Experts › Eating › Jason Fung

EatingFastingEvidence: mostly weak

Jason Fung

MD (University of Toronto) · nephrologist (kidney specialist), trained at the University of California, Los Angeles · co-founder of the Intensive Dietary Management programme · author of The Obesity Code (2016)

Weight and type 2 diabetes are driven by insulin more than by calories, so cut sugar and refined carbohydrates, stop snacking, and use fasting.

In short

  • Cut sugar and refined carbs
  • No snacking; a shorter eating window
  • 24-hour fasts 2–3 × a week (his clinic)
Build my plan in Fung’s style
Words explained (1)
Time-restricted eating
Eating all your meals within a set window of the day, for example 8am to 6pm.

At a glance

Strength 45%Easy cardio 35%Intervals 12%Mobility & stability 8%

Training focus: our estimate from what they recommend.

Fasting, less sugar
Signature habit
No fixed target
Protein
Fasting
Training style
Mostly weak
Overall evidence

Background

Jason Fung is a Canadian kidney specialist. He studied medicine at the University of Toronto and trained in nephrology at the University of California, Los Angeles.

He uses low-carbohydrate eating and fasting with patients who have obesity and type 2 diabetes, and co-founded the Intensive Dietary Management programme in Toronto, which offers fasting support. He works with Megan Ramos, who co-founded The Fasting Method.

His 2016 book The Obesity Code argues that high insulin, not calories, is the main cause of obesity. It became a bestseller and was followed by books on diabetes, fasting, longevity, cancer and, in 2026, hunger. His YouTube channel has about 1.5 million subscribers.

Known for

  • The insulin model of obesity
  • Intermittent and extended fasting
  • The Obesity Code (2016)
  • Using fasting with type 2 diabetes patients
  1. 2016Published The Obesity Code and The Complete Guide to Fasting (with Jimmy Moore)
  2. 2018Published The Diabetes Code; co-authored case reports of patients who came off insulin with therapeutic fasting
  3. 2019Published The Longevity Solution
  4. 2020Published The Cancer Code and Life in the Fasting Lane
  5. 2026Published The Hunger Code

Core ideas

  1. High insulin, driven by refined carbohydrates and constant eating, is the main cause of weight gain.
  2. Counting calories fails in the long run; when and what you eat matters more than how much.
  3. Fasting is a natural, free tool: skipping meals and longer fasts let insulin fall.
  4. Type 2 diabetes is a dietary disease that diet and fasting can often reverse.

The protocol

Fung writes about his patients and methods rather than his own routine, so we show his recommendations.

Eating

  • Cut added sugar and refined grains; eat whole foods with natural fats.Some evidence
  • Stop snacking: three meals a day, then skip one meal a day.Personal habit
  • Eat within a shorter window of the day (time-restricted eating).Some evidence
  • Fasts of 24 hours (skipping two meals in a row) two or three times a week, as used in his clinic's programme.Personal habit
  • Longer fasts of several days; in 2016 he said fasts of up to a month were fine.Doctor only
  • For type 2 diabetes: no sugary drinks, fructose or processed food, and more natural fats.Doctor only

Go deeper: Eating (Tim Spector, Chris van Tulleken)

Strong evidence = trials or large studies · Some evidence = smaller or observational studies · Personal habit = their own choice, little evidence · Doctor only = prescription or blood-test dosing, never in your plan.

Their best-known claims, fact-checked

Not supported

Cutting calories does not lead to weight loss

Controlled studies, including ones his book cites, show that eating fewer calories reliably causes weight and fat loss. Keeping it off is the hard part.

Red Pen Reviews, The Obesity Code, 2019

Partly supported

Insulin is the main cause of obesity

A minority view: insulin plays a role, but baseline insulin does not consistently predict later weight gain, and high-carb and high-fat overfeeding add similar fat at equal calories.

Red Pen Reviews, The Obesity Code, 2019

Partly supported

Intermittent fasting beats ordinary dieting for fat loss

Fasting and eating windows do cause weight loss, but trials find about the same loss as plain calorie cutting.

Liu et al., NEJM 2022; Lowe et al., JAMA Internal Medicine 2020

Not supported

Standard type 2 diabetes treatments are all wrong

A review in Clinical Diabetes called this an untrue assertion, while noting his diet approach is clearly set out.

Scibilia, Clinical Diabetes, 2019

Commercial ties: Fung co-founded the Intensive Dietary Management programme, a paid fasting-support service, and earns from his books and online content.

Fung’s style in your plan

Choose “Follow one expert” and we keep this balance of training and the methods above, then fit them to your age, your available time and your health screen.

What goes into your plan

  • Less added sugar and refined grains, more whole foods
  • No grazing: meals with gaps between them
  • An eating window of about 10–12 hours, with enough protein at each meal

What we leave out, and why

  • 24-hour and longer fasts: they make protein targets and training harder after 50, and need medical supervision with diabetes medication
  • Changing diabetes medication: only with your doctor
Build my plan in Fung’s styleCompare with other experts

The research behind it

Fasting and insulin: three case reports

Three men with type 2 diabetes stopped insulin within weeks on a supervised fasting programme. These are case reports from his own clinic, not a trial.

Furmli, Elmasry, Ramos & Fung, BMJ Case Reports, 2018

Time-restricted eating versus calorie cutting

Over 12 months, people who ate within an 8-hour window and cut calories lost about the same weight as people who cut calories alone.

Liu et al., New England Journal of Medicine, 2022

Eating window alone

Adults who ate within an 8-hour window for 12 weeks, without other changes, lost no more weight than a control group.

Lowe et al., JAMA Internal Medicine, 2020

Honest about weak spots

  • Red Pen Reviews scored The Obesity Code at 31% for scientific accuracy, and The Diabetes Code at 33%.
  • His central claim that calories do not matter is contradicted by controlled studies.
  • Much of his support comes from case reports, small older studies and his own clinic.
  • Fasting with diabetes medication such as insulin can cause dangerously low blood sugar, and long fasts cost muscle, which matters more after 50. Both need a doctor's supervision.

Where experts disagree

  • Calories: Fung says hormones, not calories, drive weight; Norton and most researchers say total calories decide weight change, with protein and food quality shaping how it feels.
  • Fasting and muscle: Fung uses long fasts; Attia moved away from them after losing muscle, and Lyon and Sims caution against fasting for active people in midlife.

Sources

Last reviewed October 2026. Spotted an error? Tell us at hello@lifelongmethod.com and we’ll correct it.

Compare Jason Fung with…

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