Quantitative aspects of phytooestrogen metabolism in sheep fed on subterranean clover (Trifolium subterraneum cultivar clare) or red clover (Trifolium pratense)

Abstract

Studies have been made of the digestion and metabolism in Merino wethers of the isoflavones in subterranean clover (Trtfolium subterraneurn cv. Clare) and red clover (T. pratense). The dietary intake of isoflavones with both clovers was about 9 g per day. With the subterranean clover, the isoflavones were predominantly genistein and biochanin A, and slight teat length increases in the wethers ingesting this clover indicated a low level of oestrogenicity. With the red clover formononetin represented 60% of the isoflavone present and the wethers on this diet exhibited maximal teat length increases indicating a high level of oestrogenicity. Less than 1 % of the daily intake of the isoflavones was excreted as such in the faeces and urine; hence most of these compounds were metabolized or retained in the sheep. The dietary isoflavones were found to disappear rapidly from the rumen, and it was estimated by using marker techniques that the removal of these compounds from the stomach (rumen, reticulum, omasum, and abomasum) was virtually complete. Equol (7,4'-dihydroxyisoflavan), a metabolite of formononetin, was the predominant phyto-oestrogen in the digesta and excreta when red clover was given. The excretion of 3.9 g/day of this compound, mainly in urine, was equivalent to 70% of the intake of formononetin. It was calculated that about 86% of the equol produced in the rumen was absorbed from that organ; the mean residence time for equol in the rumen was estimated to be 1.7 hr. The isoflavones were present in blood plasma mainly in conjugated forms. Equol predominated with both clovers. The levels of equol were much lower with the subterranean clover than with the red clover diet; the concentrations of the conjugated form were respectively 13 and 300-440 ¦g/100 ml. Equol in the free form, although not detectable with the subterranean clover, was present at 4-10 ¦g/100 ml with red clover. The data were considered to be consistent with the conclusion that equol accounts for most of the phyto-oestrogenic activity in sheep fed on clovers containing high levels of formononetin.