Studies on the mechanism by which a high intake of soybean oil depresses the apparent digestibility of fibre in horses

An increased intake of soybean oil at the expense of an iso-energetic amount of non-structural carbohydrates reduces the apparent digestibility of fibre in horses. Literature data indicate that bile acids and linoleic acid (C18:2 n − 6) may inhibit growth of pure cultures of microorganisms. In the present series of experiments, the hypotheses tested were that after extra fat intake as soybean oil more bile acids and linoleic acid would enter the caecum which depresses microbial growth and thus also fibre fermentation. Based on measurements of faecal bile acid excretion in horses, no evidence was obtained for a higher influx of bile acids into the caecum after iso-energetic substitution of dietary soybean oil for starch plus glucose. When dietary palm oil was replaced by soybean oil, which caused a six-fold increase in linoleic acid intake, fibre digestibility in horses was not lowered. The infusion of linoleic acid into the caecum of fistulated ponies increased apparent fibre digestibility. It is concluded that the results of the three experiments disprove the hypotheses tested.