Researchers have been debating this question for decades: When it comes to fat loss, are people better off doing HIIT or traditional cardio?
There’s little doubt exercise is great for helping people lose and maintain their weight. “The most successful weight-loss maintainers — those who lose weight and keep it off — often credit a regular exercise regime as a key component to keeping the weight off,” says Shelley Keating, PhD, researcher at the School of Human Movement and Nutrition Sciences at The University of Queensland, Australia.
But while exercise in general can be a helpful aid in weight loss and weight maintenance, surely one form is more effective than others, right?
Before we dive into the research on moderate-intensity versus high-intensity cardio, it may be helpful to clarify a couple of different types of interval training. And because definitions can vary depending on who you ask, here’s how Keating and her research group define two popular forms of interval training:
HIGH-INTENSITY INTERVAL TRAINING (HIIT)
This is when you perform 1–4-minute bouts of activity at an intensity equivalent to 80–100% of your maximum heart rate. These intense bouts are followed by lower-intensity recovery periods.
SPRINT INTERVAL TRAINING (SIT)
SIT is more intense than HIIT. Here, you alternate 8–60-second all-out sprints (done at 100% intensity) with a short recovery.
These two forms of interval training are typically compared to moderate-intensity cardio (often running or cycling) performed at 50–70% of maximum heart rate.
WHAT DOES THE RESEARCH SAY?
In truth, the findings on cardio and fat loss are mixed. Some studies reveal interval training is more effective than moderate-intensity cardio, while some suggest both forms are equally effective.
For example, a study in the International Journal of Obesity found that three weekly HIIT sessions helped women lose as much as 7.3 pounds over the span of 15 weeks, while women who performed steady-state cardio gained nearly 3 pounds during the study.
Meanwhile, a recent review found people lost just as much weight via interval training as they did with moderate-intensity cardio but only when the volume of training was similar. “As fat loss requires high amounts of energy to be expended, the volume of exercise is important, and therefore only HIIT and SIT protocols which have equivalent volumes of exercise to traditional training may produce similar fat loss,” says Keating, who co-authored the analysis.
See, while you burn more energy (Read: calories) per minute during interval training, you may not be able to go long enough to match the overall volume of a moderate-intensity cardio session. And while it’s true you burn more energy after HIIT and SIT than a moderate-intensity workout (known as post-exercise energy consumption or the “afterburn effect”), the amount is minimal. “The key for fat loss is how much energy you expend with each exercise session,” Keating says.
WHAT ABOUT THE “FAT-BURNING ZONE”?
If you’ve spent any time around cardio machines, you may have noticed some outline different heart rate zones. If you reach 60–70% of your maximum heart rate, you’re in what’s called the “fat-burning zone.” At this level, you should have no trouble carrying on a conversation with the person on the treadmill beside you.
But does “fat-burning” cardio (or, exercising within the “fat-burning zone”) actually burn more fat than other types of cardio?
Our bodies use three sources of fuel to power exercise and everyday activity: carbohydrates, fat and protein. When it comes to weight loss, you’ll want to use fat as fuel so you can save the other energy sources, says Jacque Crockford, MS, an exercise physiologist for the American Council on Exercise.
When you exercise at an easy, conversational pace, your body pulls primarily from your fat stores but once you increase the intensity and it becomes harder to speak in full sentences, your body starts using carbohydrates (mostly in the form of glucose). Then, if you shift into higher-intensity exercise (i.e., you’re only able to say one or two words at a time) your body switches to using another form of carbohydrates known as glycogen.
While you burn a higher proportion of fat when you exercise at a lower intensity (approximately 60% of calories burned come from fat versus 35% during high-intensity exercise), you’ll actually burn more total fat from interval training, Crockford says.
Consider this example from the American Council on Exercise: Assume you just burned 200 calories from a 30-minute, low-intensity run. Approximately 120 calories come from fat. Now, assume you just burned 400 calories exercising at a higher intensity for the same amount of time. Of those 400 calories, roughly 140 have come from stored fat.
Of course, this example assumes your exercise sessions are equal in length. If your HIIT workout is shorter than your slow cardio session, you may end up burning more fat with the slow cardio.
WHICH TYPE OF CARDIO SHOULD YOU DO?
It may sound cliché, but the answer is: the type of cardio you enjoy and will commit to in the long-term, Keating says.
While some doctors, researchers and fitpros worry that the all-out intensity of interval training scares off the people who need it most (i.e., the inactive, overweight and/or pressed-for-time), thus making them less likely to stick with it over the long-haul, new research in Medicine & Science in Sports & Exercise suggests otherwise.
Researchers from the University of British Columbia had 30 inactive men and women perform one session of moderate-intensity cardio (prolonged effort at approximately 70–75% maximum heart rate), sprint-interval training (“all-out” 20-second sprints) or HIIT (one-minute bouts at 85–90% of max heart rate) on a cycle ergometer in random order on separate days. Then, participants were asked to report whether they had logged any exercise on their own during a four-week follow-up. According to the study findings, 79% of the men and women logged HIIT sessions on their own once the study ended.
“Science is great,” Crockford says, “but it can sometimes take the humanity out of the training.” In other words, while one study may indicate HIIT is better than moderate-intensity for weight loss, you shouldn’t force yourself to do HIIT if you can’t stand it.
What’s more, researchers and fitness professionals recommend keeping high-intensity workouts at a minimum to avoid injury and burnout. So, your best bet is to perform a combination of moderate-intensity and high-intensity cardio — as long as you enjoy both forms of cardio, that is.
If you can, hire a coach or trainer to work with you one-on-one. “It’s really hard to do the weight loss thing alone,” Crockford says. She recommends working with a fitness professional who can take your lifestyle, preferences and current ability level into account to create a program that will help you achieve weight loss in the safest way possible.
A personal trainer can also guide you through the inevitable trial-and-error of weight loss: “When it comes to weight loss, there are a lot of ways to do it and there isn’t just one right way,” Crockford explains.