Cardiovascular-based exercise is a common inclusion to structured weight loss programs – and rightly so.

Meta-analyses’ consistently prove that combining exercise with appropriate diets is more effective in promoting long-term weight loss than diet-only methods [1].

This becomes obvious when realizing that weight management is just a simple balance between energy intake and expenditure [2].

Since cardiovascular exercise increases the energy expenditure side of the energy balance equation, it is clear that doing so will favor a reduction in body weight – provided energy intake does not change.

However, not many people dispute this fact. A newer and more debated concept is whether the consumption, or avoidance of certain nutrients prior to cardiovascular exercise may further enhance training adaptations and weight loss.

This has led on to what is known as “fasted cardio”, which is when people undertake cardiovascular exercise after a prolonged period without food (>6 hours), typically in the morning after sleep.

But despite an apparent theoretical basis, is there really evidence to support this in favor over exercising after a meal?

Does It Help With Fat Loss?

A key thing to note is that the body is well-adapt to utilising the type of energy that is made available to it at any given moment.

This is why people eating carbohydrate-based diets will use more carbohydrates for energy, and people eating fat-based diets will use more fat for energy. In the case of a high-carbohydrate and high-fat diet, the body will favour carbohydrates, especially during exercise.

Following on from this, starting exercise with low carbohydrate (glycogen) stores and insulin levels will naturally cause the body to shift energy utilization away from carbohydrates, and towards stored body fat or dietary fat.

In the case of fasted training where no carbohydrates have been consumed in the diet, body fat stores will be presented as the main fuel option.

Clearly for people looking to get rid of excess bodyfat, this sounds appealing.

Recent meta-analysis also support this position, concluding that aerobic exercise performed in the fasted state induces higher fat oxidation rates than exercise performed in the fed state [3].

However, there is certainly more to the puzzle. As we have discovered, changes in bodyweight are dependent on daily energy balance across a 24-hour period.

Considering this, an important question becomes whether the body compensates for the additional body fat used during fasted cardio by reducing fat utilization throughout the rest of the day.

Unfortunately, when studies have compared weekly training sessions of fasted cardio versus fed cardio, alongside an isocaloric diet, there are no clear differences in body weight changes between groups after 1-2 months [5] [6].

Therefore, based on this it seems as though the theoretical basis behind a “fat-burning advantage” to fasted exercise is focused too much on fat use during training, and ignores the dynamic nature of the human body which continually adjusts its use of substrate for fuel to maintain homeostasis.

Experts now agree that a greater utilization of fat for fuel during a given time period is compensated for by a greater carbohydrate utilization later in the day [7]. Considering this, its logical to assume that nutrient intakes over a 24-hour period if far more impactful than nutrient intakes before training.

Fat burning must be considered over the course of days — not on an hour to hour basis — to truly assess its impact on body composition.

Those seeking to lose body fat conceivably can choose to train either before or after eating based on preference [8].

However, it is worth noting that none of these studies have been performed for long durations (>3 months), and so one cannot rule out the possibility of small differences across extended periods – albeit unlikely.

It is also worth mentioning that despite no apparent direct benefits on fat loss, fasted training appears to indirectly reduce daily energy intake and may therefore be beneficial for weight management through other avenues [9].

Are There Any Benefits To Doing Fasted Cardio?

Just because fasted training does not provide additional benefit over fed training for body fat loss, does not mean to say there are no other reasons why it might be beneficial.

And in fact there are, with many elite endurance athletes utilizing fasted training as part of their training regimes.

This is due to substantial research in support of fasted training being able to upregulate training adaptations in response to cardiovascular exercise at the metabolic and cellular level.

The main benefit is the improved capability for the body to burn fat as a fuel during exercise, and thus spare carbohydrate stores in the process which are most crucial for later stages of exercise where the intensity increases.

Several studies support the idea that continuously exercising in a fed state results in the inability to efficiently allow fatty acids to enter the mitochondria to contribute towards energy production [10].

In other words, if athletes do not train their bodies to burn fat during exercise, the body will subsequently decrease the expression of genes involved in fatty acid transport and oxidation [11].

This can be detrimental for athletes in many sports as when carbohydrates become depleted is the point when fatigue begins to quickly set in.

In an ideal situation, an athlete will use fat during periods of low-intensity exercise to spare carbohydrate stores which are required to support high-intensity exercise.

Much evidence indicates that consistent exercise while fasted results in chronic molecular adaptations favorable to fat oxidation, by increasing the content of intramuscular fatty acid binding protein and uncoupling-protein-3 content to a greater extent [12].

In fact, deliberately training with reduced carbohydrate availability to enhance endurance-training adaptations has now been officially termed “train low” – a current hot topic within sport nutrition [13].

For these same reasons and mechanisms, regimented fasted training may promote superior improvements in glucose tolerance and insulin sensitivity which could, in theory, improve glycemic control and prevent diabetes [14].

Conclusion

Cardiovascular exercises such as jogging and rowing are used by people trying to lose weight as a means to burn additional calories.

However, in recent years people have also started to undertake this exercise in a fasted state in hope that they can target their body fat stores for energy use.

The scientific consensus on this theory is that it probably does not hold practical value, and any energy source used during exercise will likely be compensated for over the course of the day.

However, fasted cardio is still used in athletic purposes enhance endurance training adaptations, a concept which has been recently supported by sport nutritionists.