How much HIIT is too much?
You cant move for gyms, studios and influencers pushing HIIT workouts on us. Theres good reason to do them, of course: high-intensity training can quickly increase our fitness in a relatively short space of time. A friend of mine recently told me that she felt healthier, post-lockdown. Why? I asked, wondering whether it was because shes been able to up her exercise since the gyms reopened. Because Ive 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. Its 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 (its 30 minutes over the NHSs 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. 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 thats 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. Not everything has to be this hard all the time  too much high intensity exercise can actually make you feel less fit and healthy. When asked whether HIIT can exacerbate symptoms of stress, like bloating and fatigue, GymBox PT Arron Cook says: In short, yes. The nature of HIIT is exactly what it says on the tin  its 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 thats 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. Its 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, youll find it impossible to drift off. And then youve 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 werent receiving enough power to function optimally, and that had a knock-on effect on blood sugar and insulin production. In other words, if youre 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. You may also like Five simple ways to stress less and introduce more fun to your day  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 dont 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 youre 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, thats 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, its 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.