soy and health
not a
Mark Messina
are e
The soybean is a unique dietary source
of isoflavones, and it is primarily for
this reason that there has been so much
research interest in the health effects
of soyfoods during the past 20 years.
More than half of the nearly 2,000 soy-
related articles indexed by MEDLINE®
in 2007 involved isoflavones. This is
not to suggest that other components
of the soybean do not possess biolog-
ically interesting attributes and exert
potential health benefits. But when it
comes to the hypotheses that have at-
tracted the most attention—inhibition
of postmenopausal bone loss, allevia-
tion of hot flashes, prevention of age-
related cognitive decline, and prevention
of breast and prostate cancer—the iso-
flavones are the soybean components
of interest.
718
November 2008 inform
NOV.8.indd 718 11/19/2008 11:20:36 AM
t all isoflavones
e equol Isoflavones are, however, also most
examining the equol hypothesis
ing genistein) in soybeans. In 2002, pi- ing whether certain biological processes
often behind concerns and allegations that oneering isoflavone researcher Kenneth within cells are controlled by specific en-
soyfoods exert adverse effects under cer- D.R. Setchell proposed that equol could zymes. The result is that more than 7,000
tain conditions and in some individuals. be the key to understanding the action genistein-related papers have been pub-
Not surprisingly, therefore, people who of soy isoflavones in disease preven- lished whereas for equol, there are few-
are even only peripherally familiar with tion and treatment. This hypothesis has er than 500. But most of the latter have
soyfood research likely have some under- implications that go far beyond a sim- been published within just the past four
standing of isoflavones and recognize, for ple debate over mechanism, because not or five years.
example, that these soybean constituents all individuals possess the intestinal mi- There is no standardized, official
are classified as plant estrogens (although crobiota capable of producing equol. In method for classifying individuals as
they also have potentially relevant non- fact, estimates are that only about one- equol producers. Some investigators do
hormonal properties and in the opinion of third of Westerners are equol-producers so simply on the basis of whether urinary
this author are more accurately character- although higher percentages have been equol levels increase by a certain degree
ized as phytoselective estrogen receptor found among the Japanese and in some in response to a soy challenge, whereas
modulators rather than phytoestrogens subpopulations such as vegetarians. others have proposed using a urinary ra-
since they have estrogen-like effects). In According to the equol hypothesis, it tio of equol to daidzein because it is inde-
contrast, far fewer people are likely to is the equol-producers who benefit most pendent of isoflavone intake and minimiz-
know much of anything about equol—a from isoflavone exposure, at least in re- es interindividual variation in isoflavone
compound closely related to isoflavones lation to certain health outcomes. Impor- pharmacokinetics or differences in ana-