The Reverse J-Curve: Exercise Benefits Your Heart—Until It Doesn’t
Exercise has a hugely beneficial impact on cardiovascular health and mortality rates, but at a certain volume of exercise those benefits start to decrease.
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Alcohol is one of the most common preventable causes of disease. Excessive drinking is responsible for about 4% of deaths in the U.S. and plays an important contributing role in various diseases of the organ systems, as well as has a causative role in numerous types of cancers.
And yet, when taken in moderation—ie. less than one drink/day for woman and 1-2 drinks/day for men—alcohol, especially red wine, has been shown to confer benefits to the cardiovascular system and to decrease mortality from all causes.
So which of these depictions of the effects of alcohol are we to accept as reality? A poisonous, carcinogenic intoxicant capable of driving people to all manner of bad behaviors or a valuable addition to one’s diet that can prolong life and decrease the risk of heart attack? It turns out, that in this case, we can accept both.
Taken in moderation, alcohol does indeed confer important benefits to cardiovascular health and improves all-cause mortality. But once the intake of alcohol exceeds a certain threshold, particularly over 4 drinks per day, the results are no longer positive—rather they become very negative with the impacts described above.
This kind of relationship—where a small dose of an agent is good but more than a certain amount is bad—is referred to as a reverse J-curve and is illustrated below:
Recently, an accumulation of evidence has begun to show a similar relationship between exercise and cardiovascular health. A sedentary lifestyle, especially when coupled with obesity, are significant causes of disease and death primarily because of their roles in cardiovascular disease (CVD). Consequently, exercise has been prescribed as a means of managing patients at risk of CVD, and over the years this strategy has shown to be very successful. When compared to their sedentary counterparts, those who exercise regularly have a 30% reduction in all-cause mortality and tend to live approximately eight years longer. Even for those in whom CVD is already established, an exercise program can lengthen and improve the quality of life.
“Exercise positively affects virtually every aspect of the body and brain, and its unique ability to improve physical and mental wellbeing has convinced the general public that staying physically active is one of the best ways to stay healthy and enhance longevity. In fact, maintaining a high level of cardiovascular fitness improves prognosis independently of other therapies, and plausible arguments can be made that exercise/fitness is the most potent medical therapy we have in the battle against CVD,” say the authors of a paper that summarized the findings of how exercise impacts CVD and overall mortality.
One of the authors, Dr. Carl Lavie, a cardiologist at the John Ochsner Heart and Vascular Institute at the Ochsner Clinical School in Louisiana, spoke with me about that paper—because as much as they found that exercise was beneficial, they also noted that many studies now have suggested that there is a reverse J-curve for exercise volume and its effects on the cardiovascular system, much as there is for alcohol intake and mortality.
According to Dr. Lavie and his colleagues, the exact amount of exercise that is beneficial is unknown, though some research has suggested that as little as fifteen minutes per day is all that is needed to see an effect. On the flip side, there is some suggestion that when individuals begin to exceed somewhere in the range of 10-12 hours of exercise per week on a regular basis for many years, the benefits of exercise begin to decrease and some risks begin to increase.
The reduction in mortality benefit is not insignificant. Men who exercise moderately were seen to have a 58% reduction in mortality from CVD, while those who exercised at the higher levels of volume saw only a 34% reduction in mortality. Still, as Dr. Lavie pointed out to me, this is still an important reduction in mortality and although “there may be some loss of benefit, it is not to a point where it is worse than the non-exercisers.”
The reason for the decreased benefit is unclear, but does not appear to be related to the intensity or type of exercise, only the duration.
What then were the risks? For people who exercise at more than 10-12 hours/week for many years it seems as though the major risks relate to the likelihood of developing atrial fibrillation, myocardial fibrosis (which may be related to sudden cardiac death), and the potential for arrhythmia related to ventricular hypertrophy (enlargement of the heart) or from exercising at high intensity.
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With respect to atrial fibrillation, the risks are such that as exercise volume goes up, there is even a point at which the risk of this disorder is higher than it is in sedentary individuals. Dr. Lavie was quick to add, “It does seem however, that atrial fibrillation in athletes is associated with lower stroke risks.” (Stroke being the major complication seen in patients with atrial fibrillation.)
Sudden cardiac death is also a potential risk for those who exercise at high volume, though it remains less likely overall among those who are active than in those who aren’t. Still the subject of sudden cardiac death during exercise is one that all triathletes are acutely aware of and there is still a lot that is unknown about this issue. For example, why is it that men are so much more likely than women to succumb in this way? Per Dr. Lavie: “Women generally have lower CVD risks early in life, but start to catch up only after menopause, which is probably part of it. Certainly, there are exceptions to this, but I believe that there is the perception that women may exercise or at least compete at lower percentage of their peak heart rates (this certainly would not be the case for the elite athletes) and this may play a role.”
Given the reverse J-curve, older triathletes may find themselves wondering what this information means for them and whether or not they should consider modifying their routines. Dr. Lavie was sanguine. “The main thing is if they are doing this for health reasons, all of the health benefits occur at much lower levels (of exercise volume), but they do still occur,” he said. “However, they are likely doing this for more than just physical health. They are doing this for fun and sport, so it may just be worth the older athlete getting a coronary calcium scan and having risk factors treated vigorously. I would not encourage them to not train or compete.”
The MedTent will answer your science and medical triathlon-related questions. It’s written by Jeff Sankoff, an ER physician, triathlete, and coach who runs TriDoc coaching and hosts the TriDoc podcast.