Modulation of the biological effects of glucosinolates by inulin and oat fibre in gnotobiotic rats inoculated with a human whole faecal flora.

The influence of dietary fibre on the biological effects of glucosinolates was investigated in gnotobiotic rats harbouring a human whole faecal flora. Animals were fed for 6 wk with diets containing 12% rapeseed meal (RSM) supplemented or not supplemented with 10% inulin (INL) or oat fibre. Both fibre types enhanced the liver hypertrophy due to RSM to equal extents, but had different effects on the other glucosinolate-related toxic effects. INL partially restored a normal thyroid hormone status whereas kidney weight, goitre and growth deficit were increased on exposure to the diet containing oat fibre. Oat fibre and, to a lesser extent, INL modulated the alterations of digestive xenobiotic-metabolizing enzymes (XME) induced by RSM. They counter-balanced the induction of hepatic cytochrome P-450 and lessened the induction of uridine diphosphate-glucuronosyltransferase in the liver but did not modify depletion of its activity in the small intestine. On the other hand, they enhanced the induction of glutathione S-transferase in the liver and the large intestine but not in the small intestine. These findings give new evidence that the biological effects of naturally occurring non-nutrient compounds are closely dependent on the composition of the diet. Two mechanisms are proposed to explain the different influence of INL and oat fibre on RSM toxicity. Their different fermentative characteristics could lead to a modulation of the bacterial metabolism of glucosinolates in the caecum. Alternatively, their own action on the digestive XME could modify the subsequent metabolism of bacterial glucosinolate derivatives.