Dr. Jason Fung, bestselling author of The Obesity Code and The Complete Guide to Fasting, has dramatically shifted his thinking on fasting protocols—and what he’s discovered could change how millions approach metabolic health.
In a recent in-depth conversation, Fung revealed emerging research that’s turning conventional fasting wisdom on its head.
The benefits go far beyond weight loss, extending into inflammation control, cancer prevention, and even dementia risk reduction.
And here’s the most surprising part: you might not need to suffer through water-only fasts to reap these profound rewards.
Jump to:
- The Three-Day Threshold That Changes Everything
- Modified Fasting: The 80/50 Rule
- What You Can Actually Eat During a Fast
- The Power of Going Past 24 Hours
- Extended Fasting Protocols: How Long Is Long Enough?
- Targeting Visceral Fat: Fasting’s Secret Weapon
- Fasting and Cancer: An Adjunctive Strategy
- The Muscle Loss Myth Debunked
- Growth Hormone: Fasting’s Anabolic Secret
- The Refeed: Breaking Your Fast Right
- Making Fasting Sustainable: The Long Game
- The American Heart Association Debacle
- Practical Implementation: Your Fasting Framework
- The Evolution Continues
The Three-Day Threshold That Changes Everything
Recent research has revealed something remarkable about extended fasting that Fung himself didn’t fully appreciate years ago.
A groundbreaking study examining 7-day water fasts measured dramatic changes in protein expression throughout the body—changes that correlate with massive benefits for heart disease and other conditions.
They did a 7-day water fast and what was interesting is they measured all these proteins and there’s a humongous change in the protein expression in the body. And mostly hugely beneficial in terms of heart disease and so on. Like massive benefits.
But here’s what stunned researchers: many of these benefits don’t actually materialize until day three of fasting.
A lot of them, interestingly they point out in the study, don’t actually happen until day three. And like, wow, you really have to get to that level before you get it.
This finding suggests that popular 16:8 or even 24-hour fasting protocols, while beneficial, may be leaving significant health advantages on the table.
Modified Fasting: The 80/50 Rule
Traditional water-only fasting remains the gold standard, but Fung now advocates strongly for modified fasting approaches that make extended fasts far more accessible.
These protocols allow small amounts of specific foods while maintaining the metabolic benefits of fasting.
Think of it as Fung’s “80/50 rule”—you capture roughly 80% of the benefits while reducing difficulty by 50%.
You’re sort of get like 80% of the benefits, but it’s 50% easier, right? So, then you’re still talking about a huge plus side, right? Compared to water only fasting.
The Science Behind Modified Protocols
Modified fasting works by keeping two critical “nutrient sensors” suppressed: insulin and mTOR (a protein-sensing pathway).
When these sensors remain low, your body believes no nutrients are incoming and shifts into a restorative, fat-burning state rather than a building, storage mode.
Dr. Valter Longo’s fasting-mimicking diet pioneered this approach, restricting calories to 800-1,000 per day for five days while specifically engineering the macronutrient ratio to keep insulin and protein levels relatively suppressed.
The research supporting these modified protocols has exploded in recent years, showing benefits for inflammation, diabetes, and even cancer prevention.
What You Can Actually Eat During a Fast
Fung calls them “fasting aids” or “fasting training wheels”—foods that make extended fasting sustainable without completely derailing the process.
The key principle: keep carbohydrates and protein relatively low, as these are what trigger your body’s nutrient sensors.
Approved Fasting Aids Include:
- Avocados – high in healthy fats, minimal protein and carbs
- Olives – another fat-rich, low-carb option
- Small amounts of nuts – though watch protein content
- Bone broth – provides minerals and modest protein without breaking the fast
- Bacon – in Fung’s “avocado bacon diet” protocol
- Tea and coffee – staples since day one of Fung’s clinical practice
Bone broth deserves special attention because it solves a common fasting problem: electrolyte depletion.
What’s nice about bone broth is that you can get the minerals that you need. So, of course, during fasting, sometimes, of course, you’re taking very little sodium and so on. And some people don’t feel so good because of that.
By simmering bones, you extract not just sodium but also calcium, phosphorus, and magnesium—all while keeping protein intake modest.
The Power of Going Past 24 Hours
One-meal-a-day (OMAD) fasting has gained popularity as a 24-hour fasting protocol.
But Fung suggests pushing just slightly further—to 36 hours—produces something remarkable.
If you can actually go through the whole day, so you get to like a 36-hour fast or something. What’s interesting to me is that by the time you wake up the next morning, everything sort of resets, right? Your hunger is back down, like everything feels like just normal.
This “reset” phenomenon occurs because your body has fully transitioned into fat-burning mode.
The difficult hunger period between 16-24 hours passes, and you emerge feeling lighter and clearer rather than deprived.
Extended Fasting Protocols: How Long Is Long Enough?
For a true “full body reset,” Fung now recommends different durations depending on your approach:
- Water-only fasting: 3 days minimum
- Modified fasting: 4-5 days
- Maximum benefit studies: 7 days
Fung compares this to different running strategies—you can sprint intensely for shorter periods (water-only for 3 days) or maintain a slower pace for longer (modified for 5 days).
You could either do sprints, where you’re really really really fast, but for a shorter period, or you do sort of like, you know, slower long stuff, right? Same thing, right?
Both approaches activate the deep metabolic switches that produce inflammation reduction, enhanced autophagy, and other systemic benefits.
Targeting Visceral Fat: Fasting’s Secret Weapon
Not all body fat poses equal health risks.
Subcutaneous fat—the kind you can pinch—has minimal metabolic consequences. Visceral fat, stored around organs, drives metabolic syndrome, heart disease, and stroke risk.
Fung explains that visceral fat accumulates primarily through a process called de novo lipogenesis—when excess glucose gets converted to fat in the liver, then exported to surrounding organs.
The liver actually tries to push out that fat through triglycerides, which is VLDL, and that’s why your triglyceride levels go up when you eat too much glucose.
This explains why metabolic syndrome presents as a cluster: high blood sugar, high triglycerides, low HDL, and abdominal obesity all stem from the same root cause—hyperinsulinemia.
Why Liposuction Doesn’t Work (But Fasting Does)
Studies on liposuction reveal something fascinating: surgically removing subcutaneous fat produces zero metabolic benefit.
Fasting and low-carbohydrate diets, by contrast, preferentially target visceral fat because they deplete liver glycogen first, then force the body to mobilize fat stored in and around the liver.
Pre-surgical studies demonstrate this dramatically. Patients placed on ultra-low-calorie, ultra-low-carb diets for just five days before surgery showed:
- Total body fat: decreased 1%
- Liver fat: decreased 28%
What it’s telling you is that you’re preferentially pulling out that fat from the viscera, which is exactly what you want to do, right? Fasting is a super good way to do that.
Fasting and Cancer: An Adjunctive Strategy
Cancer cells have a metabolic vulnerability that fasting may exploit.
Research shows breast cancer cells possess 6-8 times more insulin receptors than normal breast cells, making them extraordinarily hungry for glucose in insulin-rich environments.
This breast cancer cell loves glucose because it has all this insulin. Loves insulin. So, it has six times eight times the insulin receptor. So, a little bit of insulin, that breast cancer is going to suck it all up.
The strategic implication: lower insulin through fasting to starve cancer cells of their preferred fuel.
While most body tissues can readily switch between glucose and fat for fuel, cancer cells strongly prefer glucose and function less efficiently on alternative fuel sources.
Fung emphasizes he’s not suggesting fasting as a replacement for conventional treatment, but rather as an adjunctive therapy to enhance effectiveness—particularly fasting during chemotherapy cycles.
The obesity-cancer connection strengthens this argument. Pancreatic, colorectal, and numerous other cancers show strong associations with obesity—a state defined metabolically by chronic hyperinsulinemia.
The Muscle Loss Myth Debunked
Perhaps no fasting myth frustrates Fung more than the belief that fasting burns muscle.
His counterargument relies on simple evolutionary logic.
If you think about muscle gain or loss, why do you gain muscle or why do you lose muscle? Well, it’s pretty obvious, isn’t it? You gain muscle when you exercise and you use it and you lose muscle when you don’t use it.
Muscle atrophy occurs with disuse—bed rest, space travel, immobilization—regardless of food intake. Conversely, resistance training builds muscle even in caloric deficit.
Fung asks a pointed question: Why would the human body store energy as fat, then burn functional protein tissue when that energy is needed?
Why would your body store energy in the form of glucose in the form of fat? And when you need it, you use your protein. Like that seems pretty stupid.
He compares it to storing firewood for winter, then chopping up your furniture when it gets cold—biologically nonsensical.
Muscle loss during fasting only occurs in extreme circumstances when body fat stores approach depletion (roughly 4% body fat). For the vast majority, stored fat gets mobilized first.
The Protein Powder Scam
Fung extends this critique to the fitness industry’s protein obsession.
You don’t gain muscle by eating protein. You gain muscle by exercising, okay? I would love it if all I had to do was eat protein and gain muscle, right?
The proliferation of “protein water” and “Doritos protein” represents marketing theater, not metabolic reality.
While protein plays important roles—particularly in satiety—the notion that consuming extra protein automatically builds muscle lacks biological mechanism.
Fung’s advice: prioritize whole food protein sources over ultra-processed protein powders, and recognize that muscle building requires stimulus (exercise), not just substrate (protein).
Growth Hormone: Fasting’s Anabolic Secret
Counterintuitively, fasting actually increases growth hormone levels.
This hormonal shift creates an optimal environment for muscle building when you do refeed, particularly if you’ve trained during the fasted state.
The strategic window: exercise while fasted, then refeed with nutrient-dense whole foods to maximize the anabolic response.
The Refeed: Breaking Your Fast Right
What you eat when breaking an extended fast matters, though perhaps less than many assume.
Fung’s core recommendations center on unprocessed, minimally processed foods that provide essential nutrients:
- Prioritize protein and healthy fats (essential amino acids and fatty acids)
- Include fiber-rich vegetables
- Minimize high-carbohydrate foods initially
- Avoid ultra-processed options entirely
Starting with bone broth has become traditional in Fung’s clinical practice—it’s gentle on the digestive system while providing protein, minerals, and collagen in natural proportions.
If you look at the macros of it, it’s actually like 12 g of protein and zero fat, right? So you’re just basically giving your body what it needs along with whatever salt and stuff you put into it.
Clinical observations suggest that mastering the first meal after fasting creates a ripple effect—when done well, subsequent meals naturally align with better choices, food cravings diminish, and weight loss continues or maintains.
Making Fasting Sustainable: The Long Game
Fung’s evolved philosophy centers on one critical insight: adherence trumps perfection.
By making it easy enough that it doesn’t became become a real struggle to do, right? And that’s that’s where you really have to make it so that like if if you hated every time you have to do a fast, then very soon you’re going to stop.
Using fasting aids doesn’t constitute “cheating”—it represents strategic optimization.
If bone broth, avocados, or small amounts of olives enable someone to extend from 24 to 48 hours, or from 48 to 72 hours, the net metabolic benefit far exceeds any theoretical purity gained from water-only fasting that proves unsustainable.
Fung himself continues using these aids despite years of fasting experience.
I still use them. I still use all of those things because the point is not that you know, you don’t just take them for no reason, right? You’re taking them but going longer.
The American Heart Association Debacle
Fung doesn’t hide his frustration with institutional resistance to fasting research.
A recent American Heart Association claim suggested that fasting for just 8 hours increased heart disease risk by 91%—a finding Fung calls “ridiculous” and “crap.”
Remember a couple years ago even, the American Heart Association had this big thing about how if you fasted for 8 hours, your heart risk went up by 91% or something like that. That’s the stupidest thing ever, right?
The study’s methodology was deeply flawed, yet it created waves of concern among patients and physicians.
This highlights the ongoing tension between emerging metabolic research and established nutritional dogma—dogma that, just years ago, recommended eating 10 small meals daily to “keep metabolism going.”
Practical Implementation: Your Fasting Framework
Based on Fung’s updated recommendations, here’s a practical framework for incorporating extended fasting:
For Beginners:
- Start with 16:8 time-restricted eating
- Progress to 24-hour fasts (OMAD)
- Push to 36 hours to experience the “reset”
- Use fasting aids liberally: bone broth, tea, coffee, small amounts of avocado or olives
For Metabolic Reset:
- Modified fast: 4-5 days at 800-1,000 calories/day, low carb and protein
- Water fast: 3 days minimum (with medical supervision if you have health conditions)
- Incorporate bone broth, minerals, and electrolytes throughout
- Break fast with whole foods emphasizing protein, healthy fats, and fiber
For Ongoing Health:
- Regular 36-48 hour fasts weekly or bi-weekly
- Quarterly 3-5 day extended fasts
- Exercise in fasted state when possible
- Focus on whole, unprocessed foods during eating windows
The Evolution Continues
What distinguishes Fung’s approach is his willingness to update recommendations as research evolves.
The fundamentals from The Complete Guide to Fasting remain solid—fasting powerfully addresses weight loss, insulin resistance, and metabolic dysfunction.
But the expanded evidence for extended fasting, modified protocols, and specific health conditions like cancer and inflammation represents genuine advancement, not backtracking.
The message for practitioners: fasting works, longer protocols unlock additional benefits, and strategic modifications dramatically improve sustainability without sacrificing results.
For those intimidated by multi-day water fasts, the modified approach offers an accessible entry point backed by legitimate research.
And for veterans already incorporating fasting, pushing past that 3-day threshold—whether through strict water fasting or modified protocols—may activate the deeper metabolic switches that produce system-wide regeneration.
The science of fasting continues maturing. What began as controversial now approaches mainstream acceptance, and what seemed like maximum benefit just years ago now appears to be merely scratching the surface.
