Dietary fibre, bacterial metabolism and toxicity of nitrate in the rat.

1. A semi-synthetic diet, the semi-synthetic diet plus pectin, and a stock diet were fed to rats, and three metabolic functions of the caecal bacteria (reduction of amaranth, p-nitrobenzoic acid and nitrate) were measured in vitro. 2. No consistent differences were noted between diets for the reduction of amaranth and p-nitrobenzoic acid. 3. No consistent differences in nitrate reductase activity were noted for rats maintained on a stock diet or a fibre-free diet. However, the addition of 5% pectin to the latter diet resulted in a several-fold increase in nitrite production. 4. This increased nitrite production in vitro was associated with methaemoglobinaemia following the oral administration of nitrate to rats fed the diet containing pectin. Animals receiving the basal fibre-free diet were unaffected by nitrate.