I see this question crop up fairly frequently. There seems to be the belief that doing cardio exercise after fasting is more beneficial than doing cardio exercise within a hour to a few hours of eating. I’m not sure how this got started, but I’ve been hearing it for years.
The supposition is that exercise after fasting at least eight hours (like first thing in the morning after sleeping all night) will burn more fat than exercising after eating. The bro-science here is that when you exercise on an empty stomach, your body somehow knows and switches more quickly to burning fat for energy.
Just today I read a question on a forum where a person wondered if he should get up and do cardio first thing in the morning on an empty stomach, and that doing so must be better because “that’s what the Rock does”. I’d bet the Rock does his cardio so early and first thing because he has long long days where he might otherwise not be able to fit it in.
I’ve always been curious about this myself, kind of half-believing the myth, so I finally decided to look it up. I found this study, Body composition changes associated with fasted versus non-fasted aerobic exercise, that provided a definitive answer, at least for females in their 20s, and that answer is:
It has been hypothesized that exercising when fasted forces the body to rely on using fat as a substrate rather than carbohydrate, thereby reducing body fat to a greater extent than performance of post-prandial exercise. Our results refute the veracity of this hypothesis. Although both groups lost a significant amount of weight and fat mass, no differences were seen between conditions in any outcome measure regardless of pre-exercise feeding status.
It’s worth noting the population that they tested on and the purpose of the experiment:
It has been hypothesized that performing aerobic exercise after an overnight fast accelerates the loss of body fat. The purpose of this study was to investigate changes in fat mass and fat-free mass following four weeks of volume-equated fasted versus fed aerobic exercise in young women adhering to a hypocaloric diet.
It is worth noting the criteria of the experiment:
- Subjects were between the ages of 18–35
- Not classified as obese based on a body mass index (BMI) of less than or equal to 30
- Not involved in a resistance training program at the time of the study
As for the exercise performed, it consisted of one hour of steady-state aerobic exercise performed three days per week on treadmill set at 0% grade. The participants warm-up for the first 5 minutes at an intensity equating to 50% of maximal heart rate (MHR) as determined by the 220 – age formula, then increased intensity to 70% MHR for 50 minutes, and finished with a five minute cool down at 50% MHR.
A low-to-moderate training intensity was used because it has been shown to maximize lipid oxidation during fasted aerobic exercise as compared to higher-training intensities. All training sessions were supervised by research assistants who were upper level undergraduate students in exercise science. Subjects were instructed to refrain from performing any additional structured exercise for the duration of the study.
These findings indicate that body composition changes associated with aerobic exercise in conjunction with a hypocaloric diet are similar regardless whether or not an individual is fasted prior to training.
So the take-away here about when you should do your cardio is whenever it is most convenient for you to do it. If the only way you can get it in with a certainly is getting up and knocking out first thing, then go for it. Likewise, if you aren’t able to get to it until after lunch, or even later, don’t worry about eating first.
This highlights what I and others have always said about exercise – the most important thing to do is to get it done!
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