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To HIIT or not to HIIT: the low-down on high-intensity training

Mae West once said, “Too much of a good thing can be wonderful.” But sometimes it’s not. That’s how I feel about HIIT, or high-intensity interval training, which can and should be part of almost anyone’s work-out program … in moderation.


A little review is in order. Think of your metabolism as the fuel system that powers the body. How hard you’re pushing your muscles determines the fuel mix. If, let’s say, you’re jogging or cycling at a relatively easy pace, you’re burning a lot of fat, a clean and abundant energy source, which means you won’t run out of gas and your cells won’t accumulate exercise-limiting metabolic waste products. (In a previous post, I wrote about the virtues of moderate intensity training, so-called Zone 2, easy enough to sustain for a long time, demanding enough to develop cardiovascular fitness and build up the mitochondria power plants inside the muscle cells.) The harder we push, the more glucose we burn, the sooner we have to back off.


But pushing hard for limited amounts of time confers some real physiological benefits. Over time, your body increases the amount of oxygen it’s able to use for energy, in technical terms, upping your VO2 max. Your muscle cells become more sensitive to glucose, requiring less insulin to get more fuel into the cells. The details are complex (recall the Krebs cycle from high school biology?; didn’t think so) but the take-away is simple enough -- more oxygen and more glucose equals more combustion, more ATP energy molecules, more oomph. 


Back in the ‘20s, athletic trainers and sports scientists had not yet been exposed to the relevant molecular biology but, through trial and error, they figured out what worked, namely the “overload principle” of muscular adaptation. The Finnish runner, Paavo Nurmi, “the Flying Finn,” became virtually invincible at everything from the mile to the 10K, training on a steady diet of intervals. What we now called HIIT is pretty much what Nurmi was doing: a burst of high-intensity effort, anywhere from 80 to 95% of maximum heart rate (wear an HR monitor or just gauge your perceived effort, an 8 or 9 on a 10-point scale) followed by a recovery period going at an easy 40-650%, long enough for your heart rate to calm down. Then rinse and repeat, for as long as you can or care to.


In today’s world, HIIT has become a mainstay of the fitness industry, the underlying principle behind those spandex-clad headset-wearing spin instructors exhorting their classes to go big or go home or in those spin-like-crazy-at-home Peloton ads. And it animates the work-out regimens of many of my clients, high-achievers who take an “all in” attitude to almost everything they do.  

Here's where the caution flag drops. Overdoing the hard stuff is counterproductive. There’s a reason why elite athletes spend about 80% of their time working in the lower exercise intensities, building up their muscle mitochondria in a lower-stress way, conditioning their bodies to withstand the rigors of the 20% high intensity work that allows them to extend the performance envelope. (The coach’s lament: athletes want to go too hard on their easy days and too easy on their hard days.)


An academic push back against the excesses of HIIT was occasioned by a 2021 article in Cell Metabolism which put a small group of physically active young adults through four different week-long HIIT protocols. One of the weeks was brutal, a total of five HIIT sessions, each one featuring five bouts of all-out exertion on the cycle ergometer, at maximum intensity. Afterwards, muscle biopsies and glucose tolerance tests revealed impaired mitochondrial function and glucose uptake at the cellular level, exactly the opposite of what you’d want and what HIIT promises.


HIIT adherents responded that no one except competitive endurance athletes would dream of taxing themselves to that degree, and that the negative metabolic effects were only temporary anyway. However that debate ultimately plays out, I can tell you, from personal experience and from the experiences of my patients, getting too gung-ho about HIIT training can invite over-training and that the negative effects of that overtraining can be very real: impaired immune function, poor sleep, diminished appetite and libido, irritability, the list goes on.

I recommend limiting HIIT workouts to no more than two sessions per week, starting with just one session per week during the first month. Begin with intervals of 60 seconds at 80-90% of your maximum heart rate, followed by 2 minutes of rest at 50-60%. Repeat this cycle five times, with a 5-minute warm-up and cool-down. As your fitness improves, decrease your rest time to 1 minute, which gets you to that interval sweet spot of 1:1 work-to-rest ratio. Repeat this cycle 8-10 times. And when this gets too easy, give yourself a pat on the back, then level up to a maximum of 4-minute work/rest intervals, repeated four times.

The important thing is to strive for what I call a balanced diet of exercise intensities, each type of work-out stressing different energy systems but none too much: HIIT; MICT (moderate-intensity continuous training); LSD (long slow distance). If these acronyms are Greek to you, then you might want to start by building some HIIT into your everyday life. Doing repeat laps of stairs at home or work is a great way to tap into the power of HIIT, Peloton bike not required. 


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