Can We Exercise Too Much?

We all know that exercise is critical for long term health and indeed longevity too. There are countless scientific studies across the scientific literature showing the huge benefits of exercise across a myriad of health areas, including type 2 diabetes, cardiovascular disease, obesity. neurodegenerative health and even  certain cancers. In fact, if we could put the benefits of exercise into a pill, it would be the most powerful drug ever known to man!

However, can we overdo it? Could we do too much exercise for our health? Could we push our bodies just a little too hard?

Atrial Fibrillation in Athletes

It is true that moderate physical exercise has been associated with a number of cardiovascular adaptations, including improvements in heart function and compliance, as well as enlargement of the heart's chambers. However, it is also true that excessive endurance exercise has been linked to an increased risk of atrial fibrillation (AF), which is one of the most well-documented cardiac maladaptations associated with prolonged endurance exercise.

Research has shown that endurance athletes have a higher incidence of AF than sedentary individuals, and that longer durations of endurance exercise are associated with an even higher risk of AF, even in individuals without traditional cardiovascular risk factors. While individual studies have reported varying levels of increased risk, a recent meta-analysis (which is a combination of high quality studies) found overall that the risk of AF is 500% higher in those involved in endurance sports.

It is important to note that the relationship between exercise and AF is highly complex and not fully understood, with more research needed to determine the optimal exercise dose for reducing the risk of AF while still maintaining the many health benefits associated with regular physical activity. It is in reality a balance, but the ‘dose’ of required exercise for optimum health is not really understood currently. In the meantime, individuals who engage in regular endurance exercise should be aware of this potential risk and discuss any concerns with their healthcare provider

Cardiac Fibrosis (Scarring)

Another condition associated with high end endurance exercise if ‘myocardial fibrosis’ which is a condition where the heart muscles develop scarring due to constant stress and micro-heart injuries. This leads to stiffening of the heart muscles, which can cause heart rhythm problems and other negative outcomes.

Doctors can see this fibrosis using cardiac magnetic resonance imaging (MRI), specifically with a technique called late gadolinium enhancement (LGE). Studies have shown that people who participate in chronic endurance sports or races are more likely to show scarring on their cardiac MRI scans, even if they are otherwise healthy. The more years spent training and competing in these events, the more likely scarring will occur.

During intense exercise, the heart experiences a huge strain, which along with the elevated levels of certain biochemicals, can lead to damage in the heart muscles and eventually fibrosis. While it's unclear how this fibrosis affects people’s health directly, studies have found that people with such scarring are more likely to experience problematic heart events.

Increased Atherosclerosis in Endurance Athletes

To add fuel to the fire, a recent study has strongly suggested that long-term endurance exercise could potentially lead to a paradoxical increase in coronary atherosclerosis, which is the build-up of plaque in the arteries that supply the heart with blood. The study, called the Master@Heart study, compared lifelong endurance athletes with fit and healthy individuals who had a similarly low risk of cardiovascular disease.

The study across 558 participants, compared the exercise habits of three groups of men: lifelong endurance athletes, late-onset athletes, and nonathletes who did little exercise. The endurance athletes mainly engaged in cycling and running and reported an average of 10-11 hours of exercise per week, while the nonathletes only exercised for 1 hour per week. Only 22% of the nonathletes reported no exercise at all, with the rest engaging in activities such as jogging, cycling, or nonendurance exercise like tennis.

Aerobic capacity was measured as VO2 max( peak oxygen intake) during a cycling test, whilst CT coronary angiography was used to look at both calcified and uncalcified plaque in the coronary arteries. A range of other key health metrics was also taken including blood pressure, bpm, cholesterol (both LDL and HDL), body fat and also HbA1C (a diabetic marker measuring glucose disposal efficiency).

The researchers found that the endurance athletes had more coronary plaques than non-endurance healthy people of the same age. This included both calcified plaques (CAC- coronary arterial calcium score- see graph A) and non-calcified plaques. The study was presented at the American College of Cardiology Scientific Session/World Congress of Cardiology and published in the European Heart Journal.

The researchers suggest that there may be a "reverse J-shaped" dose-response relationship between exercise and coronary atherosclerosis. This means that doing a little bit of exercise, like brisk walking or jogging up to 3 hours a week, can have the most benefit. However, after that point, an increase in coronary plaque burden may be seen. The lead researcher, Ruben De Bosscher, emphasizes that the worst thing you can do of course, is no exercise at all.

Interestingly, there was no advantage in HbA1C for the super athletes, though their heart rate was on average 5 bpm slower, indicating their VO2 max conditioning resulting from the endurance training. Their LDL cholesterol (unhealthy) was also slightly lower than the control group, despite having increased arterial plaque.

The researchers however, emphasized that despite having a very healthy lifestyle and exercising a great deal, no one is granted immunity to coronary atherosclerosis. Therefore, the most benefit seems to occur in individuals doing a moderate amount of exercise, up to about 3 hours a week. It's important to keep in mind that this study was conducted with healthy individuals who exercise, and more research is needed to understand the relationship between exercise and coronary atherosclerosis in unhealthy individuals. Nevertheless, it's still clear that staying active is important for maintaining good heart health, but moderation is key.

On a more positive note, there appears to be no evidence that this increased plaque burden later results in an increase in ischaemic heart disease in the endurance athletes. One possible explanation could be that trained endurance athletes will have wider arteries and also greater vasodilation potential, meaning that plaque to vessel ratio may still be ‘better’ in this group, though this is a theory at this time.

A heart expert named Dr Paul Thompson says that the real problem is that most people don't exercise enough. It is still vitally important to exercise moderately to get the most benefits. The current guidelines recommend 150-300 minutes of moderate exercise per week. Thompson says that people shouldn't be discouraged from doing extreme exercise if it makes them happy, but they shouldn't do it solely for their health . Amongst other ‘heart unfriendly’ behaviours such as smoking, high bp, eating poorly and stress, being sedentary plays a very significant role in poor heart health making moderate exercise highly cardio-protective. It’s all about moderation. As with anything, too much of a good thing can become a bad thing!

The researchers did add however that more research is needed to determine whether coronary events are indeed less prevalent in the high-end exercise cohort, and if so, what explains the paradox. They recommend longitudinal research at the higher end of the endurance exercise spectrum.

Take Home Message

Lifelong endurance exercise is unfortunately not associated with any more a favourable outcome when it comes to atherosclerotic plaque build-up. It may also be associated with atrial fibrillation and irreversible fibrosis or scarring of the heart muscle. Endurance exercise may be detrimental to cardiovascular health over time, though more research is required to be certain. Moderation is key to long term health.

References

1/ Training for Longevity-The Reverse J Curve: Missouri Medicine, July/August 2020,117:4, 355

2/ Am J Physiol Heart Circ Physiol. 2020, Nov, 1;319

3/ European Soc of Cardiology, European Heart Journal (2023) 00, 1–12