The effects of dietary starch and fibre on thein vitrorates of lipolysis and hydrogenation by sheep rumen digesta

Abstract

The objective of this work was to investigate the effects of changing proportions of readily fermentable carbohydrate and fibre in the diet on the rates of lipolysis and fatty acid hydrogenation by sheep rumen digesta. Three experiments were carried out using rumen-fistulated Romney wethers. In the first experiment rumen digesta from one sheep on a high-fibre diet was incubated in vitro with [l-14 C]linoleic acid and 0, 0-1, 0-2 and 0-5% sucrose. It was found that sucrose increased the rate of hydrogenation of linoleic acid by up to 40%.In the second experiment five rumen-fistulated Romney wethers were fed five diets according to a latin-square experimental design. These diets had decreasing proportions of fibre (42-8-19-5%) and increasing proportions of starch (12-2-35-7%). Lipolysis and hydrogenation rates decreased significantly with decreasing fibre content which was reflected in the amounts of stearic and octadecenoic acids in the rumen digesta.In the third experiment sheep were fed the high-fibre diet used in the first experiment and the rumen digesta incubated in the presence of 0,0-15, 0-45, 0-75 and 1-50% starch. Lipolysis rates increased but no effect of added starch on hydrogenation was observed.It was concluded that the short-term addition of energy to the rumen digesta increases lipolysis and hydrogenation rates. However, long-term replacement of fibre by starch results in a reduction of these rates.

Influence of pH on lipolysis and biohydrogenation of soybean oil by rumen contents in vitro

“…Demeyer et al (1972) found that the administration of sucrose (80 g twice daily) into the rumen of a fistulated sheep resulted in lower amounts of free fatty acids liberated from linseed oil in in vitro incubations with the rumen fluid. A similar observation was made by Gerson et al (1985), who found that the in vitro rates of lipolysis and biohydrogenation were reduced when a portion of fibre in the diet of sheep was replaced by starch. More recently, the inhibitory effect of starch supplementation on lipolysis was confirmed in experiments using the rumen simulation technique (RUSITEC) (Abel et al, 1990).…”

Effect of silage type and concentrate level on conjugated linoleic acids, trans-C18:1 isomers and fat content in milk from dairy cows

“…The fact that the concentrate level only affected trans10,cis12-CLA and trans10-C18:1 but also the cis9,trans11-CLA and trans11-C18:1 concentration in milk when maize silage was fed, suggests that increased proportions of concentrate (starch) in the diet does not severely affect the ruminal biohydrogenation pattern unless combined with a forage naturally high in starch. An increased ratio of starch to fibre in the diet has been shown to impair the rate of lipolysis of dietary lipids in vitro [17], and is an important factor controlling the formation of individual biohydrogenation intermediates in the rumen [9]. In this experiment, the daily intake of starch accounted for 67 and 54% of the variation in milk fat trans10,cis12-CLA and trans10-C18:1, respectively (Figs.…”

Targets and procedures for altering ruminant meat and milk lipids

“…The inhibition of lipolysis and/or biohydrogenation by low pH may explain the increased degree of unsaturation in rumen and duodenal lipids (Kobayashi et al 1992) and/or in body fat in animals fed on concentrate diets or treated with monensin. As shown by Gerson et al (1985), however, other factors in addition to pH, probably related to changes in microbial populations, must be involved with concentrate diets. Biohydrogenation of polyunsaturated C 20 and C 22 FA from fish oil has been shown in vivo by Doreau & Chilliard (1997a) and in vitro by Van Nevel et al (1999).…”