When do we start to actually burn fat in an exercise?
Contrary to what many people think, during physical exercise, fat burning does not start immediately. The body uses different sources of energy throughout the activity, and fat starts to be used more intensively as the time and intensity of the exercise increases. Understanding when this happens is essential for anyone looking to lose weight or improve body composition.
In the first minutes of exercise, the main source of energy used by the body is glycogen, a form of carbohydrate stored in the muscles and liver. This choice occurs because carbohydrates are more quickly mobilized and efficient to generate energy immediately, especially in more intense training, such as weight training and aerobic exercises.
After approximately 20 to 30 minutes of continuous activity, especially if it is of moderate intensity, the body begins to rely more significantly on fat as fuel. This does not mean that before this time no fat is burned, but rather that its energy contribution is lower.
"From the beginning of exercise, the body uses a mixture of carbohydrates and fat to generate energy. But fat burning becomes more prominent, generally after a few minutes of continuous exercise at a light to moderate intensity. This happens because, over time, carbohydrate stores (glycogen) are depleted, making the body need to rely more on fat as a source of energy", explains Yuri Sena Santos, exercise and sports doctor.
Another factor that influences the timing of fat burning during physical activity is the person's level of physical fitness. Athletes or more trained individuals are able to use fat as an energy source more efficiently and earlier. Those who are just starting out tend to rely more on carbohydrates in the initial stages of training.
"More trained people develop metabolic adaptations that increase the efficiency of burning fat, even at higher intensities. This happens because the cardiovascular system and mitochondrial capacity improve with training, favoring the use of fat as fuel. In sedentary people, the body tends to depend more on carbohydrates and takes longer to access fat reserves", says Rogério Sthanke, physical educator.
Intensity is also crucial. In low to moderate intensity exercises, such as walking or cycling at a steady pace, the body tends to use more fat as an energy source. In high-intensity training, the body prefers carbohydrates, as they are metabolized more quickly, although the total caloric burn is greater - which can favor weight loss when considering the expenditure throughout the day.
"So for those looking to burn fat, longer, moderate-intensity workouts are a good choice - but that doesn't mean that short or intense workouts don't have their benefits," adds Sthanke.
Plus, fat burning doesn't end when exercise ends. The body continues to spend energy to recover, especially during intense activities. This phenomenon, known as EPOC (excess post-exercise oxygen consumption), prolongs calorie burning for hours after the end of training, and may also contribute to fat oxidation during this period.
Therefore, there is no single formula to "activate" fat burning. The combination of regularity, adequate duration and intensity is what will determine results over time.
"Those who train regularly develop adaptations such as better transport of fat into the cells and greater efficiency in burning this fuel. With training we can also increase muscle mass, which ends up increasing our Basal Metabolic Rate, which is the amount of calories our body needs to function at rest. In other words, the better the conditioning, the greater the capacity to burn fat", concludes Santos.
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Source: CNN