When does the body start to burn fat during exercise?

Many people go to the gym with a goal in mind: burn fat. However, the body does not immediately begin this process as soon as the exercise begins.

Instead, the body uses a complex energy system, which varies according to the intensity of the exercise, the duration and availability of fuel.

To understand how to make your body burn fat, it is essential to understand how it uses energy.

The immediate reserve of body energy is glycogen, a carbohydrate form stored mainly in muscles and liver, explains Paulo Correia, professor of physiology at the Federal University of São Paulo (Brazil).

“It provides immediate energy for activities that require fast effort explosions, such as running 100 meters at high speed or raising heavy weights.”

When the body detects the urgent energy need for intense effort, glycogen is rapidly converted into glucose, which is used by cells to generate ATP (adenosine triphosphate), the main energy molecule.

But this reserve is limited, so high intensity activities can quickly lead to fatigue, especially in people who are not well trained.

Glycogen comes from carbohydrates we consume, including healthy options such as fruits, vegetables and whole grains, as well as less healthy choices such as candy, white bread and soda.

Although both types of carbohydrates provide energy, less healthy options tend to be more caloric and less nutritious.

Fat acts as a reserve of energy when the body does not need immediate energy and stores the surplus of calories for future use. It provides more energy by molecule than glycogen, but the process of fat conversion to usable energy is slower.

Ed Merritt, Professor of Kinesiology at Southwestern Universityin Texas, uses an analogy to explain this process.

“The candle represents the fat – it is slow and constantly, providing energy over time – while wood burns fast. Our body works similarly. When we need fast energy, we burn carbohydrates. But when energy demand is more moderate, the body (the body is mainly to fat.”

Merritt explains to BBC that this concept relates to what has been popularized as the “fat burning zone”, in which the body uses it as the main fuel in low and moderate intensity activities.

But he points out that this zone can be achieved even during sedentary activities, such as sitting at the table or watching TV – and that doesn’t mean weight loss is occurring.

“Trusting exclusively in the fat burning zone for weight loss is a mistake. The story is much more complex than that,” he adds.

A common – and wrong – idea is that cardiorespiratory exercises (such as running, walking and cycling) are the only effective way to lose fat.

Although these activities usually burn a higher number of calories when compared to others, such as bodybuilding, strength training is equally important.

Building muscle mass increases basal metabolism, as muscle tissue requires the body to burn more energy to maintain itself than fat. In other words, this helps you burn more calories even when you are not exercising.

In addition, muscle mass plays a key role in overall health, helping to prevent chronic conditions such as diabetes, heart disease and osteoporosis (bone weakening).

Intense exercises burn mainly glycogen, while fat becomes the main source of energy during moderate and prolonged walks – enough time for glycogen to run out, but not so intense as to miss the fast energy needed to maintain movement.

This range of intensity is known as “zone 2”, which corresponds to about 60-70% of its maximum heart rate. Although it has benefits for cardiovascular health and fat oxidation, it is not necessarily the most effective for fat loss.

When it comes to exercise, the key to fat loss is to burn calories – it doesn’t matter so much where they come. The body stores fat when calorie intake exceeds the expense. On the other hand, when you burn more than you consume, you lose weight.

“In low intensity, you burn more fat, but fewer total calories. In high intensity, a lower percentage of calories comes from fat, but you burn more calories in total,” says Professor Merritt.

Even High Intensity Interval Training (HIIT), which does not mainly burn fat during exercise, can help in loss of fat over time.

“After exercise, energy consumption continues – this is known as the ‘after Burn effect’. The body continues to use muscle and liver glycogen until activity levels return to normal,” explains Professor Correia.

Individual factors, such as genetics, age and level of fitness, play a significant role in the efficiency with which a person burns fat.

Genetics can cause some people to have a naturally faster metabolism, allowing them to access fat reserves more easily. As we get older, metabolism slows down, muscle mass decreases and hormonal changes affect the storage and use of fat.

How you replenish energies when ends a workout can also play a significant role.

“After exercise, the body works to replace the fast -burning fuel (glucose) that was used. If you do not feed immediately, the body can use stored fat to restore these levels,” explains Professor Merritt.

However, if your goal is to maximize performance-either by raising more weight, running faster or improving resistance-eating right after training is essential.

“This helps in recovery so that you can train with intensity again next time. In the end, it all depends on your goal. Weight loss and performance often conflict,” he adds.

Reducing carbohydrate intake can promote fat burning and be effective in some cases. However, when combined with regular exercises, this can be counterproductive.

A low carbohydrate diet can lead to fatigue, muscle weakness, and even muscle loss, as the body can begin to break muscle tissue to produce glucose when fat alone cannot meet its energy needs.

In addition, it can compromise the immune system, as glycogen is essential for the body’s recovery and defense.

Physical activity makes fat accumulation difficult, but there is a limit to the amount of calories that exercise may burn. Therefore, the diet and energy balance are fundamental to fat loss.

“Fat is only stored when excessive energy is not used,” says Professor Correia.

To put this in perspective, a kilo of fat is equivalent to approximately 7,000 calories. A 30 -minute cycling session can burn up to 300 calories, but this can be easily canceled by consuming only a slice of pizza or a brownie.

“Exercise is essential for overall health, but calories burned during exercise can easily be replaced by food,” concludes Professor Merritt.