Too many HIIT workouts could actually be bad for you – here's how often you should exercise

(Image credit: Fitbit)

last updated 2 July 2021

If any area of fitness has seen a rise in popularity in recent memory, it's HIIT (high intensity interval training). Global lockdowns and gym closures over the last year have forced people to turn to home workouts to get a sweat on, with high intensity intervals among the most popular.

Couple that with the Crossfit and functional fitness boom, and you'll be hard pressed to find a celebrity or buff instagram influencer who doesn't recommend some kind of high intensity exercise to help you burn fat.

But overdoing it with these kind of workouts may actually be doing you more harm than good. According to a recent study published on Cell Metabolism, too much high intensity exercise could have a negative effect on your body's molecular makeup.

Researchers from the Swedish School of Sport and Health Sciences studied the effects of HIIT on the body, gradually increasing the number of sessions participants completed over a period of four weeks.

They found the optimal amount of HIIT to complete in one week is a combined 90 minutes for an improvement in health and fitness, with a boost in participants' power and stamina. But any more than this and the effects will start to reverse as you'll be overtraining.

The study took 11 adult volunteers (six female, five male) through regular HIIT sessions on an exercise bike over a four week period. The participants completed short intervals of intense effort with small periods of rest.

In the first week, the group performed two sessions of four-minute intervals repeated five times on the bike, taking three minutes rest after each interval. By the end of the study, the participants were working out five times a week with up to eight-minute intervals.

Researchers found performing HIIT almost every day actually had a negative effect on the body.

Why should you try HIIT — Around 90 minutes of HIIT a week is optimal for improvements in health and fitness

While the first two weeks of the study saw participants develop an increase in their fitness levels, problems arose when participants began working out five times a week, with longer periods of all-out effort in each session. In the heaviest training week, they were doing about 152 minutes of intense exercise.

Researchers found improvements stopped at this point, and people's overall health actually started to decline. Tests showed they had worse metabolic health during the most intense period of training with mitochondrial dysfunction and less stable blood sugar levels.

Participants also showed indications of oxidative stress (cell damage) which has been linked to long term health risks like chronic illness and premature ageing, as well as fatigue and inflammation.

So next time you're planning out your training week, limit yourself to no more than 90 minutes of HIIT. Try for three 30-minute sessions and take a brisk walk on the days in between.

Why not give the ultimate HIIT workout, courtesy of top PT Keith McNiven, a go? Or tackle this 20-minute HIIT workout for the time-poor to lose weight.