Metabolism of isoflavones in ruminants

Ruminal metabolism of isoflavones has been studied mainly in sheep, and it is suggested to resemble qualitatively that in cattle (Lundh, 1995). Recent studies show that bovine ruminal bacteria can convert daidzein and genistein to dihydrodaidzein and dihydrogenistein, respectively (Wang et al, 2005), and can produce X-equol (vide infra) from daidzein through dihydrodaidzein (Wang et al, 2007). 

A summary of studies where some isoflavones and their metabolites have been analyzed from blood samples of sheep and cattle fed red clover is presented in Table 9.1. The concentrations of isoflavonoids seem to be at the same level in both sheep and cattle. Conjugated or total equol concentration varied between 33 and 770 xg per 100 mL, and free or unconjugated equol concentration was between 2 and 10 xg per 100 mL. Lundh (1995) has estimated that of the total equol concentration about 5% in cows and 1% in sheep exist as free. The plasma concentrations measured for conjugated formononetin, daidzein, genistein, or biochanin A were 1-55 jig per 100 mL. Concentrations for unconjugated forms were usually very small, less than 1 jig per 100 mL. 

Not much is known of true elimination of isoflavonoids in the ruminants. It seems that most of equol (70-80%) produced is excreted in urine and very little is found in feces (Shutt et al, 1970 Lloyd-Davies Hill, 1989). Some isoflavonoids have also been found in milk. Recently, the contents of isoflavonoids, and especially equol, in milk have been studied actively. A summary of these studies is presented in Table 9.2. Equol is the predominant isoflavonoid found in the milk samples. Equol content varies greatly, the largest concentrations being found in organic milk (411 ig L ) and in milk from red clover-fed cows (643 ig L ). The equol concentration in milk seems to be approximately one-tenth of the plasma concentration. 

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