Tldr; I felt this book raised an interesting point that over exercising can be as bad as under exercising, however I feel it was overly alarmist given the limited amount of scientific studies available for evidence and presented a largely anecdotal based view. Worth a quick read with a pinch of salt. 3.5/5. 

The Haywire Heart examines the idea that too much exercise can put undue stress on the heart and lead to some serious heart conditions later in life. It is an important read for anyone undertaking endurance training, especially those who have done it for 5-10 years or longer. 

How Does My Heart Work?

The book begins with an overview of how the heart works and the different systems that it uses to keep us alive. The authors do a nice job of explaining the workings of the heart and the different functions of its areas. It turns out that the heart is effectively two systems, plumbing and electricity as they put it. 

The plumbing is the main part of the heart, keeping the blood enclosed and flowing to the right areas. The electricity part is what keeps the heart beating. A complex series of chemical interactions in the heart cells leads to a heart beat, a small shock, which begins (usually) in the SinoAtrial (SA) node at the top and travels in a smooth wave down the heart cells, causing contraction (a heart beat). 

Adaptations in Endurance Athletes

As we train the heart undergoes changes due to the stress it is placed under. The heart will begin to grow, leading to larger chambers and a thicker wall. This enables more blood to be pumped with each stroke and leads to a lower heart beat. Whilst this sounds great, and in some ways it is, it can also lead to problems. 

As the timing of a heart beat is controlled by an interchange of sodium and calcium ions in the cells, the longer time between each heartbeat increases the possibility of one of the heart cells creating its own electrical current and contracting before it receives the signal from the SA node. This can lead to irregular heart beats, which are an issue as the heart beats without filling fully with blood making it less effective. 

Issues that Can Arrise

There are a couple of issues that can arise from being overtrained. Firstly, as explained above a long QT time (time between heart beats) increases the chance of other parts of the heart firing rather that its regular pacemaker. This can lead to arrhythmia (sustained irregular heartbeat). 

A more important issue comes from damage to the heart cells. As the heart grows and is stressed, damage and scarring to some cells can occur in the same way you can hurt any other muscle by over stressing it. In the heart, that is an issue as it can block the transmission of the electric signals and therefore cause delays in the arrival of the signal to parts of the heart. 

Scarring of the heart can lead both to arrhythmia (irregular beats) and to more serious conditions like Ventricular Fibrillation (which is where the heart beats at 300+ beats and effectively moves no blood around the body = death in minutes). Other issues such as Atherosclerosis (build up of Calcium in the arteries which can break off and cause blockages) can cause issues, but tend to build over time rather than suddenly. 

So Am I in Trouble? 

Reviewing the evidence the book presents makes for grim reading. They break down the likelihood of heart issues by age. Younger than 35 it is likely a genetic issue (and therefore structural within the heart), over 35 is more likely to be brought on by training and damaging the heart. 

One issue I take with the way the book presents its evidence is that there are no real comparative risk factors. Given the lack of ultra endurance athletes to take data from, it is quite hard to understand these risks for the general population of athletes. Whilst most of the studies the book presents suggest that ultra athletes are at an increased risk of heart issues (particularly irregular beats) with age, it doesn’t really give a great sense of the amount of increased risk athletes are taking on. Some of the studies presented have suggested that up to 81% of the athletes (vs 48% normal people) had irregular heartbeats, whilst others have come back with a number closer to 5% (vs 0.7% normal people). The differences between studies seem too great to make much sense. 

Moreover, there is little discussion of any other factors of illness or mortality that athletes are over/under exposed too. Whilst is it possible that athletes experience higher levels of heart issues, it is possible that they are experiencing these in lieu of other illnesses that they have managed to avoid at higher rates. 

The Take Aways

The main take aways from this book is that too much exercise, especially at ages over 40 can be bad for your heart. Combining volume and intensity is the biggest recipe for disaster. 

Virtually all heart problems will fall into either an electricity or plumbing based problem, with the former more common in athletes. Reducing the volume of training alongside other stressors in life can help us to protect our hearts, and it is unclear how many athletes these issues actually affect. 

The Haywire Heart is an eyeopening book, if not solely for its aggressive take on heart issues in endurance athletes. Whilst most people wouldn’t consider too much exercise bad for you, there is clearly some compelling evidence here. Equally, I would recommend taking it with a pinch of salt. Endurance sports are not the doomsday scenario that the authors pain, but it is worth keeping in mind the added risk that you are taking on. 

Learn to understand the symptoms of heart issues and know when to seek medical attention if you feel them. But don’t give up your mountain run just yet.