High-intensity interval training is great for you—but it won’t keep you from aging

Hot take: cellular-level studies are not the whole picture

Exercise is hard. That should go without saying, but it’s worth acknowledging. It’s difficult enough to instill a new habit without all the things that make exercise uniquely unpleasant at first. You generally have to go to a crowded place full of cranky strangers, share equipment in close quarters, and sweat and shower alongside them. But on top of that, oh yeah, it’s hard. A lot of the movements are weird and awkward for newcomers. Maybe you have to stick your butt up in the air, or wiggle around like a wet noodle in overpriced, stretchy clothing. It’s not surprising that so few people stick to a regular workout regimen, because to some degree, if it’s not hard, you’re not doing it right.

But we all hold out hope for a mythical exercise routine that’s short, simple, and will give us all the benefits of a truly difficult workout—plus a great diet—without actually feeling too hard. Enter: high-intensity interval training. HIIT, as it’s commonly known, involves short bursts of strenuous activity interspersed with rest. Gone are the days of 45-minute sessions on the treadmill—you just have to exercise harder for 20 minutes, and you’re set. It seems like a great deal. And that’s why headlines like “This workout reverses signs of aging, according to science” and “Study identifies best exercise to reverse signs of aging” are so clickable. It’s one thing to know that a brief period of high activity might help you shed pounds, but it’s tantalizing to think that crunching your workout into one short, sweaty burst might actually be the secret to staying forever young.

Both of those headlines refer to the same study, which came out in the journal Cell Metabolism at the beginning of March. So here’s the good news: they’re not technically wrong. HIIT does reverse signs of aging at the cellular level, which isn’t an entirely new finding though it’s still exciting to have more detailed data on how this kind of exercise benefits the body. Unfortunately, as with all exercise studies, its findings are fraught with caveats.

Let’s start with the basics. This study compared HIIT to two other kinds of exercise: resistance training (weight training) and a combination of HIIT and resistance. The HIIT group trained five days a week (with three of those devoted to high intensity biking) while the resistance group trained four days a week. The combination group did, duh, a combination, though it’s worth noting they did so at a lower intensity than either of the other groups. Everyone did their regimen for 12 weeks, before and after which the researchers looked at a bunch of cellular indicators of health and aging.

Before we get into the nitty-gritty of the findings, let’s take a moment to appreciate that this study aimed to look at cells. Yes, they were from actual people who did actual exercise, not just random cells being poked and prodded in a lab—but they’re cells all the same. It’s important that we look at how exercise affects our cells, because that’s the only way we can understand the mechanisms by which working out benefits our whole body. But it’s also important that we keep cellular findings in perspective. We can look at all the indicators of aging all we want, but if we don’t really understand how those indicators relate to lifespan, or to how good you’ll feel as an 85-year-old, it’s hard to draw meaningful conclusions from one study. Again, that’s not to say cellular findings aren’t important, it’s just that we have to keep them in perspective.

This study found that HIIT improved several general markers of health, like insulin sensitivity and muscle strength. It also reversed some changes in protein expression that are linked to aging, including some related to mitochondrial function. Mitochondria tend to function worse as we get older (like many parts of our body), so improving their efficiency in theory helps to fight off the effects of old age. Resistance and combined training didn’t seem to affect the mitochondria, though both improved some of the other markers.

Mitochondria in a cell
THE MITOCHONDRIA ARE THE POWERHOUSE OF THE CELL (also they’re the things in red) NICHD/U. Manor

All of those things are good news for HIIT fans. It’s clear that HIIT offers a range of benefits for cardiovascular health, strength, and aging, even if we don’t have the full picture yet. What’s still missing is the link between these cellular findings and meaningful outcomes for people. It’s great that HIIT helps our mitochondria to work better—it can only help our overall health. But it’s hard to say exactly what that improved mitochondrial function will mean for you personally.

Take cancer treatments, for instance. Plenty of chemotherapies do a great job at the cellular level, turning off genes that promote cancer cell growth or even turning on genes that cause cells to kill themselves. Those therapies could be very effective for helping the cells of a lot of cancer patients beat a lot of different types of cancer. But the main thing most people care about isn’t whether this chemotherapy improves the pattern of gene expression in their cancer cells. They care about whether they’ll live longer. That’s an outcome. If this treatment kills off 30 percent of my cancer cells, but it won’t give me any more time with my family, is it a good treatment? Exercise generally isn’t a life-or-death scenario, at least in the short-term, but the same principle applies: we have to look at the outcomes that matter to people.

Let’s say you’ve decided to work out, and you’re trying to decide what kind of exercise to do. Putting aside all the vain reasons you might go to the gym—losing weight, gaining muscle tone, looking better naked—you’re still probably not thinking about your mitochondria. You’re wondering if this will help you live into your 80s, and what kind of shape you’ll be in when you get there.

Those are the things that matter. Cellular studies like this are a crucial piece of the exercise puzzle—they’re just not the whole thing. If you do HIIT five days a week but eat donuts for breakfast every day, you might be worse off than someone who eats a well-rounded diet but only does moderate exercise. Aging and cardiovascular health defy simple rules and easy fixes. If they didn’t, we would have a workout-in-a-pill by now.

All this being said, HIIT is an excellent form of exercise. It’s probably not the only thing you should be doing, but it’s a great start, especially you like it enough to keep doing it. Because the best workout (really, truly, seriously)? It’s probably just the one you’ll stick with.