Does Fasted Training Burn Fat?

The term “fat burning” refers to the ability to oxidize (or burn) fat, which uses fat as fuel instead of carbohydrates. Burning fat is usually associated with weight loss or body composition changes that involve decreasing body fat and increasing lean body mass. 

What you eat and how much of each macronutrient (carbohydrate, fat and protein) you consume will influence energy metabolism and exercise performance. 

Carbohydrates are the body’s primary energy source and are readily broken down into glucose, which can be used immediately as fuel or sent to the energy stores (liver and muscle) in the form of glycogen. Since carbohydrates are the preferred energy source, eating carbohydrates before or during exercise will favor carbohydrates as the fuel source. Therefore, if you are wanting to increase fat oxidation during exercise, no carbohydrates should be consumed. However, no research so far has shown performance benefits. 

Ways to train fasted 

1.     Low carbohydrate availability 

• Exercising in an overnight-fasted state on an empty stomach (prior to breakfast)

• Restricting carbohydrate intake post-exercise and in between training sessions (two-a-days)

• Working out for several hours without carbohydrate intake

• Training late in the day and going to bed with restricted carbohydrate intake

2.     Low carbohydrate diet

• More energy coming from fat and/or protein

Low carbohydrate availability does not necessarily mean low-carbohydrate diet. It is simply altering the timing and availability of carbohydrates in the diet around specific training. Low carbohydrate availability refers to a situation where the body is deprived of carbohydrate in some way, usually in the energy stores.

What affects fat oxidation?

1.     Exercise intensity

• Oxidizing fat can happen initially with exercise then decrease at higher exercise intensities. The higher the exercise intensity, carbohydrates will be the primary source of fuel. To increase fat oxidation, exercising at low to moderate intensities are recommended 

2.     Duration of exercise

• Exercising for 30 minutes does not require carbohydrate intake. For high intensity exercise 45-75 minutes and prolonged low intensity exercise over 90 minutes, carbohydrates should be consumed to maintain or improve performance

3.     Mode of exercise

• Low to moderate impact exercises like walking and jogging have shown to oxidize fat

4.     Diet

• High carbohydrate diet will suppress fat oxidation, while a diet low in carbohydrates will result in higher fat oxidation 

5.     Nutritional supplements

• Many supplements are marketed and claim to be a fat burner. There is very little evidence supporting any supplement is truly a fat burner

6.     Gender differences 

• While there are small differences in gender, studies have shown the females have higher rates of fat oxidation than males

7.     Environment

• Hot climates and higher altitudes have shown to use carbohydrates as fuel as opposed to fat. Extremely cold conditions fat metabolism seems to take over

What about weight loss?

Weight loss can only be achieved with a calorie deficit. Therefore, you have to eat less calories than you expend regardless of the fuel used. Burning fat does not necessarily translate to fat loss. Also, with very low energy stores and prolonged recovery, muscle breakdown can occur, which would not be ideal body composition changes for weight loss.

When are fasted workouts okay? 

• Low to moderate impact exercises and intensities 

• Length of low intensity exercise does not exceed 60-90 minutes

• If certain foods are giving you gut discomfort

• Improving training adaptations 

• Improving maximal fat oxidation (utilizing fat as a fuel source)

In conclusion, training in a fed-state (consuming carbohydrates) will result in a better workout, which could lead to a greater fat loss or more desired body composition results.

Higher intakes of carbohydrates are associated with better performance. If you are looking for optimal performance to support high training volume, intensity and duration, then fueling with carbohydrates prior to training is recommended. 

Carbohydrate intake should differ depending on the different types, intensities and volume of training. Individualized recommendations for daily energy and macronutrient intakes should consider an athlete’s training or competition program, relative importance of low or high carbohydrate intake in accordance with quality of performance, enhancing training or adaptations. 

Training loads with low intensity and short duration, need less carbohydrate availability; thus, you do not have to necessarily consume carbohydrates. Work with a sports dietitian to get a personalized plan for you to determine nutritional intake for your training.