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Fasted Cardio Does NOT Enhance Fat Loss

"Fasted cardio"--i.e. performing cardiovascular exercise early in the morning on an empty stomach-- is often touted as an effective weight loss strategy. I am not sure exactly when this idea originated, but it is especially popular with bodybuilders and some personal trainers. 

On the surface, the theory seems to "make sense"--in a fasted state, blood sugar levels are lower, stored muscle glycogen tends to be lower, and you have to get your energy from somewhere, so--voila!--you tap directly into those love handles.

However, like many "common sense" recommendations for exercise and weight loss, the efficacy of "fasted cardio" is not supported by science and research.

The first reason is that, as I have stated numerous times, exercise metabolism does not exist in a vacuum. The acute changes that occur in your body in response to exercise represent only a small fraction of daily metabolism. They are not permanent and they are affected by what happens in the other 23 hours of the day. 

Research has clearly shown that the amount or percentage of fat burned during an exercise session affects fat oxidation the rest of the day. In other words, if you burn more fat during a workout, you burn less the rest of the day and vice versa (assuming equal daily calorie intake). After 24 hours, there is no difference in total fat oxidation between individuals who burned more fat during a workout and those who burned more carbohydrate. 

That alone is enough to dismiss any claimed benefits of "fasted cardio" (and the idea that "cardio burns muscle" as well).

However, if we look at the "micro" level, there is little evidence of increased fat burning as well.

Proponents of fasted cardio claim that ingestion of carbohydrates prior to exercise reduces the entry of fatty acids into the mitochondria (where fat is oxidized) and increased insulin results in less breakdown of fatty acids (lipolysis) from the tissues.

However, at least two studies have shown that, even though lipolysis was suppressed 22% after carbohydrate ingestion, there was no difference in fat oxdiation compared to those exercising in a "fasted state".

Another issue is the source of the fatty acids used during exercise. Muscle cells store droplets of lipids, called intramuscular triglycerides (IMTG). While stored adipose tissue may be "mobilized" during exercise, it is the IMTG that provide the majority of the fat that is actually used--up to 80% in trained individuals. 

What that means is that during exercise, even in a fasted state, the body uses already-available stored fat in the muscles as the primary source of fat, NOT subcutaneous stored body fat.  Body fat stores are affected primarily by daily energy balance. 

What the research literature and science DOES support that, indirectly, exercising in a fasted state can impair fat loss, not facilitate it. Studies on preexercise feedings show that performance is impaired--especially for higher-intensity exercise--when one is in a hypoglycemic state. Lower intensity often means fewer calories burned, both during and after a workout. 

Other studies have suggested that ingesting a mixed carbohydrate/protein beverage before exercise results in a higher post-exercise oxygen consumption--after both low-intensity and high-intensity workouts. 

Despite the persistent popularity of "fasted cardio", the evidence clearly shows that it is not the best strategy, either for fat loss or training quality.  

Now, there are many people who prefer exercising in a fasted state and feel that it improves their performance. Let's look at some of the possible reasons: 

1. Many people say food makes them nauseous. This can be a legitimate reason, but it has nothing to do with the topic of "fat burning" or performance. It is always brought up in the discussion, but it is really irrelevant. I will say that there might be some disagreement about what constitutes "pre exercise feeding". We're not talking about a Grand Slam breakfast--a snack or replacement beverage of 100-200 calories is the "feeding".

2. Typical workouts are not that long or high in duration. Most of the studies I have read that study the effects of "fasted vs fed" feature workouts that are 60-120 min in length.  The average 30-45 min morning workout is likely not long enough or hard enough to really tax your energy stores. So, one is not likely to see much of an effect one way or another. 

3. Hard to prove a negative. If someone always exercises in a fasted state, they may not know what they are missing.

Especially based on #2, it is probably not that important who prefers to exercise in a fasted state to change their routine.

However, people should not exercise in a fasted state out of the misguided idea that it is better for fat loss.  

 

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