Therapeutic Fasting
Dr. Jason Fung
I am a specialist physician with special interest in weight loss and reversal of type 2 diabetes. I have pioneered the therapeutic use of intermittent fasting for patients.
DR. JASON FUNG (CAN) - Nephrologist | Physician | Author | Intensive Dietary Management
Fasting is about getting optimal human performance from a simple lifestyle change. It is a way to simplify our life and gives you the power to take control of your health back into your own hands. That really is the ultimate in biohacking.
Summary
- Modern obesity and diabetes epidemics were triggered by a shift toward high-carbohydrate dietary guidelines and a significant increase in daily eating frequency since the late 1970s.
- The body alternates between a fed state, where insulin stores energy as fat, and a fasted state, where it burns fat; frequent snacking prevents the body from ever entering the fat-burning state.
- Fasting does not decrease metabolic rate or burn muscle; instead, it increases growth hormone and adrenaline, allowing the body to rejuvenate while transitioning to burning stored fat for fuel.
- Unlike calorie restriction, which increases hunger, fasting regulates the hormone ghrelin to naturally reduce hunger and eliminate cravings as the body adapts to using its own energy stores.
- Therapeutic fasting is an effective, free tool for reversing type 2 diabetes and improving mental clarity by clearing excess sugar and activating the brain’s natural focus mechanisms.
Article
Therapeutic Fasting: The Science Of Weight Loss And Type 2 Diabetes
A Radical Idea That Was Also An Ancient One
At Biohacker Summit 2018 Toronto, held in Toronto, Canada, on 15 and 16 October 2018, Dr. Jason Fung stepped onto the stage with a message that felt both provocative and oddly old-fashioned. His presentation, *Therapeutic Fasting*, argued that one of the most powerful tools for tackling obesity and type 2 diabetes was not a cutting-edge supplement, an expensive device or a pharmaceutical breakthrough, but the deliberate act of not eating for periods of time.
Fung, a specialist physician known for his work in weight loss and the reversal of type 2 diabetes, framed fasting not as a fad but as a recovery of neglected physiological wisdom. The body, he suggested, was built for cycles of feeding and fasting. Modern life had broken that rhythm.
“The best of all medicines,” he reminded the audience, borrowing from Benjamin Franklin, was “resting and fasting.”
The Epidemics That Followed Modern Dietary Advice
Fung’s case began with a historical rupture. The modern epidemics of obesity and type 2 diabetes, he said, accelerated sharply after the late 1970s. That timing mattered. It coincided not only with the rise of official low-fat, high-carbohydrate dietary advice, but also with a profound change in how often people ate.
After the 1977 Dietary Goals for the United States, carbohydrates were elevated to the centre of nutritional orthodoxy. Bread, pasta, rice and cereals became staples of respectable eating. Fat was demonised. Eggs were treated with suspicion. Butter retreated. Processed low-fat products flooded supermarket shelves.
People, Fung argued, largely did what they were told. They ate less fat, more grains and more refined carbohydrates. They also began eating far more frequently. What had once been a pattern of breakfast, lunch and dinner turned into a rolling schedule of snacks, muffins, bars and late-night grazing.
The result was not improved public health, but a dramatic rise in waistlines and a delayed but equally alarming surge in type 2 diabetes, a condition linked to heart disease, stroke, blindness, kidney failure and nerve damage.
The Body Can Only Be In One Of Two States
Fung’s central argument rested on a blunt physiological distinction. The body, he said, exists either in a fed state or a fasted state.
In the fed state, insulin rises. Its job is to store energy. Some of that energy is packed away as glycogen, but much of it, particularly in an era of chronic overconsumption, is stored as fat. In the fasted state, insulin falls and the body begins drawing on those stored reserves.
This is not an emergency mechanism, he insisted. It is normal human biology.
“If you’re in the fed state, you’re storing fat. If you’re in the fasted state, you’re burning body fat, but you can’t do both at the same time,” Fung told the audience.
The problem with modern eating, in his view, was that many people no longer left enough space between meals to ever truly enter that second state. The body was being instructed to store, store and store again.
From Glycogen To Ketosis
As fasting progresses, the body shifts fuel sources. First it uses circulating glucose from recent meals. Then it turns to glycogen stored in the liver. As glycogen falls, the body increases gluconeogenesis and begins drawing more heavily on fat stores, producing fatty acids and ketones.
For Fung, this transition was not evidence of damage. It was evidence of design.
“Your body is not that stupid,” he said. “It simply shifts its fuel sources and pumps you full of energy so that you can be sharp and activated. It is the body’s way of ensuring you have the energy you need even when you are not eating.”
That formulation cut directly against one of the most deeply embedded fears in diet culture: that skipping food would trigger metabolic collapse and muscle wasting. Fung argued the opposite. The body, he said, does not carefully store fat only to destroy valuable lean tissue the moment food is withheld. Instead, it adapts, preserving lean mass while mobilising stored energy.
Why Fasting Was Not The Same As Calorie Restriction
This was perhaps the most consequential distinction in the talk. Fung repeatedly separated therapeutic fasting from standard calorie restriction.
The old advice, he argued, was familiar and mostly futile: shave a few hundred calories off daily intake, eat constantly, stay disciplined and endure the hunger. But physiology rebelled. Metabolic rate fell. Hunger hormones rose. Weight loss became increasingly hard to sustain, not because people lacked character, but because the body fought back.
Fasting, by contrast, appeared to behave differently. Rather than slowing metabolism, Fung said, it increased counter-regulatory hormones such as noradrenaline and growth hormone. Those hormonal changes supported energy output, preserved muscle and helped the body shift into fat-burning.
He cited evidence from alternate-day fasting and multi-day fasting studies to argue that resting energy expenditure did not collapse in the way calorie-restricted diets often did. In some settings, it even rose.
The message was simple and quietly subversive: all forms of eating less were not metabolically equal.
Hunger, Cravings And The Myth Of Constant Deprivation
If calorie restriction was haunted by hunger, fasting, Fung argued, was not necessarily so.
He pointed to the behaviour of ghrelin, the hormone commonly associated with hunger. Ghrelin, he said, tended to rise in waves around habitual mealtimes, then fall again even if no food arrived. Hunger was not an endlessly escalating emergency. It was often rhythmic, temporary and trainable.
Over longer fasting periods, he argued, hunger frequently diminished rather than intensified. As the body adapted to using fat for fuel, cravings could weaken. This, too, set fasting apart from the misery of nibbling through a low-calorie diet while thinking obsessively about food.
“When you do not eat, you actually work from a mental standpoint at a much higher level,” Fung said. “Fasting allows your brain to work more clearly because your body is activated, pumping energy into the system.”
In a culture that often treats food as the prerequisite for focus, the claim landed with force. Fung painted the fasted state not as foggy and depleted, but as alert, sharpened and evolutionarily primed.
Renovation, Rejuvenation And The Anti-Ageing Appeal
The most expansive part of Fung’s case moved beyond weight loss and diabetes toward cellular renewal. Fasting, he said, was not only a metabolic reset but a biological renovation.
“When you are fasting, you are tearing down old proteins in your body and building up new ones,” he said. “You are essentially renovating and rejuvenating the body; it is a form of anti-aging available to all of us for free.”
There was a deliberate populism in that line. In a room shaped by the language of optimisation and elite self-experimentation, Fung offered something strikingly democratic. Therapeutic fasting required no membership, no shipping fee, no app-based subscription. It was available, at least in principle, to almost everyone.
He also described fasting as part of a natural cycle rather than a modern intervention.
“Fasting is part of a natural cycle of life. Your body has the ability to pull energy back out and use it. It is a healthy state that activates hormones to fight sickness and help you stay healthy for life.”
A Free Tool In An Age Of Chronic Disease
There was, underneath the metabolic detail, a broader ethical challenge in Fung’s talk. If type 2 diabetes and obesity were driven in large part by hormonal dysfunction, particularly chronically elevated insulin, then treatment could not rely solely on adding more medication while leaving meal patterns untouched.
Therapeutic fasting, he argued, directly addressed insulin resistance by lowering insulin levels and allowing the body to clear excess stored energy. It was flexible enough to fit many lifestyles, whether through time-restricted eating, a 16-hour daily fast or longer 24-hour fasting windows performed several times a week. It could complement ketogenic, whole-food or lower-carbohydrate approaches. And unlike many medical interventions, it cost nothing.
That did not mean it was for everyone. Fung noted that fasting should be used in the right context and was generally inappropriate for children, people who were underweight or malnourished, and women who were pregnant or breastfeeding. He also alluded to the real medical risks involved in refeeding severely malnourished individuals, where sudden insulin surges can trigger dangerous electrolyte shifts.
But within those boundaries, he presented fasting as both ancient and modern, intuitive and evidence-based, austere and strangely liberating.
The Challenge To A Culture That Never Stops Eating
What lingered after the talk was not merely a protocol but a critique of contemporary life. A society built around perpetual consumption had normalised the idea that the body must be constantly refuelled, that every flicker of appetite required intervention, that breakfast bars and desk snacks were marks of sensible self-care.
Fung rejected that assumption outright. He asked instead what might happen if health was restored not by adding more, but by creating space. Space between meals. Space for insulin to fall. Space for the body to do what, in his telling, it had always known how to do.
“A little starvation can do more for the average sick person than the best medicines and the best doctors,” he said, invoking an older medical tradition while challenging a deeply modern crisis. “It is about understanding the physiology so you can take care of metabolic problems without drugs or surgery.”
In Toronto, on 15 October 2018, at Biohacker Summit 2018 Toronto, Dr. Jason Fung did more than defend fasting. He recast it as a biological language the modern world had forgotten how to hear.
Part of Biohacker Summit 2018 Toronto