Endurance Athletes Face 4x Higher Irregular Heartbeat Risk

Exercise is one of the best things we can do for a healthy heart. Yet research shows that endurance athletes have up to a four times higher risk of atrial fibrillation (an irregular or fast heartbeat) than non-athletes. This heart condition increases risk of both heart failure and stroke.

If regular exercise and being fit reduces our risk of many chronic diseases and preserves mental and physical health, why is it that people who are very fit face greater risk of a potentially deadly heart condition ? Research suggests that when it comes to heart health, there may be too much of a good thing .

When we take a broad look at the evidence, it's clear that exercise plays a key role in keeping the heart healthy and lowering risk of atrial fibrillation for most of the population.

For instance, an analysis of over 400,000 people found that those who said they did between 150-300 minutes of moderate-to-vigorous intensity physical activity per week had a 10-15% lower risk of developing atrial fibrillation compared to those who were inactive.

Higher levels of exercise may only be protective in females. The study also found that exceeding these recommendations by up to three times was further protective for females but not males, with around 20% lower risk of atrial fibrillation.

Exercise is also emerging as a cornerstone treatment for patients who already have atrial fibrillation. A meta-analysis my colleagues and I conducted showed that in patients with atrial fibrillation, exercise reduced risk of arrhythmia recurrence by 30% . Exercise also improved symptoms and quality of life and fitness.

However, it was difficult to determine how much exercise was best when it came to rehabilitation, as the programme length, frequency of exercise and workout length varied substantially between participants.

So while our findings confirm that exercise plays an important role in heart health, they also highlight how little we know when it comes to the "dose" of exercise needed to optimise this protective effect. This is something we call personalised medicine.

With the growing popularity of endurance events - from marathons to mountain ultras - there's a clear need to understand what volumes of exercise may be detrimental to the heart.

Is the dose the poison?

Our previous research proposed that there's a J-shaped relationship between exercise levels and atrial fibrillation risk. This means that increasing your activity levels to the recommended guideline levels is associated with a significantly lower risk of atrial fibrillation. But when going way beyond these guidelines - such as doing ten times the recommended amount - we begin to see higher rates of atrial fibrillation.

Numerous studies have shown that heart problems can occur in athletes following long-term, intense periods of endurance training . Studies of endurance athletes' hearts have also shown some have signs of scarring , which is a potential precursor to atrial fibrillation and other heart conditions.

For instance, one meta-analysis showed that athletes had a nearly four times greater risk of atrial fibrillation compared to non-athletes. This analysis included those who had no signs or symptoms of any other heart problems. Interestingly, younger athletes had a greater risk of atrial fibrillation than older athletes - something that needs further research.

Men and women appear to have different risk profiles.

One study of 402,406 people found that men who said they did more than ten times the recommended weekly amount of physical activity had a 12% higher risk of atrial fibrillation. This is roughly the equivalent of doing seven hours of vigorous intensity exercise per week (such as running or cycling at a high intensity). However, women who did this much physical activity did not appear to have a greater risk of atrial fibrillation.

It has been suggested that this lower risk in female athletes compared to male athletes may be due to a tendency for females to have fewer structural and electrical changes in the heart in response to exercise. Oestrogen, which is known to be "cardioprotective" , may stabilise heart adaptations in response to exercise training and at rest.

It appears that an endurance athlete's atrial fibrillation risk isn't just due to the amount of exercise they're doing, but a combination of the overall load and intensity of long-term training.

For example, a Swedish study of around 52,000 cross-country skiers found those who participated in a greater number of races had a 30% higher risk of atrial fibrillation . Faster finishing times were also associated with a 20% higher risk.

The number of races an athlete competes in and the finishing time of these races likely represents an athlete's training load and intensity - with more races requiring a higher training load and faster finishing times requiring more intense training. This emphasises that both the amount and the intensity of exercise are key.

Researchers don't entirely understand the mechanisms underlying this relationship between exercise and atrial fibrillation. It's likely explained by multiple factors working together simultaneously.

For example, over many years of very high training demands, the stress placed on the heart can lead to enlargement of the atria (heart chamber) and increased stress on its walls. This can lead to scarring .

Even after a single mountain marathon , researchers have seen short, frequent spikes in inflammation and a transient slowing of the electrical conduction in the atria. Over time with repeated events and training, these cardiac stresses could be what cause an increased heart chamber size and scarring (pathological cardiac remodelling) - increasing the risk of atrial fibrillation.

While it's unlikely that the average runner is going to increase their atrial fibrillation risk while training for their marathon, it's still important to train in a smart way. Considering your overall training volume and intensity - especially if you're training for many hours per week - could help mitigate your risk of cardiac stress and atrial fibrillation.

Atrial fibrillation can be well treated and managed. Therefore, being aware of key symptoms such as an irregular pulse, palpitations or breathlessness is crucial for getting the right treatment.

Ben Buckley has received investigator initiated research funding from BMS/Pfizer, Huawei EU, NIHR, MS Society, and Research England.