Many people go to the gym with one goal in mind – to burn fat. However, the body doesn't immediately go into fat-burning mode. Instead, it relies on a complex system of energy utilization, depending on the intensity and duration of the workout and the availability of energy.
To understand how the body burns fat, you first need to know how it uses energy.
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How does the body use energy?
"The body has immediate energy reserves in the form of glycogen, a type of carbohydrate, which is stored in the muscles and liver," explains Paulo Correia, a professor of physiology at the Federal University of São Paulo, in Brazil.
"Glycogen provides rapid energy for activities that require sudden bursts of effort, such as a 100-meter sprint or heavy weightlifting," Professor Korea tells the BBC.
Glycogen comes from the carbohydrates we consume, which include healthy ones like fruits, vegetables, and whole grains, as well as less healthy ones like sweets, white bread, and sodas.
Although both types provide energy, unhealthy sources often have more calories and fewer nutrients.
Fats serve as an energy reserve when we consume more calories than we burn.
They provide more energy per molecule compared to glycogen, but the body takes longer to break them down and convert them into usable energy.
Ed Merritt, a professor of kinesiology at Southwestern University in the US state of Texas, uses the example of a candle and a piece of wood to explain this process.
"A candle represents fat - it burns slowly and steadily - while wood burns quickly and strongly, but is quickly consumed."
"Our body works in a similar way."
"If we need quick energy, like during intense exercise, we burn carbohydrates."
"But if energy demands are lower, we rely more on fat."
Professor Merritt explains to the BBC that this concept refers to the "fat burning zone" when the body uses fat as its primary source of energy during low to moderate intensity activity.
However, he emphasizes that this zone can be reached even during sedentary activities, such as working at a computer and watching television, but that does not mean that weight loss occurs.
"Relying solely on the fat burning zone for weight loss can be misleading."
"The story is much more complex," he adds.
Which exercise is best?
A common misconception is that cardio exercise is the only effective way to lose fat.
Although activities like running and cycling burn calories, strength training is equally important.
Building muscle speeds up your resting metabolism because muscle tissue requires more energy to maintain than fat tissue.
This means you burn more calories even when you're not exercising.
Muscle mass plays a key role in overall health, as it helps prevent chronic conditions like diabetes, heart disease, and osteoporosis (weakened bones).
During intense exercise, the body primarily burns glycogen.
However, during moderate-intensity activities, such as long walks, the body switches to burning more fat when glycogen stores begin to deplete.
This intensity of activity is known as "Zone 2," which is approximately 60 to 70 percent of your maximum heart rate.
While this method is beneficial for heart health and increases fat oxidation, it is not necessarily the most effective for fat loss.
If you want to lose weight, burning calories is key, and it's less important where they come from.
The body stores fat when calorie intake is greater than calorie expenditure.
And when you burn more calories than you consume, you lose weight.
"During low-intensity activity, you burn a higher percentage of fat, but fewer total calories."
"And during high-intensity activity, a smaller percentage of the calories burned come from fat, but you burn more calories overall," explains Professor Merritt.
High-intensity interval training (HIIT) may not burn fat directly during exercise, but it can contribute to fat loss over time.
"After exercise stops, increased energy expenditure continues."
"The body continues to use glycogen from the muscles and liver until activity levels return to normal," explains Professor Korea.
Individual factors, such as genetics, age, and fitness level, play a significant role in the efficiency of fat burning.
Thanks to genetics, some people may have a naturally faster metabolism, which allows them easier access to fat stores.
As we age, metabolism slows down, muscle mass decreases, and hormonal changes affect fat storage and utilization.
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Should you eat after training?
While exercise is important for fat loss, how you recover after a workout is also important.
"After a workout, your body works to restore the fast-burning fuel (glucose) you used up."
"If you don't eat immediately after training, the body can break down stored fat to restore those levels," explains Professor Merritt.
However, if your goal is to maximize performance, whether you want to lift heavier weights, run faster, or increase endurance, a meal immediately after training is crucial.
"It helps your body recover, so you can train intensely again next time."
"Ultimately, it all depends on your goal."
"Weight loss and performance improvement are often in conflict," adds Professor Merritt.
Reduced carbohydrate intake can stimulate fat burning and be effective in some cases.
However, when combined with regular exercise, it can be counterproductive.
A low-carb diet can lead to fatigue, muscle weakness, and even muscle loss, as the body may begin to break down muscle tissue to produce glucose when fat cannot meet energy needs.
It can also weaken the immune system, as glycogen plays a key role in the body's recovery and immune function.
How diet affects fat burning
Physical activity makes it harder to store fat, but the effects of exercise alone are limited in terms of the number of calories that can be burned.
Nutrition and overall energy balance are key.
"Fat is only stored when excess energy is not used," says Professor Koreja.
Let's turn everything into numbers.
A kilogram of fat is equal to approximately 7.000 calories.
A half-hour bike ride can burn about 300 calories, but that loss can easily be offset by a slice of pizza or cake.
"Exercise is essential for overall health, but the calories burned during exercise can easily be replaced through diet," concludes Professor Merritt.
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