remain.fit · When Intermittent Fasting Isn't the Right Tool

Fasting schedules are not appropriate during pregnancy or breastfeeding, in type 1 diabetes, for anyone with a history of an eating disorder, for people who are underweight, or for under-18s. Anyone taking insulin, sulfonylureas or medication that must be taken with food needs medical supervision, because hypoglycaemia is a real risk. Early side effects are hunger, irritability and poor concentration.

What we know, and what we don’t

We know:

The named groups. A clinical review lists pregnancy and breastfeeding, eating disorders, young children, advanced age, immune deficiencies and dementia among groups for whom fasting is not recommended (Vasim et al., 2022).
For type 1 and gestational diabetes, there’s insufficient data to recommend fasting at all (Varady et al., Lancet Diabetes and Endocrinology, 2026).
Hypoglycaemia is a real risk in medicated type 2 diabetes: event rates roughly doubled despite planned medication reduction (Corley et al., 2018).
The ordinary early side effects are hunger, irritability and reduced concentration, usually resolving within the first month (de Cabo and Mattson, 2019).
In trials, fasting shows no significant excess of headache, fatigue or dropout versus comparison diets (Zhong et al., 2024: 15 RCTs, 1,365 adults).
Severe, persistent energy restriction harms menstrual function and bone health in athletes (Mountjoy et al., 2018).

We don’t know:

Whether the pregnancy association is causal. The evidence is 19 observational studies and no randomised trials (Giorno et al., 2025).
Precisely how long side effects last; the “within a month” figure comes from a narrative review, not trial measurement.
Whether the 2024 “8-hour window” headline reflects a real effect. The published paper is observational with acknowledged residual confounding (Chen et al., 2025).

Who should not try intermittent fasting?

Group	Recommendation	Evidence basis
Pregnancy and breastfeeding	Not recommended	Vasim 2022 (guidance); Giorno 2025 (observational, no randomised trials)
Type 1 and gestational diabetes	Insufficient data to recommend	Varady 2026, Lancet Diabetes and Endocrinology
Eating disorder history	Avoid	Vasim 2022; Pacanowski 2015
Underweight, and under-18s	Avoid	Guidance-based; Vasim 2022 names young children
Insulin, sulfonylureas, medication taken with food	Supervision required	Corley 2018; Carter 2018
Athletes in heavy training	Watch energy availability	Mountjoy 2018 (RED-S)

One honest thing to say about this table: trials exclude most of these groups, so the contraindications rest on clinical guidance and reasoning rather than trial data. That’s not a weakness to hide. It’s why the recommendations are conservative, and why “no trial shows harm” would be the wrong way to read the absence.

Who needs supervision instead of avoidance?

People whose medication assumes food is coming. Insulin and sulfonylureas keep lowering glucose whether you’ve eaten or not, and in a 12-week study of 41 medicated adults with type 2 diabetes, fasting roughly doubled hypoglycaemic events (rate ratio 2.05, 95% CI 1.17 to 3.52) even though doses had been reduced by plan beforehand (Corley et al., Diabetic Medicine, 2018). Supervised, it’s workable: a 12-month trial ran two fast days a week in type 2 diabetes with clinician-managed medication adjustment (Carter et al., 2018). The rule that falls out is simple. If your medication can drop your blood sugar, or has to be taken with food, the fasting conversation starts at your clinician’s desk, and no medication change is ever self-directed. The insulin and glucose article has the full picture.

What are the ordinary side effects, and how long do they last?

Hunger, irritability and reduced ability to concentrate, and for most people they fade within the first month (de Cabo and Mattson, New England Journal of Medicine, 2019). Trial data adds a reassuring nuance: across 15 randomised trials, fasting groups showed no significant excess of headache, fatigue, or dropout compared with other diets (Zhong et al., Nutrition Journal, 2024). So the typical experience is a rough first couple of weeks that settles. Some people don’t adjust, and that’s information, not failure. A schedule that still feels like a fight in week six is answering your question.

What about athletes and heavy training?

The concern has a name: Relative Energy Deficiency in Sport, or RED-S. When energy intake persistently falls short of training demands, the documented consequences include impaired menstrual function and compromised bone health, particularly in women (Mountjoy et al., British Journal of Sports Medicine, 2018, the IOC consensus statement). To be precise about attribution: that statement addresses low energy availability from any cause and never names intermittent fasting. But a compressed eating window plus a heavy training load is one plausible route to exactly that shortfall. If you train hard, the question isn’t “is fasting allowed” but “am I eating enough, reliably”, and cycle changes or recurring injuries are reasons to stop and get assessed.

Is fasting during pregnancy dangerous?

The precaution is guidance-based, and here’s the actual evidence underneath it. A systematic review found 19 observational studies and no randomised trials; fasting during pregnancy, mostly Ramadan fasting, was associated with slightly lower birth weight (−94 g, 95% CI −176 to −12) and no increase in low-birth-weight rates (Giorno et al., Nutrients, 2025). Observational data can’t establish cause, and no trial will ever randomise pregnant women to fasting. Given a modest signal, no trial data, and nothing to gain that can’t wait nine months, “don’t fast while pregnant or breastfeeding” is the reasonable reading, held with appropriate honesty about where it comes from.

What about the headline that 8-hour eating nearly doubles cardiovascular death?

That headline outran its evidence twice: once as a conference abstract, and again in how it was reported. In March 2024, an American Heart Association conference abstract linked eating windows under 8 hours to a 91% higher risk of cardiovascular death, based on NHANES survey data where eating windows were inferred from two days of self-reported dietary recall. It was preliminary and unpublished, and it ran worldwide as fact. The published paper arrived in 2025 (Chen et al., Diabetes and Metabolic Syndrome): hazard ratio 2.35 (95% CI 1.39 to 3.98) for cardiovascular mortality in the under-8-hour group, with null associations for all-cause and cancer mortality, in observational data whose authors acknowledge residual confounding. People eating in very short windows may differ in illness, smoking, or a dozen unmeasured ways. It shouldn’t frighten anyone, and it shouldn’t be dismissed either: it’s an open question logged honestly, awaiting better data. The claims article keeps the longer list.

When should you stop fasting?

Four clear signals. Persistent dizziness or weakness that outlasts the adjustment weeks. Any change to your medication, which resets the safety question. Weight loss you didn’t intend. And anything that feels like the fasting is running you: rules tightening on their own, fear attaching to food, the window becoming a way to punish. That last one matters most; restriction can be a doorway to disordered eating, and the scale has the same caveat. Stopping is cheap, and a clinician conversation is cheaper than pushing through any of these.

Key points

Fasting is not recommended for specific groups: pregnancy and breastfeeding, eating disorders, young children, immune deficiencies, advanced age (Vasim et al., 2022).
Type 1 and gestational diabetes: insufficient data to recommend (Varady et al., 2026).
Hypoglycaemia is real on insulin and sulfonylureas: rate ratio 2.05 despite planned medication reduction (Corley et al., 2018).
Early side effects pass for most: hunger, irritability, poor concentration, usually within the first month (de Cabo and Mattson, 2019).
The pregnancy precaution is guidance-based: 19 observational studies, no randomised trials, −94 g birth weight, no rise in low birth weight (Giorno et al., 2025).
The “91% study” is now published and weaker than its headline: HR 2.35 for cardiovascular mortality, null for all-cause and cancer, residual confounding acknowledged (Chen et al., 2025).

Common questions

Who should not do intermittent fasting?

Pregnant and breastfeeding women; people with type 1 or gestational diabetes; anyone with an eating disorder history; people who are underweight or under 18; and anyone on insulin or sulfonylureas without clinical supervision (Vasim et al., 2022; Varady et al., 2026; Corley et al., 2018).

Is intermittent fasting safe?

For adults with overweight or obesity in trials, yes: no significant excess of headache, fatigue or dropout versus comparison diets across 15 RCTs (Zhong et al., 2024). But “generally safe” does not mean “safe for everyone”. The exclusions on this page are the point of this page.

What are the side effects of intermittent fasting?

Hunger, irritability and reduced concentration in the early weeks, usually fading within a month (de Cabo and Mattson, 2019). Persistent dizziness or weakness isn’t an ordinary side effect; it’s a reason to stop and talk to a clinician (Vasim et al., 2022).

Can I fast while taking diabetes medication?

Only with clinician supervision and a medication plan. Fasting roughly doubled hypoglycaemic events in medicated type 2 diabetes even with doses reduced in advance (Corley et al., 2018). Supervised trials managed it (Carter et al., 2018). The difference between those sentences is the supervision.

Does fasting cause cardiovascular death?

Unproven. The 2024 headline came from a conference abstract; the published paper found an association (HR 2.35) in self-reported observational data with residual confounding, and no association with all-cause or cancer mortality (Chen et al., 2025). Not a reason to fear an 8-hour window; not a settled question either.

A note on safety

This is general information, not medical advice. If you take medication or have a health condition, talk to your clinician before changing when or what you eat. Nothing here is dosing guidance, and nothing here replaces the clinician who knows your history. The evidence hub covers what fasting can actually deliver for the people it does suit.

References

Vasim I, Majeed CN, DeBoer MD. Intermittent fasting and metabolic health. Nutrients. 2022;14(3):631. doi:10.3390/nu14030631
de Cabo R, Mattson MP. Effects of intermittent fasting on health, aging, and disease. New England Journal of Medicine. 2019;381(26):2541-2551. doi:10.1056/NEJMra1905136
Zhong F, et al. Adverse events profile associated with intermittent fasting in adults with overweight or obesity: a systematic review and meta-analysis of randomized controlled trials. Nutrition Journal. 2024;23(1):72. doi:10.1186/s12937-024-00975-9
Corley BT, et al. Intermittent fasting in type 2 diabetes mellitus and the risk of hypoglycaemia: a randomized controlled trial. Diabetic Medicine. 2018;35(5):588-594. doi:10.1111/dme.13595
Carter S, Clifton PM, Keogh JB. Effect of intermittent compared with continuous energy restricted diet on glycemic control in patients with type 2 diabetes: a randomized noninferiority trial. JAMA Network Open. 2018;1(3):e180756. doi:10.1001/jamanetworkopen.2018.0756
Giorno A, et al. Intermittent fasting during pregnancy and neonatal birth weight: a systematic review and meta-analysis. Nutrients. 2025;17(22):3546. doi:10.3390/nu17223546
Varady KA, et al. Intermittent fasting to treat diabetes: time to update clinical practice guidelines. Lancet Diabetes and Endocrinology. 2026;14(9):778-786. doi:10.1016/S2213-8587(26)00119-1
Mountjoy M, et al. IOC consensus statement on relative energy deficiency in sport (RED-S): 2018 update. British Journal of Sports Medicine. 2018;52(11):687-697. doi:10.1136/bjsports-2018-099193
Chen M, et al. Association of eating duration less than 8 h with all-cause, cardiovascular, and cancer mortality. Diabetes and Metabolic Syndrome: Clinical Research and Reviews. 2025;19(7):103278. doi:10.1016/j.dsx.2025.103278
Varady KA, et al. Clinical application of intermittent fasting for weight loss: progress and future directions. Nature Reviews Endocrinology. 2022;18(5):309-321. doi:10.1038/s41574-022-00638-x

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