The new year is a convenient opportunity to start fresh and get into shape. Unfortunately, every day in the new year has the same number of hours as the days of the last. So if you had trouble finding any time in 2017 to get workouts in, turning the calendar to January 2018 might not help.
Instead of trying to find more time in 2018, you can, however, find better ways to use what you have. There are lots of ways to burn calories in a 30-minute workout. But there’s only one way that’s scientifically proven to burn the most calories in that timeframe: high intensity interval training, or HIIT workouts.
HIIT workouts involve short bursts of exercises that require both cardio and muscle engagement, like jumping lunges, burpees, squat jumps, or quick strength-training circuits that emphasize keeping your heart rate up.
These workouts are intense and arguably painful, but worth it. A 30-minute HIIT workout is more efficient than taking an hour out of your day to go for a walk or jog, which only uses some of your muscle strength but mostly relies on your cardiovascular system. Physiologically, HIIT workouts end up burning more calories because they maximize both your cardiovascular system and different muscular groups.
We all burn calories at different rates, and understanding the basic scientific principles behind the process can help you decide what kind of HIIT workout is for you, if you decide that’s what your workout routine needs.
At a cellular level, making the energy we need to burn calories through exercise all depends on how much oxygen is available.
When you work out, your cells break down the energy stored in glucose, which we get from carbs, and, eventually reserves in our fat cells, into a version it can use, called adenosine triphosphate, or ATP. It takes three separate chemical reactions to turn glucose into ATP, all of which use up oxygen. This process is called “aerobic respiration” and in it, “one liter of oxygen consumed burns about five calories,” says Rick Kreider, an exercise physiologist at Texas A&M University.
Muscles in our legs and arms, and surrounding our core, are like expandable engines attached to our bones. They have the right kind of cellular infrastructure to make ATP really quickly if they need to—more so than any other tissue in the body—which is good because they’re super thirsty for the stuff when you’re using them during an engaging exercise, like lunges or pushups.
Theoretically, if the someone worked their muscles to the fullest extent while they’re fueled on oxygen for an hour, they could conceivably burn more than a couple thousands of calories. (For context, burning a pound of fat requires expending 3,500 calories.) But this number is a rough estimate because no one can.
This is because the circulatory system has a maximum capacity of oxygen it can take up at any given time, called the “aerobic threshold.” When you cross your aerobic threshold, your body goes into anaerobic respiration, when your muscles continue to make fuel but can’t use oxygen any longer.
Anaerobic respiration doesn’t generate nearly as much ATP as aerobic respiration. Plus, instead of carbon dioxide and water and heat (which we exhale or otherwise get rid of through sweat and other bodily fluids), it makes lactic acid as a byproduct, which causes that horrible burning sensation in muscles that should be a cue to stop exercising.
Muscles can keep going in an anaerobic state for up to 180 seconds, but at that point, they stop functioning—or at least slow down a lot. This could be an evolutionary adaptation, Stephen Roth, a kinesiologist from the University of Maryland, writes in Scientific American; slowing down gives the rest of our body a chance to get more oxygen circulating again.
There are basically two factors that determine how many calories you can burn during workouts: how close you can get to your maximum aerobic threshold while maintaining energy expenditure, and your body composition.
To keep burning calories over time, you have to maintain energy expenditure at just below your maximum aerobic threshold, which requires a lot of exertion. Kreider estimates that Tour de France cyclists can reach a calorie-burn rate of 2,000 per hour because they can hold themselves at 85% to 90% of their aerobic thresholds. People who don’t hit the gym regularly can typically hit about 60% to 65% of their aerobic thresholds when they’re at a level they can sustain for roughly 30 minutes.
There are ways to improve your circulatory system’s ability to deliver oxygen to your muscles—and thus raise the level of aerobic threshold you can reach during exercise, and burn more calories as a result. Actually it’s really simple: adding in any regular cardio exercise like walking or jogging will do the trick.
To keep burning calories over time, you’ve got to do a little more than that. Eventually the number of calories you burn each time would peter out as your circulatory system improves. You’ll start to exert yourself less to do the same activity—which is why, in part, it gets easier to run the same distance at the same pace if you do it regularly.
Then there’s your body composition. “The more weight you’re carrying, the more your burning,” says Kreider. This is simply because it takes more energy to move a larger body. A person who weighs 250 pounds will burn more running than a person who weighs 150 pounds, even if they’re going the same speed and have equally matched cardio abilities.
However, what makes up those 150 or 250 pounds matters, as well. More muscle mass means larger energy-burning engines: A muscular 150-pound person would be capable of burning more calories than someone weighing 150 pounds with higher levels of fat. Most people forget that strength is an important part of fitness, Kreider says, but using muscles is the fastest way to burn energy. Plus, building up your muscles can mean you’re carrying around even more weight, which in turn lets you burning even more calories down the road.
The bottom line: if you have 30 minutes to exercise, find something that combines cardio and strength training. That way, you are working on both of the factors that determine how many calories you can burn.
HIIT workouts are some of the most time-efficient exercises, which is why they’re so hard. The good news is, you can start with whatever intervals feel hard for you at your current fitness level. Bodyweight exercises like lunges, push ups, or burpees don’t even require a gym. You can do these for short bursts—say, 20 seconds where you’re pushing yourself to your absolute limits—and then rest for another 10 seconds before repeating. Over time, you should be able to do more and more of your chosen exercise within each interval.
If bodyweight exercises aren’t your thing, you can work in HIIT training while running by going faster and adding elevation for short periods of time. “A short sprint up a flight of stairs followed by a walk back down is interval training,” writes Ryan Andrews, a registered dietician and coach at Toronto, Canada-based Precision Nutrition. Distance runners use sprint-interval training to improve their endurance.
And if you like a more formal workout, almost every gym offers HIIT classes, although sometimes they have different names like “boot camp” or “tabata.” Descriptions of the class should mention HIIT in some form.
HIIT is hard, but there’s more good news: Once you’re in the shape you want, it doesn’t take a lot for your body to stay there. “It just takes about two times a week to maintain that [fitness], and you don’t have to work out high-intensity all the time,” Kreider says.