Free and conjugated isoflavones in the plasma of sheep following ingestion of oestrogenic clover

The plasma of sheep grazing various species of oestrogenic clover contained water-soluble conjugates of daidzein, formononetin, genistein, and biochanin A, chiefly in the form of glucuronides, in concentrations up to 40 µg/100 ml. In sheep pen-fed on red clover or subterranean clover (cv. Dinninup and Yarloop) the total concentration of these four isoflavones 3–5 hr after feeding ranged from 150 to 720 µg/100 ml plasma, of which 1–2% only was present in "free" (i.e. unconjugated) form; by 24 hr the plasma level had fallen to < 5 µg/100 ml plasma. Preconditioning the pen-fed sheep to oestrogenic clover, or adding lucerne to the diet, had no significant effect on the concentration of "free" or conjugated isoflavones in the plasma and had no inhibitory effect on the uterine weight response. Comparison of the composition of circulating isoflavones to that of the clover ingested indicated partial demethylation of formononetin to daidzein and of biochanin A to genistein, and suggested that genistein and biochanin A were degraded more rapidly than formononetin or daidzein in the sheep. Equol, a metabolite previously isolated from the urine of sheep treated with formononetin, was present in the plasma at a concentration of 50–170 µg/100 ml 24 hr after feeding.