Understanding Fat Loss Through Exercise & Diet

When Does the Body Burn Fat During Exercise?

Many people turn to exercise with the goal of shedding fat. But contrary to popular belief, the body doesn’t instantly switch to burning fat the moment you start working out. Instead, it follows a complex energy-utilization process that depends on the intensity and duration of the activity, as well as your body’s existing fat reserves.

Energy Use: Glycogen vs. Fat

According to Paulo Correia, a professor of physiology at the Federal University of São Paulo, Brazil, the body’s primary and immediate source of energy is glycogen—a form of glucose derived from carbohydrates and stored in the muscles and liver.

“Glycogen provides fast energy, especially for short bursts of intense activity like sprinting or weightlifting,” explains Prof. Correia.

This glycogen comes from both healthy carbohydrates, such as fruits, vegetables, and whole grains, and less nutritious options like sugary snacks, white bread, and soft drinks. While both sources supply energy, the latter are high in calories and low in nutrients.

On the other hand, fat acts as a long-term energy reserve. When we consume more calories than we burn, the surplus is stored as fat. But accessing that fat for energy is a slower process.

The Candle and the Log Analogy

Ed Merritt, a professor of human movement at Southwestern University in Texas, uses a simple analogy: “Fat is like a candle—it burns slowly and steadily—while glycogen is like a log that ignites quickly and burns out fast. The body works in a similar way. When we need quick energy, like during intense workouts, we burn carbs. But for lower-intensity, longer-duration activity, we burn more fat.”

Merritt notes that fat becomes a primary energy source during moderate to light physical activities.

Misconceptions About Fat Burning

It’s a common misconception that sweating heavily is the best way to lose fat. While cardio workouts like running or cycling do burn calories, strength training is just as critical.

Building muscle not only improves overall health but also reduces the risk of chronic conditions like diabetes, heart disease, and osteoporosis. Muscle mass also increases post-exercise calorie burn, helping the body continue burning fat even at rest.

“The key to fat loss is a calorie deficit—burning more calories than you consume,” says Prof. Correia. “After a workout, your body continues to burn energy while it restores glycogen in the muscles and liver.”

Factors That Affect Fat Burning

Your individual traits—such as genetics, age, and fitness level—play a significant role in how efficiently your body burns fat.

Exercise and Nutrition: A Crucial Relationship

Exercise alone isn’t enough. What you eat after working out can greatly influence how your body uses and stores fat.

“After exercise, your body attempts to replenish depleted glucose stores. If you don’t eat right away, it may start converting stored fat into usable energy,” explains Prof. Merritt.

However, if performance is your goal—whether lifting heavier weights, running faster, or improving endurance—then consuming food immediately after exercise is essential for recovery and future performance.

Low-Carb Diets and Fat Metabolism

Some people turn to low-carbohydrate diets to increase fat burning. While this can be effective for some, reducing carbs during exercise can have negative effects.

Too few carbohydrates can lead to fatigue, muscle weakness, and even muscle loss, as the body may begin breaking down muscle tissue to convert into glucose when fat alone cannot meet energy demands. This can also impair the immune system, as glycogen plays a critical role in immune function.

Calorie Control Still Reigns Supreme

Exercise reduces the body’s ability to store fat, but it has its limits when it comes to burning calories.

“Fat accumulates when excess energy goes unused,” notes Prof. Correia. To put it in perspective, one pound of fat is equivalent to approximately 7,000 calories.

For instance, a 30-minute bike ride may burn up to 300 calories. But just one slice of pizza or a piece of chocolate cake can easily offset that deficit.

“Exercise is vital for overall health, but calories burned can be quickly negated by food choices,” Prof. Merritt concludes.