The effect of cellulose purified from wood pulp on wet and dry stool weights, gastrointestinal transit time (TT), frequency of defecation, and calcium and magnesium balances was tested. Seven healthy women consumed a low fiber diet of constant composition (percentage of total kcal: 23% protein, 30% fat, 47% carbohydrate) and the same metabolically controlled diet to which 16 g of refined cellulose (Solka Floc) was added. Each diet was consumed for approximately one month. The neutral detergent fiber contents of the two diets were 9.5 and 23.5 g, respectively. Cellulose consumption significantly increased mean daily wet stool weight from 74.6 +/- 23.4 (SD) to 130.5 +/- 29.4 g, mean daily dry fecal weight from 19.1 +/- 4.2 to 39.5 +/- 7.7 g, and frequency of defecatin from 0.85 +/- 0.2 to 1.10 +/- 0.29/day. Cellulose effectively shortened TT of two subjects with initially slow TT (7 days), but mean TT of all subjects was not significantly affected when fiber was added to the diet. Fecal excretions of calcium and magnesium were significantly greater when the diet containing cellulose was fed, and only calcium balance was significantly more negative. These results suggest that moderate levels of refined cellulose adversely increased fecal losses of calcium and magnesium. All other indices of bowel function remained within normal ranges.