- High-intensity interval training, or HIIT, is beneficial for fitness and general health, studies show.
- But new research suggests that too much HIIT can hinder athletic performance and metabolism.
- About 90 minutes a week is a safe amount of HIIT for healthy people, according to a new study.
- Visit the Insider home page for more stories.
Too much high-intensity interval training (HIIT) can backfire if your goal is to improve your health and fitness, new research suggests.
While a little bit of HIIT can improve your stamina, long hours of this training style can overwhelm your body and dampen your metabolism, according to a small study published on March 18 in Cell Metabolism.
Researchers at the Swedish School of Sports and Health Sciences recruited 11 healthy adult volunteers (six women, five men) for regular HIIT sessions on an exercise bike. Participants completed short pedaling intervals with total effort – with short rest periods – and gradually increased the time they spent exercising.
The study found that moderate amounts of HIIT improved exercise performance, but long-term HIIT workouts almost every day caused participants’ stagnant fitness gains and deteriorated health.
The findings suggest that while HIIT has benefits, much can be harmful rather than useful for health, impairing performance and stressing 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 in their exercise performance. They pedaled with 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 strength during training and had better endurance. They also showed improvements in their mitochondria, the part of the cells that generates energy.
This supports previous evidence that HIIT can be an efficient way for people to improve their athletic performance and overall health, through short, intense workouts with little rest (usually in a format known as Tabata).
Too much HIIT can stress the body and disrupt metabolism, study concluded
The researchers in this study wanted to explore how much was too much for excessive exercise.
They found that the problems started when the participants started exercising almost every day, with longer periods of total effort in each session. In total, they did about 152 minutes of intense exercise each week.
At that point, they stopped improving the bike. Tests showed that they had worse metabolic health during the most intense training period and less stable blood sugar levels and mitochondrial dysfunction.
They also showed markers of oxidative stress, a type of cellular damage linked to long-term health risks, such as chronic diseases and premature aging, as well as short-term symptoms such as fatigue and inflammation.
The good news is that the participants were able to recover after a week of less frequent exercise and started making progress on the bike again. However, their mitochondria were still not working as well as they did before overtraining.
Use HIIT sparingly, especially if you are just starting out
While these findings are not a reason to abandon HIIT altogether, it is a good reminder to exercise in moderation, with plenty of time to recover.
And if you’re an exercise novice, this can be especially important. The researchers in this study warned that their healthy participants could handle 90 minutes of exercise without a problem, but people with existing health problems or who are new to fitness may benefit from less, at least to begin with.