High-intensity interval training (HIIT) has become the popular way to exercise in recent years, with studios and gyms taking advantage of this efficient workout method. HIIT involves alternating between intense bursts of activity, like sprints, and periods of less-intense active recovery or rest. Supporters of this highly efficient workout claim that it effectively burns calories, targets fat, and builds muscle in less time compared to a moderate-intensity run. (More: Your Comprehensive HIIT Workout Guide)
Another claimed benefit of HIIT is the body’s ability to continue boosting metabolism long after the workout is over. This phenomenon, known as the afterburn effect, is said to increase metabolism and calorie burn for up to 24 hours post-exercise. Many HIIT-based studios promote this as a significant health benefit of their workouts, suggesting that you’ll enjoy the effects of a one-hour class for days.
But are these benefits too good to be true? Is the afterburn effect real? Here’s what the science has to say about it.
The afterburn process, scientifically called excess post-exercise oxygen consumption (EPOC), refers to the oxygen required by the body to return to its pre-workout state or resting metabolism. While exercising, the body utilizes oxygen to produce adenosine triphosphate (ATP), which acts as fuel for the muscles. However, it can also tap into stored energy sources that don’t require additional oxygen. HIIT workouts often rely more on these energy sources compared to steady-state exercise, leading to a higher oxygen requirement after the workout. This is why short bursts of intense exercise are associated with the afterburn effect.
“Oxygen and calorie burn are closely linked after exercise,” explains Tedd Keating, Ph.D., C.S.E.S., an associate professor of kinesiology at Manhattan College. “Metabolism remains elevated for a period of time post-exercise.” However, the number of calories burned during the afterburn is typically only 6 to 15 percent of the total calories burned during the workout. For example, if you burned 300 calories during your workout, the afterburn would only account for approximately 18 to 45 calories.
(Physical activity isn’t the sole approach to sustain a rapid metabolism, by the way. Adjustments in one’s way of life such as consuming an adequate quantity of protein might also be beneficial.)
EPOC is not restricted to HIIT workouts — it is present after all aerobic exercise — but the degree of exercise intensity does play a role in how many calories your body burns after sweating. For low-intensity activity — for instance, a treadmill jog — you can expect a lower afterburn effect (around 6 percent calorie burn, toward the lower end of the range), but with HIIT workouts and exercises like Tabata and speed drills, you could experience an afterburn closer to that 15 percent mark, emphasizes Keating.
There are several other factors that influence your individual afterburn benefits as well: weight, fitness level, and muscle mass all have an impact. “People who are more aerobically fit, their bodies are going to be better at burning fat overall,” says Keating. You can anticipate these individuals to have a more prolonged afterburn effect.
Does the Afterburn Effect Last for 24 Hours?
In brief, the assertion that your body continues to burn calories for up to 24 hours after a workout is misleading. Although individuals who are more physically fit may experience longer benefits from the EPOC effect, it is typically only up to a couple of hours, at most.
A study from Colorado State University assessed participants who engaged in sprint interval training and then measured their post-workout calorie burn. Although the results did demonstrate energy expenditure within 24 hours after the workout, most of the calorie burning was observed during and immediately after exercise, with the figures declining significantly from there. (And it should be noted that your body is continually burning energy/calories even at rest.)
So does that mean the 24-hour EPOC effect is simply a myth? “Never say never. There might be an exceptionally rigorous exercise program that can achieve it,” says Keating. “But your typical high-intensity sprint interval stuff that is currently being recommended, like four 30-second bursts or something milder like 10 one-minute bursts, probably will not provide people with that substantial 24-hour afterburn,” he adds.
But HIIT Benefits Go Beyond the Afterburn Effect
Even though HIIT is one of the most effective workouts for burning fat, the potential afterburn effect is only part of the equation, says Keating. During high-intensity workouts, your body produces more epinephrine and human growth hormone (HGH), both of which have fat-reducing capabilities; one study found that a 30-second sprint on the stationary bike led to a nearly 450-percent increase in HGH production. Other reported benefits of HIIT are improved insulin sensitivity and blood pressure, along with being an efficient way to exercise.
Interval training is also superior at preserving lean body mass, says Keating. And the more muscle you have, the more calories you will burn while at rest.
Plus, as you become more physically fit, you’ll enhance your VO2 maximum, which is a numerical indication of how effectively your body utilizes oxygen during physical activity.
In summary: Although HIIT exercise is an excellent means of improving your fitness, be cautious of fitness studios that promise to miraculously transform your body into a metabolism-boosting machine long after you’ve finished your workout. However, this doesn’t imply that you should abandon your preferred boot camp (remember, HIIT training still enhances endurance and body composition). Rather, the afterburn effect should not be the sole factor in selecting a specific type of exercise. (Coming up: The Distinction Between Muscular Endurance and Muscular Strength, Explained)
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