The high intensity cardio versus low intensity cardio debate has resolved itself into a state of “it depends,” with the different styles having opposing advantages and disadvantages. Low intensity cardio is generally best for people who can’t recover from the impact of high intensity training, have large blocks of infrequent training time during the week that would accommodate extended training sessions, and/or have a pastime they can attach to it (e.g., hiking).
Much has been said about ways to enhance the benefits of complexes, circuits, sprint intervals, and the like, though ways to enhance jogs and treadmill sessions are rare in lifting-focused publications. To help balance things, I want to take a fresh look at the topic of low intensity cardio and how it might be enhanced by fasting. Because the actual training adaptations of fasted cardio aren’t well documented, the following concepts are inferences.
It’s already established that the metabolic outcomes of fasted and fed cardio are similar: fasted is more likely to use fat stores as a substrate (Iwayama 2012), but the overall caloric deficit in fed cardio is greater due to increased energy expenditure. As John Meadows notes, there are specific situations where these differences could matter, but they involve a complex evaluation of the trainee.
What may be more useful in a general sense is looking at differences beyond burned calories. For example, cardio has been demonstrated to temporarily blunt the appetite in some individuals (Schubert 2013). While probably multifactorial, one likely culprit is leptin production. Leptin blunts appetite and elevates metabolism. Because fed cardio has been demonstrated to elevate leptin production above baseline, fed cardio could be advantageous to a dieter.
On the other hand, fasted cardio might be an interesting tool for someone looking to gain weight while maintaining aerobic capacity. De Bock found that two hours on a bike lowered leptin production perhaps by as much as 50 percent of baseline. This means that it may actually stimulate appetite not just in comparison to fed cardio but also when compared to everyday life. If you find that you have a hard time meeting your calorie goals because of post-cardio appetite suppression, performing it while fasted may be a cure. Deighton and colleagues came to a similar conclusion using only sixty minutes of cardio. The fact that insulin levels explode four hours after fasted cardio may make this more even more effective for mass seekers (De Bock 2005).
Fasting might also reduce some of the unpleasant sensations of cardio. Van Proyen and colleagues showed that blood glucose levels are actually more stable during extended bouts of fasted cardio than in otherwise identical fed cardio sessions. This could improve everything from your sense of well-being to basic balance.
Cortisol is likewise influenced. De Bock and colleagues documented that cortisol was higher fasted versus fed during cardio but lower than baseline afterward. Because cortisol is an immunosuppressant that inhibits protein synthesis, there might be some benefits here. During the cardio session, the extra cortisol could temper inflamed and painful tissues, such as joints and strained muscles. Afterward, the immune system is free to do its job scavenging dead cells and healing injuries while benefiting from enhanced muscle creation. While there isn't any reason to think that the immune suppressing factor is more potent than an ibuprofen tablet or aspirin gel, there may be a measurable effect here when combined with the slight reduction in body weight that accompanies fasting.
As mentioned above, while these factors may produce beneficial training outcomes, there isn’t much evidence yet in the labs. The same seems to be true in terms of anecdote. If you have any experiences on the topic, feel free to share them below.
References
- De Bock, et al (2005) “Exercise in the fasted state facilitates fibre type-specific intramyocellular lipid breakdown and stimulates glycogen resynthesis in humans.” The Journal of Physiology 564(2):649–60.
- Deighton, et al (2012) “Appetite, energy intake and resting metabolic responses to 60 min treadmill running performed in a fasted versus a postprandial state.” Appetite 58(3):946–54.
- Iwayama and Tokuyama (2012) “Exercise in a metabolic chamber—effects of exercise on 24-h fat oxidation.” The Journal of Physical Fitness and Sports Medicine 1(2)307–16.
- Schubert, et al (2013) “Acute exercise and subsequent energy intake. A meta-analysis.” Appetite 1(63):92–104.
- Van Proyen, et al (2011) “Beneficial metabolic adaptations due to endurance exercise training in the fasted state.” Journal of Applied Physiology 110(1):236–45.