Dr. Jason Fung: Fasting, Insulin and The Obesity Code

Jason Fung, MD, is a Canadian nephrologist — a kidney specialist — who has practised in Toronto since 2001. Since the early 2010s he has written and lectured that obesity and type 2 diabetes are disorders of too much insulin (hyperinsulinemia) rather than of calorie balance, and that therapeutic and intermittent fasting is the most direct way to bring insulin down. He set out that case for general readers in The Obesity Code (2016), The Diabetes Code (2018) and The Cancer Code (2020), and in 2012 he co-founded a fasting program with Megan Ramos whose patients became the subjects of his published case reports.

This hub gathers the six deep-dive articles on his work: his life and career, his hormonal theory of obesity, his account of fasting, his view of type 2 diabetes, his evolutionary theory of cancer, and his books and legacy. Below the article cards are a short account of who he is, his argument on one page, a dated timeline, and his most central papers. Everything here is reported as he states it, attributed to him; where later research on the same question exists, the deep-dive articles describe what those papers found.

Deep-Dive Articles

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Jason Fung: Life and Career

A Toronto upbringing, a medical degree at the University of Toronto, nephrology training in Los Angeles, two decades of kidney medicine in Scarborough, the 2008 trials he calls turning points, and the fasting program he co-founded in 2012.

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The Obesity Code: Jason Fung’s Hormonal Theory of Obesity

Why he rejects calories-in, calories-out, names insulin as the cause of weight gain and describes a “Body Set Weight” thermostat — with the insulin-dose studies he cites and the later diet and metabolic-ward trials on the same question.

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Jason Fung on Intermittent and Therapeutic Fasting

The stages of a fast as he describes them, the brain’s shift to ketones, growth hormone, metabolic rate, muscle and minerals, the 382-day fast he cites, who he says is not a candidate for fasting, and what the main fasting trials found.

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The Diabetes Code: Jason Fung on Type 2 Diabetes

Type 2 diabetes as “overflow” rather than starvation, the twin cycles of fatty liver and fatty pancreas, insulin resistance as protection, his case for reversibility, his case reports and the remission trials.

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The Cancer Code: Jason Fung’s Evolutionary Theory of Cancer

His “Cancer Paradigm 3.0”: beyond the somatic mutation theory, seed and soil, convergent evolution, the atavistic model of Paul Davies and Charles Lineweaver, and insulin and obesity as the “soil” in which cancer grows.

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Jason Fung: Books, Clinic and Legacy

Every book he has written or co-written from 2016 to 2026 with years and co-authors, the fasting program he co-founded with Megan Ramos, its case reports, his lectures and video channel, and later research on the same subjects.

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Who Jason Fung Is

Fung was born and raised in Toronto. He considered engineering, completed two years of biochemistry, and then earned his medical degree at the University of Toronto. He trained in nephrology at the University of California, Los Angeles, from 1999 to 2001, and chose the specialty, he has said, because it is the most mathematical branch of medicine: “In nephrology, you deal with more equations than in other types of medicine and I really like that.”

He returned to Toronto in 2001 to practise kidney medicine in Scarborough. His name appears on clinic studies from the Scarborough nephrology division on chronic kidney disease, peritoneal dialysis and dialysis catheters, with author affiliations at Scarborough General Hospital and later the Scarborough Health Network. For his first seven years in practice, by his own account, he followed conventional advice: low-fat diets and strict blood-sugar control.

He names three 2008 trials as the point where that changed: a two-year trial in the New England Journal of Medicine in which a low-carbohydrate diet produced the most weight loss, and the ACCORD and ADVANCE trials, in which intensive lowering of blood glucose in type 2 diabetes did not deliver the hoped-for benefit. Around 2011–2012 he began lecturing colleagues and patients on insulin and diet, and in 2012 he and Megan Ramos set up a fasting program in Toronto that, in its own description, was the first intermittent-fasting clinic in North America. About 2014 a publisher approached him after a lecture, which led to The Obesity Code.

His Argument on One Page

Obesity is hormonal. Fung writes: “It appears that insulin is not merely associated with obesity but causes obesity.” He describes insulin as “the hormonal signal to the body to increase weight” and the body as defending a “Body Set Weight”, much as a thermostat defends a temperature. In his summary, “Raising insulin levels causes weight gain. Lowering insulin levels causes weight loss.” He points to the weight gain seen with insulin treatment in type 2 diabetes — “The more insulin you take, the more weight you gain” — and concludes that “The question is NOT how to balance calories, the question is how to balance our hormones.”

Fasting lowers insulin. In his account of fasting physiology, liver glycogen lasts roughly a day, after which the body makes its own glucose and then shifts to burning fat and ketones; he writes that after four days of fasting about 75% of the brain’s energy comes from ketones. “Fasting is the most efficient and consistent strategy to decrease insulin levels,” he writes, adding that regular fasting improves insulin sensitivity. He also states who should not fast: “patients that are severely underweight or malnourished should not fast”, because of the risk of refeeding syndrome.

Type 2 diabetes is reversible. Fung calls type 2 diabetes a disease of too much insulin and describes it as “overflow”: cells already packed with glucose, rather than starved of it. Building on the “twin cycle” work of the British diabetes researcher Roy Taylor, he argues that fat in the liver drives insulin resistance and fat in the pancreas impairs insulin release, and that “both defects of insulin resistance and the beta cell dysfunction are entirely reversible by removal of the excess fat clogging the organs.” He holds that treatments which raise insulin do not address the cause; that is his attributed view, and this page gives no advice on medication.

Cancer is evolutionary and metabolic. In The Cancer Code and the blog series before it, he argues that cancer is “almost certainly NOT a genetic disease” in the usual sense, revives the nineteenth-century “seed and soil” idea, and adopts the atavistic model in which cancer is a reversion to a more primitive, single-celled way of life. He describes high insulin and obesity as the fertile “soil”. He does not claim that fasting treats cancer, and neither does this page.

Timeline at a Glance

Key Research Papers

  1. Furmli S, Elmasry R, Ramos M, Fung J. Therapeutic use of intermittent fasting for people with type 2 diabetes as an alternative to insulin. BMJ Case Rep. 2018;2018:bcr2017221854. — PubMed PMID: 30301822
  2. Lichtash C, Fung J, Ostoich KC, Ramos M. Therapeutic use of intermittent fasting and ketogenic diet as an alternative treatment for type 2 diabetes in a normal weight woman: a 14-month case study. BMJ Case Rep. 2020;13(7):e234223. — PubMed PMID: 32641437
  3. Fung J, Berger A. Hyperinsulinemia and Insulin Resistance: Scope of the Problem. Journal of Metabolic Health. 2016;1(1). — DOI: 10.4102/jir.v1i1.18
  4. Taskapan H, Tam P, Au V, Chow S, Fung J, Nagai G, et al. Improvement in eGFR in patients with chronic kidney disease attending a nephrology clinic. Int Urol Nephrol. 2008;40(3):841-8. — PubMed PMID: 18386153
  5. Shai I, Schwarzfuchs D, Henkin Y, Shahar DR, Witkow S, Greenberg I, et al. Weight loss with a low-carbohydrate, Mediterranean, or low-fat diet. N Engl J Med. 2008;359(3):229-41. — PubMed PMID: 18635428
  6. Action to Control Cardiovascular Risk in Diabetes Study Group, Gerstein HC, Miller ME, Byington RP, Goff DC Jr, Bigger JT, et al. Effects of intensive glucose lowering in type 2 diabetes. N Engl J Med. 2008;358(24):2545-59. — PubMed PMID: 18539917
  7. ADVANCE Collaborative Group, Patel A, MacMahon S, Chalmers J, Neal B, Billot L, et al. Intensive blood glucose control and vascular outcomes in patients with type 2 diabetes. N Engl J Med. 2008;358(24):2560-72. — PubMed PMID: 18539916
  8. UK Prospective Diabetes Study (UKPDS) Group. Intensive blood-glucose control with sulphonylureas or insulin compared with conventional treatment and risk of complications in patients with type 2 diabetes (UKPDS 33). Lancet. 1998;352(9131):837-53. — PubMed PMID: 9742976
  9. Owen OE, Morgan AP, Kemp HG, Sullivan JM, Herrera MG, Cahill GF Jr. Brain metabolism during fasting. J Clin Invest. 1967;46(10):1589-95. — PubMed PMID: 6061736
  10. Stewart WK, Fleming LW. Features of a successful therapeutic fast of 382 days’ duration. Postgrad Med J. 1973;49(569):203-9. — PubMed PMID: 4803438
  11. Taylor R. Pathogenesis of type 2 diabetes: tracing the reverse route from cure to cause. Diabetologia. 2008;51(10):1781-9. — PubMed PMID: 18726585
  12. Davies PC, Lineweaver CH. Cancer tumors as Metazoa 1.0: tapping genes of ancient ancestors. Phys Biol. 2011;8(1):015001. — PubMed PMID: 21301065

PubMed Topic Searches

  1. PubMed papers by Fung J with a Scarborough affiliation
  2. Intermittent fasting and type 2 diabetes
  3. Hyperinsulinemia as a cause of obesity
  4. The atavistic model of cancer

Further Reading

Connections