Who Should Not Do Intermittent Fasting

Most fasting pages bury this list at the bottom. It belongs first, because for several of these groups the risk is not theoretical discomfort — it is hypoglycemia, growth, or a psychiatric illness with real mortality. Each entry below comes with the source that says so.

1. Anyone taking insulin, a sulfonylurea or a meglitinide

This is the entry with a mechanism behind it rather than a caution. NIDDK’s page on low blood glucose names fasting explicitly as a cause: continuing to take medicines that lower glucose while fasting increases the risk of blood sugar dropping too low. The drug classes it names are insulin, sulfonylureas (usually taken once or twice a day, raising insulin over several hours) and meglitinides.

Low blood glucose means a reading under about 70 mg/dL, and severe hypoglycemia is defined as a drop so low you cannot treat it yourself. NIDDK reports that in a large global study of people with diabetes taking insulin, four in five with type 1 and nearly half with type 2 had a low blood sugar event within a four-week period — before anyone added a 16-hour fast.

This does not mean fasting is forbidden with diabetes. It means the medication has to be adjusted by the person who prescribed it, in advance, and never by the patient reading a calculator.

2. Pregnancy and breastfeeding

A clinical commentary in BMC Clinical Diabetes and Endocrinology states directly that pregnant and lactating women should not participate in intermittent fasting, because research on safety and efficacy in those populations is lacking. These are the two life stages with the highest nutrient demands and the least margin for an experimental eating schedule.

3. Children and adolescents

Two separate reasons. The review of human fasting trials notes that fasting is not recommended for children under the age of 12, because of the potential to negatively affect puberty and growth. Above that age the concern shifts: survey data covering 2,762 adolescents and young adults found fasting was already widespread — 47.7% of young women, 38.4% of young men and 52.0% of transgender and gender non-conforming participants in the previous year — and associated with disordered eating behaviours, most consistently in young women.

4. Anyone with a history of disordered eating

Eating disorders carry high morbidity and mortality and a lifetime prevalence around 9%, and fasting is a practice built out of restriction rules. The commentary above argues the risk has been badly under-examined in fasting research and recommends that screening for disordered eating be routine, and paramount for any patient a clinician is thinking of putting on a fasting schedule.

The specific finding worth knowing: in a study of 682 college students, combining a low-carb diet with intermittent fasting was associated with a substantially higher likelihood of binge eating and food cravings than either practice alone. NIMH’s page on eating disorders is the place to start if any of this is familiar territory.

5. Older adults

The same commentary places elderly patients alongside children and pregnancy in the “should not participate” group, on the same grounds: too little research on safety and efficacy. StatPearls agrees that data concerning underweight, geriatric and pediatric patients is still lacking. Add the ordinary risks of aging — reduced appetite, sarcopenia, polypharmacy — and a protocol that shortens eating hours is working against the goal.

6. Anyone whose condition or medication requires food on a schedule

Unstable chronic conditions and drugs that must be taken with food at fixed times are practical disqualifiers. This is also where StatPearls’ general position belongs on the page: fasting should be performed under the supervision of a physician, ideally in a clinical setting — a stricter line than any consumer fasting content draws.

Signs to stop, even if you are in none of these groups

• Strong dizziness, weakness or feeling unwell.
• Any sign of low blood glucose, if you take diabetes medication.
• Menstrual cycle changes — see intermittent fasting for women.
• Bingeing, food cravings that dominate the day, or preoccupation with eating rules.
• Persistent fatigue that a normal week does not explain.

Fasting headache is common enough to expect rather than fear: StatPearls describes it as a notable effect at the beginning of fasting, with hypoglycemia, dehydration and caffeine withdrawal all proposed as contributors.

Fasting is optional

Nothing about intermittent fasting is required for health. It is one dietary approach among several, and the trials do not put it above the others — a point covered on the intermittent fasting calculator page, and in more detail for longer fasts, where every risk on this page is sharper.

If you have read the list and none of it applies, this is the schedule tool:

This content is informational and does not replace evaluation by a physician or dietitian.