The dietary fiber content and composition of 25 foods were related to their glycemic index (GI) to see whether the individual components of dietary fiber, specifically soluble fiber, would provide a better predictive capability of the glycemic response than their total dietary fiber content. Total dietary fiber was significantly related to GI (r = 0.461, p less than 0.05). Soluble fiber was not significantly related to GI (r = 0.308), and uronic acids in insoluble fiber were most closely related to GI (r = 0.584, p less than 0.01). Multiple-regression analysis showed that more variation of GI was explained by uronic acids in insoluble fiber (34%) than by total dietary fiber alone (21%, p = 0.05). The combination of pentoses, hexoses, and uronic acids in soluble and insoluble fiber explained only 50% of GI variability. In determining the potential metabolic effects of diet, physiologic assessment of foods is a useful supplement to chemical analysis of their dietary fiber content and composition.