Burning Fats vs Carbohydrates (4 Things You Must Know)

The main difference between fat burning and carbohydrate burning is in metabolic pathways of digesting, breaking down, and extracting energy from these two sources.

See, the carbs you eat are either transformed into blood glucose or stored as a backup in the form of glycogen - a large branched and chained molecule made of hundreds of glucose units [1].

Glycogen is stored in muscles and the liver, but when stores reach the maximum capacity, glucose gets transformed into fat in a process called 'de novo lipogenesis' [2].

Meanwhile, fat as a fuel comes from healthy fat sources in foods that get broken down into fatty acids - the building blocks of the fat molecules, with an excess stored as energy backup in fat cells.

Naturally, the body has a greater capacity for storing fat than carbs since fat is more calorie-dense and, therefore, more practical. That's why excess carbs are also stored as fat.

Generally, the body burns fat as a primary source of fuel. However, when glucose is elevated, the body secretes insulin to deal with that sugar, shutting down other energy sources and putting burning fat on pause in the process.

So, depending on your diet (but exercise and metabolism as well), your body is probably more adapted to use one of these sources more efficiently.

"Carbohydrates are broken down into glucose, which may be used as an energy source by our cells, tissues, and organs."

- Aastha Kalra, DO, Founder of Weight Zero MD 

Energy for exercises comes from both carbohydrates and fats, but how many calories come from what source depends on the stored body fat, stored glycogen, and type of workout.

During the workout, your body burns both fat and carbs but at different times.

Since glucose is a fast energy source, the body will recruit the carb stores - glycogen in your liver and muscles as fuel in a process called glycolysis [4].

This fuel is depleted quickly, as glucose has only four calories of energy per gram, so it's activated in the first 30 minutes of exercises or when performing high-intensity exercise movements (anaerobic) like lifting a heavy object or sprinting [5].

After around 30 minutes, glycogen levels begin to decline and your body begins to use fat as a fuel source.

Even though fat has more calories per gram (nine), this happens later on because the process of converting fat into readily available energy is much slower.

Therefore fat metabolism is often activated during longer-lasting (aerobic exercises) like cycling, walking, and running to burn stomach fat [6].

When you combine regular aerobic exercise with good diet, you can help your body lose fat and burn fat, which leads to weight loss.

Aerobic exercises and other fat burning activities boost your body's metabolism, which helps break down and use stored body fat. This lowers your total body fat percentage and makes you look leaner.

The 'fat-burning zone' principle is developed based on the switching between fat vs. carb stores.

To work out in the fat-burning zone means keeping your heart rate relatively low while performing a continuous lower-intensity cardio exercise.

It is presumed that a large number of calories you burn will come from fat.

However, overall calorie expenditure will still be higher with high-intensity workouts, meaning, you will burn more calories and lose more weight performing higher intensity exercises than lower intensity ones.

Moreover, there are studies suggesting that HIIT workouts for fat loss provide better results in terms of individual calorie burn and fat store depletion compared to continuous, moderate-intensity cardio [7].

Yet another study showed increased fat metabolism and decreased carbohydrate metabolism after six weeks of continuous training [8].