By: Tana Mardian
If you look up “Does fasted cardio work?” or “Does fasted cardio burn more fat?” you’ll find a long list of various articles discussing this latest fitness trend. Fasted cardio involves performing aerobic exercise after forgoing energy intake (i.e. not consuming food or drinks with calories) for a period of time. The average person does an “overnight fast” for about 8-12 hours, and there are metabolic changes that happen during these hours when we don’t take in energy. When we’re in our overnight fast, the body tends to rely more on fat as a fuel source and tries to spare the carbohydrate in our bodies (Maughan et al., 2010). The idea behind fasted cardio is that this shift in preferential energy source could continue in an exercise session.
In a systematic review and meta-analysis, Vieira et al looked over a multitude of studies that investigated aerobic exercise in a fasted vs. fed state and specifically the effects of these states on carbohydrate and fat metabolism. After sorting through over 10,000 studies, they settled on 23 that fit their criteria. These articles provide evidence that low-to-moderate intensity aerobic exercise done in the fasted state increases fat oxidation during the exercise session. In other words, if someone is doing low/moderate intensity exercise while fasted, most of the energy for that workout is coming from fat stores in the body. Despite this, the fuel source for high intensity exercise (about 70% or greater of an individual’s maximal exercise capacity) is still carbohydrate for both fasted and fed states. (Vieira et al., 2016). So what does this mean for us and our exercise sessions?
This information has important implications especially for people with obesity and insulin-resistance. In a study looking at a physical activity and weight loss intervention in obese individuals, the researchers found that improved fasting rates of fat oxidation was the strongest predictor for enhanced insulin sensitivity. Essentially, exercise and weight loss increased fat oxidation in the fasted state, which seems to be a crucial component of boosting insulin sensitivity. (Goodpaster et al., 2003). Issues with fat metabolism are involved in the development of obesity and diabetes, so overall, exercise and enhanced fat oxidation may have potent benefits for these clinical populations (Melanson et al., 2009).
To be clear, this doesn’t mean that you should only do low intensity exercise on an empty stomach in the morning. There is further evidence that other exercise regimens improve fat oxidation, like endurance training and interval training in particular, and exercise in general may increase fat metabolism (Melanson et al., 2009). With that being said, the best exercise program is the one that you enjoy and can stick to. When figuring out our individual exercise routines, it is essential to pay attention to your body and what you are feeling each day, not just what will “burn the most fat.” This could mean that one day you need a slow yoga stretch, whereas other days may call for a heavy sweat session with high intensity interval training (HIIT). While it is helpful to understand the science behind our workouts, it’s important to remember that the benefits of exercise are widespread and exist far beyond the notion of fat metabolism.
Goodpaster, B. H., Katsiaras, A., & Kelley, D. E. (2003, September 1). Enhanced Fat Oxidation Through Physical Activity Is Associated With Improvements in Insulin Sensitivity in Obesity. https://diabetes.diabetesjournals.org/content/52/9/2191.
Maughan, R. J., Fallah, J., & Coyle, E. F. (2010, June 1). The effects of fasting on metabolism and performance. British Journal of Sports Medicine. https://bjsm.bmj.com/content/44/7/490.long.
Melanson, E. L., MacLean, P. S., & Hill, J. O. (2009, April). Exercise Improves Fat Metabolism in Muscle but Does Not Increase 24-h Fat Oxidation. Exercise and sport sciences reviews. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2885974/.
Vieira, A. F., Costa, R. R., Macedo, R. C. O., Coconcelli, L., & Kruel, L. F. M. (2016, September 9). Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis: British Journal of Nutrition. Cambridge Core. https://www.cambridge.org/core/journals/british-journal-of-nutrition/article/effects-of-aerobic-exercise-performed-in-fasted-v-fed-state-on-fat-and-carbohydrate-metabolism-in-adults-a-systematic-review-and-metaanalysis/0EA2328A0FF91703C95FD39A38716811.