ONAs a researcher at the Swedish School of Sport and Health Sciences, Filip Larsen heard anecdotes about the disadvantages of exercising too much – a phenomenon that was common enough but still puzzled him. “All athletes know that if you exercise too much, something happens. . . . Your legs feel awful after a while, and then if you just keep going you have these mental disorders too, like mood disorders, ”he says. “It hasn’t really been described in the literature – nobody knows exactly what’s going on.”
To find out, Larsen and his colleagues recruited eleven healthy young people and went through a four-week, increasingly intense session on a stationary bike while monitoring their glucose tolerance and mitochondrial function. During the toughest week, the subjects exhibited insulin resistance and other harmful metabolic changes, the team reported in last week (March 18) Cell metabolism.
“It’s a very impressive study,” says Thijs Eijsvogels, an exercise physiology researcher at Radboud University Medical Center who was not involved in the work. Typically, cardio-metabolic health improves as exercise volume increases, and the results show that there’s a point where these benefits cease, he notes.
In fact, the subjects’ mitochondria, collected via muscle biopsies, showed improved capacity in the first two weeks of the training plan. During high-intensity interval training (HIIT), subjects were warmed up and then asked to maximize their performance in intervals of 4 or 8 minutes, interrupted by 3-minute breaks. The workout began relatively easily with a total of 36 minutes of intense intervals spread over the week, with no warm-up or rest periods. In the following moderate week, the subjects completed a total of 90 minute intervals. Among other things, the researchers found that a measure of metabolic efficiency known as intrinsic mitochondrial breathing improved during this time, as did physiological parameters such as oxygen consumption.
This changed on week three to represent over-training, with participants doing grueling 152-minute intervals over the course of the week. After that, the subjects’ intrinsic mitochondrial breathing decreased by an average of 40 percent compared to the samples taken at the end of the week with moderate intensity, the researchers report.
It’s pretty similar to the changes you see in people who start developing diabetes or insulin resistance.
—Filip Larsen, Swedish School of Sport and Health Sciences
In addition, subjects’ glucose tolerance – as measured by their glucose levels before and after consuming a sweet drink – decreased between the week of light exercise and the end of the week of excessive exercise (no oral glucose test was performed after the moderate week). “It’s pretty similar to the changes you see in people who start developing diabetes or insulin resistance,” says Larsen.
Most of the measures recovered after a recovery phase in which the participants divided 53 minute intervals over the week. The oxygen consumption and power output of the test subjects during training, measured by how hard they pedaled, were higher after recovery than, for example, at the beginning or at another point in time during the experiment. However, intrinsic mitochondrial breathing had not fully recovered by the end of the experiment and remained 25 percent lower after recovery than after the moderate week.
In a second component of the experiment, the researchers monitored blood glucose levels in 15 elite athletes who did not undergo intervention and in coordinated non-athlete controls. On average, the two groups’ readings were roughly the same over a period of 24 hours, but the athletes spent more time with glucose levels that were either above or below normal, the team reports. Eijsvogels names the orientation of the in vitro measurements that the team carried out on the biopsies of the test subjects, with this observation result, as one of the strengths of the study. “I think that putting these results together gives a really strong message about the effects of exercise training on glucose tolerance,” he says.
Although the study did not examine the long-term health consequences of excessive exercise, Larsen views the effects of the results as primarily academic. After all, top athletes tend to be a “really healthy” bunch, he says, and too much exercise is a far more common problem than too much.
Brent Ruby, an exercise researcher at the University of Montana, calls the study “incredibly well designed,” but asks whether the exercise level in the third week of the study is applicable to anyone in real life. “Even the most narcissistic exercise addict probably wouldn’t put themselves in this position,” he says.
Linda Pescatello, who studies the health effects of exercise at the University of Connecticut and was not involved in the study, suggests that the results on the effects of overexertion actually have real-life effects, with people having different thresholds for overexertion based on their threshold have their fitness. “These extreme forms of exercise in this article don’t apply to the general recreational trainer, but I think they are the overarching principles of exercise,” she says.
She refers to a review article co-authored by Eijsvogels in 2020, in which connections between very high levels of stress and what the authors described as “potential cardiac maladjustments”, such as B. Coronary artery calcification. “I think the bottom line, especially for the average person, is all in moderation if you want to maximize the health benefits of exercise,” she says.
Study co-author Mikael Flockhart, also at the Swedish School of Sports and Health Sciences, says it is not clear what the “tolerable exercise limit” is, especially because the study shows that overexertion does not necessarily result in a decrease in actual athletic activity Performance leads. Knowing where that limit is would be helpful for athletes and their coaches.
M. Flockhart et al., “Excessive exercise leads to impairment of mitochondrial function and decreases glucose tolerance in healthy volunteers.” Cell Metab, doi: 10.1016 / j.cmet.2021.02.017, 2021.