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It has long been believed, especially in the bodybuilding and physique world, that doing cardio fasted (for example first thing in the morning before eating) would increase fat loss compared to doing it fed. In fact, you can count me among the many people who once strongly promoted fasted cardio for improved fat loss.
Fasted cardio certainly works for fat loss – no one disputes that. But for decades, questions have always remained about whether fasted cardio works better for fat loss than fed cardio.
The reason a strong verdict was never in for so many years is because the right kind of research had never been done to support or refute the fasted cardio hypothesis. All we had to go on was opinion, anecdote and hypothesis-generating research.
Previous studies showed that fat oxidation is increased while doing cardio in a fasted state. Many people assumed that meant more loss of body fat. However, fat oxidation (a measure of what fuel is being burned at a given time) is not the same thing as fat loss (a change in body composition).
For example, a study may have measured the amount of fat burned during a workout, which might only be 30 minutes or 1 hour out of a whole day. Or perhaps a study might measure fat oxidation for a full 24 hours. Yet those kinds of studies were all very short term. They didn’t measure fat mass or body fat percentage at all, let alone after weeks or months of workouts.
Fasted cardio has been recommended for fat loss for at least 25 years or more, but it wasn’t until fairly recently that for the first time a study was conducted that actually measured the effect of fasted vs fed cardio on had body composition (fat loss) over time.
The research was headed by Brad Schoenfeld and the results were published in the Journal of the International Society of Sports Nutrition. The conditions were similar to what recreational exercisers might do in the real world (an hour of steady state cardio 3 times a week). One group did cardio fasted, the other fed.
There was no difference in the fat loss (measured over 4 weeks); the cardio worked either way – fed or fasted.
This was only the first study on the subject to measure body composition over time and more research is needed before there is a truly definitive answer (ideally, studies with larger sample sizes, longer duration – 12-16 weeks would be great – and also using different types of cardio, using both male and female subjects of different ages, and perhaps subjects with different levels of starting body fat).
It seems there have always been and still are a lot of anecdotal reports, especially from bodybuilders and physique competitors, of better fat loss with fasted cardio, and it’s still possible that in some cases, fasted cardio might provide fat loss advantages.
For example, it has been proposed, based on other lines of research (looking at the adrenergic receptor system and fat mobilization, not just fat oxidation), that people who are already very lean and down to the last pockets of stubborn fat might be the ones who benefit from fasted cardio the most . This makes some sense, as the fasted cardio practice originally emerged in the bodybuilding world, where physique athletes are already lean to begin with.
However, based on this newer evidence that measures actual fat loss over time, it’s looking more and more like it just doesn’t matter that much whether you do cardio fasted or fed. If it helps doing cardio fasted, the difference is probably pretty small.
You also have to consider that some people will have less energy to train when fasted and that could produce less, not more fat loss due to reduced exercise performance. People are different in this regard, however, so blanket statements can’t be made for everyone – some people will report having no energy and “bonking” completely when training fasted, others will say they feel fine, or even “great” doing cardio fasted, so this should be left to the individual to decide.
There is also a possibility that the risk of losing lean body mass might be increased. However, muscle loss while doing fasted cardio is likely only a problem if a person is doing only cardio and no weight training, and or if protein is too low. Muscle loss is also unlikely to be a concern unless someone is already dieted down and trying to get even leaner, then lean body mass should be monitored just to be sure.
Bottom line: unless you’re already lean, trying to get even leaner and or you’re a physique athlete attempting to get every bit of fat loss possible, it probably doesn’t matter when you do your cardio or whether you do it fed or fasted. This decision should be left up to personal preference, and ideally, cardio should be done when you are most likely to do it consistently and when you think you have the most energy.
Scientific reference:
Body composition changes associated with fasted versus non-fasted aerobic exercise. Schoenfeld B, Aragon A, Journal of the International Society of Sports Nutrition. 18;11(1):54.
Train hard and expect success!
Tom Venuto,
Founder, Burn the Fat Inner Circle
Author, Burn the Fat, Feed the Muscle
The post Fasted Cardio Revisited: Does Aerobic Exercise On An Empty Stomach Really Burn More Fat? appeared first on Burn The Fat Blog.
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