If you’ve spent any time in the fitness or nutrition world, you’ve likely heard the same warning over and over:
“If you don’t eat, you’ll lose muscle.”
Especially from me.
And that fear makes sense. Muscle is essential for metabolism, recovery, strength, and aging well. It’s the last thing you want to lose during any fat loss phase or in pursuit of longevity.
But what if that assumption isn’t the whole story?
According to Canadian nephrologist and fasting researcher Dr. Jason Fung, longer fasts defined as 48 to 72 hours or more do not automatically trigger muscle loss.
In fact, under the right hormonal conditions, fasting may actually help preserve muscle tissue while your body lives off stored fat and ketones.
Here’s what the science and Dr. Fung’s clinical experience actually show.
The Traditional View: Starvation Equals Muscle Loss
Most people assume that if you stop eating, your body will quickly run out of fuel and start breaking down muscle tissue for energy. This is rooted in a misunderstanding of how the body prioritizes fuel sources.
In reality, your body is designed to protect muscle, especially in the early stages of fasting.
💡 Key Takeaway: Your body’s first instinct during fasting isn’t to break down muscle. It’s to conserve it by shifting fuel sources.
What Actually Happens During a Multi-Day Fast
0–24 hours:
- Your body uses stored glycogen from your liver and muscles for energy
- Insulin begins to drop
- Fat burning starts to increase
24–48 hours:
- Glycogen stores are mostly depleted
- The body shifts toward burning triglycerides (stored fat)
- Ketone production begins, fueling the brain and muscles
- Growth hormone (GH) surges, which helps preserve lean tissue (1)
48–72+ hours:
- Ketones become the primary fuel source
- GH remains elevated
- The body suppresses proteolysis (muscle protein breakdown) to conserve muscle
- Autophagy and cellular repair intensify (2)
💡 Key Takeaway: Fasting doesn’t make your body eat muscle. It makes it smarter about preserving it, especially when ketones and growth hormone rise together.
Dr. Fung’s Argument: Fasting Is Not Starvation
Dr. Fung has worked extensively with patients using extended fasts for metabolic improvement, especially in cases of obesity and type 2 diabetes. One of his most repeated claims is:
According to Dr. Jason Fung, fasting triggers hormonal changes that promote fat burning while preserving muscle, a response distinct from chronic calorie restriction (3).
He cites three key mechanisms:
- Elevated growth hormone during fasting helps maintain lean mass
- Ketone bodies (especially beta-hydroxybutyrate) spare muscle protein by fueling organs and the brain
- Fat oxidation provides sufficient energy from triglyceride stores, reducing the need for amino acids (4)
💡 Key Takeaway: Longer fasting may appear extreme—but metabolically, it’s a natural, structured response. The body adapts by sparing muscle and burning fat.
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What About Exercise and Protein?
Fasting alone, through hormonal shifts like elevated growth hormone and ketone production, helps protect muscle during extended fasts, according to Dr. Jason Fung’s research (5).
However, maintaining and building muscle over the long term still requires consistent strength training and adequate protein intake during non-fasting periods. This is supported broadly across metabolic and fitness research, even though Dr. Fung’s fasting protocol focuses primarily on metabolic healing, not on fitness-specific goals.
In simple terms:
Fasting can help preserve muscle during the fast itself, but preserving and optimizing lean mass over months and years still depends on how you train and recover when not fasting.
💡 Key Takeaway: Fasting may protect muscle hormonally during fasts—but smart strength training and nutrition are essential for lifelong muscle maintenance.
Important Clarification from the PlateauBreaker Perspective
At PlateauBreaker, extended fasting is not required for fat loss. We recognize its potential as a strategic tool for cellular repair and longevity when used appropriately.
We support educating people on how fasting works and why it’s not automatically a muscle-wasting process.
In the PlateauBreaker Diet book, I share my own personal experience with fasting. I’ve been practicing structured fasts for nearly two years, and I’ve done it while maintaining muscle, strength, and recovery.
That said:
Longer fasts (48–72+ hours) are not appropriate for everyone.
They should be done only with medical supervision—especially if you are taking medications or have any pre-existing medical conditions.
So… Can You Fast Without Losing Muscle?
Yes—under the right circumstances.
If your body is:
- Well-nourished heading into the fast
- Able to generate and use ketones efficiently
- Supported with resistance training and protein during non-fasting periods
- And you keep fasts within a safe window (48–72 hours max) unless medically supervised
Then you may be able to fast without losing muscle, and possibly with some added long-term metabolic benefits.
✏︎ The Bottom Line
Fasting has been misunderstood for decades.
It’s not about starving your body or wrecking your muscles. When done properly, it’s a controlled metabolic shift that prioritizes fat for fuel, raises growth hormone, and even protects lean tissue.
It’s not for everyone. And it’s not part of every fat loss strategy. But understanding how it works gives you options and shows that muscle loss isn’t a guaranteed outcome.
Want to learn more about how your body adapts to real-world fat loss, not fitness myths?
👉 Sign up for PlateauBreaker and get science-based strategies to support sustainable transformation, whether or not fasting is part of your plan.
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Bibliography
- Ho, K Y et al. “Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man.” The Journal of clinical investigation vol. 81,4 (1988): 968-75. doi:10.1172/JCI113450. https://pmc.ncbi.nlm.nih.gov/articles/PMC329619/
- Cahill, George F Jr. “Fuel metabolism in starvation.” Annual review of nutrition vol. 26 (2006): 1-22. doi:10.1146/annurev.nutr.26.061505.111258. https://pubmed.ncbi.nlm.nih.gov/16848698/
- Nørrelund, Helene et al. “Effects of GH on protein metabolism during dietary restriction in man.” Growth hormone & IGF research : official journal of the Growth Hormone Research Society and the International IGF Research Society vol. 12,4 (2002): 198-207. doi:10.1016/s1096-6374(02)00043-6. https://pubmed.ncbi.nlm.nih.gov/12175652/
- Nørrelund, Helene et al. “Effects of GH on protein metabolism during dietary restriction in man.” Growth hormone & IGF research : official journal of the Growth Hormone Research Society and the International IGF Research Society vol. 12,4 (2002): 198-207. doi:10.1016/s1096-6374(02)00043-6. https://pubmed.ncbi.nlm.nih.gov/12175652/
- Longo, Valter D, and Satchidananda Panda. “Fasting, Circadian Rhythms, and Time-Restricted Feeding in Healthy Lifespan.” Cell metabolism vol. 23,6 (2016): 1048-1059. doi:10.1016/j.cmet.2016.06.001. https://pmc.ncbi.nlm.nih.gov/articles/PMC5388543/
