Does Fasting Burn Muscle? What MrBeast's 14-Day Fast Actually Shows

A clip of MrBeast talking about his 14-day water fast is making the rounds again. The detail that got people arguing is the scan. He says a DEXA scan showed he lost about 13 pounds of fat and around six or seven pounds of muscle (UNILAD). One post putting it as "1 in every 3 pounds he lost wasn't fat" passed a million views on X this week (X).

The quotes aren't new. They come from his 2025 appearance on Theo Von's podcast. But the debate they reopened is a good one for anyone who lifts: does fasting burn muscle, and is it a smart way to lose fat?

Extended water fasting does cost you lean mass, more than most people expect. Some of that loss is water and stored carbs, not muscle. But not all of it. And if your goal is to lose fat while keeping the muscle you've built, there's a better-tested way to do it.

What MrBeast actually said

This is one person's account, from memory, on a podcast. It isn't a published case report. In the interview he said he lost "20 something pounds," that he "lost like six pounds of muscle," and that "even after you start eating, the muscle didn't just come back." He also said he never got the mental clarity fasting fans promise: "I was too tired to have clarity" (UNILAD).

He was medically supervised and has said not to try it without that. It's one data point. The question is whether the research agrees.

What the research on prolonged fasting shows

The best summary we have is a 2023 narrative review in Nutrition Reviews from Krista Varady's lab at the University of Illinois Chicago, which looked at human trials of water fasting lasting 5 to 20 days. People lost 2% to 10% of their body weight. Roughly two-thirds of that weight was lean mass and only one-third was fat. The authors wrote that the amount of lean mass lost suggests prolonged fasting may increase the breakdown of muscle protein, which they called "a concern" (Ezpeleta et al., PubMed).

Two more findings from that review matter here:

• The metabolic benefits didn't last. In the trials that followed people after they started eating again, improvements like lower blood pressure and better blood sugar were gone three to four months later, even when the weight stayed off.
• The exact two-thirds ratio rests on thin data. Only a handful of trials measured body composition, and some used methods that are easily thrown off by hydration.

The direction is clear, even if the exact number isn't. A long fast is not a muscle-sparing way to lose weight.



"It was just water weight"

The main pushback from fasting advocates is that a DEXA scan can't tell muscle apart from the water and carbohydrate stored inside it. That part is true.

DEXA counts everything that isn't fat or bone as "lean mass." When you stop eating, your muscles burn through their stored carbohydrate, called glycogen, and the water bound to it goes too. A 2017 study at the Australian Institute of Sport showed how much this moves the reading. In 18 trained male cyclists, two days of carb loading raised DEXA estimates of lean mass by about 2% to 3%, over a window too short for muscle protein to change much. The authors concluded that shifts in glycogen and water alter DEXA lean mass estimates (Bone et al., PubMed).

Physician Ken Berry made the same point on X this week, using that study to argue MrBeast's scan overstated his muscle loss (X). Nutrition researcher Alex Leaf argued the other side, that older fasting research found real loss of body protein, not just water (X).

Both are partly right. Some of those six or seven pounds were almost certainly glycogen and water, and that comes back when you eat. But "it was all water" doesn't fit the review data, and it doesn't fit MrBeast's own report that the muscle was slow to return. Part of it was water, part of it was muscle, and one scan can't tell you how much of each.

What actually protects muscle while you lose fat

If the goal is a leaner body with your strength intact, the research points somewhere much less dramatic than two weeks of water.

A 2016 randomized trial at McMaster University put 40 young men on a steep diet, about 40% below what they needed, for four weeks. All of them lifted and did interval training six days a week. One group ate about 0.55 grams of protein per pound of body weight a day. The other ate about 1.1 grams per pound. The higher-protein group gained about 2.6 pounds of lean mass and lost about 10.6 pounds of fat. The lower-protein group held their lean mass roughly steady and lost about 7.7 pounds of fat (Longland et al., PubMed).

It was a short lab study with meals provided, so don't expect those exact results at home. But it shows the principle. Eat enough protein and keep lifting, and you can lose fat in a real deficit without giving up muscle.

For most lifters, a daily target of about 0.7 grams of protein per pound of body weight is a sensible floor. That's where a large 2018 meta-analysis found extra protein stopped adding muscle (Morton et al., PubMed). We went through that number in detail in How Much Protein Do You Actually Need to Build Muscle?

How to use this

Pick a moderate deficit. Aim to lose fat over months, not days. A steady deficit you can hold beats a crash you have to recover from.

Hit your protein. Around 0.7 grams per pound of body weight a day. For a 180 lb man, that's about 126 grams, spread over three or four meals.

Keep lifting. Training is the signal that tells your body the muscle is worth keeping. In the Doctrine of Becoming, that's the Forge. The Citadel, your nutrition, supports it.

Keep moving. Daily walking helps a deficit without eating into recovery. Our post on whether you really need 10,000 steps a day covers how much is enough.

Treat fasting as a tool, not a test. A shorter eating window is fine if you still reach your protein. A two-week water fast to prove you can costs more than it gives.

There's a mindset point under all of this. Discipline isn't the most extreme thing you can endure for a few days. It's the standard you can keep on an ordinary Tuesday. That's the idea behind You Don't Need a Reset. You Need a Standard.

Build it into something bigger

If you want a starting structure for training, nutrition and mindset, download The Doctrine of Becoming, a free 21-day reset of practices you adapt into your own lifestyle. Get it here.

And if you'd rather have someone build your cut with you and keep you accountable, apply for 1:1 coaching at the $99/month founding rate.

This post is for education, not medical advice. Prolonged fasting can be risky, especially if you have diabetes, a digestive condition, a history of disordered eating or take regular medication. Talk to your doctor before trying it.