Eating certain veggies not only supplies key
nutrients, it may also influence hormone levels and
behaviors such as aggression and sexual activity,
says a new study led by researchers at the
University of California, Berkeley, that could shed
light on the role of diet in human evolution.
The research is the first to observe the connection
between plant-based estrogenic compounds, or
phytoestrogens, and behavior in wild primates in
this case, a group of red colobus monkeys in Uganda.
The more the male red colobus monkeys dined on the
leaves of Millettia dura, a tropical tree containing
estrogen-like compounds, the higher their levels of
estradiol and cortisol. They also found that with
the altered hormone levels came more acts of
aggression and sex, and less time spent grooming —
an important behavior for social bonding in primates.
The study, published in the current issue of the
journal Hormones and Behavior, suggests how
potentially important consuming phytoestrogens is in
primate ecology and evolution.
“It’s one of the first studies done in a natural
setting providing evidence that plant chemicals can
directly affect a wild primate’s physiology and
behavior by acting on the endocrine system,” said
study lead author Michael Wasserman, who conducted
the research as a graduate student at UC Berkeley’s
Department of Environmental Science, Policy and
Management. “By altering hormone levels and social
behaviors important to reproduction and health,
plants may have played a large role in the evolution
of primate — including human — biology in ways that
have been underappreciated.”
For 11 months, the researchers followed a group of
red colobus monkeys in Uganda’s Kibale National Park
and recorded what the primates ate. For behavioral
observations, the researchers focused on aggression,
as marked by the number of chases and fights, the
frequency of mating and time spent grooming.
To assess changes in hormone levels, the researchers
collected fecal samples once a week from each of 10
adult males in the group (a separate study examining
phytoestrogens in females is ongoing). More than 407
samples were collected and analyzed for estradiol
and cortisol levels.
The researchers found seasonal variation in the
consumption of estrogenic plants, which made up 0.7
percent to as much as 32.4 percent of the red
colobus diet in any given week. For red colobus
adult males, higher consumption of estrogenic plants
corresponded to higher levels of estradiol and
cortisol, two steroid hormones important to
reproduction and the stress response.
Phytoestrogens are also found in human foods,
especially soy and soy-based products. Millettia
dura, the tropical tree that was most important to
red colobus monkey hormone levels and social
behaviors, is a close relative of soy.
“With all of the concern today about phytoestrogen
intake by humans through soy products, it is very
useful to find out more about the exposure to such
compounds in living primates and, by analogy, human
ancestors,” said study co-author Katharine Milton,
professor in UC Berkeley’s Department of
Environmental Science, Policy and Management and an
expert on the dietary ecology of primates. “This is
particularly true when determining the influence of
phytoestrogens on reproductive behavior, which is
the whole keystone of natural selection.”
The study authors cautioned against overinterpreting
the power of phytoestrogens in altering behavior,
however. They emphasized that estrogenic plant
consumption is just one of multiple factors
influencing primate hormone levels and behavior.
Notably, the primates’ own endogenous hormone levels
were the stronger predictor of certain behaviors,
while phytoestrogens played a secondary role.
The researchers noted that the tendency for certain
behaviors to occur can be affected by complex
interactions between endogenous hormones and
phytoestrogens, in addition to factors such as the
quality and quantity of food, competition for
resources and mates and predation.
Nonetheless, previous research in laboratory and
agricultural settings found that eating estrogenic
plants could disrupt fertility and affect behavior
in animals such as rodents, monkeys and sheep.
Effects of phytoestrogen consumption in other
studies have included more aggression, less body
contact, more isolation, higher anxiety and impaired
reproduction.
To expand on this possibility, Wasserman and his
colleagues are now examining the relationship
between phytoestrogens and other primate species,
including our closest-living relative, the
chimpanzee, to determine how common estrogenic
plants are in the diets of wild primates.
“Human ancestors took most of their diet from wild
tropical plants, and our biology has changed little
since this time, so similar relationships as those
found here are expected to have occurred over our
evolutionary history,” said Wasserman, now a
post-doctoral scholar at McGill University’s
Department of Anthropology in Montreal, Canada.
However, the researchers noted that the red colobus
diet contains a high percentage of leaves, while the
diet of chimpanzees, other apes and human ancestors
consists primarily of fruits. Thus, one of
Wasserman’s current goals is to compare the presence
of phytoestrogens in wild leaves and fruits.
“If phytoestrogens make up a significant proportion
of a fruit-eating primate’s diet, and that
consumption has similar physiological and behavioral
effects as those observed in the red colobus, then
estrogenic plants likely played an important role in
human evolution,” said Wasserman. “After studying
the effects of phytoestrogens in apes and
fruit-eating primates, we can then get a better
sense of how these estrogenic compounds may
influence human health and behavior.”
Other co-authors of the study are Colin Chapman and
Jan Gogarten from McGill University, and Daniel
Wittwer and Toni Ziegler from the Wisconsin National
Primate Research Center at the University of
Wisconsin-Madison.
The National Science Foundation and the
International Primatological Society helped support
this research.
For more information
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