Dr. Jason Fung on Cancer as a Disease of the Soil, and Why Fasting Beats Snacking

High-Level Overview

In the first of a two-part conversation, Lewis Howes talks with Dr. Jason Fung, the physician and author of The Obesity Code, The Diabetes Code and The Complete Guide to Fasting, and co-founder of The Fasting Method, about his new book The Cancer Code. Fung explains what we actually know about the causes of cancer, why obesity and type 2 diabetes turned out to be major risk factors, and his central metaphor: the seed of cancer may exist in all of us, but we control the soil. From there the conversation shifts to fasting, including why the modern habit of eating all day keeps growth signals switched on, how the body moves between fed and fasted states, and the myths about muscle loss, starvation mode and hunger.

The tone is clear, practical and optimistic, with Fung using everyday analogies (firewood and sofas, bathroom renovations, hybrid cars) to explain physiology, and Howes adding his own experience of a four-day water fast. The conversation moves from the science of cancer to the mechanics of fasting and ends with a list of fasting's benefits.

The Discussion Explores:

• The biggest known causes of cancer: tobacco first, diet close behind
• The decades of failed theories about fiber, fat and vitamin supplements
• Obesity and type 2 diabetes as major, only recently recognized cancer risk factors
• The seed and soil model of cancer, and why lifestyle matters even with genes like BRCA
• Cancer cells as selfish, almost separate organisms that respond to growth signals like insulin and mTOR
• What to cut: sugar, refined grains, refined oils and meats, and constant eating
• The fed and fasted states, the rise of snacking and the 14-hour eating day
• Fasting myths about muscle loss and metabolism, managing hunger, autophagy and fasting's core benefits

Deep Dive into Main Ideas

What Actually Causes Cancer

Fung pushes back on the idea that we don't know what causes cancer. The World Health Organization keeps a long list of carcinogens, and studies estimating their contribution put tobacco smoke first at about 35 percent (as of 2015 estimates), with diet a close second. Together these two far outstrip things people often worry about, such as radiation, sunscreens, chemicals and pesticides, which do cause cancer but contribute little across a whole population.

The hard part was figuring out which part of the diet mattered. In the 1970s researchers suspected too little fiber; in the 1980s and 1990s, dietary fat. Neither held up. Then came decades and millions of dollars of randomized trials of vitamins and supplements: vitamin A, B vitamins, folic acid, vitamins C, D and E, selenium and omega-3 fatty acids. None reduced cancer. By the mid-2000s the picture became clear: starting with definitive studies around 2003, obesity and type 2 diabetes emerged as major risk factors. Fung notes that when he was in medical school nobody thought obesity caused cancer, yet the WHO now lists 13 obesity-related cancers, including breast and colorectal cancer, though obesity plays almost no role in cancers like lung cancer or asbestos-linked mesothelioma.

The Seed and the Soil

Fung says one big mistake was focusing so heavily on genetics and random mutation, which makes cancer seem like bad luck. He gives the example of the BRCA gene: women carrying it in the 1930s, 40s and 50s had roughly a 30 percent risk of breast cancer, compared with about 80 percent in modern America. Same genes, different lifestyle. The seed of cancer exists in all our cells, and in all multicellular animals, but we can change the soil it grows in.

He points to history for evidence: the Inuit were once thought to be immune to cancer, with Queen's University sending yearly expeditions to study why, until Western foods like sugar and white flour arrived. Missionary doctor Denis Burkitt observed that Africans living traditionally had almost no colon cancer until they adopted Western diets, which is why these were called diseases of civilization. And when women from Japan or Shanghai move to San Francisco, breast cancer risk roughly triples within a couple of generations. That, Fung says, is hopeful: if lifestyle drives it, lifestyle can reduce it. He is careful to add that once cancer has developed, diet alone is not enough and patients need the drugs developed over the past decades; diet and fasting are about prevention.

Cancer as a Selfish Organism

Fung explains that cancer has been hard to treat partly because we never understood what it is. Heart disease has a clear cause (blocked arteries), which led to blood thinners, balloons and imaging. Cancer makes no evolutionary sense at first: it arises from our own cells and ultimately kills itself along with us. His view is that a cancer cell evolves almost as a separate species, no longer a team player but, like a player padding his stats, out only for itself, growing at the expense of its neighbors and spreading around the body. The immune system even recognizes cancer cells as foreign.

Crucially, cancer cells grow or not depending on growth signals. The body's nutrient sensors, including insulin and mTOR, rise when we eat and tell cells to grow. Fung notes that breast cancer cells grown in the lab wither and die without insulin. That is why obesity and type 2 diabetes, both conditions of too much insulin, are so tightly linked to cancer, and why he thinks vitamin supplements disappointed: in trials, high doses of folic acid and beta carotene were associated with more cancer, not less, like spreading fertilizer on an empty field and growing weeds. Most North Americans, he says, are not vitamin deficient.

What to Cut and Why Frequency Matters

Asked for five things to eliminate, Fung names sugar and refined grains like white bread first, then refined foods more broadly: refined oils (eat natural oils instead) and processed meats, contrasting bologna full of chemicals with grass-finished beef. He notes the evidence on refined fats and proteins is weaker. His fifth factor is how often we eat, because constant eating keeps growth signals switched on and provides fertile soil.

He compares it to a daily coffee habit: a small amount paid every day adds up. In 1970 people ate three meals with no snacks; mothers refused after-school snacks because you would ruin your dinner. Today studies show people eat five or six times a day, schools recommend packing two snacks, and parents hand out cookies at halftime of kids' soccer games. Fung argues nobody in human history has eaten this way, because our great-grandparents had work to do.

The Fed State, the Fasted State and the 14-Hour Eating Day

The body, Fung explains, is always in one of two states. In the fed state, insulin rises and tells the body to store calories as glycogen or body fat. In the fasted state (any time you are not eating), insulin falls and the body pulls calories out of storage, which is why we don't die in our sleep. You can't do both at once. Studies show the average person now eats across a window of about 14 hours and 45 minutes, say from 8 a.m. to 10:45 p.m., and it takes about four hours after eating to switch into the fasted state. So many people never get there, their insulin stays high and fat burning is blocked.

The fix is simple: increase the time you are not eating. That is all intermittent fasting is, whether one meal a day or an eight- or four-hour window. Fung himself rarely eats breakfast, a habit from medical school when he rolled out of bed and left, usually eats within a six-to-eight-hour window, does occasional 24-hour fasts when busy, and schedules longer fasts after holidays; after overeating on a cruise, a three- or four-day fast had him back to normal weight within a week. He notes a pound of fat holds about 3,500 calories, so someone 100 pounds overweight has a huge reserve, and that longer fasts are physically easier than people think but socially disruptive, which is why religious traditions like Lent, Ramadan and Yom Kippur fasted together.

Busting the Muscle and Starvation Mode Myths

On muscle loss, Fung asks why the body would store energy as fat and then burn muscle the minute it needs energy, like saving firewood all winter and then chopping up your sofa for the fire. Muscle is built by exercise, not eating. There is a brief period of protein breakdown during fasting, but it tends to consume excess tissue like the loose skin seen after rapid surgical weight loss, while the body maintains the muscle you use.

On starvation mode, he explains that simple calorie cutting (500 fewer calories a day) typically causes the body to lower its metabolic rate by about the same amount, so weight loss stalls, like cutting spending when your income drops. But fasting is different: insulin falls, the body switches fuel sources like a hybrid car moving from gas to electric, and suddenly it has access to a vast reserve of body fat. In a study of a four-day fast, people burned about 10 percent more calories and had 10 percent higher VO2 than when eating, because noradrenaline rises. He frames it as a survival response: a hungry caveman needed energy and focus to hunt.

Hunger, Focus and How to Get Through It

Fung says mental sharpness increases when fasting; think of how sluggish you feel after Thanksgiving dinner, or the difference between a lounging lion and a hungry wolf. He cites the book Unbroken, in which starving prisoners of war performed remarkable mental feats, recalling one who remarked that it was "simply the mental clarity of starvation," and notes Pythagoras required students to fast before class.

Hunger, he explains, is largely learned: ghrelin, the hunger hormone, peaks at usual mealtimes, then falls back to baseline within a couple of hours if you don't eat, because the body takes that meal from body fat. So hunger comes in waves you can let pass. His strategies are to stay busy and to drink green tea or coffee. Over multi-day fasts, ghrelin drops after about day two, and by days four and five physical hunger almost disappears, even if pizza still looks tempting. Supportive communities help, as they always have in religious fasts. Howes describes his own four-day fast on water and black coffee: tough for two days, then in the zone by day three, seven and a half pounds lighter and feeling more confident, almost a spiritual experience.

Autophagy and the Benefits of Fasting

Fung explains autophagy, the process recognized with the 2016 Nobel Prize in medicine awarded to Yoshinori Ohsumi, in which the body breaks down and recycles old cellular parts when nutrient sensors are turned down. Breaking things down sounds bad, but like renovating a bathroom, you have to rip out the avocado-green tub before putting in a new one. Growth hormone rises roughly four-fold during a 24-hour fast, helping rebuild what you still use once you eat. He adds that caloric restriction is the one intervention that reliably extends lifespan in lab animals, and that fasting has historically been seen as cleansing and renewal, only recently recast as dangerous. Fellow doctors report feeling their brains on fire, and a cardiologist friend could finally play a piano piece he had never managed.

His summary of fasting's benefits: it is free, it works with any diet (vegetarian, keto, paleo or even fast food), it is flexible (skip it at Christmas, extend it afterward), it is simple (water, tea, coffee and nothing else), and it is powerful, because you can't go lower than zero calories. He warns it is for people who are not underweight or malnourished, and that anyone on medications needing adjustment should take care. He and Howes also note that businesses profit from selling more food and more medicine, not less; Fung mentions dietitian conferences sponsored by breakfast cereal makers and reality-show dieters forced to drink foul protein shakes. In his patients, he says, the results have been remarkable: people coming off medications, reversing type 2 diabetes and losing weight.

Conclusion

Dr. Jason Fung's message is that cancer, obesity and type 2 diabetes share common soil: too much insulin and constant growth signaling driven by refined foods and eating all day. We can't change the seed, but we can change the soil by cutting sugar and refined foods and, above all, by giving the body regular time in the fasted state. His case for fasting rests on physiology rather than hype: the body is built to live off stored fat, metabolism and focus rise rather than fall, hunger passes in waves, and autophagy renews cells. As he puts it at the start, "The secret to managing all of these things is not more medicine and surgery. It's less", and because "fasting is free to every single person on earth," it is a tool anyone can start using, with the usual caution for those who are underweight or on medication.

This is a summary of "The Secret to Managing Disease Is Less, Not More | Jason Fung" from The School of Greatness, giving you the GIST so you can decide if you want the REST. Listen to the full episode.