Being fat has long been seen as a personal problem, fixed only
by
struggling against the proliferation of fast food restaurants,
unlucky
genes, and a sedentary life.
But could something in the environment also be making Americans
fat in
epidemic numbers?
Animal studies in recent years raise the possibility that
prenatal
exposure to minuscule amounts of common chemicals - found in
everything
from baby bottles to toys - could predispose a body to a life of
weight
gain. The chemicals, known as endocrine disrupters, mimic natural
hormones
that help regulate, for example, how many fat cells a body makes
and how
much fat to store in them.
These findings have led some scientists to put forth a
provocative
argument: They say diet and too little exercise clearly are key
reasons
for
the worldwide rise in obesity in the past 20 years, but they may
not be
the
only ones. Food intake and exercise just haven't changed that much
in that
period, they argue. And while genetics obviously play a role - just
think
of someone you know who can eat three Big Macs a day and never gain
an
ounce - these researchers say it would be impossible to see such
widespread
genetic change in just two decades, giving them more reason to
suspect the
environment.
"This is a really new area ... but from multiple labs on
multiple
levels
we are getting preliminary data that all say the same thing:
Chemicals can
play a role," said Jerry Heindel, a program administrator for the
National
Institute of Environmental Health Sciences. "We know that
nutrition and
exercise are very, very important, but underlying that could be
environmental exposures during development that alter your
physiology,
including how you respond to food and exercise."
Thousands of chemicals have come on the market in the past 30
years, and
some of them are showing up in people's bodies in low levels.
Scientists
studying obesity are focusing on endocrine disrupters - which have
already
been linked to reproductive problems in animals and humans -
because they
have become so common in the environment and are known to affect
fat
cells.
One key researcher in the field, Bruce Blumberg of the
University of
California, Irvine, has even coined a new word for chemicals that
can make
you fat: Obesogens.
A recent US Centers for Disease Control and Prevention study
found that
about 93 percent of the US population had bisphenol A, a chemical
that can
be found in canned goods and in hard, clear plastic items such as
baby
bottles and hiking containers, in their body. A study at the
University of
Missouri-Columbia showed that mice fed bisphenol A during early
development
- at lower amounts than what would have resulted in the levels
found in
most people in the CDC study - become markedly more obese as adults
than
those that weren't fed the chemical. Tufts University scientists
observed
similar phenomenon in rats.
The chemical industry, however, disputes those studies and says
dozens of
others that examined bisphenol A showed no weight gain.
"The scientific evidence shows that bisphenol A ... does not
have any
effect on body weight," said Steven Hentges, executive director of
the
polycarbonate/BPA global group of the American Chemistry Council,
which
represents chemical manufacturers.
Bisphenol A is only one of the chemicals scientists are
studying.
Blumberg's lab has also studied tributyltin, an endocrine disrupter
that
is
used as an antifungal agent in agriculture and in marine paints to
keep
ship hulls free of barnacles. Female mollusks exposed to the
chemical were
seen to grow male sex organs. Lab mice exposed to tiny levels of
tributyltin during prenatal development became fatter adults than
those
not
given the chemical.
"It predisposed them for life," said Blumberg.
These scientists are focusing on early development because it is
a
critical time for determining a baby's long-term health and weight.
For
example, studies show that babies born underweight are likely to be
fatter
later in life, possibly because undernourished fetuses learn to use
fat
cells more efficiently - and it gets embedded in their physiology.
Researchers suspect the same thing may be taking place with
chemical
exposures.
Exposure "can be critical on the front end of one's life where
the rest
of your life's physiology is being programmed," said Frederick vom
Saal,
a
biological scientist at the University of Missouri-Columbia who
studies
bisphenol A.
His lab is studying genes in the fat cells of mice to better
understand
why the animals became fatter when exposed to the chemical.
Growing up with more fat cells isn't necessarily a problem if
you are
running around a lot, says Pete Myers, chief scientist for
Environmental
Health Sciences, which publishes the online journal Environmental
Health
News. But in a world where exercise is down and poor diets abound,
it
could
exacerbate a weight problem.
Vom Saal says as people become adults, they may be able to shake
off the
weight with extreme diet and exercise, but it won't be easy. "It
is a
very
intractable thing to change," he said.
Scientists who study obesity's link to chemicals say the
research is
still
in its infancy. Among the many unanswered questions that remain:
How do
the
changes happen? What about the combined impact of exposure to many
chemicals? Are humans affected by the chemicals the same way as
animals?
For those who don't want to wait until all the evidence is in,
there is
another question: How to avoid these chemicals now?
"It can be difficult," said Felix Grun, assistant researcher
in the
department of developmental and cell biology at the University of
California who works with Blumberg. To minimize exposure to
bisphenol A,
Grun said people can avoid buying plastics with the recycling
number 7
marked on the bottom, but similar types of chemicals abound in
other
products, too. "These compounds are everywhere, the carpet fibers,
the
PVC
piping, etc," he said.
Scientists say years of research into a once-popular synthetic
hormone -
diethylstilbestrol (DES) - also bolsters their belief that chemical
exposure during early development can affect weight later in life.
DES was
once given to women to prevent miscarriages until it was linked to
cancer
in female offspring. Now, research by Retha R. Newbold at the
National
Institute of Environmental Health Sciences shows that mice exposed
to DES
in utero are fatter than those not given the chemical.
Ana Soto, a Tufts University professor who studies endocrine
disrupters
and development, says scientists already know that the most serious
health
problems of DES impact mice and humans similarly. Now that mice
exposed to
low levels of bisphenol A are behaving much the same way they do
when
exposed to DES, it makes sense to conclude that humans may be at
risk too.
She wants the chemicals like bisphenol A to be regulated by the
federal
government.
"What else are we waiting for?" Soto asked. "There is
evidence these
chemicals have a multitude of deleterious effects in animals... We
should
be worried."
Beth Daley can be reached at bdaley@globe.com
c.2007 The Boston Globe