Will fasted cardio benefit my training performance?

Have you wondered about fasted cardio for enhancing your endurance performance?

Table of contents:

• How can nutrition impact my training adaptations?
• How could fasted cardio enhance endurance performance?
• What does the research show?
• Could there be downsides to training in a fasted state?
• Take aways

How can nutrition impact my training adaptations?(some science)

The manipulation of nutritional intake can promote training adaptations. Training low is a term used to describe training in a state of low carbohydrate availability.

The term low carbohydrate availability may describe: low muscle glycogen, low liver glycogen, low-carbohydrate intake pre/during/after exercise, or any of these combinations.

Low carbohydrate availability, which may occur with fasted cardio, may be associated with beneficial training adaptations. Research shows certain genes to be expressed that make proteins associated with improved training and performance (1).

How could fasted cardio enhance endurance performance?

Fasted cardio may be a tool for training low. During training, a stress signal is stimulated, initiating certain proteins to be made, which over a period of time result in training adaptations taking place.

Nutrient availability may also influence signals of stress and training adaptations. For example, levels of glycogen influence levels of signalling substances, such as AMPK (AMP activated protein kinase), which may act as a signal for the regulation of certain genes and subsequent proteins involved in levels of performance.

What does the research show?

Fasted cardio depletes liver glycogen levels rather than muscle glycogen. Studies show that versus training in a fed state, fasted cardio may be superior at inducing training adaptations (2).

Training fasted may improve how effectively the muscles can utilise oxygen to generate energy!

Science: it is associated with the upregulation of specific markers involved in this process (citrate synthase and 3-hydroxyacyl-CoA dehydrogenase) (2,3) – this could theoretically translate into improved performance.

Other beneficial adaptations shown include the use of fat as fuel within muscle cells, improved blood glucose regulation and glycogen resynthesis (2,3). However, another study showed no improvements from training in a fasted state (4).

Intensity matters:

During near maximal intensity training, being in a fed state is likely more beneficial than being fasted training for enhancing adaptations (5). Therefore, the potential benefits of fasted training are more likely to occur at lower intensities of cardio exercise.

Gender may matter:

The research also indicates that the beneficial training adaptations that occur for males whilst training fasted, may not occur for females. Females may in fact experience better training adaptations when exercising in a fed state (6).

So what?

Further research is required to understand the impact of fasted training better, along with longer term studies to see whether these metabolic adaptations translate into performance benefits over time. Both the intensity and duration for which fasted training is most beneficial, would also need to be addressed, as well as a closer focus on the different impacts on males versus females.

In practise, working with a trained professional who can provide specific advice on your own individual situation will be most effective in making fasted cardio work for your health and performance!

Could there be downsides to training in a fasted state?

Health:

For anyone who trains regularly, performs at a high level, or is on the leaner side, they are already at increased risk of low energy availability. Within-day energy deficiencies from training fasted may increase the risk of this.

Cumulatively, this may increase risk of developing Relative Energy Deficiency Syndrome (RED-S), associated with numerous negative health impacts, including a weakened immune system, reduced cardiovascular health and osteoporosis.

Research indicates that even within-day energy deficiencies, which may occur with fasted training, may be associated with irregular/absent menstrual cycles and reduced levels of sex hormones for both females and males (7,8).

Additionally, training in a fasted state is associated with being in a catabolic (breakdown) state. This may be associated with increased levels of the stress hormone cortisol.

Since training is already a stressor, depending upon the individual and volume of fasted training, fasted cardio may result in chronic elevation of cortisol. This may impact health in a variety of ways, including levels of sex hormones, immune function, mood and sleep.

Performance:

Fasted training may be counterproductive for improving performance at higher intensities of cardio where glucose is the preferred fuel source for generating energy.

Fasted training may be counterproductive for gaining muscle strength or size, due to enhanced muscle protein breakdown. If muscle protein synthesis is insufficient to compensate for muscle breakdown, then gaining lean muscle mass or strength will not be possible.

Whilst further research is needed to show whether fasted training translates into impairing muscle gains, ultimately this will depend upon the individual, their body fat levels, energy availability, length of fasting, total protein intake and distribution, as well as training volume and stimulus.

Recovery:

Because fasted training may increase levels of muscle protein breakdown and reduce nutrient availability, this may impact the capacity for muscle protein repair. Insufficient nutrient availability may also increase levels of the stress hormone cortisol, impairing recovery.  

Females:

In general, the considerations within Health may be more of an issue for females. Females may be more sensitive to stressors and changes in energy availability.  This will obviously depend upon the female – factors such as genetics, lifestyle and body fat levels will all influence this.

However, there is research to also indicate that training adaptations for females may be more beneficial when cardio is undertaken in a fed state (6).

Appropriate energy and nutrient intake can reduce these risks and help avoid these potential downsides from occurring.  A well-planned diet and nutrient intake determines whether the fasted cardio is potentially beneficial or detrimental.

Take aways

Opt for fasted cardio during lower intensity training: since it may be beneficial for lower intensities of cardio, whereas it is unlikely to provide benefits at higher intensities and may be detrimental. Therefore opt for walking/jogging/cycling/yoga in a fasted state.

Follow a periodised nutrition plan: just like your training plan is periodised, do the same for your nutrition! Regardless of whether you are interested in fasted cardio for enhancing training adaptations, nutrient availability and sufficiency is critical to optimising recovery and performance. A nutrition and lifestyle program to suit your specific training regime and goals is a must!

Be aware of long-term impacts: if you do consider fasted cardio, then ensure that you have a longer-term nutrition plan to account for this, alongside your training. This is key to reducing risks around detrimental health. Deficiencies from nutrition often take time to manifest – this delay can be mistakenly perceived as “I feel really great from what I’m doing / everything is working well”.

Choose what is optimal for you: depending on your health, training, lifestyle (stress) and goals, fasted cardio may or may not be beneficial for you. What is optimal for your performance may not always be perfectly aligned with what is optimal for your health, however, finding a balance that supports your health will enable you to engage in performing for the longer term!

Related articles

Nutrient timing – what’s the deal and does it matter when you eat?

Pros and cons of fasted cardio: is fasted cardio right for you?

Fasted cardio for weight loss?

What are your thoughts on this – do you do fasted cardio to improve your performance? What are your experiences?

References

1. Pilegaard et al. (2002). Influence of pre-exercise muscle glycogen content on exercise induced transcriptional regulation of metabolic genes.
2. De Bock et al. (2005). Exercise in the fasted state facilitates fibre type-specific intramyocellular lipid breakdown and stimulates glycogen resynthesis in humans.
3. Van Proeyen et al. (2011). Beneficial metabolic adaptations due to endurance training exercise training in fasted state.
4. De Bock et al. (2008). Effect of training in the fasted state on metabolic responses during exercise with carbohydrate intake
5. Achten & Jeukendrup (2003). The effects of pre-exercise carbohydrate feedings on the intensity that elicits maximal fat oxidation.
6. Stannard et al. (2010). Adaptations to skeletal muscle with endurance exercise training in the acutely fed versus overnight fasted state.
7. Farenholtz et al. (2018). Within-day energy deficiency and reproductive function in female endurance athletes.
8. Torstveit et al. (2018). Within-day energy deficiency and metabolic perturbation in male endurance athletes.