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The facts on fasted training

Last Modified: April 22, 2022

Sports & Exercise


This post was written by Sami Kauffman, MA, RD, CD, sports dietitian, Parkview Sports Medicine.

Among the many debates in the fitness world, fasted exercise remains a polarizing topic. The current research available on the subject is limited and mixed. Let’s take a closer look at what we know about this approach.

What does it mean to be fasted?

A fasted state occurs when you have not eaten for 10-12 hours prior to exercise. This allows for glycogen stores to be fully depleted and means that the body has stopped breaking down food. In theory, this would facilitate increased fat burning by lowering insulin levels and depleting glycogen stores enough that body fat would be broken down and burned as free fatty acids for fuel.

What the research says

A beta-analysis from 2017 looking at five studies on fasted versus fed exercise concluded that working out fasted does not result in greater weight loss or changes in lean muscle mass or fat mass. Although fasted workouts do increase fat burning, it does not translate to decreased body fat levels like changes in long-term caloric balance does.

What the experts say

In addition to not seeing significant effects on fat loss from fasted training, the tactic could result in poor performance and longer recovery times. There is an increase in protein breakdown that occurs as the body attempts to find fuel before it seeks fat as a fuel source.

However, we do see that there are a few scenarios in which fasted training can be beneficial. For those individuals who are training for ultra-endurance events and will have limited access to fuel during the event, this type of training can prepare them for prolonged exercise on minimal fuel. Additionally, performing low-intensity exercise, such as walking, in a fasted state may be better as eating beforehand can blunt the fat burning effects that occur as a result.

Fasting and exercise can both assist in increasing growth hormone (HGH) production, which can aid in weight loss and muscle building, but the effect ceases when the fast ends. So, to take advantage of this benefit, you would need to implement a regular weight training program, eat enough protein and carbohydrates, and fast regularly to keep HGH elevated.

Several studies also show a link between fasted cardio and improved endurance. The mechanism through which this occurs is an increase in VO2 max, which is the maximum amount of oxygen used during exercise. Fasted cardio has been shown to increase your ability to take in and deliver oxygen to your muscles, allowing you to train harder, especially during high intensity training sessions.

What does work for changing body composition?

The most important things to consider when attempting to lose weight or change your body composition are total caloric deficit and consistent training.

Is fasted training right for you?

So, should you work out in a fasted state? As a fat loss technique, fasting is a trend. There is limited research that supports it as more beneficial than other techniques and it could hinder performance, preventing you from maximizing caloric burn. There are some benefits in the conditions, described above, and some people just prefer to train that way. As long as it’s not hindering your recovery and performance, there is no harm in doing so, but if you don’t enjoy it or feel good doing it there’s no reason to continue doing so. If you choose to do work out fasted, be sure that your meal the night before consists of plenty of protein and carbohydrates to ensure your body is prepared for the session and be sure to eat a protein- and carbohydrate-rich meal right afterward.


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