From Joe Wicks to FIIT, HIIT workouts are everywhere online and gyms still offer plenty of high-intensity classes. That’s great if you’re time-poor but what impact is all that fast and furious training having on our health?
If you’re time-poor or don’t exactly love exercising, then HIIT can be brilliant. You only have to workout for 20 minutes to reap the same health benefits as running for double the time, so it’s no wonder that so many gyms, studios and influencers pushing HIIT workouts on us.
But despite its time-saving credentials, more and more women are moving away from high-intensity training in favour of slower workouts. Why? Because we’re starting to realise that after years of jump lunging, squatting and burpeeing like crazy, there really is such a thing as too much HIIT.
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A friend of mine recently told me that she felt healthier, post-lockdown. “Why?” I asked, wondering whether it was because she’s been able to up her exercise since the gyms reopened. “Because I’ve stopped doing HIIT,” she said. She went on to explain that over lockdown, she noticed a “huge surge in influencers pushing HIIT content online,” which she decided to try. Five or six days a week, she followed a 30-minute HIIT plan and after a few months, started to notice that by 11am, her mood and energy were through the floor.
“I found myself getting into a bit of a tizz over this 30-minute blitz of exercise, and even though I was increasingly becoming irritable and tired, I found it totally addictive.” It’s easy to become addicted to HIIT, she warns, while hating it at the same time.
At the rate she was going, my pal was doing up to 180 minutes or three hours of HIIT a week. That might not sound like loads (it’s 30 minutes over the NHS’s exercise recommendations), but according to new research, anything over 90 minutes of HIIT a week is “unsafe”.
Released back in March, the report, published in the journal Cell Metabolism, found that spending hours doing this kind of training had the potential to overstress the body and dampen metabolism. Researchers found that people who did a moderate amount of HIIT improved their physical performance in general, but that near-daily HIIT workouts actually hampered participants’ health and caused their fitness gains to plateau. So why is that? Well, the study found that anything over 152 minutes of HIIT a week can cause oxidative stress, a type of cell damage linked to chronic illness, fatigue and inflammation – something that’s easily reversible by cutting down on the amount of high-intensity exercise you do.
That makes total sense; pre-pandemic, I used to go to a HIIT class after work every day. While my overall fitness was pretty good, I used to struggle with the most chronic bloating. Nothing seemed to make it better, despite experimenting with eating different foods. My sleep was all over the place, too. Both stopped when I cut down on gym time and started changing up my weekly routine to include cycling, easy runs and pilates. Coincidence?
When asked whether HIIT can exacerbate symptoms of stress, like bloating and fatigue, GymBox PT Aaron Cook says: “In short, yes. The nature of HIIT is exactly what it says on the tin – it’s high intensity. This, therefore, requires a certain response from our body. One of the many responses that our body goes through is a huge increase of cortisol, triggering our ‘fight or flight’ sympathetic nervous system.”
He says that the issues really start when you combine HIIT with other stresses in life, whether that’s a heavy workload, a stressful home life or something more innocuous that causes a degree of anxiety. “It could potentially be too much,” he tells Stylist.
It’s not just that intense workouts can exacerbate stress that already exists in the body. Numerous studies have claimed that excessive HIIT can cause our glycogen stores to deplete, leaving us feeling slower and weaker during workouts and less able to recover effectively after exercise. Because of that cortisol increase, sleep can become disrupted. If adrenal hormones are always firing on all cylinders without a natural fall, you’ll find it impossible to drift off.
And then you’ve got the fact that HIIT can cause our metabolism to go haywire. Participants who took part in that 2021 study showed mitochondrial impairment from doing HIIT five days a week. That means that the cells weren’t receiving enough power to function optimally, and that had a knock-on effect on blood sugar and insulin production. In other words, if you’re doing tonnes of HIIT in the hope of tweaking your body composition, you may find that you actually start to go backwards in terms of gains, with the body clinging to energy and refusing to grow lean muscle.
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However, Cook says that HIIT is a brilliant style of training that should have a place in most fitness routines and it can be a great addition if we don’t overdo it, have proper recovery protocols in place and are able to manage out-of-gym stressors.
If HIIT is the only form of exercise that you do, then Cook says that yes, there is such a thing as “too much” HIIT. “This would mean that you’re neglecting true strength elements, injury prevention and rehab-style exercises, but most importantly, your longer duration aerobic system.” He explains that over the past 18 months of people working out from home, people have done these classes that contain the same exercises and the same ratio of low-to-high impact movements. No doubt, that’s led to injuries (my friend says that her knees have been shot to shreds by all the jump squats, burpees and plyo lunges). “There is a big difference in terms of physical stress on the body doing bike intervals in a gym versus a home class with 200 squat jumps,” says Cook.
That being said, it’s hard to specify what an ideal mix of exercise might look like. What might suit you in terms of fitness and health is going to depend on your goals, existing fitness, stress levels, sleep quality and more. In an ideal world, however, we want to focus on being well-rounded, with a focus on building a good level of strength alongside a sturdy aerobic system and mobility accessibility for longevity.
Need a HIIT break? Why not try one of our 15-minute mobility workouts designed to strengthen, lengthen and calm the body?
Miranda Larbi is the editor of Strong Women and Strong Women Training Club. A qualified personal trainer and vegan runner, she can usually be found training for the next marathon, seeking out vegan treats or cycling across London on a pond-green Tokyo bike.