Moderate evidenceGut health

Galactooligosaccharides (GOS)

A prebiotic fiber that reliably increases bifidobacteria in the colon. Whether that translates into better symptoms, immunity or wellbeing is much less certain.

Evidence last reviewed: August 2026

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What the science says

Increase in bifidobacteria Moderate evidence
A randomized trial in prefrail older and healthy adults found that GOS clearly increased fecal bifidobacteria. The bifidogenic effect is one of the most reproducible findings in the prebiotic literature.
Immune function Limited evidence
In the same randomized trial, the rise in bifidobacteria was not accompanied by any measurable change in immune function or oxidative stress markers.
Bowel symptoms and IBS Limited evidence
A meta-analysis of randomized trials of prebiotics in IBS and functional bowel disorders found no consistent improvement in overall symptoms, and a separate trial showed that adding β-GOS to a low-FODMAP diet improved symptoms without restoring fecal bifidobacteria.

Dosage & safety

Studied doseTrials typically used 3.6–5.5 g per day of GOS powder, often split across meals.

SafetyDose-dependent gas, bloating and flatulence, particularly above 5 g per day. High-FODMAP by definition, so usually avoided by people following a low-FODMAP diet for IBS. Most commercial GOS is made from lactose and may contain traces of milk.

Scientific references

Galacto-oligosaccharides supplementation in prefrail older and healthy adults increased faecal bifidobacteria, but did not impact immune function and oxidative stress
Wilms E, An R, Smolinska A, Stevens Y, Weseler AR, Elizalde M, Drittij MJ, Ioannou A, van Schooten FJ, Smidt H, Masclee AAM, Zoetendal EG, Jonkers DMAE — Clin Nutr, 2021
Prebiotics in irritable bowel syndrome and other functional bowel disorders in adults: a systematic review and meta-analysis of randomized controlled trials
Wilson B, Rossi M, Dimidi E, Whelan K — Am J Clin Nutr, 2019
β-Galactooligosaccharide in Conjunction With Low FODMAP Diet Improves Irritable Bowel Syndrome Symptoms but Reduces Fecal Bifidobacteria
Wilson B, Rossi M, Kanno T, Parkes GC, Anderson S, Mason AJ, Irving PM, Lomer MC, Whelan K — Am J Gastroenterol, 2020

Where to buy

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