So, you’ve been told fasted cardio will turn your body into a fat burning machine and have you leaning down in no time. But how much truth is there to this tale? And is it really worth adding those early morning runs into your weekly training routine?
Let’s take a look.
What is fasted cardio?
Fasted cardio refers to exercising in a fasted state without any recent food in your system, usually first thing in the morning before breakfast. Some people also consider fasted cardio any exercise you complete after a fasting window of at least 3-4 hours, so it can also include exercising at a different time of day provided you still have this same fasting window.
Does fasted cardio burn more fat?
In order to understand this, we first need to take a look at basic energy utilisation and storage processes, and how the body actually works.
When we consume food and drinks containing energy, this moves into our bloodstream and can be used by the body right away, while any leftover energy is stored in the body as body fat or glycogen for use at a later time. This process is particularly important as it is what allows us to keep our cells alive and functioning during times of fasting, such as overnight, or between meals.
So when do we use these stores? Well, we use this stored energy all the time and dip into it constantly throughout the day to keep our blood glucose levels nice and stable.
When it comes to exercise, research shows that when we complete exercise during a fed state, our body primarily gets its fuel directly from the blood stream in the form of glucose, either from the carbohydrates in the food we have eaten or from our stored energy depending on the duration and intensity of the exercise. However, in a fasted state, the body needs to rely more heavily on those energy stores to keep your blood glucose levels stable.
This means that completing cardio in a fasted state does result in an increase in fat oxidation or burning more body fat as fuel for the duration of the exercise compared to training after consuming a meal. However, there’s a bit more to the story as this is also impacted by the intensity of the exercise, with higher intensity exercise or cardio favouring carbohydrate fuel sources, including the breakdown of glycogen rather than body fat.
This also doesn’t consider the most important factor during any fat loss phase which is your overall energy balance at the end of the day.
In order to see an overall reduction in body fat you need to be sitting in a negative energy balance (or a calorie deficit) over a period of time. This means that your energy in (from food and drink) needs to be less than your energy out (from exercise, daily activities + basal metabolic rate) to see an overall reduction in body fat.
So while lower intensity fasted cardio may have you burning more fat while you are exercising, over the course of the day it is your overall energy balance that determines any net fat loss or gain you may experience.
In addition, you will likely see a greater increase in total daily energy expenditure over time following a well structured resistance training program that focuses on increasing your muscle mass rather than heading straight to that treadmill every morning. So arguably, fasted cardio may not be the most sustainable solution to any long term fat loss goals.
The bottom line
While it’s true that fasted cardio does utilise more fat for fuel for the duration of the exercise, it is definitely not superior for your fat loss long term and nothing can outweigh a consistent calorie deficit over time.
Remember, cardio (fasted or not!) is still a fantastic tool we can use for recovery and to optimise our heart health, so it’s definitely worth keeping in your weekly routine - just don’t rely on fasted cardio as the magical fat loss secret just yet.
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