One of the biggest concerns people have about Fasting is that they will lose muscle instead of body fat. It comes up again and again in the Fasting communities, especially among people who are new to Fasting.

It’s a legitimate concern. After all, if you’re not eating, where does your body get its energy? And it doesn’t help that there are nutritional “experts” out there promoting the idea that Fasting causes your body to burn through muscle.

The reality is that your body is much smarter than that. 

You have more than one fuel source

Your body can use several different sources of fuel. After you eat, glucose from food is readily available for energy. Some of that glucose is stored as glycogen in your liver and muscles, creating an easily accessible supply for later.  

As you Fast, insulin levels fall and glycogen stores begin to decline. Your metabolism then starts shifting toward other sources of energy. This is where your stored body fat comes in. 

Fat is an excellent energy source. Stored fat can be broken down into fatty acids, which can then be used for energy. Your liver can also turn some fatty acids into ketones, which provide another important fuel source during Fasting. 

In other words, your body does not simply “freak out” when food stops coming in and immediately starts burning muscle for energy. It does exactly what it’s supposed to do and looks for stored fat. 

Remember, your body is designed to keep you alive. Survival is the goal. When there is plenty of stored fat available, using that energy source makes much more sense than immediately relying on muscle tissue. 

What happens to muscle during a fast?

This is where things get interesting. 

When insulin falls during a Fast, other “counter-regulatory” hormones rise. One of these is Human Growth Hormone, or HGH. HGH plays several important roles in the body, including helping regulate fat metabolism and supporting the preservation of lean tissue.

Fasting can cause massive spikes in HGH (Human Growth Hormone). During a 24 hour Fast, HGH in women can increase by up to 1,300 percent, and in men, up to 2,000 percent. That’s huge! 

But my DEXA scan shows I lost muscle!

Don’t panic. 

Changes in hydration and glycogen can affect measurements of lean mass, including those seen on a DEXA scan. Glycogen is stored along with water, so when glycogen levels decrease during Fasting, some of the associated water is lost as well. 

When Fasting is actually not safe

Fasting is not appropriate for everyone.

If someone has very little stored body fat, the body has less energy available to draw from, and prolonged Fasting can increase the risk of losing lean tissue. This is one reason people who are already very lean should be cautious about Extended Fasting. 

If you are unsure whether Fasting is appropriate for you, talk with your healthcare provider. 

How much body fat is not enough? 

A BMI below 18 is considered underweight, but BMI does not tell us how much body fat someone has or whether Fasting is safe for that individual. Fat percentage of lower than 10% for women and 5% for men is also considered below a healthy weight. Anyone in this weight bracket should not Fast. 

The closer you are to being underweight or extremely lean, the more cautious you should be about Extended Fasting. 

Tips for preserving muscle

While Fasting encourages greater use of stored fat, muscle preservation still matters. 

Eating enough protein during your eating periods can provide your body with the amino acids it needs to maintain and repair muscle. Resistance training is also important because your muscles respond to the demand placed on them. 

Your overall approach matters, too. More Fasting is not automatically better. Very aggressive or prolonged Fasting should be approached thoughtfully, especially as you get closer to your goal weight.  The goal is to use Fasting strategically while giving your body what it needs when you do eat. 

If you practice Extended Fasting, take breaks and focus on nutrient-dense refeeds with adequate protein, and as you get closer to your goal weight, transitioning  to Intermittent Fasting may be better long-term. Check out our Refeed product here.

Always check in with your healthcare provider if you have any concerns about muscle loss. 

Fasting burns body fat and supports muscle

The myth that Fasting causes your body to “eat your muscles” is just that, it’s a myth, not a scientific fact. When Fasting is combined with adequate nutrition, protein, and resistance training, plus a little wisdom, you can put that fear to rest. 

For more information on this subject, please click here: https://fung.substack.com/p/does-fasting-cause-muscle-loss