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Not medical advice. The Atlas is not a substitute for a clinician who can examine you.

Fat Fasting

Bulletproof-Style · Fat-Only Window

Evidence grade 1 of 5, Untested. Read how the Atlas grades evidence.

0 human trials, about 8 studies, Anecdotal. The written rationale for this grade is still being prepared.

FDA
Not a regulated intervention
WADA
Not prohibited

Reference only. Not medical advice.

What can go wrong

This is a very-low-calorie, nutritionally narrow regimen that omits protein, fiber, and most micronutrients, so it is only described by its own proponents as appropriate for a few days at most and not as a way of eating. Rapid early weight change is largely water, which can mean dehydration and electrolyte shifts. MCT oil commonly causes cramping, nausea, and loose stools at higher doses. People with diabetes or on glucose-lowering or blood-pressure medication can face hypoglycemia or hypotension and should not attempt it without medical oversight. It is not appropriate during pregnancy or breastfeeding, for children or adolescents, or for anyone with a current or past eating disorder or a disordered relationship with food; restrictive 'fasting' framing can reinforce harmful patterns. Anyone considering it, and especially anyone in those groups, should first consult a qualified physician or registered dietitian.

Fat fasting is a short, very-high-fat near-zero-carbohydrate eating pattern, around 1000 to 1200 calories a day with roughly 80 to 90 percent of calories from fat, run for only a few days. Proponents describe it as a way to re-enter or deepen ketosis, blunt hunger, and break a low-carb weight-loss stall. It is not a true fast, since food is eaten, and despite the name it is a calorie-restricted ketogenic regimen rather than abstention. The signature claim, that fat 'does not break a fast,' is partly defensible for insulin and ketosis but contested for autophagy, where no direct human evidence settles the question. Controlled evidence specific to this practice does not exist; what is cited is mid-century observational work that later metabolic studies challenged.

Add fat to extend a fast. Eases ketosis and hunger, but the "fat does not break a fast" claim is contested.

What people take it for

These are the reasons people give for taking it. They are not effects it has been shown to have.

  • Reported by proponents to help re-enter or deepen nutritional ketosis after a carbohydrate slip
  • Used by some keto dieters as a short, self-limited attempt to break a weight-loss plateau
  • Described as a hunger-management approach because high fat intake is satiating
  • Promoted as a transitional or keto-adaptation aid by low-carb authors

What people get wrong

  • Often saidA fat fast is a type of fasting

    ActuallyDespite the name it is eating, specifically a short calorie-restricted ketogenic diet of roughly 1000 to 1200 calories at about 80 to 90 percent fat. Atkins and modern keto educators present it as a stall-breaking tool, not abstention from food (Atkins.com; KetoDiet Blog).

  • Often saidFat does not break a fast, period

    ActuallyIt depends which benefit you mean. For insulin and blood-glucose stability, dietary fat has minimal effect, so ketosis is preserved. For autophagy, the answer is unresolved: there is no direct human study, and mechanistic arguments cut both ways, since fat carries no leucine to activate mTOR strongly, while the calories themselves and ketone-driven AMPK signaling pull in opposite directions (Bubs Naturals review of mechanism; mTOR-autophagy literature).

  • Often saidThe fat fast melts a pound of fat a day

    ActuallyThe dramatic early losses cited by Atkins and 1950s reports are mostly water and glycogen-bound water, not body fat. Yang and Van Itallie (J Clin Invest 1976) found most early loss on low-calorie ketogenic diets was water, and modern proponents concede the original studies were flawed (KetoDiet Blog).

  • Often saidIt is backed by clinical research

    ActuallyNo randomized or controlled human trial tests the fat fast as an intervention. The supporting data is one mid-century observational inpatient series plus mechanistic extrapolation; the broader RCT literature covers intermittent fasting and alternate-day fasting, not this protocol.

Who profits, and who pushes back

  • Theodore B. Van Itallie

    Pushes backObesity researcher, Columbia / St. Luke's, co-author of the metabolic-ward weight-loss composition study

    Greater total weight loss on a low-calorie ketogenic diet consisted largely of water rather than fat tissue, with the mixed diet achieving superior fat loss as a fraction of weight lost, evidence against a metabolic advantage. (Yang and Van Itallie, J Clin Invest 1976)

  • Martina Slajerova

    Pushes backKetoDiet author, reviewed by Franziska Spritzler RD CDE (sympathetic source conceding the evidence gap)

    The studies from the 1950s and 1960s that the fat fast leans on have been shown to be largely flawed, and no study seems to take keto-adaptation into consideration.

  • Cleveland Clinic dietetics

    Pushes backAcademic medical center nutrition commentary on fat-in-coffee fasting

    There is not enough research to back up the touted benefits of high-fat coffee as an everyday or fasting practice, and it should not be treated as a proven health intervention.

  • Dave Asprey

    Sells this. Has a commercial interest in a product sold as this compound, as of July 2026.

    Promotes itBulletproof

    Popularized fat-in-coffee fasting; sells MCT oil and related products. Direct commercial interest in the claim.

  • Keto community

    Promotes itMixed

    Uses fat fasting for ketosis maintenance and hunger control.

Where the research comes from

Most of the published research on this comes from one place. 58% of it is from USA. Findings that no unrelated group has reproduced are weaker than the study count makes them look.

Share of published research by country of origin.
PlaceShare of published research
USA58%
Other42%

Research published from: USA, Global.

How we got here

The fat fast is an American clinical-nutrition technique, not a traditional fasting practice. Its empirical root is British: at the Middlesex Hospital in London, Professor Alan Kekwick and Dr Gaston Pawan reported in The Lancet in 1956 that obese inpatients on roughly 1000-calorie diets lost weight fastest when about 90 percent of calories came from fat, and they later described a putative 'fat-mobilizing substance.' Dr Robert Atkins adapted that observation in his 1972 book Dr. Atkins' Diet Revolution, naming it the 'fat fast' and reserving it for severely 'metabolically resistant' patients stalled in his induction phase, under supervision and for only three to five days. Low-carb author Dana Carpender revived it for the modern keto audience with her 2013 Fat Fast Cookbook. Separately, Dave Asprey's Bulletproof coffee (fat in coffee, launched around 2011 to 2013) popularized fat-in-the-fasting-window for a wider audience, a related but distinct commercial phenomenon.

  1. 1956Kekwick and Pawan publish in The Lancet that obese inpatients lose weight fastest on roughly 1000-calorie, about 90 percent fat diets, the observational basis later borrowed for the fat fast
  2. 1967Kekwick and Pawan describe a putative 'fat-mobilizing substance' (FMS); the finding is never independently replicated
  3. 1972Robert Atkins names and publishes the 'fat fast' in Dr. Atkins' Diet Revolution as a supervised tool for metabolically resistant patients
  4. 1976Yang and Van Itallie (J Clin Invest) report the early extra weight loss on low-calorie ketogenic diets is largely water, challenging the 'metabolic advantage' the fat fast assumed
  5. 2011-2013Dave Asprey popularizes Bulletproof coffee, spreading fat-in-the-fasting-window to a mainstream audience as a separate commercial product line
  6. 2013Dana Carpender's Fat Fast Cookbook revives the technique for the modern ketogenic community
  7. 2010s-2020sOnline keto and intermittent-fasting communities debate whether added fat or MCT 'breaks a fast,' a dispute that turns on which fasting benefit is being measured and that remains untested in humans

At a glance

Form
Fat only (MCT, butter)
Insulin
Minimal but non-zero
Autophagy
Blunted by MCT
Claim
"Does not break a fast"

What this entry does not yet cover

What follows is unfinished work on our side, not a finding about Fat Fasting. A short entry is not a safe entry.

  • Cost and market. We have not documented what it sells for or who sells it.
  • Supply chain. We have not mapped where it is made.
  • Who uses it. We have not documented the user groups.
  • What could change. No forward-looking summary yet.
  • How it compares. We have not set it against the alternatives.
  • Children and teenagers. We have not checked what is known about younger people.

Sources