Constipation is often caused by insufficient fluid and fibre in the diet, resulting in low stool volume and inhibition of bowel evacuation due to the consequent lack of stimulus to peristalsis in the colon. Bulk-forming laxatives provide the closest approximation to the natural process of increasing faecal volume, and are the first-line recommendation for functional constipation.

They contain natural or semi-synthetic polysaccharides or cellulose derivatives that pass through the gastrointestinal tract undigested. Their main action is to add directly to the volume of the intestinal contents. They also add to faecal mass by acting as substrates for the growth of colonic bacteria.

Bulk-forming laxatives contain one of the following:

• Ispaghula husk, the seed coats of various species of plantago, a plantain
  Sterculia (also known as Indian tragacanth or karaya gum), from the tropical shrub Sterculia urens
• Methylcellulose, a semi-synthetic hydrophilic colloid.

All contain mucilloid constituents that bind water and swell in the colonic lumen to form a gel, thereby softening the faeces and increasing their bulk.