1. Various methods of estimating intestinal mass and cell proliferation were used to compare the effects of a pelleted laboratory diet (diet A), an elemental diet (Flexical; diet B), Flexical plus kaolin (diet C) and Flexical plus cellulose (diet D) on the mucosa and other tissues of the mouse small intestine and colon.
2. The weight of the distal third of the small intestine was significantly decreased in mice given diets B, C and D (P < 0·001). The length of the colon was significantly decreased in the mice given diets B (P < 0·001), C (P < 0·01) and D (P < 0·05); however, the weight of the colon was only decreased in the mice given diet B or diet C (P < 0·001).
Similar changes were noted in the weights of the intestinal muscle and serosa layers; however, no such changes were noted in the weight or DNA content of the mucosa.
3. No changes were observed in the crypt cell production rate in the small intestine, apart from an increase in the mid-region (P < 0·01) of mice given diet C, which was associated with a similar increase in the mucosal content of DNA.
Crypt-cell production was significantly decreased (P < 0·01, 0·001) in the two sites of the colon studied in the mice given diets B or C, but the crypt-cell production rates were not significantly different from control levels at either site in the mice given diet D.
4. The length of the colonic crypts was significantly decreased (P < 0·05, 0·02) in mice given diets B and C, but not in those given diet D; total cell number showed a similar change (P < 0·01, 0·05).
The correlation coefficient between the length of the colonic crypts and absolute cell number was 0·513.
5. The conclusion of the present study was that dietary fibre (cellulose), but not inert bulk (kaolin), prevents mucosal atrophy of the colon of mice given a fibre-free diet, and that many of the observed changes in intestinal weight are due to reduction in the bulk of the muscle layer.