What You Should Know about HIIT

HIIT training is one of the hottest trends in fitness, because it's been marketed as a way to torch calories and burn fat fast. More recently, there have been studies published showing that HIIT might not be all it's been hyped up to be, and might even be harmful.

So what's the deal?

First, let's talk about what HIIT is and what it isn't. Lots of workouts, especially on social media, are promoted as "HIIT" when they're not actually HIIT.

HIIT stands for "high intensity interval training," emphasis on the "high intensity" part. It means exercising really, really hard for a short to moderate amount of time with rest periods in between. The exact work-to-rest ratio can vary, but it's usually 20 seconds (Tabata) to 4 minutes of nearly maximal work, with rest from 10 seconds to a few minutes.

"Nearly maximal" means 85-200% of your maximum heart rate, oxygen uptake (VO2 max), and power output. You're pushing past your anaerobic threshold, aka getting breathless, on each working set, and you're going legitimately "all out," meaning you physically could not give one more second of effort before the rest time.

If you're using a heart rate tracker, you're above 85% on the work intervals. If you're skipping the rest periods or feel like you don't need them, you're not doing HIIT -- that's just interval training. (And there's nothing wrong with that! But HIIT gets thrown around a lot when it's not actually what people are doing.)

HIIT, when done properly, can have a ton of benefits, which is why it's become so popular in the fitness world. It increases aerobic and anaerobic fitness levels, can reduce body fat and insulin sensitivity, and it can pack a lot of work into a short amount of time. Who wants to hang out on the elliptical for an hour or more when you can up the intensity and save time in the gym??

That being said, if you've been reading my emails for awhile, you might have caught on to the fact that when it comes to fitness and nutrition, there's no magic bullet. There's no diet or type of workout that is the holy grail of getting fitter, healthier or leaner, no matter what magazine articles and social media influencers will tell you.

HIIT can be a great addition to your fitness programming, but it shouldn't be the only type of exercise you do.

Due to the intensity of true HIIT workouts, it's generally recommended that they be performed no more than 1-3 times per week. If you're trying to do HIIT workouts every day, your muscles won't have had the recovery time needed to push you into that nearly maximal range.

Plus, any balanced training program includes strength, cardio, and flexibility components, and skipping out on the strength and the flexibility will likely limit your power output, muscular endurance, and range of motion during HIIT or other interval or circuit training.

Recent research has shown that too much HIIT can also affect the mitochondria, or the energy powerhouses, of the cells. In a short, small-scale study, healthy athletes who increased their volume of HIIT training showed decreased power output as the frequency and time of the exercise intervals increased. Their mitochondria produced less energy and their blood sugar levels were destabilized.

This is similar to an unpublished study Les Mills completed in 2018, also with a small group of exercisers, that measured cortisol levels of participants. It showed that 30-40 minutes of high-intensity interval training per week was the "sweet spot" to maximize the benefits of HIIT without overtraining or causing a drop in power output.

A few things to note: Many studies (not just related to fitness!) are conducted with only male participants, because it's easier for researchers to not have to account for menstrual cycle fluctuations. This limits the data available on potential differences between women and men. Both the studies referenced above included a small number of participants over a short period of time, which also limits the available data. And participants were monitored closely during the study, so they were definitely pushing into their 85-200% max effort zone. For people working out on their own who might not be doing true HIIT workouts in that range, the same data wouldn't apply.

So is HIIT good or bad?

You should know by now that my answer for nearly all fitness and nutrition related questions is "It depends."

It has benefits, but there can be "too much of a good thing" when it comes to HIIT.

My advice is to do HIIT if you enjoy it, but don't make it your only form of exercise. Lower-intensity and moderate intensity training days are incredibly valuable to help your body recover so you can get the most from each session, regardless of the mode of training, without feeling beat up and undoing the benefits of the harder work. Remember, feeling sweaty or absolutely crushed doesn't necessarily mean it was a productive workout.

No matter what the latest clickbait headline or your favorite influencer says, HIIT is not necessarily dangerous, but it's also not the end-all and be-all for burning fat and getting lean. Like most things in the fitness and nutrition world, the truth is somewhere in between. HIIT is just one way to exercise, and our bodies were designed to move in many different ways, so mix it up!