What characterizes insoluble fiber?

Insoluble fiber is characterized by its inability to dissolve in water and its resistance to fermentation by bacteria in the gastrointestinal tract. This means that it passes through the digestive system largely intact, contributing to bulk and aiding in the regularity of bowel movements. Because it's not broken down by bacteria, insoluble fiber plays a crucial role in digestive health by helping to move waste through the intestines effectively.

The other characteristics mentioned in the options, such as dissolving in water or being broken down by bacteria, pertain to soluble fiber instead. Soluble fiber does dissolve in water and may be fermented by gut bacteria, producing beneficial short-chain fatty acids. While insoluble fiber can promote satiety indirectly by adding bulk to the diet and improving digestive health, it does not directly break down in the GI tract, distinguishing it from soluble fiber.