top of page
  • Writer's pictureMaija E

Fasted cardio, does it work?

Updated: Jul 3, 2019

Lately, we've had clients ask us about working out while fasted. Specifically,"fasted cardio" is a strategy some people use to drop the pounds faster. The logic is that since our body does not have readily available glucose, it will burn through glycogen stores more quickly, and start to utilize fat as an energy source. But does it work?

While it is true that your body will utilize fat stores when there is a lack of glycogen, it will also utilize protein (amino acids). This means that you will also start to break down muscle tissue for energy, which is not ideal.

When you work out hard, your body releases a hormone called cortisol. If glycogen levels are low, cortisol acts to mobilize fatty acids and amino acids for energy. It signals your body to take amino acids from muscle tissue, and send them to the liver where they undergo a process known as gluconeogenesis. This literally means "glucose formation." Your body will do whatever it takes to create the glucose necessary to fuel your workout.

Liver, site of gluconeogenesis (glucose formation)

Studies show that it is true more fat will be broken down during cardio sessions while fasted. However, this effect levels off during the rest of the day. After you finally eat, your body will utilize more glucose to make up for the extra fat breakdown. A study by the International Journal of Sport Nutrition and Exercise Metabolism showed that athletes who performed moderate intensity cardio after consuming a light meal increased their VO2 and burned more fat over the course of 24 hours than the fasted athletes. Let's talk about the importance of VO2.

What is VO2?

It is the amount of oxygen your body consumes, transports and utilizes. You might have heard of VO2 max, which is the maximum rate at which your body can take in and process oxygen. It is used in a lab setting to measure cardiorespiratory fitness (trained athletes will have a higher VO2 max than a sedentary person). It is also the "strongest independent predictor of future life expectancy in both healthy and cardiorespiratory-diseased individuals" according to researchers from the Division of Medical Biochemistry in Innsbruck, Austria.

Exercise induces something called excess postexercise oxygen consumption (EPOC). EPOC is responsible for increased energy utilization (i.e. calories burned) in the hours after a workout. This overall increase in metabolism is more important to weight control than the actual calories burned during training.

The fat burning zone makes it seems like you should workout lighter to lose more.

What about the "Fat Burning Zone?"

You'll see this label on treadmills and cardio machines throughout gyms across America. When we train at lower intensities (under aerobic conditions), our bodies will primarily rely on fat oxidation for energy. At higher intensities (anaerobic conditions), we start to rely on glucose. Despite this, studies show that high intensity workouts (closer to your max heart rate) are best for weight loss. Why is this?

High intensity interval training forces our body to adapt to the stressful conditions. As a result, it increases oxygen consumption and transport (VO2). This revs up our metabolism in the hours following a workout, which means that you're burning more calories over a 24 hour period. That's why studies consistently show better fat loss results amongst participants assigned to high intensity interval training (HIIT) than those who exercise at a slow and steady pace.

Moreover, when you feed yourself adequately for your workout, you can push your body harder. This means you will create a more effective workout, and will likely burn more calories, build more muscle and do more to change your overall body composition than the person who hangs out in the "fat burning zone" and hasn't eaten all day long.

And, as always there is another side to consider

There is some evidence to suggest that fasted training may benefit athletes looking to increase their endurance. Some studies show that in a low-glycogen state, fasted training induces increased blood supply to the muscles, more mitochondria (which provide our bodies with ATP, our molecular energy source), and increased fatty acid breakdown.

They note that the benefit is only gained when carbohydrates are restricted sometimes. Training consistently in a low-glycogen state impairs performance, hurts immune function, and leads to muscle wasting.

It's tough, we get it.

Remember, you can't get around working hard to change your body composition. It is difficult, and you will need a strategy and social support to reach your goals. That's why people who work with trainers are more likely to increase their lean body mass and stick to a consistent routine. It is so easy to push yourself hard for a couple of weeks then burn out and give up.

Many people quit after the first few months because the results aren't immediate. Working with a trainer ensures you are getting the most out of your time at the gym. Nobody wants to waste hours doing fasted cardio or performing poor technique that could lead to an injury.

We can help you reach your fitness goals, and the initial consultation is completely free! Just let us know a little bit about yourself here.

bottom of page