Overdoing It Could Reverse Your HIIT Gains

Lead balloon, awful good, small crowd, open secret… The list of oxymorons has a new member. Moderate high-intensity interval exercise.

It’s what you should be doing if you are apt to overdo it. HIIT is good, but not too much of it.

High-intensity interval exercise—HIIT—alternates a short burst of maximum effort with a rest interval done at a lower rate. For example, pedaling at top speed for 30 seconds followed by a minute at low speed.  It’s anything but moderate when it comes to peak effort.

Those intense go-for-broke intervals are why HIIT has numerous benefits to your health, aerobic capacity, even muscle building.  What motivates many of us, though, is how much you can accomplish in a short time.

And that is exactly what may create a problem for some people who use HIIT workouts according to a new study.


Enough Extra Time to Get In Trouble

To backtrack a bit, HIIT comes in infinite variations.

You can run, pedal, row, lift weights, jump rope, do chin-ups, or dance. There’s even, somewhat counter-intuitively, a yoga HIIT workout.

But whichever activity you choose, a HIIT workout can meet your aerobic exercise goals in sessions that are as short as 4 minutes for a Tabata workout, up to 20-30 minutes. Most people do their HIIT sessions two or three times a week. 

Whichever variant you choose, a HIIT practice cuts time compared to the usual recommendation of150 minutes a week or more for aerobic activity. And you get results that equal and surpass the traditional approach.

That leaves a lot of extra time for people who like to work out. And benefits like muscle gains and fat loss also make it tempting to double down to get faster results.

That would be immoderate.


A Big HIIT Is a Bad Choice

If you choose to do HIIT too often, you will lose the best benefits. In fact, you could slow your mitochondria down so much you get no benefit at all and end up in worse condition.

This is a new finding because in recent years numerous experiments have concentrated on finding out how short HIIT sessions can be to reap big improvements. We learned a lot about the lower end of the time commitment, but almost nothing about the upper limit until now.

In the March (2021) issue of Cell Metabolism, researchers at Sweden’s Karolinska Institute and the Swedish School of Sport and Health Sciences published results that finally begin to answer that question.

The researchers put a group of volunteers through a four-week HIIT program to see what would happen as they increased their workouts.

The first week, volunteers did two HIIT sessions a week. They worked out on an exercise bike alternating five-minute intense intervals with three -minute relaxed intervals  four times.  

The second week, the volunteers did this workout three times a week and increased some intervals to eight minutes.

The third week, volunteers did HIIT five days a week. 

In week four, they rested by working out at only half their prior intensity.


Cell-Deep, Mitochondria Like a Break

After each session, scientists took biopsies of the exercisers’ leg muscles and measured blood sugar. The biopsies gave them a chance to look at the mitochondria in the exercisers muscle cells. Mitochondria control cell energy and respiration. They are also related to overall health and longevity.

Now we have our first glimmer of how much is too much. When exercisers worked out two or three times a week, they showed significant improvements in blood sugar control and their cells had gained mitochondria. They were noticeably fitter as well.

When they upped their regime to five days a week, not only did they fail to improve, they went backwards. They were unable to increase power when cycling. At the cellular level, their mitochondria produced only 60% as much energy as earlier. They did improve when they took a restful approach in the fourth week, but even then they were worse off. They still had 25% less energy than they did in the second week (3X a week routine) of the experiment.

The researchers have not discovered why this happens. They can’t pinpoint exact biochemical changes, but we do know that if you want great benefits from exercise, HIIT is excellent—as long as you keep your intensity moderate.

Interested in HIIT benefits? See  4 Amazing, Unexpected Benefits of HIIT That You Probably Don’t Know.


Revised March 2021



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