Doing too many HIIT workouts a week can hurt your metabolism and destabilize your blood sugar, a small study suggests

• High intensity interval training, or HIIT, is beneficial for fitness and overall health, studies show.
• But new research suggests too much HIIT could disrupt athletic performance and metabolism.
• About 90 minutes a week is a safe amount of HIIT for healthy people, according to a new study.

Too much high intensity interval training (HIIT) can backfire if your goal is to improve your health and fitness, new research suggests.

While a bit of HIIT can improve your endurance, long hours of this style of training can overstress your body and dampen your metabolism, according to a small study published March 18 in Cell Metabolism.

Researchers from the Swedish School of Sport and Health Sciences recruited 11 healthy adult volunteers (six female, five male) for regular HIIT sessions on an exercise bike. Participants completed short intervals of all-out effort pedaling - with short periods of rest - and gradually increased the time they spent working out.

The study found that moderate amounts of HIIT improved exercise performance, but long HIIT workouts nearly every day caused participants' fitness gains to stagnate and their health to deteriorate.

Findings suggests that while HIIT has benefits, too much could be harmful instead of helpful for health, stalling performance and stressing out the body.

HIIT has proven benefits if done correctly, up to 90 minutes a week

The study found that for two weeks, participants showed improvements to their exercise performance. They cycled maximum effort for five intervals of four to eight minutes at a time, with three minutes of rest between rounds.

Over time, they were able to generate more power during the workout and had better stamina. They also showed improvements to their mitochondria, the part of cells that generates energy.

That supports previous evidence that HIIT can be an efficient way for people to improve their athleticism and overall health, through short, intense workouts with little rest (often in a format known as Tabata).

Too much HIIT can stress the body and disrupt metabolism, the study found

Researchers in this study wanted to explore how much was too much for excessive exercise.

They found that problems started when participants began working out nearly every day, with more long periods of all-out effort in each session. In total, they were doing about 152 minutes of intense exercise a week.

At this point, they stopped improving on the bike. Tests showed they had worse metabolic health during the most intense period of training and less stable blood sugar levels and mitochondrial dysfunction.

They also showed markers of oxidative stress, a type of cell damage linked to long-term health risks like chronic illness and premature aging, as well as short-term symptoms like fatigue and inflammation.

The good news is, participants were able to recover after a week of less frequent exercise, and started making progress on the bike again. However, their mitochondria still weren't working quite as well as before the overtraining.

Use HIIT sparingly, especially if you're just getting started

While these findings aren't a reason to swear off HIIT completely, it's a good reminder to pursue exercise in moderation, with plenty of time to recover.

And if you're an exercise novice, this may be especially important. The researchers in this study cautioned that their healthy participants could handle 90 minutes of exercise without a problem, but people with existing health issues or who are new to fitness might benefit from less, at least to start.