KBW | How to Lose Weight with Fasted Cardio

Fasted cardio is kind of a hot topic of debate in the wellness world. Does it help you lose weight and burn fat? Is it just burning carbs? How do you do it right? Does it have to be a specific type of cardio workout, or is any one type of workout more effective than the others?

Here’s my take on the technique and how you can put it to work for you, no personal trainer is needed.

 Does Fasted Cardio Burn Fat? 

The Direct Effects

To understand the direct effects of fasted cardio, we’ve got to get a handle on the two types of fuel our bodies use. There are only two options here, and your body will use one or the other. We have glucose, which is broken down from sugar and carbs and can be stored in the form of glycogen in the liver, muscle, and fat cells, and we have ketone bodies, which are made in the liver from fat. 

When we are sleeping, we aren't taking in any fuel; this allows our body to enter a fasted state (consequently, this is why we have the term breakfast...you're breaking your fast!). Throughout our nightly fast, the body will begin to use stored glycogen for fuel.

When you do cardio in the morning, your insulin and glycogen levels are naturally low after being depleted through the night. These levels are further lowered by your cardio efforts, which allows your body to tap into its alternate fuel source: fat (via ketone bodies).  

The Indirect Effects

One of the positive indirect effects of fasted cardio is the Intermittent fasting that comes along with it. If you’re doing fasted morning cardio, you’ll naturally be subscribing to the compressed feeding window that is the hallmark of intermittent fasting, and intermittent fasting has been shown to help with weight loss. 

Fasted cardio seems to lead to a naturally lower calorie intake as well, likely due to that naturally compressed feeding window (aka intermittent fasting) we just discussed. In a 2019 study published in the British Journal of Nutrition, a pool of male test subjects was split into two groups, fasted cardio, and fed cardio. The researchers found out that, due in part to sticking to a compressed feeding window, the men in the fasted cardio group consumed an average of 400 calories less per day. This led to a pound a week in weight loss in the fasted cardio group. 

A Case Study

I’ve seen success with fasted cardio for myself and my clients. Here’s just one success story from a professional athlete I work with. 

This NBA player came to me with the goal to burn fat while maintaining muscle mass and athletic performance. In addition to making some small dietary changes, I put him on a very simple fasted cardio routine. He stopped eating at 7 pm and woke up around 7 am to do his morning cardio workout. He then ate breakfast at about 9 am, leaving him with a 14-hour fasting window.

I bet you're thinking this athlete did some super intense workouts... but actually, the workout itself was very easygoing. I asked him to take a mere 30-minute walk outside. If that didn’t do the trick, our next step was to up the intensity to a light jog, but we never needed to!

In the end, he achieved his body composition goals, all the while maintaining his high level of performance without taking any drastic steps. He lost .5 lbs a week doing simple fasted cardio and healthy eating. This wasn’t strenuous, it didn’t leave him feeling starved, and at the end of 3 months, he was leaner than he’d ever been, a total fasted cardio win.  

Doing Fasted Cardio Right

First, it’s important to note that fasted cardio may not be appropriate for everyone. Women with severe adrenal fatigue, for example, need to heal their adrenals before starting a fasted cardio routine.

Remember, you don't need a really intense workout for your fasted cardio. Running or biking for miles and miles, high-intensity spin classes, and CrossFit aren't necessary. 

I always suggest starting with a low-intensity workout outside. It’s as simple as taking a walk outside each morning. An additional benefit of a daily morning walk is the sunlight helps reset your circadian rhythm.

It’s enough to get your heart rate up a little, but nothing too strenuous because high-intensity workouts while practicing fasted cardio can actually have the opposite effect of what is intended. When you ask your body to perform high-intensity workouts without any sustenance, cortisol is released. Cortisol is often referred to as a stress hormone, and it can cause your body to hang on to stubborn belly fat, which no one wants. 

 Fasted Cardio Guidelines

There are some simple guidelines that you can follow to practice effective fasted cardio: 

• Start with a 12-hour feeding window. This time window doesn’t even seem like fasting since you’re asleep for most of it. If you eat dinner at 7 pm, you’ll achieve a 12-hour feeding window by not having breakfast until 7 am the next morning. As long as you’re getting your workout in as a part of your morning routine, it’s as simple as avoiding midnight snacks if you’re aiming for a 12-hour compressed feeding window.
• Start with about half an hour of cardio that isn’t too strenuous. Like I told the client above, even a simple walk will do the trick. Work your way up to a longer workout if you’re not seeing the (reasonable) results you’re hoping for.
• Optimal benefits stop at an hour of fasted cardio, so there’s no need for a long workout. A half-hour to an hour is plenty of time.
• Bioindividuality is SO important. Our bodies are all different, so that the same thing won’t work for everyone. That’s totally normal, and it’s totally fine. You’ve got to find the routine that works for you. If a bedtime to breakfast fasting window doesn’t feel right or fit into your schedule, it’s okay to make adjustments.

If you’d like to learn more about how I approach fasting and exercise while fasting over on my YouTube Channel. Be sure to check out the video below in particular and be sure to subscribe while you’re there so you never miss a video!