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Doing This Popular Workout Can Damage Your Body, Says New Study

An eye-opening new study sheds light on the harmful effects of too much interval training.

Your mitochondria are tiny hardworking organelles that exist deep in your cells that are largely responsible for turning the substrates your body strips from the food you eat into energy. In metabolism terms, it's the mitochrondria that actually perform the crucial act of "burning off"—taking in calories and turning them into heat. They're also why exercising and muscle growth are essential to sustained weight loss and health, as your muscles are your body's hotbeds of mitochondria. Put simply: The more muscle you have, the more mitochondria you have to burn calories.

But according to a new study just published last week in the journal Cell Metabolism, if you're engaged in the wrong form of exercise, you can actually damage your mitochondria's ability to do its job effectively. Read on for more about this study, and for more news from the cutting edge of fitness science, make sure you're aware of the One Major Side Effect of Sitting on the Couch Too Much, Says New Study.

Too Much HIIT

hiit workout

The research was led by Filip Larsen, of the Swedish School of Sport and Health Science, who wanted to study the effects of overtraining on the body. He and his colleagues tested 11 young people over the course of four weeks using a stationary bike, increasing the workout intensity as they progressed. Over the course of the trial, the researchers monitored their insulin resistance and their body's mitochondrial function.

During week one the participants performed light high-intensity interval training for only 36 minutes total. During week two, they went up to 90 minutes. During week three, they went up to a leg-melting 152 minutes. Week four was a recovery period, with only 53 minutes of training.

What happened to the participants' bodies.

hiit workout class

During the first two weeks of the study, the participants experienced all of the usual effects one can expect from high-intensity training. Their mitochondrial function improved, among other things.

But in the third week of training, the participants' mitochondrial function dropped by an average of 40% compared to week two. "Following the week with the highest exercise load, we found a striking reduction in intrinsic mitochondrial function that coincided with a disturbance in glucose tolerance and insulin secretion," says the study.

That's right: The subjects' insulin resistance actually went up. "It's quite similar to the changes that you see in people that are starting to develop diabetes or insulin resistance," Larsen explained to The Scientist.

The volunteers' ability to generate power on their cycles also plateaued.

What happened in week four.

hiit workout

After the difficult third week of the study, the participants entered into a recovery phase. In this period, in which they completed only 53 minutes of exercise, their bodies largely returned to normal, but the mitochondrial function remained at 25% less than it was during the second week of the study. For more news from the cutting edge of science, see why Drinking This 30 Minutes Before Exercise Torches Fat, Says New Study.

Proceed with caution.

hiit class

Though the study was relatively small and the long-term effects of hardcore exercise aren't clear, the researchers conclude: "HIIT exercise should not be excessive if increased health is a desired outcome," Mikael Flockhart, a researcher and doctoral student at the Swedish School of Sport and Health Sciences, explained to The New York Times.

According to the Mayo Clinic, you should get roughly 150 minutes of moderate exercise or 75 minutes of vigorous exercise every week. If you're a vigorous exerciser, this new study shows the benefits of reaching 90 minutes per week. If you're going as high as 152 minutes, you could be doing more harm than good. And if you're in the market for more moderate-exercise routines, check out What Walking for Just 20 Minutes a Day Does to Your Body, According to Science.

William Mayle
William Mayle is a UK-based writer who specializes in science, health, fitness, and other lifestyle topics. Read more about William