Intermittent Fasting and Gallstones: Risks

Intermittent fasting and gallstones are increasingly linked as more people adopt fasting-based eating patterns for weight management. While intermittent fasting does not inevitably cause gallstones, prolonged fasting periods and rapid weight loss can reduce gallbladder contractions, allowing bile to stagnate and cholesterol to crystallise. Understanding why this happens, who is most at risk, and how to reduce that risk allows most people to fast safely. This article explains the key mechanisms, warning signs to watch for, and practical steps to protect your gallbladder health during intermittent fasting.

Can intermittent fasting cause gallstones?

Intermittent fasting can increase gallstone risk by reducing gallbladder contractions during fasting periods, allowing bile to stagnate and cholesterol to crystallise, though it does not inevitably cause gallstones in everyone.

Intermittent fasting can increase the risk of gallstone formation, particularly when fasting periods are prolonged or weight loss occurs rapidly. Gallstones are solid deposits, most commonly made of cholesterol, that form inside the gallbladder. The gallbladder normally contracts and empties bile into the digestive tract when you eat fat, but during extended fasting periods this contraction happens less frequently. When bile sits stagnant in the gallbladder for longer than usual, cholesterol and other substances can crystallise and gradually form stones.

The link between fasting and gallstones is well established in the context of very low calorie diets (VLCDs) and prolonged fasting, where reduced frequency of gallbladder contractions promotes bile stasis. This is a recognised physiological mechanism described in NHS and NICE guidance on gallstone disease. Whether the same degree of risk applies to common intermittent fasting patterns such as 16:8 (sixteen hours fasting, eight hours eating) or 5:2 (five days of normal eating, two days of significant calorie restriction) is less certain, as most of the supporting evidence comes from VLCD and rapid weight-loss studies rather than from trials of these specific regimens. It is therefore more accurate to describe the risk associated with longer daily fasting windows and more severe calorie restriction as theoretically greater, rather than as a firmly established dose-response relationship.

Intermittent fasting does not inevitably cause gallstones, and many people follow these eating patterns without developing any gallbladder problems. The overall risk depends on a combination of individual factors including existing metabolic health, dietary composition during eating windows, and the rate of any associated weight loss. Understanding the mechanism helps explain why certain precautions, discussed later in this article, can meaningfully reduce risk without requiring people to abandon their chosen eating pattern entirely.

Why does rapid weight loss increase gallstone risk?

Rapid weight loss causes the liver to secrete excess cholesterol into bile while simultaneously reducing gallbladder motility, creating conditions that strongly favour gallstone formation.

Rapid weight loss increases gallstone risk primarily because it causes the liver to secrete more cholesterol into bile, which can overwhelm the bile's capacity to keep cholesterol dissolved.^[5] When the body breaks down fat stores quickly, excess cholesterol is released into the biliary system. At the same time, reduced food intake means the gallbladder contracts less often, allowing this cholesterol-rich bile to pool and eventually crystallise. This combination of supersaturated bile and reduced gallbladder motility creates conditions that favour stone formation.

Evidence from studies of VLCDs and bariatric surgery populations suggests that losing weight at a rate greater than approximately 1 to 1.5 kg per week is associated with a substantially higher risk of developing gallstones compared with slower, more gradual weight loss, though the precise threshold varies between studies. This is relevant to intermittent fasting because some protocols, particularly those involving very low calorie intake on fasting days, can produce rapid initial weight loss, especially in the first few weeks. The risk appears to be highest during the active weight-loss phase rather than during weight maintenance, based on data from these higher-intensity dietary interventions.

The composition of the diet during eating windows also matters. Diets very low in fat reduce the stimulus for gallbladder contraction even further, compounding the problem. Conversely, including a modest amount of healthy fat at each meal encourages the gallbladder to empty regularly, which helps prevent bile from becoming stagnant. This is one reason why extremely low-fat diets, sometimes combined with fasting, carry a particularly elevated theoretical gallstone risk. Gradual, steady weight loss supported by a nutritionally balanced diet remains the approach most consistent with minimising gallbladder complications, in line with NHS guidance on safe rates of weight loss.

Who is most at risk of developing gallstones while fasting?

People at greatest risk include women, those over 40, individuals with obesity, and anyone with a personal or family history of gallstones, type 2 diabetes, or previous rapid weight loss.

People with pre-existing risk factors for gallstones face the greatest likelihood of developing them during intermittent fasting. Well-established epidemiological risk factors include being female, being over the age of forty, living with obesity, and having been pregnant. Women, particularly those who have been pregnant or who use oestrogen-containing contraception or hormone therapy, have higher baseline rates of gallstone formation because oestrogen increases cholesterol secretion into bile.^[6][7] People with obesity or a history of significant weight fluctuation are also at elevated risk, as recognised in NICE guidance on gallstone disease.

Additional risk factors include:

• Type 2 diabetes or insulin resistance, which alters bile composition and gallbladder motility

• A personal or family history of gallstones, which significantly raises individual susceptibility

• Rapid previous weight loss, for example following bariatric surgery or very low calorie diets

• Crohn's disease or other conditions affecting the terminal ileum, which impairs bile salt reabsorption^[20]

• Haemolytic anaemias, which increase pigment stone formation

• Certain medications, including fibrates used to lower triglycerides and some hormone therapies

This list reflects commonly cited risk factors but is not exhaustive. People who are already living with overweight or obesity and who begin intermittent fasting with the aim of losing weight quickly may face a compounded risk: their baseline risk is higher, and the rate of weight loss may be faster in the early weeks. This does not mean fasting is contraindicated for these groups, but it does mean that a conversation with a GP or dietitian before starting is particularly worthwhile. Tailoring the fasting protocol to encourage slower, steadier weight loss can substantially reduce the risk of gallbladder complications.

What are the warning signs of gallstones you should not ignore?

The key warning sign is severe upper right abdominal pain (biliary colic); fever, jaundice, dark urine, or pale stools alongside pain require emergency assessment immediately.

The most characteristic warning sign of gallstones is biliary colic, a severe, cramping pain in the upper right abdomen or the centre of the abdomen, often radiating to the right shoulder or between the shoulder blades. This pain typically comes on suddenly, often after a fatty meal, and can last from around thirty minutes to several hours before easing, as described in NHS patient information on gallstones. It is frequently accompanied by nausea and vomiting.

Not all gallstones cause symptoms. A significant proportion of people have what are called silent gallstones, discovered incidentally on an ultrasound performed for another reason. However, once symptoms begin, they tend to recur and can progress to more serious complications if left unaddressed. Warning signs that require prompt medical attention include:

• Persistent or worsening upper abdominal pain lasting more than a few hours

• Fever and chills alongside abdominal pain, which may indicate infection of the bile duct (cholangitis) or gallbladder (cholecystitis)

• Yellowing of the skin or whites of the eyes (jaundice), which suggests a stone may be blocking the bile duct

• Dark urine and pale stools, which can accompany jaundice

• Rapid heart rate combined with severe abdominal pain, which warrants urgent assessment

The combination of fever, jaundice, and severe upper abdominal pain is a recognised pattern of serious bile duct infection and requires emergency assessment.^[1][9] If you develop any of these features, go to your nearest accident and emergency department or call 999 immediately. Milder or intermittent symptoms should be discussed with your GP without delay.

How to reduce your gallstone risk during intermittent fasting

Including a source of healthy fat at every meal, aiming for gradual weight loss of no more than 0.5 to 1 kg per week, and eating regular meals during eating windows all meaningfully reduce gallstone risk.

Reducing gallstone risk during intermittent fasting is achievable through a combination of dietary choices, pacing of weight loss, and attention to gallbladder health. The most important practical step is to include a source of healthy fat at each meal during your eating window. Even a small amount of fat, such as olive oil, nuts, avocado or oily fish, stimulates the gallbladder to contract and empty, which prevents bile from becoming stagnant. Completely fat-free meals during eating windows remove this protective stimulus and should be avoided.

Aiming for a gradual rate of weight loss, ideally no more than 0.5 to 1 kg per week in line with NHS guidance on safe weight loss, reduces the surge of cholesterol into bile that accompanies rapid fat breakdown. If you are following a 5:2 or similar protocol, ensuring that fasting days still include some dietary fat, even in small quantities, can help maintain gallbladder motility. Staying well hydrated throughout the day, including during fasting periods, supports overall digestive function.

Other evidence-informed strategies include:

• Eating regular meals during your eating window rather than one very large meal, to encourage more frequent gallbladder contractions

• Including fibre-rich foods such as vegetables, legumes and wholegrains, which help regulate cholesterol metabolism

• Limiting refined carbohydrates and added sugars, which can raise triglyceride levels and alter bile composition

• Maintaining physical activity, which supports healthy weight loss and may improve gallbladder motility

Certain groups should seek medical advice before starting any fasting regimen. These include people with diabetes who take insulin or sulfonylureas (where fasting carries a risk of hypoglycaemia), and people who are pregnant or breastfeeding.^[15][16] If you have known risk factors for gallstones, speaking with your GP or a registered dietitian before starting intermittent fasting allows you to tailor your approach safely. Bolt Pharmacy is a GPhC-registered UK online pharmacy offering online weight-management consultations, with prescriber review and pharmacist support, where clinically appropriate options can be discussed.

When should you see a GP about gallstone symptoms?

See your GP promptly for recurring upper abdominal pain after eating; a GP can arrange an abdominal ultrasound and blood tests in line with NICE CG188 to confirm or exclude gallstone disease.

You should see your GP if you experience recurring upper abdominal pain, particularly after eating, that you suspect may be related to your gallbladder. In line with NICE guidance (CG188: Gallstone disease), a GP can arrange an abdominal ultrasound, which is the standard first-line investigation for suspected gallstones, and can assess whether your symptoms are consistent with biliary colic or another cause.^[2][3] Blood tests including liver function tests and inflammatory markers may also be requested to check for complications such as bile duct obstruction or infection.

If your GP confirms gallstones, management will depend on whether the stones are causing symptoms and whether complications are present. Asymptomatic gallstones often require no immediate treatment, but your GP will advise on monitoring and lifestyle measures. Symptomatic gallstones are typically referred to a surgical team for consideration of laparoscopic cholecystectomy (keyhole removal of the gallbladder), which is the definitive treatment as set out in NICE CG188. Ursodeoxycholic acid, a bile acid medication, is occasionally used in selected patients who are not suitable for surgery, though its use is limited to specific circumstances and it is less commonly prescribed than surgery.^[12][13]

For urgent symptoms, the correct escalation route depends on severity. If you develop fever, jaundice, or severe unrelenting pain, go to accident and emergency or call 999 immediately. For symptoms that are concerning but not immediately life-threatening, such as a new episode of significant abdominal pain without fever or jaundice, contact NHS 111 for advice on the appropriate next step. Routine or recurring mild symptoms should be discussed with your GP at the earliest convenient appointment. If you suspect that a medication you are taking alongside your fasting regimen has caused a side effect, you can report this to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk.

Scientific References

1. Gallstones.
2. Gallstone disease: diagnosis and management (CG188).
3. Gallstone disease: diagnosis and management (CG188) – Chapter 1 Recommendations.
4. Gallstone disease: Appendix A – Summary of evidence from surveillance.
5. Gallstones in obesity and weight loss.
6. New insights into the molecular mechanisms underlying effects of estrogen on cholesterol gallstone formation.
7. Gender and gallstone disease.
8. Role of fibrates and HMG-CoA reductase inhibitors in gallstone formation.
9. Acute cholecystitis.
10. Tips to help you lose weight.
11. Overweight and obesity in adults.
12. Gallstones – Treatment summaries.
13. Ursodeoxycholic Acid 250 mg hard capsule – Patient Information Leaflet.
14. Gallstones – approach to medical management.
15. Is Time-Restricted Eating Safe in the Treatment of Type 2 Diabetes? A Review of Intervention Studies.
16. Intermittent fasting and 'metabolic switch': Effects on metabolic syndrome, prediabetes and type 2 diabetes.
17. Lack of endogenous cholecystokinin promotes cholelithogenesis in mice.
18. Review article: in vitro studies of gall-bladder smooth muscle function. Relevance in cholesterol gallstone disease.
19. Gallbladder motility and cholesterol crystallization in bile from patients with pigment and cholesterol gallstones.
20. Crohn's disease.
21. Insulin resistance causes human gallbladder dysmotility.

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