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A ketogenic diet and fasting are contraindicated in inherited disorders of fatty-acid oxidation and ketogenesis - including CPT1 and CPT2 deficiency, carnitine and carnitine-acylcarnitine translocase deficiency, other beta-oxidation defects and pyruvate carboxylase deficiency - so these must be screened for and excluded before a ketogenic diet is recommended.
If you have an inherited condition that stops your body burning fat or making glucose normally, a keto diet or fasting can be life-threatening. These are rare, but they must be ruled out first.
Safety. Harmful if misapplied. The conditions above are not optional detail.
Only when
in pyruvate carboxylase deficiency impaired gluconeogenesis creates dependence on dietary glucose; carbohydrate restriction can worsen lactic acidosis and ketoacidosis, and a carbohydrate- and protein-rich diet with frequent meals is used instead
in CPT1 and CPT2 deficiency fat cannot be converted to energy efficiently; a carbohydrate-rich, fat-poor diet with frequent meals is used instead, with intravenous dextrose during severe hypoglycaemic episodes
the described consequences of getting this wrong are hypoglycaemia, coma and death
acute intermittent porphyria is named alongside these as a further contraindication
these conditions are rare - pyruvate carboxylase deficiency around 1 in 250,000, CPT2 deficiency roughly 1-9 per 100,000, CPT1 deficiency approximately 60 reported cases worldwide
Who
anyone being considered for a ketogenic diet, and specifically people with a known or suspected inborn error of fatty-acid metabolism, pyruvate carboxylase deficiency or acute intermittent porphyria
Moderate confidence
Three independent first authors in peer-reviewed sources converge exactly, and the direction is uncontested clinical practice; capped below high because the strongest evidence tier present is clinical experience and the rarity of the conditions means no trial evidence exists or ever will.
Adaptive Ketogenic-Mediterranean Protocol (AKMP) in Real Clinical Practice: 14-Week Pre-Post Cohort Study on Glucolipid Markers and Safety. · Nutrients · 2025 · doi:10.3390/nu17223559
Scientific evidence underlying contraindications to the ketogenic diet: An update. · Obesity reviews : an official journal of the International Association for the Study of Obesity · 2020 · doi:10.1111/obr.13053
The ketogenic diet, along with other low-carbohydrate diets and fasting, is an absolute contraindication in people with porphyria (especially acute intermittent porphyria), because carbohydrate restriction increases heme precursor production and can trigger neurotoxic symptoms and acute porphyric attacks.
If you have porphyria, avoid keto, very-low-carb, or fasting diets -- cutting carbs can trigger a dangerous attack. Talk to your doctor about diet, and expect them to recommend more carbohydrate, not less.
Safety. Harmful if misapplied. The conditions above are not optional detail.
Only when
Porphyria is a group of inherited heme-synthesis-pathway disorders; porphyria cutanea tarda affects about 5-10 per 100,000 people.
Low-carbohydrate diets in general -- not only the ketogenic diet -- are implicated in triggering attacks, via increased mitochondrial ALA synthase activity and heme precursor production.
Also avoid fasting and very-low-calorie diets in this population; recommended alternative is a high-carbohydrate diet, roughly 60-70% of energy.
Who
People with porphyria, including acute intermittent porphyria
Moderate confidence
Two independent authors agree with a stated mechanism, which is consistent with established medical knowledge about porphyria. Evidence types are clinical_experience and author_assertion (the two weakest tiers), so this is kept at moderate rather than high despite being a well-recognized contraindication.
Scientific evidence underlying contraindications to the ketogenic diet: An update. · Obesity reviews : an official journal of the International Association for the Study of Obesity · 2020 · doi:10.1111/obr.13053
The ketogenic diet, and fasting, are contraindicated in inherited fatty-acid oxidation disorders (including CACT, MHADD, LCHAD, MCADD, and VLCADD deficiencies) because these conditions impair the body's ability to use fat for energy, raising the risk of acute metabolic decompensation; a carbohydrate-based diet with restricted long-chain fat is recommended instead.
People with certain rare inherited disorders that block the body from using fat for fuel should never try a ketogenic diet or fasting -- it can trigger a dangerous metabolic crisis. These conditions need a doctor-supervised, carbohydrate-based diet instead.
Safety. Harmful if misapplied. The conditions above are not optional detail.
Only when
CACT deficiency: high-carbohydrate diet (>60% of energy), long-chain fat under 10% of energy, triheptanoin or MCT oil as fat sources
MHADD: affected individuals cannot effectively switch to fat as their main energy source, so keto is contraindicated
LCHAD deficiency: also avoid fasting, caloric deficit under stress, extreme environmental exposure, intense exercise, and long-chain-fat-containing anaesthetics (e.g. propofol); use a low-fat diet, frequent meals, MCT or triheptanoin supplementation
MCADD: contraindication is driven mainly by a lack of safety data and good response to glucose-based metabolism, not a demonstrated mechanism of harm; use a standard carbohydrate-based diet with fat capped at 30% of energy, avoid fasting, shorten feeding intervals in infancy
VLCADD: use a carbohydrate-based diet minimizing long-chain fat, favoring medium-chain fat and triheptanoin, with shorter meal intervals, and avoid fasting
Who
people with an inherited fatty-acid oxidation disorder (CACT, MHADD, LCHAD, MCADD, or VLCADD deficiency)
Moderate confidence
Single voice within this corpus, and the underlying evidence type is clinical_experience rather than trial data, but this reflects well-established management guidance for recognized inborn errors of fatty-acid metabolism rather than one author's personal opinion. No independent replication exists in this corpus.
This is a summary of published research, not medical advice. It cannot account for your medications, conditions or history. Talk to a clinician before acting on any of it.