Do not start without medical supervision
For these groups the answer is not "be careful" — it is "speak to a doctor first".
- You are pregnant or breastfeeding. Energy and micronutrient needs are elevated and consistent intake matters. This is not the time.
- You have type 1 diabetes, or take insulin, sulfonylureas, or other glucose-lowering medication for any type of diabetes. Fasting while on these carries a real risk of hypoglycaemia. Some people with type 2 diabetes do fast successfully, but doses generally need adjusting first, and that requires your diabetes team.
- You have or have had an eating disorder, or any pattern of binge eating, purging, or compulsive restriction. Fasting protocols reliably reactivate these. This holds even if the illness was years ago and even if you feel entirely recovered.
- You are underweight (BMI under about 18.5) or have been losing weight without trying.
- You are under 18. Growth and development require consistent intake, and restriction in adolescence is a well-established risk factor for eating disorders.
- You are frail or over 75. Loss of muscle mass is the dominant risk in older age, and fasting protocols make adequate protein intake harder.
If you take any regular medication, check before starting. Some drugs must be taken with food; some have doses timed to meals; some are affected by dehydration. A pharmacist can answer this quickly and free of charge. Never skip or delay a dose to preserve a fasting window.
Grey areas — get advice, then decide
- Type 2 diabetes without insulin. Often compatible and sometimes beneficial, but medication may need adjusting and blood glucose needs monitoring at first.
- Low blood pressure or a tendency to faint. Fasting lowers blood pressure further and shifts fluid balance.
- A history of gallstones. Rapid weight loss and long gaps without eating are both associated with gallstone formation.
- Gastro-oesophageal reflux. Larger meals in a compressed window makes reflux worse for many people.
- Thyroid conditions. Levothyroxine absorption is affected by food timing; the schedule needs coordinating with the dose, not the reverse.
- Menstrual irregularity. If periods become irregular or stop after starting, that is a signal of insufficient energy availability. Stop and see a doctor.
- Shift work. Not unsafe as such, but circadian disruption plus an eating schedule fighting your rota tends to be counterproductive.
Warning signs that mean stop now
These are not things to push through, and none of them indicate that you need to try harder:
- Fainting, near-fainting, or repeated dizziness
- Heart palpitations or an unusually fast resting pulse
- Confusion, difficulty concentrating beyond the first week, or slurred speech
- Persistent nausea or vomiting
- Hair loss, feeling cold constantly, periods stopping
- Sleep that gets worse and stays worse
- Thinking about food for most of the fasting hours
- Eating past fullness, or feeling out of control, when the window opens
- Guilt, shame, or anxiety when the schedule slips
The last three are the ones people are most likely to dismiss and least should. They describe a relationship with food that is heading somewhere bad, and continuing to fast will accelerate it rather than resolve it.
If this page describes you, the next step is a phone call, not a decision. Most of the entries above are “get advice first” rather than “never”. Book the conversation, get your medication or your history properly accounted for, and then come back — the schedules on this site will still be here.
Getting help
If any of this has raised a concern about your own eating, talk to your GP. In the UK, Beat runs a free helpline. In the US, the National Association of Anorexia Nervosa and Associated Disorders operates a free helpline. Both are staffed by people who will take the question seriously.
Get cleared, then get the benefits
Being on this page does not automatically mean fasting is off the table for you. For a great many people it means one conversation first — and the reason that conversation is worth having is what sits on the other side of it.
It works alongside the medication you already take
Nineteen adults with metabolic syndrome, most of them on a statin or a blood-pressure drug, moved to a ten-hour eating window for twelve weeks while their treatment continued as normal. Weight came down. So did blood pressure and atherogenic lipids.1
That is the case for asking rather than for avoiding. Fasting can move blood pressure and blood glucose — which is exactly why someone whose dose was set on the assumption of three meals a day needs that dose reviewed, not abandoned. Your doctor adjusts the medication; you keep the benefit.
And the benefits are not only about weight
In a trial where participants were fed enough to hold body weight completely constant, an early eating window still improved insulin sensitivity, beta-cell responsiveness, blood pressure and oxidative stress.2 If your reason for being careful here is a metabolic condition, that is precisely the population these effects were measured in — with supervision.
None of this overrides the lists above. If you are pregnant or breastfeeding, take insulin or another glucose-lowering medication, have any history of disordered eating, are underweight, or are under 18, the answer is still to speak to a doctor before starting — not to start and see what happens.
References
- Wilkinson MJ, Manoogian ENC, Zadourian A, et al. Ten-Hour Time-Restricted Eating Reduces Weight, Blood Pressure, and Atherogenic Lipids in Patients with Metabolic Syndrome. Cell Metab. 2020;31(1):92–104.e5. doi:10.1016/j.cmet.2019.11.004
- 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. doi:10.1016/j.cmet.2018.04.010