More Evidence That Pollutants
Play A Major Role In Obesity
If true, it could turn the conventional wisdom of how obesity
causes diabetes on its head. Emerging evidence suggests that pollutants
stored in body fat may be contributing to the ongoing rise of
type 2 diabetes.
While obesity is still thought to be a major cause, there is
more and more evidence to suggest that persistent organic pollutants
(POPs) also play a key role.
While obesity is thought
to be a major cause, there is more and more evidence to suggest
that pollutants also play a key role
POPs are synthetic chemicals that can accumulate in the fatty
tissue of animals. Many POPs - such as polychlorinated biphenyls
(PCBs), which were used as coolants in electrical equipment, and
pesticides such as DDT - have been banned in developed countries,
but they remain in the food chain and often end up in people.
Last year, Duk-Hee Lee at Kyungpook National University in Daegu,
South Korea, and her colleagues reported that people with higher
levels of six different POPs were more likely to have diabetes
than people with low levels of POPs (New Scientist, 30
September 2006, p 18).
Now, a follow-up study published last month suggests an association
in non-diabetic people between certain pesticides, PCBs and insulin
resistance - a precursor to diabetes (Diabetes Care, vol
30, p 622). Fat people with POPs in their blood were more likely
to develop insulin resistance than thin people with POPs, but
the expected association between obesity and insulin resistance
disappeared in people with no POPs. "This suggests the possibility
that POPs stored in fat tissue, not obesity itself, may be a key
factor for the development of type 2 diabetes," says Lee.
The information used by the researchers was designed to be representative
of the general population in the US. "That's the somewhat shocking
implication," says David Jacobs at the University of Minnesota,
Minneapolis, who supervised Lee's research. "The association exists
at everyday background levels of POPs."
The precise mechanism by which POPs could contribute to diabetes
remains a mystery. Some PCBs, such as dioxin, are known to interfere
with genes that control insulin sensitivity, says Lee, although
dioxin was not studied by her group.
However, it could also be the other way around, says Matthew
Longnecker, an epidemiologist at the National Institute of Environmental
Health Sciences in Research Triangle Park, North Carolina. "People
with diabetes or a pre-diabetic condition may clear POPs from
their system at a slower rate," he says. This would lead to increased
concentrations of POPs over time.
Lee acknowledges that this is a possibility but thinks it is
unlikely. Long-term studies will be needed to pin down the sequence
of events, but previous studies have also shown a causal link
between POPs and diabetes, she says. For example, US air force
pilots who sprayed Agent Orange - which contains dioxin - during
the Vietnam war are more likely to develop diabetes, as are people
who live near waste sites contaminated with POPs.
Robert Lustig, a paediatric endocrinologist at the University
of California, San Francisco, agrees, but warns that the relationship
could be more complicated yet, as animal studies have shown that
environmental toxins in the womb may cause obesity in later life,
which could then lead to diabetes. "We just don't know," he says.
"But is there reason to be concerned? You bet."
From issue 2599 of New
Scientist magazine, 12 April 2007, page 1.