ISSA Nutritionist Practice Exam 2025 - Complete Preparation Guide

Gut bacteria primarily prefer fiber because it serves as a substrate for fermentation, a process by which these bacteria break down complex carbohydrates. Fiber, particularly soluble fiber found in foods such as fruits, vegetables, legumes, and whole grains, is not easily digestible by human enzymes but can be metabolized by gut bacteria. This fermentation process produces short-chain fatty acids and other beneficial compounds that can have positive effects on gut health and overall well-being.

While simple sugars, cellulose, and starch also play roles in digestion and nutrition, they do not serve the same purpose for gut bacteria as fiber. Simple sugars can be rapidly absorbed in the small intestine and thus may not reach the large intestine where gut bacteria are primarily located. Cellulose, although a type of fiber, is more resistant to fermentation compared to other soluble fibers and may not be as readily utilized by all gut bacteria. Starch can be digested by human enzymes before reaching the gut bacteria, making it less available for their fermentation compared to fiber. Therefore, fiber is uniquely suited to support the growth of beneficial gut bacteria and contribute to a healthy microbiome.