What dual components drive the celebrated digestive regularity effect promoted by prunes?

Dietary fiber and sorbitol

The promotion of healthy and predictable bowel movements associated with prune consumption results from a powerful synergistic action involving two distinct elements: dietary fiber and sorbitol. Dietary fiber, present in both soluble and insoluble forms, plays a critical role; the insoluble type physically adds bulk to the stool, which facilitates faster transit through the digestive tract, while the soluble type absorbs water to create a gel, softening the stool for easier passage. Complementing this mechanical action is sorbitol, which is a naturally occurring sugar alcohol that is poorly absorbed by the body. Because it remains largely unabsorbed, sorbitol functions osmotically, drawing substantial amounts of water into the large intestine. This influx of water lubricates the colon walls and provides a strong laxative encouragement, making the combination of these two factors highly effective for maintaining regularity.