In a fast-paced city like Hong Kong, high-intensity interval training (HIIT) has been loved by many for its efficient way of burning more calories in shorter periods of time. However, if your goals are to improve your health and fitness, a new study finds that too much HIIT training may do more harm than good. 

Although occasional HIIT workouts can improve your exercise endurance, near-daily strenuous HIIT training can overstress the body and cause metabolic issues, according to a new study on HIIT that was published March 18. 

In the study, 11 healthy adult participants were asked to do regular HIIT training on an exercise bike for a month, starting from two sessions a week and progressively increased to five sessions a week in the third week. The fourth week acted as a recovery period and the exercise intensity was halved. Participants’ leg muscles, fitness levels and 24-hour blood sugar control were recorded each week to see how their bodies change per week. 

While the initial findings in the first two weeks were encouraging, with increased fitness levels, improved blood-sugar control and more total mitochondria—a subunit of cells that are responsible for about 90 per cent of chemical energy for cell survival—the good news stopped coming in week three. 

Instead, researchers found extreme function decline in participants’ mitochondria, as well as fitness stagnation and early signs of blood sugar dysfunction. Tests also showed oxidative stress in participants, a phenomenon that can negatively affect multiple cellular structures that may lead to chronic illnesses and premature ageing. During the recovery week, participants’ mitochondria function and blood-sugar levels improved, however, it did not return to the same extent as week two. 

If you’re an avid HIIT believer, now might be a good time to reevaluate your workout schedule and accept that you can achieve more by doing less. While this study shouldn’t discourage you from eliminating all HIIT workouts from your routine completely, it might be the nudge you need to give yourself plenty of rest days to recover. 

See also: This Is The Best Time of Day to Exercise