IBS-C (Constipation-Predominant): The Evidence-Based Supplement Protocol

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IBS-C combines the abdominal pain of IBS with a constipation-dominant bowel habit. Differs from chronic functional constipation in that pain and bloating dominate the symptom picture. Supplements that work for IBS-C may worsen IBS-D and vice versa — strain-level specificity matters.

Psyllium Husk, 10–20 g Daily

Psyllium is the only fiber with both Rome IV and ACG guideline support for IBS-C. A 2018 meta-analysis confirmed efficacy for stool frequency, consistency, and global symptom relief. Start at 5 g daily and ramp over 2 weeks to minimize the initial bloating phase. See psyllium piece.

Magnesium Oxide, 400–800 mg Daily

Osmotic laxative effect from poorly-absorbed magnesium. Effective alternative to PEG in IBS-C with the added benefit of magnesium repletion. Reasonable first or second-line. Avoid in advanced CKD. See magnesium forms piece.

Kiwifruit, 2 Green Kiwifruit Daily

A 2018 RCT showed 2 green kiwifruit daily was non-inferior to psyllium for IBS-C symptom improvement, with substantially better tolerability. Actinidin enzymes plus soluble fiber appear to drive the effect. Food, not supplement, but evidence-based and worth including.

Partially Hydrolysed Guar Gum (PHGG / Sunfiber), 5–10 g Daily

PHGG is gentler than psyllium with less bloating in sensitive guts. Particularly useful in IBS-C patients who don't tolerate psyllium ramping. See PHGG piece.

What NOT to Take

Avoid fermentable fibers (inulin, FOS, GOS) at high dose in IBS-C — they often worsen bloating without improving constipation. Skip "colon detox" stimulant laxative blends — tolerance and dependence risk. Avoid lactulose if FODMAP-sensitive — increases gas and bloating. Don't replace prescription prokinetics (linaclotide, prucalopride) in moderate-severe disease with supplements alone. Avoid IBS-D-targeted probiotics (B. infantis 35624 dominantly tested for IBS-D pain, less so IBS-C).

How to Run the Protocol

Low-FODMAP diet trial first (the strongest single dietary intervention in IBS). Layer psyllium 10 g daily ramping over 2 weeks. If inadequate, add magnesium oxide 400 mg at bedtime. Substitute PHGG for psyllium if tolerability is an issue. Re-evaluate at 8 weeks; if unresolved, consider linaclotide or plecanatide prescription therapy. See the general IBS page and the IBS-D protocol for the diarrhea-predominant subtype.