Best Probiotics and Prebiotics for Constipation: Evidence-Graded 2026 Guide
Updated April 2026 · Sources: Cochrane, NICE, NIH ODS, Monash University
Understanding Constipation
Constipation is broadly defined as fewer than 3 bowel movements per week, with stools that are hard, dry, or difficult to pass. The Rome IV diagnostic criteria for functional constipation require 2 or more of the following for at least 3 months: straining during 25%+ of defaecations, lumpy/hard stools (BSS types 1-2) in 25%+ of defaecations, sensation of incomplete evacuation, sensation of anorectal obstruction, or manual manoeuvres to facilitate defaecation, and fewer than 3 spontaneous bowel movements per week.
Constipation is extremely common: approximately 16% of adults globally meet diagnostic criteria, rising to 33% in adults over 60. The gut microbiome is directly implicated: constipated individuals typically have reduced Bifidobacterium and Lactobacillus counts, increased methane-producing archaea (which slow colonic transit), and reduced short-chain fatty acid production (particularly butyrate, which stimulates peristalsis).
For Bristol Stool Scale type reference and identification of your constipation pattern, see bristolstoolchart.com.
First-Line Interventions Before Supplements
Before reaching for any supplement, evidence-based first-line interventions are:
- Hydration: 1.5-2 litres water daily. Dehydration is a primary driver of hard stools (BSS types 1-2).
- Dietary fibre: 25-30 g total dietary fibre daily. UK average is 18 g. Prioritise whole grains, legumes, and vegetables.
- Physical activity: Regular exercise (even 30 min daily walking) significantly improves colonic transit time.
- Establish regular bowel routine: Respond to the urge to defaecate promptly; attempt to use the toilet at the same time each morning.
- Elevated footrest during defaecation (squatting position): Strong evidence for reducing straining by straightening the anorectal angle.
Evidence-Graded Recommendations
Psyllium Husk (Metamucil, Konsyl) - Prebiotic + Bulk-Forming
Strong EvidencePsyllium is the most evidence-backed supplement for constipation and has the unusual dual property of being both a prebiotic fibre (feeds Bifidobacterium) and a bulk-forming agent (physically draws water into the stool). Multiple RCTs and a 2014 Cochrane review (Ford et al.) confirm that psyllium significantly increases stool frequency and softens stool consistency in functional constipation. NICE guidelines include psyllium as a first-line recommendation.
Cite: Ford et al., Am J Gastroenterol 2014; NICE IBS/Constipation guidance 2017
B. lactis BB-12 (Danisco/IFF) - Probiotic
Strong EvidenceB. lactis BB-12 is the most commercially distributed Bifidobacterium strain and has the strongest probiotic-specific evidence for constipation. Multiple RCTs in adults, the elderly, and pregnant women show significant improvements in stool frequency and consistency. Mechanisms include stimulation of intestinal motility via serotonin (5-HT) pathways and increased production of butyrate, which stimulates peristalsis. Commonly found in Activia yogurt and many supplement products.
Cite: Guyonnet et al., Aliment Pharmacol Ther 2007; Ojetti et al., Eur J Gastroenterol 2010
L. reuteri DSM 17938 (BioGaia) - Particularly Paediatric
Strong EvidenceL. reuteri DSM 17938 (BioGaia Protectis) has its strongest constipation evidence in infants and children, where it has been shown to increase bowel movement frequency and reduce the need for laxatives. For adult constipation, evidence is emerging rather than strong. A 2018 meta-analysis (Machado et al., World J Gastroenterol) confirmed effect in paediatric functional constipation.
Cite: Machado et al., World J Gastroenterol 2018; Guerra et al., J Pediatr Gastroenterol 2011
Kiwifruit (2 medium kiwis/day) - Dietary
Emerging EvidenceA notable emerging non-supplement option: two medium green kiwifruit per day. A 2020 RCT (Gearry et al., Am J Gastroenterol) in 184 constipated adults found that daily kiwifruit consumption significantly increased stool frequency, softened stools, and improved straining compared to psyllium. The mechanism involves both fibre (pectin) and actinidin enzyme, which promotes gastric emptying.
Cite: Gearry et al., Am J Gastroenterol 2020
Prebiotic Fibres for Constipation
Beyond psyllium, other prebiotic fibres can improve constipation by increasing stool bulk, feeding beneficial bacteria, and stimulating colonic peristalsis through SCFA production:
- Beta-glucan (oats, barley): Strong Evidence for stool regularity. 3-6 g/day from oat/barley sources or supplements.
- Resistant starch (cooked and cooled potato/rice): Emerging Evidence for colonic transit improvement and SCFA production.
- Inulin and FOS: Emerging Evidence for constipation, but high-FODMAP - may worsen bloating before improving transit.
- Acacia fibre: Emerging Evidence. Low-FODMAP, gentle, well-tolerated. Good option if inulin/FOS cause bloating.
When to See a Clinician
Most constipation responds to dietary and lifestyle changes within 2-4 weeks. Seek medical assessment if:
- Constipation is new onset and persistent (over 6 weeks) without dietary explanation
- Blood in stool or on toilet paper (must be excluded as a symptom of colorectal cancer or haemorrhoids)
- Significant unintentional weight loss
- Nocturnal symptoms waking you from sleep
- Alternating constipation and diarrhoea (IBS pattern - may need Rome IV assessment)
- No response to first-line dietary changes after 4-6 weeks