Fasting Evidence

Safety

Who should not fast

Fasting is safe for most healthy adults and genuinely unsafe for a specific minority. Knowing which you are matters more than the protocol.

Updated 3 min read 10 citations Evidence strength 4/5

Do not fast without medical advice if any of these apply

Contraindications and strong cautions
SituationWhyWhat to do
Insulin or sulfonylurea therapyReal risk of hypoglycaemiaOnly with supervised dose adjustment
Type 1 diabetesHypoglycaemia and ketoacidosis riskSpecialist supervision only
Pregnancy or breastfeedingNutrient and energy demandsDo not fast
History of an eating disorderRestriction can reactivate the disorderDo not fast; seek support
Under 18Growth and developmentDo not fast
Underweight or frailLean mass and nutrient adequacyDo not fast
Advanced kidney or liver diseaseAltered metabolic handlingMedical advice first
Cancer treatmentDocumented interactions [3]Oncologist decision
Medication requiring foodAbsorption and tolerabilityPharmacist advice

The diabetes case in detail

This deserves more than a table row because it is where people get hurt. INTERFAST-2 tested intermittent fasting in people with insulin-treated type 2 diabetes and reported on efficacy and safety [8] — it can be done, and it was done with medication management built into the protocol. A trial comparing intermittent fasting plus early time-restricted eating against calorie restriction and standard care in adults at risk was similarly structured [7].

Meta-analysis comparing intermittent fasting with continuous energy restriction in type 2 diabetes found comparable outcomes [5] — meaning there is no efficacy reason to accept the additional hypoglycaemia risk unsupervised.

Eating disorders

This is the risk least often taken seriously in fasting content. Structured restriction, rules about permitted hours, and the moral framing that grows around them are close to the mechanics of restrictive eating disorders. For someone with that history, or with an active tendency toward binge-restrict cycles, a fasting protocol can be the thing that reopens it.

The warning signs worth naming: distress when the window shifts, escalating window length, compensating for eating with extra fasting, and secrecy. If any of that is familiar, the right intervention is support, not a different protocol.

The common, non-dangerous side effects

  • Headaches in the first week or two, usually settling.
  • Irritability and poor concentration during fasting hours, particularly early on.
  • Constipation, largely from reduced total intake and fibre.
  • Poor sleep if the window ends very late or hunger is severe.
  • Cold intolerance with sustained energy deficit.

Most resolve with adaptation, adequate hydration and electrolytes, and not making the deficit unnecessarily aggressive.

Surgery

Perioperative fasting has its own consensus guidance and it is not the same thing as intermittent fasting. If you are having a procedure, follow the anaesthetic team's instructions rather than your protocol.

Common questions

Is fasting safe long term?
For healthy adults, the trials show no alarming signal, but the longest are measured in months to a couple of years. Cochrane's verdict on cardiovascular prevention is "insufficient evidence" [4].
Can I fast on metformin?
Metformin alone rarely causes hypoglycaemia, so the risk is lower than with insulin or sulfonylureas. Still worth confirming with your prescriber.
What about gallstones?
Rapid weight loss by any means increases gallstone risk. This is a weight-loss-rate issue rather than a fasting-specific one.
Should older adults fast?
With more caution. Reviews specifically flag the ageing cardiovascular system and lean-mass concerns — see metabolic effects.

References

Every citation below links to the original peer-reviewed record on PubMed or via DOI. Nothing here is a substitute for medical advice.

  1. Intermittent fasting for adults with overweight or obesity Garegnani LI, Oltra G, Ivaldi D, et al. · The Cochrane database of systematic reviews · 2026 · Meta-analysis DOIPubMed 41692034Full text
  2. Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomised clinical trials Semnani-Azad Z, Khan TA, Chiavaroli L, et al. · BMJ (Clinical research ed.) · 2025 · Meta-analysis DOIPubMed 40533200Full text
  3. Therapeutic Fasting in Reducing Chemotherapy Side Effects in Cancer Patients: A Systematic Review and Meta-Analysis Ferro Y, Maurotti S, Tarsitano MG, et al. · Nutrients · 2023 · Meta-analysis DOIPubMed 37375570Full text
  4. Intermittent fasting for the prevention of cardiovascular disease Allaf M, Elghazaly H, Mohamed OG, et al. · The Cochrane database of systematic reviews · 2021 · Systematic review DOIPubMed 33512717Full text
  5. Intermittent fasting versus continuous energy-restricted diet for patients with type 2 diabetes mellitus and metabolic syndrome for glycemic control: A systematic review and meta-analysis of randomized controlled trials Wang X, Li Q, Liu Y, et al. · Diabetes research and clinical practice · 2021 · Meta-analysis DOIPubMed 34391831
  6. Intermittent fasting and weight loss: Systematic review Welton S, Minty R, O'Driscoll T, et al. · Canadian family physician Medecin de famille canadien · 2020 · Systematic review PubMed 32060194Full text
  7. Intermittent fasting plus early time-restricted eating versus calorie restriction and standard care in adults at risk of type 2 diabetes: a randomized controlled trial Teong XT, Liu K, Vincent AD, et al. · Nature medicine · 2023 · Randomised controlled trial DOIPubMed 37024596
  8. Efficacy and Safety of Intermittent Fasting in People With Insulin-Treated Type 2 Diabetes (INTERFAST-2)-A Randomized Controlled Trial Obermayer A, Tripolt NJ, Pferschy PN, et al. · Diabetes care · 2023 · Randomised controlled trial DOIPubMed 36508320Full text
  9. Ketogenic Diet: A Nutritional Therapeutic Tool for Lipedema? Verde L, Camajani E, Annunziata G, et al. · Current obesity reports · 2023 · Review DOIPubMed 37924422Full text
  10. Clinical application of intermittent fasting for weight loss: progress and future directions Varady KA, Cienfuegos S, Ezpeleta M, et al. · Nature reviews. Endocrinology · 2022 · Review DOIPubMed 35194176