Fasting Tracker

Quick answers

Constipation while fasting: what helps

Fewer meals mean less to pass. If it becomes uncomfortable, fibre, water and movement usually help

Why it happens

Your gut handles what you give it. Fewer meals often mean less food and less fibre passing through, so you may go less often. That's expected. Constipation is something else: fewer than three bowel movements a week, or stools that are hard, dry, lumpy or painful to pass, or a feeling that you haven't finished 3.

How common is it? In an 8-week trial of alternate-day fasting, 17% of people reported constipation, but the same share had reported it before they started 4. So fasting doesn't always cause it. A short eating window can make it easier to fall short on fibre and fluids, though, and that's what to fix.

MYTH
You need a bowel movement every day.
FACT
Normal varies widely from person to person. Doctors usually call it constipation when you go fewer than three times a week or stools are hard and difficult to pass, not because you skip a day 3.

What helps

Fibre is the best-studied fix. A review of 16 randomized trials in adults with chronic constipation found that 66% of people improved with fibre supplements, compared with 41% with a placebo. Psyllium worked best, at doses of more than 10 g a day for at least 4 weeks. The catch: more wind 1.

Key study

Water made fibre work better

  • Randomized trial
  • 117 adults with constipation
  • 2 months

Everyone ate about 25 g of fibre a day. Those asked to drink 2 litres of water a day had more frequent bowel movements and used fewer laxatives than those who drank as they liked, about 1.1 litres.

Anti et al., Hepatogastroenterology 1998

Keeping things moving

  1. Build every meal in your window around vegetables, fruit, legumes, whole grains, nuts or seeds.
  2. Add fibre gradually over a week or two, so your gut can adjust.
  3. Drink steadily through the day and the fast: water, plain tea and black coffee all count 5.
  4. Move every day. A walk after your first meal is a good habit.
  5. Don't ignore the urge to go, and give yourself time in the morning.
  6. If you're on a very short window, such as 20:4 or one meal a day, try a longer one for a week.

Our guide to your gut while you fast explains how fasting affects your gut, and what to eat during intermittent fasting gives meal ideas. The water intake calculator suggests a daily target.

When to see a doctor

See a doctor if constipation doesn't improve with these changes, or if you have a family history of bowel or rectal cancer 3. Ask before using laxatives regularly, and before stopping any medication you suspect.

  • If you have a digestive condition such as IBS or inflammatory bowel disease, talk to your doctor before changing how you eat.
  • Fibre supplements need plenty of water. If you have trouble swallowing, ask a doctor or pharmacist first.

Sources

  1. van der Schoot A, Drysdale C, Whelan K, et al. The Effect of Fiber Supplementation on Chronic Constipation in Adults: An Updated Systematic Review and Meta-Analysis of Randomized Controlled Trials. Am J Clin Nutr. 2022;116(4):953-969. doi.org/10.1093/ajcn/nqac184
  2. Anti M, Pignataro G, Armuzzi A, et al. Water supplementation enhances the effect of high-fiber diet on stool frequency and laxative consumption in adult patients with functional constipation. Hepatogastroenterology. 1998;45(21):727-32. pubmed.ncbi.nlm.nih.gov/9684123/
  3. National Institute of Diabetes and Digestive and Kidney Diseases. Symptoms & Causes of Constipation. U.S. National Institutes of Health. www.niddk.nih.gov/health-information/digestive-diseases/constipation/symptoms-causes
  4. Hoddy KK, Kroeger CM, Trepanowski JF, et al. Safety of alternate day fasting and effect on disordered eating behaviors. Nutr J. 2015;14:44. doi.org/10.1186/s12937-015-0029-9
  5. Killer SC, Blannin AK, Jeukendrup AE. No evidence of dehydration with moderate daily coffee intake: a counterbalanced cross-over study in a free-living population. PLoS One. 2014;9(1):e84154. doi.org/10.1371/journal.pone.0084154

A note on health

Fasting Tracker is designed for general wellness and is not a medical device. Fasting is not right for everyone. If you are pregnant, diabetic, have a history of eating disorders or take medication, talk to your doctor first, and before any extended fast.