Equol

History

(S)-Equol was first isolated from horse urine in 1932, and the name was suggested by this equine connection. Since then, equol has been found in the urine or plasma of many other animal species, although these animals have significant differences in their effectiveness in metabolizing the soy isoflavone daidzein into equol. In 1980, scientists reporting the discovery of equol in humans. The ability of (S)-equol to play a role in the treatment of estrogen- or androgen-mediated diseases or disorders was first proposed in 1984.

Chemical structure

Equol is a compound that can exist in two mirror image forms known as the enantiomers, (S)-equol and (R)-equol. However, only (S)-equol is produced in humans and animals with the ability to produce equol after soy isoflavone consumption. (S)-Equol is not of plant origin. It is a metabolite of the soy isoflavone daidzein. (S)-equol thus is characterized as an isoflavan. In contrast, R-equol is not made in humans, but can be chemically synthesized, such as in the laboratory. The molecular and physical structure of (S)-equol is similar to that of the hormone estradiol.

Production in humans

Not all humans can produce (S)-equol after soy consumption. The ability to do so depends on having certain strains of bacteria living within the intestine. Twenty-one different strains of intestinal bacteria cultured from humans have the ability to transform daidzein into (S)-equol or a related intermediate compound. Several studies indicate that only 25 to 30 percent of the adult population of Western countries produces (S)-equol after eating soy foods containing isoflavones, significantly lower than the reported 50 to 60 percent frequency of equol-producers in adults from Japan, Korea, or China. Vegetarians are more capable of transforming daidzein in this substance too. In research studies, the ability of a person to produce (S)-equol is determined with a standardized test in which the person, who has not had antibiotics for at least a month prior to testing, drinks two 240 milliliter glasses of soy milk or eats a soy food equivalent for three days followed by measurement of (S)-equol concentrations in their urine on the test's fourth day. Seaweed and dairy consumption enhances the production of equol.

(S)-equol producing bacteria

While many more bacteria are involved in the related intermediate process of (S)-equol production, such as conversion of daidzin to daidzein, or genistein to 5-Hydroxy-equol, the bacteria that can produce a complete conversion of daidzein to (S)-equol, are the following:[1]

However, the Bifidobacterium conversion has only been claimed once by Tsangalis et al. 2002, and not reproduced since.[2] Mixed cultures such as Lactobacillus sp. Niu-O16 and Eggerthella sp. Julong 732 can also produce (S)-equol.[3] Some equol producing bacteria, as implied by their nomenclature, are Adlercreutzia equolifaciens, Slackia equolifaciens and Slackia isoflavoniconvertens.

Estrogen receptor binding

(S)-Equol has about 2% of the affinity for the human estrogen receptor alpha (ERα) estrogen compared to estradiol. (S)-Equol has a stronger affinity for the human estrogen receptor beta (ERβ), yet this affinity is still just 20% that of estradiol. The preferential binding of (S)-equol to ERβ, compared to ERα and to that of estradiol, indicates the molecule may share some of the characteristics of a selective estrogen receptor modulator (SERM).

Pharmacokinetics

(S)-Equol is a very stable molecule that essentially remains unchanged when digested, and this lack of further metabolism explains its very quick absorption and high bioavailability. When (S)-equol is consumed, it is rapidly absorbed and achieves a T_max (rate of peak plasma concentration) in two to three hours. In comparison, the T_max of the daidzein is 4 to 10 hours because it occurs in a glycoside (with a glucose (sugar) side chain) form and the body must, in order to use daidzein, convert daidzein to its aglycone form (without the glucose side chain), achieved through removal of the sugar during digestion. If consumed directly in aglycone form, daidzein has a T_max of one to three hours. Also, the percent fractional elimination of (S)-equol in urine after oral administration is extremely high and, in some adults, can be close to 100 percent, which is far higher than the percent fractional eliminations of either daidzein (30 to 40 percent) or genistein (7 to 15 percent).

Equol has been found to act as an agonist of the GPER (GPR30).