Fasting safety: who should not fast or needs a clinician first

What it is

This topic gathers what studies and professional bodies say about harms of fasting across all types, and about groups for whom fasting carries extra risk. It covers diabetes medicines, blood pressure and water pills, pregnancy and breastfeeding, eating disorders, older adults and muscle, gallstones, gout and medicine timing.

What the research says

Harms reported: hypoglycaemia with sulfonylureas and insulin, euglycaemic diabetic ketoacidosis with SGLT2 inhibitors, dehydration linked to SGLT2 inhibitors and diuretics, lean mass loss, minor side effects such as constipation and dizziness, a threefold higher risk of symptomatic gallstones with a 500 kcal/day diet than a 1200-1500 kcal/day diet, and associations between intermittent fasting and eating disorder symptoms in young people. Null findings: no rise in binge eating or purging in an alternate-day fasting trial, no rise in uric acid or gout attacks during Ramadan, no change in breast milk composition or infant weight gain. Professional positions list pregnancy and nursing, eating disorders, cachexia and advanced kidney or liver disease as contraindications to medical fasting, and list blood pressure medicines, diuretics and diabetes medicines as needing adjustment.

Evidence grade: Limited evidence.

The schedule

• Pattern: periodic

Why people choose it

This is not a schedule to choose but a list of groups where the studies found harm or where guidelines ask for a clinician's involvement. The strongest signals are hypoglycaemia with sulfonylureas and insulin, ketoacidosis with SGLT2 inhibitors, loss of lean mass in a time-restricted eating trial, and associations with eating disorder symptoms in young people. Some feared harms were not seen in the studies that looked: disordered eating did not increase in an alternate-day fasting trial, gout attacks did not rise during Ramadan, and breast milk composition did not change.

Cautions

• People taking insulin or sulfonylureas: These medicines are linked to hypoglycaemia during fasting; the IDF-DAR guidelines flag people on both as at increased risk. (Source 1)
• People taking SGLT2 inhibitors (for example dapagliflozin, empagliflozin): Euglycaemic diabetic ketoacidosis has been reported during prolonged daily fasting; the case report authors recommend counselling and possible medicine adjustment. (Source 2)
• People taking blood pressure medicines or diuretics (water pills): A fasting guideline lists these among medicines needing adjustment; NIDDK notes diuretics add to dehydration risk. (Source 3)
• Older adults: A review raises concern about blood pressure swings, dizziness on standing and falls, and about electrolyte problems; a 12-week trial found more lean mass loss with time-restricted eating. (Source 4)
• Pregnant women: Fasting during pregnancy in Ramadan was associated with poorer child and adult health in a life-course meta-analysis, although birth weight was unaffected. (Source 5)
• People with a history of eating disorders: Intermittent fasting was associated with eating disorder psychopathology in a large survey of young people; a fasting guideline lists eating disorders as a contraindication. (Source 6)
• People taking medicines on a set schedule or with food: An educational framework for clinicians notes that patients change or stop medicines during Ramadan. (Source 7)
• People losing weight very fast (very-low-calorie approaches): A 500 kcal/day diet carried three times the gallstone risk of a 1200-1500 kcal/day diet. (Source 8)

What goes wrong

In the TREAT trial of 16:8 time-restricted eating, the fasting group lost more appendicular lean mass (arm and leg muscle) than the group eating three meals a day. (Source 9)

• Randomized trial, Certainty not rated.
• Size: 116 randomised; 50 in the in-person body composition cohort (25 per group)
• Who: Men and women aged 18 to 64 with BMI 27 to 43 in the United States.
• How long: 12 weeks.
• Result: Difference in appendicular lean mass index between groups -0.16 kg/m2 (95% CI -0.27 to -0.05; P = .005)
• Funding: not stated.

Limit of this finding: Lean mass was a secondary outcome measured in a subset of 50; participants were under 65, so older adults were not studied.

There was also a significant difference in appendicular lean mass index between groups

In people with type 2 diabetes fasting during Ramadan, sulfonylureas were linked to more hypoglycaemic events than other diabetes medicines. (Source 10)

• Systematic review, Certainty not rated.
• Size: 68 studies (35 RCTs, 33 observational)
• Who: Adults with type 2 diabetes.
• How long: Ramadan month.
• Result: Fewer hypoglycaemic events with non-sulfonylurea medicines than with sulfonylureas.
• Funding: not stated.

Non-sulfonylureas hypoglycemic medications showed superior effects in lowering the incidence of HE over sulfonylureas hypoglycemic medications.

A case report describes euglycaemic diabetic ketoacidosis (dangerous acid build-up with only moderately raised glucose) in a man taking dapagliflozin, an SGLT2 inhibitor, while fasting about 14 hours a day. (Source 2)

• Case report, Certainty not rated.
• Size: 1 patient.
• Who: 51-year-old man with type 2 diabetes on dapagliflozin and metformin, fasting during Ramadan.
• How long: About 30 days of daily fasting.
• Result: Severe high anion gap metabolic acidosis with ketonuria at a glucose of 13.5 mmol/L (243 mg/dL); treated with fluids and insulin infusion.
• Funding: not stated.

Limit of this finding: A single case cannot show how often this happens.

Dapagliflozin is associated with euglycemic diabetic ketoacidosis in the setting of prolonged fasting.

In an 8-week alternate-day fasting study, some participants reported constipation, dizziness and general weakness. (Source 11)

• Cohort study, Certainty not rated.
• Size: 59 adults with obesity.
• Who: Adults with obesity in Chicago.
• How long: 8 weeks.
• Result: Constipation 17%, water retention 2%, dizziness under 20%, general weakness under 15%; body weight -4.2%.
• Funding: not stated.

Limit of this finding: Single-arm study without a control group.

Some subjects reported constipation (17%), water retention (2%), dizziness (<20%), and general weakness (<15%).

In a Canadian national survey of adolescents and young adults, intermittent fasting in the past year or month was associated with eating disorder psychopathology in women, men and gender-diverse participants. (Source 6)

• Survey study, Certainty not rated.
• Size: 2,762 adolescents and young adults.
• Who: Canadian Study of Adolescent Health Behaviors participants.
• How long: Past 12 months and past 30 days.
• Result: Intermittent fasting reported by 47.7% of women, 38.4% of men and 52.0% of TGNC participants in the past 12 months; significant associations with EDE-Q psychopathology.
• Funding: not stated.

Limit of this finding: Cross-sectional: shows an association only, not which came first; young people, not adults over 50.

Intermittent fasting in the past 12 months and 30 days was significantly associated with eating disorder psychopathology among women, men, and TGNC participants.

In a matched cohort from a commercial weight-loss programme, a 500 kcal/day very-low-calorie diet was associated with three times the risk of gallstones needing hospital care compared with a 1200-1500 kcal/day diet, though absolute risk was low. (Source 8)

• Cohort study, Certainty not rated.
• Size: 6,640 adults (3,320 matched pairs)
• Who: Adults in a Swedish commercial weight-loss programme.
• How long: 3-month weight-loss phase plus 9-month maintenance.
• Result: 48 vs 14 gallstone events; 152 vs 44 per 10,000 person-years; HR 3.4 (95% CI 1.8 to 6.3; P < 0.001)
• Funding: not stated.

Limit of this finding: Indirect: this was a continuous very-low-calorie diet with rapid weight loss, not intermittent or religious fasting.

The risk of symptomatic gallstones requiring hospitalization or cholecystectomy, albeit low, was 3-fold greater with VLCD than LCD during the 1-year commercial weight loss program.

What the evidence does not support

In the same trial, time-restricted eating was not more effective for weight loss than eating throughout the day. (Source 9)

• Randomized trial, Certainty not rated.
• Size: 116 adults.
• Who: Men and women aged 18 to 64 with BMI 27 to 43.
• How long: 12 weeks.
• Result: Between-group weight difference -0.26 kg (95% CI -1.30 to 0.78; P = .63)
• Funding: not stated.

Time-restricted eating, in the absence of other interventions, is not more effective in weight loss than eating throughout the day.

In the same alternate-day fasting study, depression and binge eating decreased, and purging and fear of fatness did not change. (Source 11)

• Cohort study, Certainty not rated.
• Size: 59 adults with obesity.
• Who: Adults with obesity; people with eating disorders are typically excluded from such trials.
• How long: 8 weeks.
• Result: Depression and binge eating decreased (P < 0.01); purgative behaviour and fear of fatness unchanged; restrictive eating increased (P < 0.01)
• Funding: not stated.

Limit of this finding: Short, single-arm, and restrictive eating scores did rise.

Purgative behavior and fear of fatness remained unchanged.

In people with gout, uric acid levels and gout or kidney stone attacks did not rise significantly during Ramadan fasting compared with before it or with non-fasting controls. (Source 12)

• Cohort study, Certainty not rated.
• Size: 43 patients with gout (21 fasting, 22 non-fasting)
• Who: Muslim patients with gout from a hospital registry in Nazareth.
• How long: Ramadan month.
• Result: Serum uric acid 8.11 mg/dL at end of Ramadan vs 7.92 mg/dL before (P = 0.707); no difference in attacks between groups.
• Funding: not stated.

Limit of this finding: Very small; the authors note earlier studies in people without gout reported higher uric acid during Ramadan.

There was no risk for a significant increase in gouty arthritic/renal calculi attacks or serum uric acid in patients with gout during Ramadan fast.

In a small study of breastfeeding mothers, breast milk energy and macronutrient content and infant weight gain did not differ between mothers who fasted during Ramadan and those who did not. (Source 13)

• Cohort study, Certainty not rated.
• Size: 48 exclusively breastfeeding mothers (21 fasting, 27 not fasting)
• Who: Mothers in Turkey during Ramadan 2021.
• How long: Ramadan month.
• Result: No significant difference in milk energy, carbohydrate, protein or lipid; no difference in infant weight gain.
• Funding: not stated.

Limit of this finding: Very small; did not measure micronutrients in milk or longer-term infant outcomes.

There was no significant difference between the energy, carbohydrate, protein, and lipid composition of breast milk in fasting and nonfasting mothers.

Where the evidence is mixed

A review on fasting and the ageing heart states it is unknown whether intermittent fasting preserves muscle during weight loss or worsens muscle protein breakdown, and raises concern about blood pressure swings, dizziness on standing and falls in older people. (Source 4)

• Expert review, not systematic, Certainty not rated.
• Size: Not applicable.
• Who: Older adults.
• How long: Not applicable.
• Result: Not applicable.
• Funding: not stated.

Limit of this finding: Narrative review; the authors also note some studies suggested decreased lean mass with time-restricted eating and 5:2 diets, but state the general consensus is that intermittent fasting has no harmful effect on lean mass in otherwise healthy people and in people with insulin-treated diabetes.

The effect of IF on muscle health is a matter of debate because it is unknown whether IF preserves lean muscle during body weight loss or it exacerbates muscle protein breakdown and net catabolism.

What official bodies say

An NIDDK expert interview states that SGLT2 inhibitors and diuretic medicines add to the risk of dehydration during fasting because they increase urine output. (Source 3)

• Official position, Certainty not rated.
• Size: Not applicable.
• Who: People with diabetes who fast.
• How long: Not applicable.
• Result: Not applicable.
• Funding: US government (NIDDK)

Limit of this finding: Blog interview with one clinician (Dr Martin Grajower) published by NIDDK in 2020; an expert view, not a guideline.

The dehydration could be due to not drinking or because some diabetes medicines, such as the SGLT-2 inhibitors and diuretic medicines, induce diuresis.

The same NIDDK interview states that sulfonylureas, short-acting meglitinides and insulin are the diabetes medicines associated with hypoglycaemia during fasting. (Source 3)

• Official position, Certainty not rated.
• Size: Not applicable.
• Who: People with diabetes who fast.
• How long: Not applicable.
• Result: Not applicable.
• Funding: US government (NIDDK)

Sulfonylureas, the short-acting meglitinides, and insulin, are associated with hypoglycemia.

A European expert-panel guideline on medical fasting lists blood pressure medicines, especially beta-blockers and diuretics, among medicines needing adjustment during fasting, and lists pregnancy and nursing, cachexia, eating disorders and advanced liver or kidney failure as contraindications. (Source 14)

• Official position, Certainty not rated.
• Size: Not applicable.
• Who: People undergoing medically supervised fasting (fasting therapy)
• How long: Not applicable.
• Result: Not applicable.
• Funding: not stated.

Limit of this finding: Held by a German-led expert panel on fasting therapy (Wilhelmi de Toledo et al.), 2013; applies to prolonged medical fasting, several authors are linked to fasting clinics.

Antihypertensives (especially beta-blockers and diuretics)

Where the research disagrees

Whether intermittent fasting worsens disordered eating

• Hoddy et al. 2015 (alternate-day fasting trial), cohort: Binge eating and depression decreased; purging and fear of fatness did not change. (Source 11)
• Ganson et al. 2022 (Canadian survey), cross-sectional: Intermittent fasting was associated with eating disorder psychopathology in young people. (Source 6)

Whether fasting leads to loss of lean mass

• Lowe et al. 2020 (TREAT trial), rct: Greater loss of appendicular lean mass with 16:8 time-restricted eating than with three meals a day. (Source 9)
• Ozcan et al. 2024 review, narrative-review: Unknown whether intermittent fasting preserves muscle; the general consensus is no harmful effect on lean mass in otherwise healthy people. (Source 4)

A common belief, and what the research shows

The belief: Fasting is just skipping food, so it is safe for anyone.

What the research shows: Studies report hypoglycaemia with sulfonylureas and insulin, ketoacidosis with SGLT2 inhibitors, lean mass loss, and associations with eating disorder symptoms; guidelines list pregnancy, eating disorders and some medicines as reasons for caution.

References

1. Diabetes Research and Clinical Practice; International Diabetes Federation (IDF) and Diabetes and Ramadan (DaR) International Alliance. Diabetes and Ramadan: Practical guidelines 2021. 2022. DOI 10.1016/j.diabres.2021.109185. Read the source
2. Journal of Medical Case Reports. Euglycemic diabetic ketoacidosis induced by sodium-glucose cotransporter 2 inhibitor in the setting of prolonged fasting: a case report. 2022. DOI 10.1186/s13256-022-03347-1. Read the source
3. National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), US National Institutes of Health. Fasting Safely with Diabetes (Diabetes Discoveries & Practice blog, interview with Dr. Martin M. Grajower). 2020. Read the source
4. Canadian Journal of Cardiology (Elsevier). Risks and Benefits of Intermittent Fasting for the Aging Cardiovascular System. 2024. DOI 10.1016/j.cjca.2024.02.004. Read the source
5. Human Reproduction Update (Oxford University Press). Ramadan during pregnancy and offspring health outcomes over the life course: a systematic review and meta-analysis. 2024. PMID 39178355, DOI 10.1093/humupd/dmae026. Read the source
6. Eating Behaviors (Elsevier). Intermittent fasting: Describing engagement and associations with eating disorder behaviors and psychopathology among Canadian adolescents and young adults. 2022. DOI 10.1016/j.eatbeh.2022.101681. Read the source
7. MedEdPORTAL (AAMC). Clinical Considerations While Providing Care for Patients During Ramadan: A Framework for Health Care Professionals. 2026. DOI 10.15766/mep_2374-8265.11614. Read the source
8. International Journal of Obesity (Nature). Risk of symptomatic gallstones and cholecystectomy after a very-low-calorie diet or low-calorie diet in a commercial weight loss program: 1-year matched cohort study. 2014. DOI 10.1038/ijo.2013.83. Read the source
9. JAMA Internal Medicine. Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical Trial. 2020. PMID 32986097, DOI 10.1001/jamainternmed.2020.4153. Read the source
10. Frontiers in Endocrinology. Impact of Ramadan Diurnal Intermittent Fasting on Hypoglycemic Events in Patients With Type 2 Diabetes: A Systematic Review of Randomized Controlled Trials and Observational Studies. 2021. PMID 33763029, DOI 10.3389/fendo.2021.624423. Read the source
11. Nutrition Journal. Safety of alternate day fasting and effect on disordered eating behaviors. 2015. DOI 10.1186/s12937-015-0029-9. Read the source
12. Journal of Clinical Rheumatology. The impact of Ramadan fast on patients with gout. 2014. PMID 25275760, DOI 10.1097/RHU.0000000000000172. Read the source
13. Breastfeeding Medicine. Effect of Ramadan Fasting on Breast Milk. 2023. DOI 10.1089/bfm.2023.0144. Read the source
14. Forschende Komplementärmedizin / Research in Complementary Medicine (Karger). Fasting Therapy - an Expert Panel Update of the 2002 Consensus Guidelines. 2013. PMID 24434758, DOI 10.1159/000357602. Read the source