Is fasting cardio recommended?

Is a morning jog without breakfast really the ideal way to start the day? Running on an empty stomach burns more fat, but it also promotes muscle loss. According to the majority of results: weight loss (fat) is equivalent between fasting and after-meal cardio. 

In other words, if running on an empty stomach doesn't hurt, this routine won't help you lose weight faster. Let's see why:

What physiological mechanisms are involved in fasting cardio?

Fasting cardio modifies the body's energy metabolism: after a night's sleep, glycogen reserves are partially depleted, and blood insulin levels are low.

In this state, the body consumes fatty acids to produce energy in the form of ATP during exercise: this process of fat oxidation is also known as lipolysis.

A 2016 ^study1demonstrated that around 3g more fat is burned during an endurance session performed on an empty stomach than during a session performed after a meal.

However, these results need to be qualified.

Firstly, because while training on an empty stomach promotes the elimination of body fat, it also risks promoting muscle loss.

To counter this, protein powders are commonly consumed by athletes during dry periods. The aim is to limit protein breakdown while effectively controlling calorie intake.

However, regular training during a fasting period has long-term benefits:

Daily fasting training over several weeks can induce interesting metabolic adaptations.

For example, six weeks of cardio training carried out systematically on an empty stomach led to an increase in the amount of intramuscular fatty acid transport and oxidation proteins, compared to the same training carried out with ^food3.

In other words, fasting cardio stimulates the utilization of stored fat and promotes metabolic adaptation for a better transition from glycolysis to lipolysis. However, these metabolic benefits do not necessarily translate into better long-term weight loss.

Having established this metabolic principle, let's move on to the next section, which looks at the real effects on performance and weight loss.

What are the real effects of fasting on performance and weight loss?

Effects on weight loss

Exercising on an empty stomach in the morning raises many hopes for effective weight loss. Today, however, a large body of scientific research qualifies the beneficial effects of training on an empty stomach.

The authors of a 2018 meta-analysis of several studies comparing the impact of fasting aerobic exercise on weight loss claim that weight and fat loss is equivalent between fasting and post-meal cardio.

The authors argue that exercise-related weight and fat loss is more likely to be improved by creating a significant caloric deficit over a period of time, rather than by exercising in a fasted or fed state.

Some studies also suggest that doing cardio after a meal may slightly increase total energy expenditure via an increase in thermogenesis.

Eating before exercise stimulates the metabolism (thermogenic effect of the meal) and can result in a higher post-exercise caloric expenditure compared to exercise on an empty stomach.

Effects on performance

In terms of performance, the intensity of a sports session can be compromised when training on an empty stomach.

With depleted glycogen reserves, it becomes difficult to maintain prolonged high-intensity sporting activity.

• High-intensity cardio: for high-intensity or short-duration workouts, fasting cardio may be less effective than exercising after eating, as the body primarily uses carbohydrates as an energy source for these types of efforts.
  
• Endurance : For endurance exercise, such as long-distance running or cycling, fasting exercise can be beneficial in terms of metabolic adaptation, as it forces the body to efficiently use fat as an energy source. Again, beware, without circulating amino acids, the body can draw on muscle tissue, particularly during prolonged efforts.

In any case, while sport in the morning poses no problem, for best performance it's advisable to have eaten before your physical activity, to ensure your glycogen reserves are full.

What is the recommended approach to muscle toning?

The principle of muscle drying is simple: lose body fat while preserving muscle tissue.

But what is the most effective method?

Once again, unsurprisingly, the way in which calories are spent is less important than the calorie deficit to be respected. The important thing is to optimize recovery and choose an approach that is sustainable over time.

• Fasting cardio: practised by many before breakfast, we have shown above that the fat loss achieved will be equivalent to that of cardio performed at another time if the calories expended and ingested are the same. In terms of recovery, fasting cardio is often performed at low intensity and doesn't interfere too much with muscle recovery, which means it can be done frequently.
  
• Fractional cardio training, also known as HIIT (High Intensity Interval Training), is an interesting alternative. This method alternates periods of intense effort with phases of recovery, mainly soliciting the anaerobic system. A 2019 meta-analysis revealed that HIIT and continuous moderate cardio produce equivalent reductions in body fat, with the advantage that HIIT requires less time for a session. The post-HIIT EPOC (Excess Post-exercise Oxygen Consumption) effect prolongs calorie burning for several hours after exercise. This high post-exercise metabolism contributes to total energy expenditure^.4

It is essential to understand that the determining factor for rapid drying is the caloric deficit, and that the best approach is one that suits your preferences and lifestyle.

Even though the benefits of cardio are numerous, weight loss combined with resistance training is often more effective than weight loss combined with endurance training in terms of energy expenditure.

The objective is always to aim for a progressive approach with a balanced diet, including all the essential macronutrients, optimized training, good hydration and respect for recovery times.

What about intermittent fasting? 

Intermittent fasting is an interesting approach. This diet consists of alternating periods of fasting and eating, and has shown positive effects on reducing fat percentage while maintaining muscle mass.

A 2024 study shows that participants practicing intermittent fasting spontaneously reduced their calorie intake, creating a deficit favorable to fat loss without feeling severely restricted.

The article suggests that intermittent fasting, unlike prolonged fasting or strict low-calorie diets, does not appear to compromise sporting performance. Despite periods of fasting, the body maintains blood glucose levels sufficient for performance. ⁵

Conclusion

In short, fasting cardio is just one way of losing fat: it effectively mobilizes fat during exercise, but scientific studies show that it offers no clear advantage over other strategies in terms of weight loss or muscle definition: creating a caloric deficit while maintaining training and good nutrition remains the determining factor.

High-intensity split cardio (HIIT), on the other hand, burns mostly carbohydrates during exercise, but induces post-exercise lipid expenditure, and ultimately results in a similar fat reduction for a shorter training time.

The choice between these methods therefore depends above all on personal preference, intensity tolerance and fatigue management.