Recent research reveals that our bodies continue to burn extra energy well after our workout session has ended. Scientists are uncovering the mechanisms behind this phenomenon and, most importantly, how to activate this effect. Fat continues to burn even during rest periods.
In our bodies, there are substances not typically favored by doctors, such as uric acid and cholesterol, or a protein called interleukin-6 (shortened to IL-6), which can induce inflammation and plays a key role in rheumatism, diabetes, and severe coronavirus infections. However, IL-6 can also have a positive impact. Eduardo Ropelle from the University of Applied Sciences in Limeira, Brazil, explains, "In certain situations, it fulfills different functions—such as burning muscle fat during and after exercise."
Molecular biologist Eduardo Ropelle studies the physiological causes of obesity and seeks ways to combat it. Recently, his focus has been on the phenomenon known as "excess post-exercise oxygen consumption" (EPOC), which is the body's ability to burn more calories not only during but also after exercise.
The existence of this effect is undisputed; however, its sufficiency in aiding weight loss remains unclear. Ropelle and his team believe they have found the key in IL-6. The release of this protein can be "up to 100 times higher during and after exercise compared to when the body is at rest," according to Ropelle.
The Body Cools Down Longer Than an Engine
To understand the role of IL-6, researchers conducted an experiment on mice, injecting the protein into their muscles (specifically, the hind leg) and brains. It turned out that fat was only burned in mice that received the brain injection. The scientists suspected the hypothalamus, which has IL-6 receptors, was responsible.
This hypothesis needed verification. In a drastic experiment, researchers severed the sciatic nerve, which transmits information from the legs to the spinal cord and up, in the mice. Then, they injected IL-6 into the animals' brains again. As expected, nothing happened.
"The burning of muscle fat is indeed due to the neural connection between the hypothalamus and muscles," the scientists concluded. This also explains why the afterburn effect can last for hours. The level of IL-6 in muscles drops soon after exercise, allowing the hypothalamus to continue sending neural signals to the muscles for hours post-exercise.
This discovery also clarified the phenomenon often referred to in sports science as "homeostasis disruption." Theodor Stemper, a sports scientist from the University of Wuppertal, explains, "Any sports activity can be seen as a disruption of balance since our body is normally at rest." After the disruption, the body must return to a state of equilibrium, which involves an accelerated metabolism and occurs even after the sports activity has ended. "Our body doesn't cool down like a car engine, which takes about half an hour after being turned off," Stemper adds. "The restoration of homeostasis takes much longer for us."
How to Activate the Afterburn Effect
The duration of this effect largely depends on the intensity of the exercise. A light walk won't activate the afterburn effect. It requires utilizing more than 60%, ideally even 90%, of maximum performance for at least 30 minutes. The afterburn effect can last for 24 hours or more, significantly impacting calorie consumption.
In a study by Queens University in Charlotte, ten healthy men were allowed to pedal on a stationary bike for 45 minutes at just below 60% of their maximum performance. Using a metabolic chamber, researchers recorded the participants' energy expenditure and compared it to their resting calorie burn.
The result: participants on bikes burned over 500 additional kilocalories. And in the 14 hours following their workout, they burned another 200 kcal thanks to the afterburn effect. With three workout sessions a week, this adds up. For comparison, the recommended daily calorie intake is 2,400 kcal for men and 1,900 kcal for women.
An even greater afterburn effect can be achieved by pushing training intensity to the limit. American scientists discovered a particularly high EPOC following high-intensity interval training (HIIT), where 30-second periods of very intense exercises (at least 85% of maximum performance) are alternated with equally long periods of less intense activity. This can be achieved, for example, by sprinting on a treadmill followed by a steady-state endurance run.
"EPOC increases linearly with the duration of the workout," says study lead Jeffrey Janot from the University of Wisconsin. "But it grows exponentially with the increase in training intensity." The American sports scientist's advice for those looking to lose weight is as follows: if you can dedicate 30 minutes to exercise three times a week, you should opt for high-intensity interval training over endurance running.
In the first few minutes, the afterburn effect is marked by classic sweating. "However, afterward, you hardly feel anything, even though calorie consumption remains elevated," Stemper says. This means you can take a shower and relax at home in front of the TV—and continue burning calories. It's especially helpful to skip the peanuts or gummy bears at this time.
Mass Matters Too
Fundamentally, there's no difference between sexes in the afterburn effect. "But of course, it depends on muscle mass," Stemper explains. "Men usually have an advantage here." The level of an individual's training is even more crucial; inexperienced athletes lack the energy for intense training. Their muscles have fewer mitochondria, the so-called cellular power plants that provide energy for movement.
This is evident not only during sports activity but also in the long phase of returning to homeostasis—trained individuals benefit more than beginners. Beginners, therefore, need patience and a systematic training program until they experience the first successes in losing weight through exercise. This could take months or even years.
From the perspective of those looking to lose weight, intense sports activity has another advantage: muscles grow excellently. And muscle mass consumes more calories than fat tissue—not only during activity but also at rest. "If I replace one kilogram of fat with one kilogram of muscle mass, I automatically triple my metabolic turnover," Stemper says.
In practice, this only means an additional 13 kcal per kilogram of body mass per day, as muscles only use a fraction of the energy burned by other organs, such as the brain, liver, and heart. "However, this is a building block that, together with higher energy consumption during and after exercise, constitutes an effective weight control tool," the scientist adds.
Not to mention that exercise not only increases the body's energy expenditure but also— as recently discovered at Stanford University School of Medicine—reduces calorie intake. This is thanks to a compound called N-lactoyl-phenylalanine (shortened to Lac-Phe), which acts as an appetite suppressant in the body.
Lac-Phe is produced from lactic acid, which every athlete knows all too well, as it can cause muscle fatigue. And the more intense the sports activity, the greater the fatigue. Stanford scientists found that the most Lac-Phe is produced after interval training, followed by strength training. 90 minutes of endurance training on a stationary bike had almost no effect on appetite suppression. So, there's no way around it: if you want to lose weight through sports, you need to sweat and get out of breath. Not for long, but time and again.
