Fasted Cardio & Fat Loss
Cardio is often considered synonymous with fat loss, and while it certainly can be used for that function, there is some confusion as to if fasted cardiovascular exercise is superior to fed cardiovascular exercise for fat lass. Many claim cardio is cardio and it does not matter if it is fed or fasted. In this article, we will examine these claims by understanding the distinction between these two conditions, the rationale for the arguments in one camp or another, and finally, the evidence in an attempt to supply an answer.
What does the term "fasted" mean?
To begin, we need to understand what “fasting” or being “fasted” means. The term is a bit ambiguous, because in reality, any time nutrients are not being consumed is technically being “fasted”. On the other hand, if you have ever been to the doctor for certain procedures and tests, fasting means not consuming food for 12 hours or more. In this context, we will use the term “fasting” as a period of 8+ hours without food – so, roughly, the time (some of us wish!) we are asleep and pre-breakfast or having skipped breakfast; this means, no calorie dense nutrients of any sort – water is alright to consume.
Before we investigate what the research states, let us first look into the thought process for why fasted aerobic exercise (walking, jogging, etc.) are thought to offer an advantage when it comes to fat loss. One of those reasons is due to a general understanding of physiology and how the body adapts to various circumstances. The idea goes something like this, basically:
If we consume calories (from food, duh) before we go jogging, we elevate blood glucose (sugar) levels as these calorie associated nutrients are absorbed into the body. This, in turn, prompts the body to release the hormone insulin. Insulin, then, shuttles this excess blood glucose into the cells to be, likely, broken down into energy . However, as a consequence, the body is in a “fed” state and as it is in an energetically high state (due to the food), it slows down lipolysis, the breakdown of fat, from the fat cells to use the energy that has been ingested recently .
On the other hand, the opposite is true if a person does not consume calories for breakfast, for example, and goes jogging, then there is no elevation in blood glucose, and the body has no need to release insulin as it is in an energy deprived state. As a result, the body begins increasing lipolysis (breakdown of fat) in the fat cells to then release those fat molecules to be reabsorbed by the muscle cells to offer energy for continued activity through an energy pathway called fat oxidation . It is also possible that this increase of fat molecules in the blood, from increased lipolysis in the fat cells, leads to increased blood glucose due to increased gluconeogenesis (synthesis of glucose from non-glucose sources), although this point is still debated . It is in this (fasted) case that we see an increase in fat use.
Does fasted cardio lead to greater fat loss?
Now that we understand the reasoning for why fasted cardio does cause greater fat use for energy sustenance for activity, it might seem a moot point to discuss it again, but the reality is far from intuitive. Fasted cardio does not lead to greater fat loss than fed cardio – wait, what?
That is correct. A study investigating this exact question found no difference between fed and fasted cardio in terms of overall fat loss . Now, how does that make any sense after discussing how the body does use greater amounts of fat for energy when in a fasted state? Well, the answer is in the time line.
Fasted cardio still, undeniably, increases fat substrate use, but that is only while cardio is being performed fasted. If all things are equal, like they should be in a fair comparison of this type, where calories and macronutrients consumed throughout the day are still eventually consumed, then the body must still compensate accordingly – this is best explained with an example using what we’ve learned in the last section.
If person A and person B consume the same number of calories at, let’s say, 2500 calories each day, and they both do cardio the same length of time and at the same intensity and all other variables are the same; however, person A uses fasted cardio and person B uses fed cardio. Person A wakes up at 5 am (he’s crazy) to do 1 hour of fasted cardio, and person B wakes up at 4:30 am (he’s crazier) to eat a 500 calorie breakfast before going for his 6 am 1 hour cardio session. Person A will burn more fat in that hour. Then, person A and B consume a 1000 calorie lunch, insulin and decreased fat release are greater for person B than person A, because person A did not eat breakfast. However, if they continue throughout their day and person B has finished his calories by the end of the day at 2500 calories, his body has finished dealing with the intake of nutrients and insulin will eventually reduce back to normal and lipolysis will continue like normal; however, person A still has 500 calories to consume to match person B’s calorie intake (which, coincidentally, is also his calorie intake) and therefor insulin rises again and lipolysis slows. So, really, it is simply a delay of when lipolysis decreases and increases, but the net effect at the end of the day remains the same.
In more concise, scientific terms - fed cardio increases respiratory exchange ratio in favor of carbohydrate use (RER = .96) while fasted cardio favor fat use (RER = .84), but later in the day, the body compensates by favoring fat use in fed cardio individuals and carbohydrate use in fasted cardio individuals .
There we have it, fasted cardio does use more fat, during exercise, than non-fasted cardio. In the short term, it seems clear that a bout of fasted cardio will garner better fat loss than fed state (i.e. post-breakfast) cardio. However, in the long term, there is no difference between the two as fat utilization increases later in the day, in fed cardio condition, to compensate – this, in turn, renders the entire argument useless that fasted cardio is superior for fat loss as both end up losing the same amount of fat at the end of the day if conditions are equal (calorie deficit). So, use the cardio you prefer – it does not matter unless you simply perform worse on an empty stomach.
Writer: Nicolas Verhoeven
 Van Proeyen, K., Szlufcik, K., Nielens, H., Pelgrim, K., Deldicque, L., Hesselink, M., … Hespel, P. (2010). Training in the fasted state improves glucose tolerance during fat-rich diet. The Journal of Physiology, 588(21), 4289-4302. Retrieved from
 Schoenfeld, B., Aragon, A., Wilborn, C. D., Krieger, J. W., & Sonmez, G. T. (2014). Body composition changes associated with fasted versus non-fasted aerobic exercise. J Int Soc Sports Nutr, 11(1), 54. Retrieved from
 Horowitz, J. F., Mora-Rodriguez, R., Byerley, L. O., & Coyle, E. F. (1996). LIPOLYTIC SUPPRESSION FOLLOWING CARBOHYDRATE INGESTION LIMITS FAT OXIDATION DURING EXERCISE 442. Medicine & Science in Sports & Exercise,28(Supplement), 74. Retrieved from
 Chen, X., Iqbal, N., & Boden, G. (1999). The effects of free fatty acids on gluconeogenesis and glycogenolysis in normal subjects. Journal of Clinical Investigation, 103(3), 365-372. Retrieved from
Rui, L. (2014). Energy Metabolism in the Liver. Comprehensive Physiology, 177-197. Retrieved from
 Bowen, R. (2009, August 1). Physiologic Effects of Insulin. Retrieved from
 Verboeket-van de Venne, W. P. (1991). Influence of the feeding frequency on nutrient utilization in man: consequences for energy metabolism. European Journal of Clinical Nutrition,45(3), 161-169. Retrieved from
 Aragon, A. (2006). Myths Under the Microscope Part 1: The Low Intensity Fat Burning Zone - AlanAragon.com - Fitness Based on Science & Experience. Retrieved from
 Schoenfeld, B. (2011). Does Cardio After an Overnight Fast Maximize Fat Loss?. Retrieved from National Strength and Conditioning Association website: