Some believe that simply by exercising first thing in the morning on an empty stomach will burn more body fat, but if it were that simple; we wouldn’t have an overweight/obesity epidemic in our country.
The truth is, human physiology is a bit more complex than this.
The average person’s total exercise for a week is less than 5%; which is approximately 3-6 hours of exercising per week. The total amount of energy expended during exercise represents a small percentage (~17%) of the grand total expended over a 24-hour period. Therefore, the type of macronutrient actually utilized for fuel during the workout is inconsequential.
In addition; we have known for years that intense exercise after fasting (such as 8-hours of sleep) promotes catabolism of precious muscle tissue and creates a metabolic environment that makes improvements virtually impossible (1).
Which means this catabolic environment is something we should take all precautions to avoid in order to build and/or preserve as much muscle tissue as possible!
So the question is:
If you’re looking to optimize fat loss, is it better to do your cardio in a fed or fasted state?
Let’s take a look at what the evidence has to say...
As mentioned, one strategy thought to accelerate body fat loss is to do your cardio first thing in the morning before breakfast (2).
In theory, the low glycogen and insulin levels after an overnight fast cause the body to shift energy utilization away from carbohydrates and allow greater mobilization of stored fat for fuel.
Whereas exercise in a fed state results in a reduced entry of long-chain fatty acids in the mitochondria, which corresponds to a decrease in fat breakdown (3).
This decrease in fat oxidation after eating is attributed to an insulin-mediated attenuation of adipose tissue lipolysis (4). Adipose tissue lipolysis is the catabolic process that leads to the breakdown of triglycerides stored in fat cells and release of fatty acids and glycerol.
There is also evidence that consistently performing cardio in a fasted state results in molecular adaptations that favor fat breakdown (5).
Despite an apparent theoretical basis, there is little evidence as to whether fasted aerobic exercise results in long-term fat loss compared to exercising after eating.
Schoenfeld et al. were the first to directly investigate this topic during periods of energy restriction. They investigated changes in fat mass and fat-free mass following four weeks of aerobic exercise in the fasted vs fed state while also remaining in an energy deficit (6).
Results showed that both the fasted and fed groups lost a significant amount of weight and fat mass with no differences seen between conditions in any outcome measures regardless of pre-exercise feeding status.
The theoretical basis behind a fat-burning advantage to fasted exercise is built on increasing fat breakdown during an exercise bout.
However, this ignores the dynamic nature of the human body, which continually adjusts its use of substrate for fuel. There is evidence that a greater utilization of fat for fuel during a given time period is compensated by a greater carbohydrate utilization later in the day (7).
To meaningfully assess its impact on body composition; fat burning must be considered chronically over the course of days instead of acutely on an hour to hour basis (8).
In support of this contention, differences were compared in 24-hour fat metabolism associated with performance of moderate-intensity treadmill exercise in a fasted vs fed state. The amount and quality of food was identical between conditions over the 24-hour recovery period.
When compared to fasting; consuming breakfast caused a significant increase in respiratory exchange ratio (RER), which is a measure of how much carbohydrate or fat is being utilized to fuel the body during exercise.
However, at the 12-hour post-exercise timepoint, RER was significantly lower in the fed vs fasting condition and this difference remained significant after 24 hours (9).
Interestingly, any potential increases in fat oxidation from fasted exercise might be neutralized by an increase in the thermic effect of exercise seen from eating pre-exercise.
Results of recent research show that consuming a mixed meal (carb/protein) increased post-exercise oxygen consumption significantly greater than exercise performed while fasted. This occurred in both high and low intensity exercise conditions (10).
This demonstrated that more calories were expended from and increased in post-exercise oxygen consumption which can be attributed to the pre-exercise meal.
Generally speaking, a higher intensity level can be reached when one is not feeling hungry, sluggish, and suffering from low blood glucose.
So, while performing cardio in a fasted state may result in greater percentage of fat utilization during the exercise period; it may also result in fewer calories in total being burned over time.
You can indeed find some studies suggesting that fasted training will enhance fat oxidation and even improve insulin sensitivity. 
Nonetheless, as is always the case with research, you must consider the population studies and the type of training they engaged in because chances are the results of the study might not even apply to you.
While fat oxidation may be increased at certain times, or due to certain circumstances while exercising, this has little effect on 24-hour fat oxidation and does not mean that we are necessarily burning body fat over time.
So where does this leave you in your journey for a lean sculpted physique?
Here are two salient points we know from research on this topic:
Bottom line is that if you train hard with medium to higher intensity cardio; you are creating an intermittent stressor on your body.
Essentially, you are breaking down some muscle fibers and causing metabolic stresses that your body will respond to this acute stress by building back muscles that are bigger, stronger and ready to tackle the next challenge.
By training at high intensities in a fasted state you are introducing yet another stressor that will likely put your muscle in a catabolic state, and a robust body of research suggests that smart fueling before a grueling workout will only do you good.
Pre-workout meals will not only allow you to perform better and preserve lean muscle mass, (11) they will also increase thermogenesis and fat oxidation during the recovery period, (10) which is when the adaptations actually take place.
After reviewing the literature on this topic; the main takeaway is that fasting and hard training do not go hand in hand according to the current body of evidence.
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2.Kang, J., Raines, E., Rosenberg, J., Ratamess, N., Naclerio, F., and Faigenbaum, A. (2013) Metabolic responses during postprandial exercise. Res Sports Med 21, 240-252
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