Is Fasted Cardio Superior for Weight Loss or Just a Fad?

The fitness industry is constantly jumping from trend to trend. As coaches we need to develop core values based upon scientific principles and stick to them. When new stuff comes up, we need to think about it, see where it can fit in with our clients, apply it, and collect data to see whether it is working or not.

One of the current trends is training in a fasted state for weight loss. To reemphasize that last part, this tactic is used only for weight loss. Those of you looking to increase performance are best served not applying this approach. For those of you looking to drop a few pounds, let us look at what the science says.

What the Science Says About Fasted Cardio

A study performed in 2010 compared fasted training to fed training over a six-week period. Researchers looked at some key areas and compared the two groups. Training was always performed in the morning and consisted of two sixty-minute sessions and two ninety-minute sessions. 

“Both the fasted and the fed groups increased their glucose tolerance, but the fasted group increased it substantially more.”

The fed group was given a breakfast of 675 calories, made up of 70% carbohydrate, 15% protein, and 15% fat. On top of that, they were given 1g of maltodextrin per kg of bodyweight about ninety minutes before training. The fasted group received this exact same cocktail in the midafternoon. The areas the researchers compared were glucose tolerance, GLUT4, muscle glycogen, metabolic enzymes, body composition, and exercise capacity. The results of the study were quite interesting.

Both the fasted and the fed groups increased their glucose tolerance, but the fasted group increased it substantially more. GLUT4 is a glucose transporter that allows glucose to enter muscle cells. The more of these we have, the more glucose gets taken up into the muscles, as opposed to being stored as fat. GLUT4 transporters increased by 28% in the fasted group and only 2-3% in the fed group.

Glycogen stores increased more in the fasted group when compared to the fed group, but there was no significant difference between intramuscular lipid stores. Increased intramuscular lipid stores are seen in obesity and can lead to insulin resistance and mitochondrial dysfunction.¹ 

“Glycogen stores increased more in the fasted group when compared to the fed group, but there was no significant difference between intramuscular lipid stores.”

The only differences in metabolic enzymes were seen in translocase/CD36 and carnitine palmitoyltransferase¹. These two enzymes play a role in our abilities to metabolize both fat and glucose. The fasted group saw an increase of approximately 30% and the fed group did not see an increase. All other metabolic enzymes tested remained the same.

As expected, the fed group had a greater increase in VO2max, but the fasted group showed a greater increase in fat oxidation. As I stated previously, fasted cardio is not a way to increase performance, but instead to lose weight. With less glucose present, it is no surprise that the fasted group showed greater fatty acid oxidation for energy.

The Practical Side of Fasted Cardio

There are other studies that show similar results as the one presented in this article. After reading that study and hearing that other studies back up these findings, you may be ready to jump right into a fasted cardio weight-loss routine – but it’s not for everyone.

“[K]ey areas to assess are sleep, vitamin D, stress levels, exercise routine, and diet. If all of these are squared away, then give fasted cardio a try.”

I have messed around with the fasted cardio component with myself, as well as with clients, and I have experienced a mixed bag of results. For some it worked great, but for others it didn’t. This is something you may have to mess around with for a period to see if it helps. Before doing that, I encourage you to assess your lifestyle and improve upon areas that may be weak. Some key areas to assess are sleep, vitamin D, stress levels, exercise routine, and diet. If all of these are squared away, then give fasted cardio a try.

There are some key points to keep in mind if you do decide to try it:

• This study was performed in the morning, and I encourage you to do the same. Waiting until the afternoon or evening to work out while fasting all day may be too much of a stressor and could have the opposite of the desired effects.
• I also encourage you not to make the fasted cardio something intense. Low-level aerobic activity between 70-80% of VO2max should suffice. Anything more intense may also become a stressor.
• Make sure you take in carbohydrates with your first meal. I ran into this problem with quite a few clients. We use a paleo diet template, and I encourage my clients to get their carbs in the afternoon or evening. This created the habit of them having a large salad with meat and very little carbohydrates for lunch. This didn’t work well for coming off a fast. So, if this sounds like your current lunch make sure to add some fruit or sweet potatoes to it.

Those of you who have tinkered with fasted cardio I would like to hear from you. Let us know how you implemented it, for how long and how well it worked for you. Your experiences may be able to help someone else trying to lose a few extra pounds.

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