Not ALL fibres are bulk-forming laxatives

There is confusion. A fibre confusion.

Question, which fibre supplement is clinically indicated to treat constipation?

  • A) Psyllium
  • (Metamucil®)
  • B) Wheat dextrin (Benefiber®)
  • C) Wheat bran⁣ D) A,B,C⁣ ⁣

Hint: the answer isn’t D).⁣

Confused? Don’t worry, I know why. ⁣ In the AMH (Australian Medicine Handbook) under ‘Bulk-forming laxatives,’ it writes, ‘agents include ispaghula husk (Fybogel®), psyllium (Metamucil®) and sterculia (Normacol Plus®).’ From this it would be easy to conclude that ‘Bulk-forming laxatives’ is just a fancy word for fibre supplement.

The AMH also writes that ‘adequate dietary fibre intake’ is one of the usual first steps to treat constipation.

Similarly the APF (Australian Pharmaceutical Formulary) under constipation it writes: ‘Advise on dietary and lifestyle modifications (e.g. increased fibre and fluid intake, exercise) if there is no underlying cause.’

No wonder you have been confused. Not all fibres are the same, yet we use the word as if they were.

Fibre can be water INSOLUBLE (e.g. wheat, vegetables, legumes, wholegrain rye meal, brown rice and seeds) or water SOLUBLE (e.g. psyllium, ispaghula, vegetables, sterculia and fruit skins) and these can also be FERMENTING or NON-fermenting.

Wheat bran is an insoluble fibre that is not fermented. Its laxative effect is due to mechanical irritation of the mucosa, causing secretion of water and mucous, leading to bulky/soft/easy-to-pass stools. Because of its mechanical mechanism of action the size of the wheat bran particle matters. Large/coarse particles can have a profound laxative effect, but small/smooth particles have no effect (and can even be constipating).

Wheat dextrin (Benefiber®), is a soluble fibre. However, it is fermented (not present in stool throughout the large bowel) and has no water-holding capacity and thus do not provide a laxative benefit at its recommended dose.

Psyllium is a soluble fibre that is not fermented. So it retains its waterholding gelled structure throughout the large bowel. This gel allows psyllium to softens hard stool in constipation

TLDR: Only soluble non fermenting, gel-forming fibre (i.e. a bulk-forming laxative) has clinical indication in treating constipation.

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There is confusion. A fibre confusion. Question, which fibre supplement is clinically indicated to treat…

Posted by Memorise Medicine on Monday, 13 April 2020