Is Industrial Pollution Making America Fat?
[Rachel's Introduction: Dr, Bruce Blumberg believes the obesity epidemic actually is due, in part, to industrial pollution -- specifically to low levels of toxic compounds he calls "obesogens."]
In the United States, the percentage of adults who are obese -- defined by the National Institutes of Health as a body-mass index exceeding 30 -- has doubled since 1990, climbing from 12 percent to a whopping 24 percent in 2005, closely tracking Oregon figures, according to the Oregon Health Division.
For the most part, the blame for the obesity epidemic has fallen on diet and exercise, with particular emphasis on familiar evils such as the proliferation of junk food, the advent of the remote control, trans fat, ever-longer commutes and even the disappearance of physical education in schools.
But now some researchers have identified a new suspect: pollution.
Attributing obesity to diet and exercise is "practically scientific dogma at this point," says Bruce Blumberg, associate professor of developmental and cell biology at UC Irvine. But, he continues, "diet and exercise are simply not adequate to explain the explosion of obesity in Western countries."
Instead, Blumberg believes the obesity epidemic actually is due, in part, to industrial pollution -- specifically to low levels of toxic compounds he calls "obesogens."
Just as exposure to carcinogens can trigger cancer, Blumberg and other researchers say exposure to obesogens can trigger a dramatic increase in the amount of fat produced in a person's body, leading to excess weight and obesity.
The precise mechanism by which these obesogens operate remains dimly understood. They belong to a class of compounds known as "endocrine disrupters" because they block or pervert the operation of the hormones that govern crucial biological processes such as growth, reproduction, sexual development and behavior.
Five years ago, Blumberg was studying the biological effects of various marine pollutants -- in particular, tributyl tin, or TBT, a pesticide notorious for its toxic properties, such as bizarre mutations in the shells of mollusks and the sex organs of sea snails.
Blumberg and his co-workers exposed female frogs to extremely low levels of TBT; as expected, TBT did indeed cause sexual mutation among frogs. But what was really striking, he says, was that the hapless amphibians got fat -- really fat.
"To be honest, I will have to say we stumbled on this," he says.
Tiny doses had a big effect
Although most of the research on endocrine disrupters has focused on their potential effects on sexual development, fat production also is regulated by the hormone system and is, theoretically at least, just as susceptible to disruption.
Blumberg injected mice with TBT and observed similar results: fat rodents. Even more significant, the compound triggered obesity in ridiculously min-uscule quantities. In fact, Blumberg and his colleagues demonstrated effects from TBT at 27 parts per billion -- the rough equivalent of 4 tablespoons in an Olympic-sized swimming pool.
Blumberg concluded that the fattening effects of TBT and a group of similar compounds known as organotins are so profound that even trace amounts could trigger an increase in weight. "The introduction of organotins is likely to be a contributing factor to the obesity epidemic," Blumberg says.
Toxicology experts concur that some compounds are so potent that they can indeed trigger changes at minute concentrations, at least in the test tube.
"It sounds absurd, but it's not inconsistent with what we see in the lab," says Fred Berman, director of the Toxicology Information Center at the Center for Research on Occupational and Environmental Toxicology at Oregon Health & Science University.
Organotins are everywhere
The disruptive effects of organotins stem from their propensity to stimulate a particular hormone receptor that plays a key role in maintaining the body's metabolism, in effect telling the body which kind of cells are in short supply and need to be grown.
Organotins somehow encourage that receptor to manufacture fat cells -- which in turn promotes that ominous abdominal bulge feared by statisticians and movie stars alike.
Organotins first came into widespread use in the 1960s in the shipbuilding industry, where they were mixed with paint to deter barnacles and mollusks from accumulating on the hulls of ships.
They also have been used as soil fungicides for crops such as nuts, potatoes, rice and celery; as "slimicide" to clean up the goop that accumulates in underground water wells; and in the manufacture of polyvinyl chloride, or PVC, a hard plastic found in drainpipes, vinyl flooring, window frames and hundreds of other places.
These widespread uses suggest several possible routes of human exposure, Blumberg says. Organotins may contaminate crops, seep into wells or leach into drinking water from PVC pipes.
Link to obesity stays unclear
It is worth pointing out, however, that little research has been conducted into actual levels of organotins in the average household.
Moreover, many basic questions about the link between organotins and fat remain unexplored -- for example, whether people who might encounter high levels of organotins as a result of their occupation, such as farmworkers or shipyard workers, suffer higher rates of obesity.
The pernicious effects of organotins on the marine environment are well-established, and they now are banned as anti-barnacle agents.
Nonetheless, they continue to be used in agriculture and in the manufacture of plastics. "At my house, we minimize the use of plastic to store food," Blumberg says. "We use glass or stainless steel instead, and in general we try to eat fresh, local, organic food with as little packaging as possible."
OHSU's Berman reckons that Blumberg's research raises "a real concern" about the role of organotins and other possible pollutants in the obesity epidemic. "We need to be looking at this," he says. "But we don't know for sure. We need to do more studies to see if there is a real effect in the real world."
Blumberg is careful to note that obesity is a complex phenomenon stemming from many factors, and that obesogens probably are only part of the story.
In addition, he points out that people who are exposed to obesogens are not doomed to a lifetime of corpulence -- they simply have to work harder than others to shed weight.