Jason Fung on Intermittent and Therapeutic Fasting

Table of Contents

1. 1. Why Fung Turned to Fasting
2. 2. The Stages of a Fast as Fung Describes Them
3. 3. Ketones and the Fasting Brain
4. 4. Growth Hormone, Adrenaline and Metabolic Rate
5. 5. How Fung Answers the Muscle Question
6. 6. Minerals and the 382-Day Fast
7. 7. Fasting Schedules in His Case Reports
8. 8. Who Fung Says Is Not a Candidate, and Refeeding Syndrome
9. 9. Intermittent Fasting and Time-Restricted Eating Trials
10. 10. Pooled Evidence and Overviews
11. Key Research Papers
12. Connections
13. Featured Videos

1. Why Fung Turned to Fasting

Fung’s fasting work grows directly out of his insulin theory of weight gain, set out on the Obesity Code page. If, as he argues, high insulin drives fat storage and raises the body’s defended weight, then the practical question becomes how to lower insulin. His answer is that every meal raises insulin and every period without food lets it fall, so the length of time between meals matters as much as what is eaten.

In his essay “Fasting Physiology” (2021) he puts it plainly: “Fasting is the most efficient and consistent strategy to decrease insulin levels.” He adds: “Regular fasting also improves insulin sensitivity significantly.” In his clinic, fasting is paired with a low-carbohydrate way of eating on the eating days; he presents the two as working on the same hormone from two sides.

He describes fasting not as starvation but as the normal other half of eating: the body stores food energy when insulin is high and draws that energy back out when insulin is low. In his telling, modern eating patterns—three meals plus snacks, from waking to bedtime—leave very little time in the second state.

A 2005 Danish study is one human test of the insulin-sensitivity part of this idea. Eight healthy young men fasted for 20 hours every second day for 15 days while keeping their weight steady. Measured by the glucose-clamp method, insulin-mediated whole-body glucose uptake rose from 6.3 to 7.3 mg per kilogram per minute, and the authors called it the first experiment in humans to show that intermittent fasting increases insulin-mediated glucose uptake.

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2. The Stages of a Fast as Fung Describes Them

Fung divides the hours and days after the last meal into five stages. His timings, as he gives them in “Fasting Physiology”, are:

1. Feeding. Insulin rises; sugar is burned for energy, and the surplus is stored as glycogen in the liver and muscle and, once those stores are full, turned into fat.
2. Post-absorptive phase, about 6 to 24 hours. Insulin falls and the liver breaks down glycogen to keep blood sugar up. He writes that liver glycogen lasts “roughly 24 hours”.
3. Gluconeogenesis, about 24 hours to 2 days. With glycogen running low, the liver makes new glucose from other materials, including amino acids and glycerol.
4. Ketosis, about 2 to 3 days. Low insulin releases stored fat; fatty acids are burned directly and the liver turns part of them into ketone bodies that many tissues, including the brain, can use.
5. Protein conservation, beyond about 5 days. Rising growth hormone and fat-derived fuel take over, and, in his account, the body spares protein.

He also explains why weight often drops quickly in the first days. Part of it, he writes, is water: “high insulin causes salt and water retention in the kidney,” so when insulin falls the kidneys let go of salt and water. As a nephrologist he gives this kidney point particular weight.

The underlying physiology of these stages was mapped by the Harvard physician George Cahill, whose 2006 review “Fuel metabolism in starvation” describes how studies of prolonged starvation characterised the roles of insulin, free fatty acids, glycerol, glucose and lactate, and how beta-hydroxybutyrate used by the brain “has permitted man to survive prolonged starvation”. The site’s fat-burning and ketosis animation shows the same switch.

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3. Ketones and the Fasting Brain

A common objection to fasting is that the brain needs sugar. Fung answers it with a specific figure: “After four days of fasting, approximately 75% of the energy used by the brain is provided by ketones.” The rest, in his account, comes from glucose made by the liver during gluconeogenesis, so the brain is never left without fuel.

He draws on two classic sources. The first is the 1967 study by Owen, Cahill and colleagues in the Journal of Clinical Investigation. They placed catheters in the brain blood vessels of three obese patients who were undergoing five to six weeks of starvation and found that beta-hydroxybutyrate and acetoacetate—the two main ketone bodies—had replaced glucose as the brain’s predominant fuel. The second is Cahill’s 2006 review, which credits D-beta-hydroxybutyric acid with allowing humans to survive prolonged starvation and notes interest in its efficiency as an energy source.

The 1967 study was small (three patients) and covered fasts far longer than four days; the 75% figure is Fung’s own summary of this body of work rather than a number stated in the Owen abstract.

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4. Growth Hormone, Adrenaline and Metabolic Rate

Growth hormone

Fung writes: “One of the most potent stimuli to growth hormone secretion is fasting. Over a five-day fasting period growth hormone secretion more than doubled.” In a separate 2021 post he says fasting “naturally boosts growth hormone”. In his model, growth hormone is one of the signals that tells the body to hold on to lean tissue while it burns fat.

The study behind this point is Ho and colleagues, 1988, in the Journal of Clinical Investigation. Six healthy men had blood drawn repeatedly over 24 hours on a normal fed day and on the first and fifth days of a five-day fast. By the fifth day, growth hormone pulses were more frequent (5.8 to 9.9 pulses per 24 hours), the peak pulse was higher (5.9 to 12.3 ng/mL), and the 24-hour integrated growth hormone concentration rose from 2.82 to 8.75 micrograms·min/mL. Over the same days, blood glucose and somatomedin C (insulin-like growth factor 1) fell, while free fatty acids and acetoacetate rose sharply.

Adrenaline and metabolic rate

The second half of Fung’s argument is that short fasts do not slow the metabolism. He writes that “48 hours of fasting produces a 3.6% increase in metabolic rate” and that after a four-day fast “resting energy expenditure increased up to 14%”, concluding that resting metabolism is not decreased. He attributes this to the surge of counter-regulatory hormones—noradrenaline (norepinephrine), adrenaline, cortisol and growth hormone—that keep the body alert and active while food is absent. He contrasts this with daily calorie cutting, which, in his account, lowers metabolism.

The 4-day figure matches Zauner and colleagues, 2000, in the American Journal of Clinical Nutrition. In 11 healthy, lean people fasting for 84 hours, resting energy expenditure rose from 3.97 kJ per minute on day 1 to 4.53 kJ per minute on day 3, alongside a rise in noradrenaline from about 1,716 to 3,728 pmol/L by day 4; blood glucose fell and insulin did not change significantly. The authors concluded that resting energy expenditure increases in early starvation and that the noradrenaline rise “may be the initial signal” for the metabolic changes of early starvation.

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5. How Fung Answers the Muscle Question

In “Does Fasting burn muscle?” (2018), Fung argues that repeated fasting does not burn muscle. His reasoning runs as follows: gluconeogenesis only starts about 24 hours after the last meal, so a daily 16-hour fast or a 24-hour fast barely touches protein; and during longer fasts the surge of adrenaline, noradrenaline, cortisol and growth hormone acts to preserve lean tissue while the body switches to fat and ketones. He describes the body as built to store energy as fat precisely so that it does not need to break down muscle between meals.

Later controlled trials measured body composition directly, with mixed results:

• In a 2016 pilot trial in adults with obesity, zero-calorie alternate-day fasting and daily calorie restriction produced similar weight loss over 8 weeks (8.2 kg and 7.1 kg); after 24 weeks of unsupervised follow-up, the authors reported that changes from baseline in percentage fat mass and lean mass were more favourable in the fasting group.
• In a 2021 University of Bath trial in lean, healthy adults, alternate-day 24-hour fasting with 150% intake on eating days lost about the same weight as daily 25% energy restriction over 3 weeks, but less of that loss was fat (0.74 kg versus 1.75 kg); the authors wrote that alternate-day fasting “less effectively reduces body fat mass” than a matched daily energy restriction.
• In the 2020 TREAT trial of a 16:8 eating window, the in-person group showed a between-group difference in appendicular lean mass index (−0.16 kg/m²) with time-restricted eating.

These trials used different fasting patterns, people and durations from those Fung writes about, and none tested his clinic’s combination of fasting with a low-carbohydrate diet.

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6. Minerals and the 382-Day Fast

Fung writes that during fasting potassium “may decrease slightly”, while magnesium, calcium and phosphorus stay stable. As his most extreme example he cites a Scottish case published in 1973: a man who fasted for 382 days under medical supervision, taking a multivitamin, as he describes it. He uses the case to show how far the body’s fat stores can carry a person who is heavily overweight.

The paper itself, by Stewart and Fleming in the Postgraduate Medical Journal, reports a 27-year-old man who fasted under supervision for 382 days and afterwards maintained his normal weight. In the last eight months, blood glucose readings around 30 mg/100 mL were recorded consistently while he was walking about as an out-patient. After an initial decrease was corrected, plasma potassium stayed normal without supplementation. A temporary period of high calcium occurred toward the end of the fast, and the abstract states that “decreased plasma magnesium concentrations were a consistent feature from the first month onwards”. After 100 days, the urine showed a marked rise in mineral and phosphate losses. The authors concluded that prolonged fasting in this patient had no ill-effects.

The case is a single supervised patient, not a trial. On magnesium, the paper’s own abstract records a consistent fall, which differs from the “stable” summary in Fung’s essay.

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7. Fasting Schedules in His Case Reports

Fung’s clinic has published several case reports, and these are the clearest written record of the schedules it used. They describe what was done for particular patients; they are case reports, not trials.

Three patients off insulin (2018)

A 2018 case series in BMJ Case Reports (Furmli, Elmasry, Ramos and Fung) documents three patients with insulin-treated type 2 diabetes referred to the Intensive Dietary Management clinic in Toronto. The authors report that therapeutic fasting reversed their insulin resistance and that all three stopped insulin while keeping blood sugar under control, while also losing a significant amount of body weight, reducing waist circumference and lowering their glycated hemoglobin.

A normal-weight woman over 14 months (2020)

A 2020 report (Lichtash, Fung, Ostoich and Ramos) follows a 57-year-old normal-weight woman with type 2 diabetes treated with metformin and a standard diabetic diet whose hemoglobin A1c (HbA1c) was 9.3%. The fasting schedule changed in steps:

• 24-hour fasts three times a week;
• then 42-hour fasts three times a week;
• then 42-hour fasts twice a week plus one 16-hour fast;
• from month 8, a maintenance phase of a 16-hour daily fast with 24-hour fasts three times a month, when metformin was restarted.

Combined with a ketogenic diet, she reached a hemoglobin A1c of 6.4% within four months, off medication, and 5.8% at 14 months, with her body mass index minimally changed.

A four-month case study (2017)

A 2017 paper in the Journal of Metabolic Health (Ku, Ramos and Fung) presents a four-month case study of therapeutic fasting as a potential treatment for type 2 diabetes. More on the diabetes side of his work is on the Diabetes Code page.

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8. Who Fung Says Is Not a Candidate, and Refeeding Syndrome

In “Refeeding syndromes – Fasting 20” (2016), Fung writes that “patients that are severely underweight or malnourished should not fast”. His reasoning is that refeeding syndrome belongs mainly to starvation and wasting—people with no reserves left—rather than to controlled, voluntary fasting in people with ample body fat, and that refeeding becomes a concern mainly with extended fasts, which he places beyond about five days.

Refeeding syndrome is described in a 2008 BMJ review by Mehanna, Moledina and Travis as a set of dangerous shifts in fluids and electrolytes—above all a fall in blood phosphate, with low potassium and magnesium—that can occur when a malnourished person starts eating again after a period of little or no food. It can affect the heart, breathing and nerves and can be fatal. The site’s Refeeding Strategy page covers it in more detail.

Medicines and low blood sugar

Insulin and the sulphonylurea drugs lower blood sugar whether or not a person has eaten. Fasting while taking them carries a risk of hypoglycaemia (dangerously low blood sugar), which can cause shakiness, confusion, seizures and loss of consciousness. In Fung’s published cases, medication was changed as fasting began: the 2018 patients came off insulin, and the 2020 patient came off metformin and later restarted it, within his clinic’s care.

What the trials recorded

The 2016 Colorado pilot trial reported no adverse effects attributed to zero-calorie alternate-day fasting, with 93% completing 8 weeks. The 2017 year-long alternate-day fasting trial had the highest drop-out in the fasting group (38%, against 29% for daily calorie restriction), and by month 12 its participants had higher low-density lipoprotein cholesterol than the calorie-restriction group (+11.5 mg/dL). In a 2005 study of alternate-day fasting in non-obese people, hunger rose on the first fasting day and stayed high for the three weeks of the study.

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9. Intermittent Fasting and Time-Restricted Eating Trials

Since Fung began writing, fasting patterns have been tested in a growing number of controlled studies. The main ones are summarised here as their abstracts report them. Most compare fasting with daily calorie restriction rather than with ordinary eating, and none tested his exact clinic approach.

Alternate-day fasting

• Heilbronn and colleagues, 2005. Sixteen non-obese adults fasted every other day for 22 days. They lost 2.5% of body weight and 4% of fat mass; fasting insulin fell by 57%, and fat burning rose. The authors noted that hunger on fasting days did not ease, suggesting the pattern might be hard to continue for long.
• Catenacci and colleagues, 2016. Twenty-six adults with obesity were randomised to zero-calorie alternate-day fasting or a daily 400-calorie cut for 8 weeks. Weight loss, body composition, lipids and insulin sensitivity did not differ significantly between groups; the authors called alternate-day fasting safe and tolerable.
• Trepanowski and colleagues, 2017. One hundred adults with obesity were randomised for a year to alternate-day fasting (25% of energy needs on fast days, 125% on feast days), daily calorie restriction or no intervention. Weight loss was similar in the two diet groups at 6 months (6.8% each) and 12 months (6.0% and 5.3%). Fasting participants ate more than prescribed on fast days and less on feast days. The authors concluded that alternate-day fasting did not produce superior adherence, weight loss, weight maintenance or cardioprotection compared with daily calorie restriction.
• Templeman and colleagues, 2021. In lean adults over 3 weeks, fasting with a matched energy deficit lost less fat than daily restriction, and fasting without an energy deficit did not significantly reduce body or fat mass; the authors found no evidence of fasting-specific effects on metabolic or cardiovascular health.

Two-day-a-week fasting

• Harvie and colleagues, 2011. In 107 overweight or obese premenopausal women over 6 months, a 25% energy restriction taken as two very-low-energy days a week produced weight loss similar to the same restriction spread over seven days (6.4 kg versus 5.6 kg). Fasting insulin and insulin resistance fell modestly in both groups but more with the intermittent approach. The authors concluded the intermittent pattern was as effective as continuous restriction.

Time-restricted eating

• Sutton and colleagues, 2018. Men with prediabetes ate within a 6-hour early window (dinner before 3 p.m.) or a 12-hour window for 5 weeks each, in a crossover design, with enough food to keep their weight steady. The early window improved insulin sensitivity, beta-cell responsiveness, blood pressure, oxidative stress and appetite. The authors wrote that this showed for the first time in humans that the benefits are not solely due to weight loss—the point closest to Fung’s insulin argument.
• The TREAT trial, Lowe and colleagues, 2020. In 116 adults with overweight or obesity over 12 weeks, eating only between noon and 8 p.m. was compared with three structured meals a day. Weight change did not differ significantly between groups, and the authors concluded that time-restricted eating alone was not more effective for weight loss than eating throughout the day.
• Liu and colleagues, 2022. In 139 adults with obesity on a calorie-restricted diet for 12 months, adding an 8 a.m. to 4 p.m. eating window produced 8.0 kg weight loss against 6.3 kg for calorie restriction alone, a difference that was not statistically significant.
• Jamshed and colleagues, 2022. In 90 adults with obesity receiving weight-loss treatment for 14 weeks, an early 8-hour window (7 a.m. to 3 p.m.) produced 2.3 kg more weight loss than a window of 12 hours or more, and lower diastolic blood pressure, but did not significantly change body fat in the main analysis.

The site’s 16:8 time-restricted eating and 24-to-72-hour fasts pages cover these patterns in more depth.

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10. Pooled Evidence and Overviews

A 2018 systematic review and meta-analysis by Cioffi and colleagues pooled 11 randomised trials lasting 8 to 24 weeks. Intermittent energy restriction gave weight loss comparable to continuous restriction; fasting insulin was slightly lower with the intermittent regimens (by about 0.89 µU/mL), though the authors described the clinical relevance as uncertain, and they wrote that long-term trials are needed.

In a 2019 review in the New England Journal of Medicine, Rafael de Cabo and Mark Mattson of the National Institute on Aging reviewed the effects of intermittent fasting on health, aging and disease, including the body’s switch from glucose to fat and ketones as fuel during a fast—the same switch at the centre of Fung’s account of the stages of a fast.

A 2022 review in Nature Reviews Endocrinology by Krista Varady and colleagues describes the three most studied forms—alternate-day fasting, the 5:2 pattern and time-restricted eating—and notes that, despite their popularity, only a few studies have examined their health benefits in humans. It also assesses the safety of these regimens.

Fung’s own position, as he writes it, is that fasting acts directly on insulin, the hormone he places at the centre of obesity and type 2 diabetes. The trials above measured mainly weight, often against an equal calorie cut, and largely found the two approaches similar. More of the site’s coverage is on the Fasting and Insulin Resistance pages.

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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. Ku M, Ramos M, Fung J. Therapeutic fasting as a potential effective treatment for type 2 diabetes: A 4-month case study. Journal of Metabolic Health. 2017. — DOI: 10.4102/jir.v2i1.31
4. 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
5. Cahill GF Jr. Fuel metabolism in starvation. Annu Rev Nutr. 2006;26:1-22. — PubMed PMID: 16848698
6. Ho KY, Veldhuis JD, Johnson ML, Furlanetto R, Evans WS, Alberti KG, et al. Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man. J Clin Invest. 1988;81(4):968-75. — PubMed PMID: 3127426
7. Zauner C, Schneeweiss B, Kranz A, Madl C, Ratheiser K, Kramer L, et al. Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine. Am J Clin Nutr. 2000;71(6):1511-5. — PubMed PMID: 10837292
8. 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
9. Mehanna HM, Moledina J, Travis J. Refeeding syndrome: what it is, and how to prevent and treat it. BMJ. 2008;336(7659):1495-8. — PubMed PMID: 18583681
10. Halberg N, Henriksen M, Söderhamn N, Stallknecht B, Ploug T, Schjerling P, et al. Effect of intermittent fasting and refeeding on insulin action in healthy men. J Appl Physiol (1985). 2005;99(6):2128-36. — PubMed PMID: 16051710
11. Heilbronn LK, Smith SR, Martin CK, Anton SD, Ravussin E. Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism. Am J Clin Nutr. 2005;81(1):69-73. — PubMed PMID: 15640462
12. Harvie MN, Pegington M, Mattson MP, Frystyk J, Dillon B, Evans G, et al. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. Int J Obes (Lond). 2011;35(5):714-27. — PubMed PMID: 20921964
13. Catenacci VA, Pan Z, Ostendorf D, Brannon S, Gozansky WS, Mattson MP, et al. A randomized pilot study comparing zero-calorie alternate-day fasting to daily caloric restriction in adults with obesity. Obesity (Silver Spring). 2016;24(9):1874-83. — PubMed PMID: 27569118
14. Trepanowski JF, Kroeger CM, Barnosky A, Klempel MC, Bhutani S, Hoddy KK, et al. Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial. JAMA Intern Med. 2017;177(7):930-938. — PubMed PMID: 28459931
15. Sutton EF, Beyl R, Early KS, Cefalu WT, Ravussin E, Peterson CM. Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. Cell Metab. 2018;27(6):1212-1221.e3. — PubMed PMID: 29754952
16. Lowe DA, Wu N, Rohdin-Bibby L, Moore AH, Kelly N, Liu YE, et al. Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical Trial. JAMA Intern Med. 2020;180(11):1491-1499. — PubMed PMID: 32986097
17. Liu D, Huang Y, Huang C, Yang S, Wei X, Zhang P, et al. Calorie Restriction with or without Time-Restricted Eating in Weight Loss. N Engl J Med. 2022;386(16):1495-1504. — PubMed PMID: 35443107
18. Jamshed H, Steger FL, Bryan DR, Richman JS, Warriner AH, Hanick CJ, et al. Effectiveness of Early Time-Restricted Eating for Weight Loss, Fat Loss, and Cardiometabolic Health in Adults With Obesity: A Randomized Clinical Trial. JAMA Intern Med. 2022;182(9):953-962. — PubMed PMID: 35939311
19. Templeman I, Smith HA, Chowdhury E, Chen YC, Carroll H, Johnson-Bonson D, et al. A randomized controlled trial to isolate the effects of fasting and energy restriction on weight loss and metabolic health in lean adults. Sci Transl Med. 2021;13(598):eabd8034. — PubMed PMID: 34135111
20. Cioffi I, Evangelista A, Ponzo V, Ciccone G, Soldati L, Santarpia L, et al. Intermittent versus continuous energy restriction on weight loss and cardiometabolic outcomes: a systematic review and meta-analysis of randomized controlled trials. J Transl Med. 2018;16(1):371. — PubMed PMID: 30583725
21. de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. N Engl J Med. 2019;381(26):2541-2551. — PubMed PMID: 31881139
22. Varady KA, Cienfuegos S, Ezpeleta M, Gabel K. Clinical application of intermittent fasting for weight loss: progress and future directions. Nat Rev Endocrinol. 2022;18(5):309-321. — PubMed PMID: 35194176

PubMed Topic Searches

1. Therapeutic fasting and type 2 diabetes
2. Intermittent fasting and insulin sensitivity
3. Time-restricted eating randomised trials
4. Fasting and growth hormone secretion
5. Refeeding syndrome

Further Reading

• Jason Fung, “Fasting Physiology” (2021) — thefastingmethod.com/fasting-physiology/
• Jason Fung, “Fasting and Growth Hormone” (2021) — thefastingmethod.com/fasting-and-growth-hormone/
• Jason Fung, “Does Fasting burn muscle?” (2018) — thefastingmethod.com/does-fasting-burn-muscle/
• Jason Fung, “Refeeding syndromes – Fasting 20” (2016) — thefastingmethod.com/refeeding-syndromes-fasting-20/

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Connections

• Dr. Jason Fung: Fasting, Insulin and The Obesity Code
• Jason Fung: Life and Career
• The Obesity Code: Jason Fung’s Hormonal Theory of Obesity
• The Diabetes Code: Jason Fung on Type 2 Diabetes
• The Cancer Code: Jason Fung’s Evolutionary Theory of Cancer
• Jason Fung: Books, Clinic and Legacy
• Contemporary Practitioners
• Fasting
• Extended Fasts of 24 to 72 Hours
• Refeeding Strategy
• Time-Restricted Eating 16:8
• Fat Burning and Ketosis (animation)

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