Many will tell you that hitting some cardio first thing in the morning, before eating anything, leads to greater fat loss. As it turns out, this very likely isn’t the case, but there still may be a reason or two for you to include fasted cardio in your training.
Why Do People Think Fasted Cardio Burns More Fat?
If you do a cardio session in the morning, before eating anything, your body WILL preferentially burn stored fat to fuel that activity. This is well established in the research.
A recent meta-analysis comparing fasted vs. fed cardio clearly demonstrates this phenomenon as a scientific reality based on data from 273 participants across 27 studies.
The hypothesis for why this happens is, when you haven’t had anything to eat in the morning, your glycogen (stored carbohydrate) levels are somewhat depleted and insulin levels are lower, thus your body will preferentially liberate stored fat for energy.
But, it turns out, if you take a step back and look at body composition, there is no advantage from fasted cardio.
Is There Any Effect On Body Composition?
A study performed on 20 fit females had them all go on a hypocaloric diet for 4 weeks – aka they were eating less than they burned. Half of the women had a shake before an hour-long treadmill session in the morning. The other half waited to have that same shake until after their morning run. In the end, both groups consumed the same amount of calories – one group just did their exercise fasted and the other didn’t.
The conclusion shouldn’t be a surprise – I mentioned it in the first paragraph of this article, there was no statistically significant difference in fat loss between the groups. Both fasted and fed groups lost an average of 2lbs of body fat after the 4-week long intervention.
So, even though we know fat oxidation increases acutely after fasted cardio, it doesn’t seem to affect overall body composition.
I think we can all agree that the body is a fairly complex machine. We try to isolate very specific mechanisms when conducting scientific research, like what happens to fat burning right after fasted cardio and then extrapolate that out to more global outcomes (like change in body composition).
What we really care about in most cases though is the bigger picture, not what’s happens acutely. Who cares about an increase in short-term fat oxidation? You just want to drop some of that jiggle from your belly.
Why doesn’t this brief increase in fat oxidation lead to a reduction in body fat?
Well, the body seems to respond to the increased morning fat burn, by compensating later in the day with a decrease. Over the course of 24 hours, it all seems to even out.
In fact, there is even research that suggests when looking over a 24 hour period, fat burning is slightly higher with fed cardio when compared to fasting.
This finding doesn’t mean I’m endorsing fed cardio as a superior method for fat loss though. From the whole of the research, it appears that the impact of fed vs. fasted cardio on body composition is equivocal at best.
This conclusion is consistent with the findings outlined above as well as those presented in a recent meta-analysis that aimed to pool the results of all body composition related research on this topic.
But, up top, I mentioned there may still be a reason to include fasted cardio in your training. What’s the deal with that?
Are There Any Good Reasons To Do Fasted Cardio?
First, regular fasted training has been shown to enhance whole-body glucose tolerance and increase insulin sensitivity.
So, if you have blood sugar regulation issues, e.g. exercise-induced hypoglycemia, type 2 diabetes, pre-diabetes, etc. – this may very well help improve those conditions. Quick disclaimer – this isn’t meant to be medical advice, talk to your doctor, blah, blah.
Second, long term fasted cardio appears to lead to chronic molecular adaptations favoring fat oxidation. Now, this probably has no impact on body composition as I’ve outlined, but it may be of interest to athletes in sports where preferential fuel substrate utilization can give them an edge.
Mechanistically this implies that fasted training may help athletes like those that compete in endurance events since fat is an important fuel for those activities OR those that compete in a sport like CrossFit that requires some flexibility in fuel substrate utilization.
This is somewhat hypothetical at this point though. I’m not aware of existing research that demonstrates a real effect.
And that’s it.
Let’s Wrap It Up
For the most part, fasted cardio is just another one of those hacks people use to distract themselves from the things that actually make a real difference in body composition.
Based on the current data, unless you fall into one of the camps I outlined in the previous section, do fasted cardio or don’t – it doesn’t really matter.
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