Fasted Cardio: Does it Speed Up Fat Burning? 

Have you heard that doing cardio before breakfast can actually speed up fat burning? The theory makes sense. If you work out when you don’t have much sugar in your body (like after 8 hours of sleep), your body will naturally turn to fat.

But does it actually work? Let’s examine that.

What Happens in Your Body When You do Fasted Cardio? 

Your body basically has 2 main fuel sources that it uses just to stay alive: sugar (AKA glucose) and fat. How much of each you’re burning depends on a number of things. The intensity of exercise matters, and what you eat (or don’t eat) matters.

The higher the intensity of the exercise, the more sugar you burn. The lower the intensity of the exercise, the less sugar you burn.

If you ate a carbohydrate-rich meal within 2-4 hours of your workout, your body will burn more sugar for fuel than if you didn’t have a carbohydrate-rich meal within 2-4 hours of the workout.

So theoretically, if you’re doing cardio before breakfast, you will not have eaten in more than 8 hours, and your body should use more fat for fuel than sugar.

Does it Actually Work?

The theory makes a lot of sense, but can we actually say that specifically because a person was doing fasted cardio they lost body fat faster?

For the most part, people lose body fat at a rate of 1-2 pounds per week. Can I actually say that due to fasted cardio, clients started losing body fat at a rate of 2.5 pounds per week? Having measured hundreds of people, some of whom have done fasted cardio and some who haven’t, I can’t say that I’ve actually seen faster fat loss from fasted cardio.

            I suspect that there are a couple of reasons for that. First, when you do cardio in a fasted state, you have less “high octane” fuel (that’s carbohydrates) so you either go slower or you go for a shorter distance.

Another consideration is that if you have low blood sugar when you are doing cardio, yes, fat will be the major fuel source, but the amount of calories you burn from protein (ie muscle) increases quite a bit. So if you burn more muscle, you slow down your metabolism. 

The Final Verdict: Should You Do Fasted Cardio?

So as you can tell, I haven’t found results to be any better with fasted cardio, but I haven’t found them to be any worse with fasted cardio.

My conclusion for you is if the most convenient time to do your cardio is first thing in the morning, do it then. Or if other times are more convenient, do it at other times. In either case, it won’t make much of a difference.

The bottom line: just do it!

Any Questions?

If you have any questions, just leave them in the comments section below, and I’ll be sure to answer them.

Quick Summary

  • The body burns 2 fuel sources just to stay alive: sugar and fat. The theory behind fasted cardio is that if you have less sugar in you (because you haven’t eaten in 8+hours), you’ll burn more fat.
  • Practically, if you have less fuel, you go slower or for a shorter distance.
  • If you don’t have carbs in you, your body will use more protein (that’s muscle) to burn fat.

Additional Resources

Aerobic Exercise: Too Much Can Make You Fat

Aerobic Exercise or Strength Training: What’s More Important for Health?

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