Effects of oat fibre on nutrient absorption and sterol metabolism. Short-term studies in diabetic and ileostomy subjects
Oats are unique among the cereals due to its high content of mixed-linked (1-3),(1-4) ß-D-glucan (ß-glucan), a highly viscous dietary fibre component which may have several physiological effects.The aims of the thesis were to study the glycemic response and gastric emptying rate of an oat fibre test meal in diabetic subjects, to elucidate the mechanisms behind the serum cholesterol lowering effects of oat fibre, and to study the small bowel excretion of energy supplying nutrients as well as iron absorption from diets with oat bran, barley fibre and wheat bread.The specific effect of oat bran ß-glucan was studied by comparing the physiological effects of oat bran bread with added ß-glucanase with the effects of normal oat bran bread. Controlled diets were served in two-day periods to ileostomy subjects or as test meals with postprandial blood sampling. Ileostomy effluent was frequently sampled and immediately frozen. Radioisotope-technique was used for measurement of gastric emptying rate and of cholesterol and iron absorption.A portion of oat bran porridge (4 g ß-glucan) did not influence glycemic and insulinemic responses or gastric emptying rate compared to a low-fibre control meal in NIDDM subjects. Oat bran ß-glucan (8.7 g/d) increased 24-h bile acid excretion by 83%, and by 93% after the oat bran test meal (6 g ß-glucan), high in fat and cholesterol. Cholesterol absorption was unchanged while the excretion of endogenous cholesterol decreased after the oat fibre test meal. The changes in serum lathosterol concentration, reflecting cholesterol synthesis, correlated with changes in sterol excretion. Fat excretion increased and there seemed to be a delay in the lipid solubilization process observed as a reduced increase in serum chylomicron lipids. The oat ß-glucan did not influence small bowel nutrient excretion. Less than 50% of the energy excreted with the oat and barley diets was supplied by the fibre per se. Iron absorption was lower from the oat bran diets compared to a low-fibre, low-phytate wheat diet. The iron absorption was related to the phytate content of the oat, barley and wheat diets independent of the amount of ß-glucan.In conclusion, the main physiological effect of oat fibre is an increase in bile acid excretion mediated by the ß-glucan. This induces several changes in cholesterol metabolism which most probably explains the cholesterol lowering effect of oat fibre. Oat fibre does not seem to decrease nutrient absorption from the small intestine but may delay the lipid absorption process.
Göteborgs universitet/University of Gothenburg
Department of Clinical Nutrition
Avdelningen för klinisk näringslära
Date of defence