. Consider using AI-powered training apps to help structure your polarized approach.
• Periodize your intensity — Build hard training phases into your calendar, but balance them with genuine recovery periods. Your heart needs cycles of stress and adaptation, not perpetual high output.
• Reconsider your race calendar — More A-races doesn't always mean better outcomes. A focused, intentional schedule that allows for full recovery between peaks may serve your long-term health better than back-to-back race seasons.
• Prioritize sleep and recovery — Emerging research consistently identifies sleep as one of the most powerful recovery tools available. Chronic sleep deprivation amplifies the stress response to training. Support your recovery with proper magnesium supplementation to improve sleep quality and muscle recovery.

Work With Providers Who Understand Athletes

Not every general practitioner is equipped to interpret the health profile of a veteran endurance athlete. Seek out sports medicine physicians or sports cardiologists who understand the nuances of training-adapted physiology. They'll be better positioned to interpret your results in context and guide decisions about screening frequency, test selection, and training modifications.

If you're over 35 and haven't had a baseline cardiovascular assessment, scheduling one is the single most impactful step you can take today. Track your training metrics with reliable tools like the Garmin Forerunner 55 to share accurate data with your healthcare provider.

The Bottom Line: Train for the Long Game

Nothing in this research argues against being an endurance athlete. The science remains clear that regular, sustained physical activity is among the most powerful health interventions available to human beings. The study from Radboud University doesn't undermine that — it refines it.

What it tells us is this: at the extreme end of the training spectrum, sustained over many years, the relationship between more training and better health becomes less linear than we assumed. For athletes over 35 who have spent a decade or more pushing hard, that nuance matters.

The takeaways are practical and manageable:

• Exercise remains your most powerful health tool — don't let this research push you toward inactivity
• Understand where you sit on the U-shaped curve — 3 to 5 hours per week of varied intensity training represents a genuinely powerful sweet spot
• Monitor the warning signs — sudden performance drops and unusual breathlessness deserve clinical attention, not dismissal
• Build regular health screening into your routine — every five years minimum after 35, covering cholesterol, blood sugar, and blood pressure
• Work with healthcare providers who understand your sport — context matters when interpreting athlete health data

The athletes who will thrive longest in this sport are not necessarily those who train the most. They're the ones who train intelligently, listen carefully to what their bodies are telling them, and treat long-term health as a performance variable — not an afterthought.

For those looking to optimize their training approach, explore game-changing triathlon drills that build fitness efficiently without excessive volume. And if you're considering your first or next race, check out our guide to beginner-friendly Ironman races to find events that match your current fitness level.

Join the Conversation

Have you adjusted your training approach as you've gotten older? Are you working with a sports medicine professional to monitor your heart health? Share your experience in the comments — and if this raised questions about your own training, it's worth a conversation with your doctor.

Sources: Radboud University research findings as reported by NOS and Triathlon Today (April 2026). Readers are encouraged to consult the original study and speak with qualified sports medicine professionals about their individual health profiles.