Can Intermittent Fasting Cause Constipation?

Intermittent fasting can cause constipation in some people, particularly during the early weeks of a new eating pattern. Changes to meal timing, food volume and fluid intake can disrupt the digestive signals the gut relies on for regular bowel movements. However, constipation linked to intermittent fasting is often temporary and manageable with straightforward adjustments. This article explains why fasting slows digestion, who is most at risk, practical steps to relieve symptoms, and when it is important to speak to a GP.

Can intermittent fasting cause constipation?

Yes, intermittent fasting can cause constipation by disrupting meal timing, food volume and fluid intake, though it is often temporary and resolves with dietary adjustments.

Yes, intermittent fasting can cause constipation in some people, particularly during the early weeks of a new eating pattern. The NHS describes constipation as passing fewer than three stools per week, or experiencing stools that are hard, dry or difficult to pass.^[1] While intermittent fasting is not inherently harmful to bowel function, the changes it introduces to meal timing, food volume and fluid intake can disrupt the regularity that the digestive system relies on.

The relationship between fasting and constipation is not universal. Many people adopt intermittent fasting without any change to their bowel habits, while others notice a marked slowdown, especially in the first few weeks. The pattern of eating, the quality of food consumed during eating windows, and individual differences in gut motility all play a role in determining whether constipation develops.

Constipation during intermittent fasting is often temporary and manageable with straightforward dietary and lifestyle adjustments. Understanding why it occurs is the first step towards preventing or relieving it effectively.

Why does your digestive system slow down during fasting?

Fewer meals mean less frequent gastrocolic reflex stimulation and peristalsis, allowing stool to dry out in the colon; reduced fibre and fluid intake compound this effect.

Intermittent fasting can slow digestion primarily because reduced meal frequency means the gut receives fewer mechanical and hormonal signals to contract and move contents forward. The digestive system relies on a process called peristalsis, a series of rhythmic muscle contractions that propel food and waste through the intestines.^[13][18] Eating also triggers the gastrocolic reflex, a physiological response in which the colon contracts in response to food entering the stomach.^[13] When meals are compressed into a shorter window, these contractions occur less frequently, which can allow stool to remain in the colon for longer. As the colon progressively absorbs water from stool, it becomes harder and more difficult to pass.

Fibre intake is another key factor. People who restrict their eating to a narrow window sometimes consume fewer fruits, vegetables, wholegrains and legumes than they would across a full day of eating. Dietary fibre adds bulk to stool and draws water into the bowel, both of which support regular transit.^[2][18] A reduction in fibre, even unintentional, can quickly contribute to constipation.

Dehydration is also a significant contributor. During fasting periods, some people drink less fluid than usual, either because they associate eating and drinking or because appetite cues that normally prompt fluid intake are absent. When overall hydration is low, the colon absorbs more water from stool, producing drier, harder stools.

Finally, changes to the gut microbiome, the community of bacteria living in the large intestine, may play a role. Shifts in meal timing and food variety can alter microbial populations that influence bowel transit time, though research in this area is still developing and firm conclusions cannot yet be drawn.

Who is most likely to be affected?

People with IBS-C, hypothyroidism, older adults, women, and those on constipating medicines are most susceptible, as are those following very restrictive eating windows.

People who already have a tendency towards sluggish bowel function are most likely to experience constipation when they begin intermittent fasting. Those with a history of irritable bowel syndrome with constipation (IBS-C), hypothyroidism, or other conditions that affect gut motility may find that fasting exacerbates pre-existing symptoms. Similarly, individuals who take medicines known to slow bowel transit, such as certain opioid-based painkillers, iron supplements, some antidepressants or calcium channel blockers, may be at greater risk.^[12]

Older adults are also more susceptible, as gut motility naturally slows with age and fluid intake often decreases. Women are statistically more likely to experience constipation than men, and hormonal fluctuations across the menstrual cycle can further influence bowel regularity, meaning that fasting may interact with these changes in unpredictable ways.^[15]

Pregnant and postpartum women are another group at higher risk of constipation, and anyone who is pregnant or breastfeeding should seek advice from their GP or midwife before starting intermittent fasting, as fasting may not be appropriate during these periods.

People who adopt very restrictive eating windows, such as a one-meal-a-day approach, may be at higher risk than those following a more moderate pattern such as 16:8 (fasting for 16 hours and eating within an 8-hour window), simply because the opportunity to consume adequate fibre and fluid is more limited. Those who significantly reduce their overall calorie intake alongside fasting, rather than simply shifting when they eat, may also notice a greater impact on bowel function, as lower food volume means less material to stimulate the bowel.

People with a history of an eating disorder, or those taking medicines for diabetes that lower blood glucose (such as insulin or sulphonylureas), should speak to their GP or specialist before starting intermittent fasting, as fasting carries additional risks in these groups.^[16][17]

How can you relieve constipation while intermittent fasting?

Drinking six to eight glasses of fluid daily and prioritising high-fibre foods during eating windows are the most effective first steps; bulk-forming or osmotic laxatives can help short-term.

Increasing fluid intake is one of the most effective and immediate steps for relieving constipation during intermittent fasting. Drinking water, herbal teas or diluted squash throughout both fasting and eating periods helps keep stool soft and supports normal bowel transit. NHS guidance recommends around six to eight glasses of fluid per day, though individual needs vary with activity level and climate.^[3]

Prioritising dietary fibre during eating windows is equally important. Practical ways to increase fibre include:

• Wholegrains: choosing wholemeal bread, oats, brown rice or wholegrain pasta over refined alternatives

• Vegetables: aiming for a variety of colours and types, including leafy greens, root vegetables and cruciferous vegetables such as broccoli

• Legumes: incorporating lentils, chickpeas and beans into meals

• Fruit: eating whole fruit rather than juice to retain fibre content

• Seeds: adding ground flaxseed or chia seeds to yoghurt, porridge or smoothies

Physical activity also stimulates gut motility. Even a 20 to 30 minute walk after eating can help move contents through the bowel more efficiently. Using a small footstool to raise your feet while on the toilet can also help by improving the natural position for passing stool.

If dietary changes alone are insufficient, an over-the-counter bulk-forming laxative such as ispaghula husk (for example, Fybogel) can be used short-term and is generally well tolerated. It is important to take bulk-forming laxatives with a full glass of water and to maintain adequate fluid intake throughout the day, as taking them without enough fluid can worsen a blockage. Osmotic laxatives such as macrogol (for example, Movicol) are another option available without prescription.^[8] It is advisable to speak to a pharmacist before starting any laxative to ensure it is appropriate for your circumstances. Stimulant laxatives should generally be reserved for short-term use and on professional advice.

When should you speak to a GP about your symptoms?

See a GP if constipation lasts several weeks despite self-care, or if accompanied by blood in stool, black tarry stools, unexplained weight loss or severe abdominal pain.

Most cases of constipation related to intermittent fasting resolve with the dietary and lifestyle measures described above, but certain symptoms warrant a conversation with your GP. You should contact your GP if constipation lasts for several weeks and does not improve with self-care, or if it is accompanied by any of the following:

• Blood in your stool or on the toilet paper, which should always be assessed by a clinician

• Black, tarry stools (melaena), which may indicate bleeding higher in the digestive tract and require prompt assessment

• Unexplained weight loss that is not related to your intentional dietary changes

• Severe or persistent abdominal pain or bloating

• A noticeable change in your usual bowel habit that has lasted more than a few weeks, particularly if you are aged 60 or over

• Nausea or vomiting alongside constipation, especially if you are also unable to pass wind, as this may suggest a bowel obstruction

These symptoms can occasionally indicate an underlying condition that requires investigation, such as colorectal disease, thyroid dysfunction or another gastrointestinal disorder. NICE guidance on suspected cancer (NG12) sets out the criteria that GPs use when deciding whether to refer adults urgently for investigation of possible colorectal cancer, including combinations of symptoms such as rectal bleeding alongside a change in bowel habit. Your GP can assess whether further tests, such as blood tests or a referral to a gastroenterologist, are appropriate.

If you are unsure whether your symptoms need urgent attention, NHS 111 can provide guidance. Contact 999 or go to A&E only if you develop sudden, severe abdominal pain that is worsening rapidly, as this may indicate a more serious condition requiring emergency assessment. Suspected side effects from any medicine that may be contributing to constipation can be reported to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk.

Scientific References

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3. Water, drinks and hydration.
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5. How and when to take Fybogel.
6. Ispaghula husk | Drugs | BNF – NICE.
7. Fybogel Orange Granules – Summary of Product Characteristics.
8. Macrogol: a laxative medicine to treat constipation.
9. Suspected cancer: recognition and referral (NG12).
10. The Yellow Card scheme: guidance for healthcare professionals, patients and the public.
11. Adverse reactions to drugs | Medicines guidance – BNF – NICE.
12. Fentanyl | Drugs – BNF – NICE.
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16. Is Time-Restricted Eating Safe in the Treatment of Type 2 Diabetes? A Review of Intervention Studies. Nutrients..
17. Intermittent fasting and 'metabolic switch': Effects on metabolic syndrome, prediabetes and type 2 diabetes. Diabetes Obes Metab..
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