Study: Is Too Much HIIT Training Detrimental?

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A new research study[1] shows that extreme exercise training causes mitochondrial functional impairment and lowers glucose tolerance in healthy volunteers.

The right amount of exercise training absolutely influences metabolic health through enhanced mitochondrial oxidative potential and increased glucose regulation and is also the preferred treatment in various metabolic complications. But too much of anything is not good, and it also applies in exercising.

Air Bikes can be used for LISS - Low Intensity Steady State exercise, but HIIT is where they shine.
Air Bikes can be used for LISS – Low Intensity Steady State exercise, but HIIT is where they shine. But is too much HIIT exercise bad for you?

What was the Goal of the Research Study?

Some studies show that intense blasts of exercise improve the number of mitochondria in our muscle cells, and more mitochondria provide better cellular and metabolic health. But there was also research where people did HIIT three times a week for six weeks, but there were no changes in their blood pressure or body fat compared to people who exercised five times a week. The researchers suspected that not working out in the four-day rest may have sabotaged the effect of the workout they did for three days. The studies were not clear whether doing HIIT sessions harder or more often can affect health.

Battle ropes are a great thing to do in Tabata or HIIT style training
HIIT training is short, but intense. It’s very demanding.

What were the Research Methods of the Study?

A new study was conducted to find clarity in the inconsistencies of the previous studies. The researchers at the Swedish School of Sport and Health Sciences and the Karolinska Institute recruited eleven healthy men and women who trained but were not aggressive athletes. The volunteers had their current fitness, and metabolic health tested, including blood sugar levels during the day. Afterward, they started an aggressive exercise program. During the first week, they completed two HIIT sessions, five times on a stationary bike repeating four-minute-long intervals with three minutes of rest in between. The researchers observed the volunteers’ power output as they pedaled as hard as possible during each four-minute surge. The volunteers added a third HIIT session and increased the length of their intervals to a draining eight minutes on week two. On the third week, they worked out five times, with four-minute and eight-minute explosions of all-out pedaling. For recovery in week four, they halved the amount and severity of their exercise. The researchers repeated all testing each week.

What were the Results and Conclusions of the Findings?

By the end of week two, the volunteers seemed fitter, with better daily blood-sugar control and more total mitochondria in their muscle cells, which was more efficient now, producing more significant amounts of energy than at the beginning. On week three, though, the volunteers’ mitochondria produced only about 60 percent as much energy as during the first week. When they decreased their workout to 50% on week four, their mitochondria began to recover, giving more energy, but still 25% less than during week two. The monthlong research implies that “HIIT exercise should not be excessive if increased health is the desired outcome,” says Mikael Flockhart, a doctoral student at the Swedish School of Sport and Health Sciences, one of the researchers who administered the study.

Fit woman working out with class on stationary bike at gym. Portrait of young beautiful woman smiling and looking at camera while cycling. Happy cheerful athletes in a row training on exercise bike.
HIIT Workout

Recommendations for Fitness Trainees Based on the Research

This study was limited and short, with hardly a week of each exercise routine. The study also had healthy volunteers rather than people with existing metabolic problems which does not show whether results would be the same, better, or worse. Still, the findings firmly recommend that anyone interested in high-intensity interval training start small, Mr. Flockhart says. Practice a few times a week and on the remaining days, take a walk.


1. “Excessive exercise training causes mitochondrial functional impairment and decreases glucose tolerance in healthy volunteers”

2. “Too Much High-Intensity Exercise May Be Bad for Your Health”