People are well aware of the heart protective effects of exercise, and recommendations that people increase exercise quantity and intensity are common. But as I’ve written about previously, evidence from observational studies suggests that the sweet spot is reached at just 35 minutes of intense exercise per day (or two hours of moderately intense exercise), after which exercise confers no additional health benefit. After an hour of intense exercise, you are actively harming your health.
There are plenty of examples of people who were thought to be supremely healthy up to the point they dropped dead of heart disease. One such example is that frequently comes to my mind is Micah True, the ultra-runner and central character in the book “Born to Run”, who died of a heart attack at the age of 58. Another is Bob Harper, the celebrity fitness trainer and host of Biggest Loser, who had a cardiac arrest at the age of 52 (but survived thanks to rapid intervention by bystanders).
It’s not hard to imagine why more is not always better when it comes to intense exercise. During a bout of intense exercise, the systolic blood pressure is often over 200 mmHg, a level that would in other situations immediately result in a person being placed on multiple blood pressure lowering drugs. The strain on the heart leads to increased blood levels of troponins, molecules that normally exist inside heart muscle cells, but which are released when they suffer damage, and which are used clinically as a means to detect heart attacks. And intense exercise increases oxygen needs massively, resulting in a dramatic increase in free radicals. The longer the exercise goes on for, the harder it is for the body to maintain sufficient mechanisms to neutralize the free radicals, and the greater the probability that they will succeed in causing damage.
It’s been known for the last decade or so that professional athletes have an increased risk of developing atrial fibrillation, a condition in which the atria of the heart stop contracting synchronously and instead “wobble” in a disorganized manner. Blood clots frequently form in the fibrillating atria, and can travel from there to the brain, causing a stroke – which is why people with atrial fibrillation are usually put on blood thinners. None of this is controversial. What is it about prolonged intense exercise that causes atrial fibrillation?
Heart muscle damage, which leads to scar tissue, which leads to disorganized pathways for the cell-to-cell signals that cause the heart to contract.
Unfortunately it’s not just the heart muscle that gets damaged by too much exercise. The arteries take a hit too. Multiple studies have found that people who engage in a lot of high intensity exercise have an increased risk of having significant coronary artery calcification, even when compared with people who don’t exercise at all. The lowest risk of having significant coronary artery calcification across the studies was found among those who engage in a moderate amount of exercise. The reason this matters is because a higher amount of calcification usually means an increased risk of experiencing a heart attack.
What can we conclude? Some exercise is good, a lot of exercise is bad. As mentioned above, the optimal health benefits appear at around 35 minutes of intense exercise per day (or 2 hours of moderately intense exercise, such as walking, if you don’t like getting sweaty).