May 29, 2009 -- Low-level exposures to environmental toxins may partially explain the increasing problem of liver disease in U.S. adults, says a Kentucky researcher.
"Liver disease is a rapidly growing problem for the U.S. population," says Matthew Cave, MD, assistant professor of medicine at the University of Louisville, who is scheduled to present his study June 1 at the Digestive Disease Week meeting in Chicago. As obesity has increased, so has liver disease.
More than one in three U.S. adults has liver disease, Cave found in his study. That's after excluding people with traditional risk factors for liver disease, such as hepatitis and alcoholism. He bases the one in three figure on the percentage of people he evaluated that had abnormal levels of a liver enzyme associated with liver injury.
Some of these cases, he says, are linked with environmental pollutants, such as pesticides and heavy metals.
Environmental Toxins & Liver Disease: Study Details
Cave and his colleagues extracted data from the 2003-2004 National Health and Nutrition Examination survey (NHANES), a database often used by researchers.
About 200 pollutants were measured by either blood or urine test in the NHANE participants. Cave's team narrowed it down -- and, he says, "ended up with a list of 111 chemical pollutants which were commonly found in at least 60% of NHANES subjects."
Among the chemicals studied were lead, mercury, and organochlorine pesticides. "And, so from this list, we found several chemicals associated with a dose-dependent increased risk for abnormal liver enzymes," he says.
Organochlorine pesticides have been banned from use in the U.S. since the 1980s, but they can linger in the environment. Exposure to organochlorines can still occur when people eat fatty foods such as fish or dairy products contaminated with these long-lasting pesticides. A pregnant woman can pass them to her unborn child through the placenta or to her child during breastfeeding, according to the CDC.
Cave evaluated the NHANE participants' levels of the enzyme alanine aminotransferase, or ALT, found in the highest amount in the liver. When the liver is injured, ALT is released into the blood.
He found that 34.1% of the 4,582 participants had abnormal ALT levels.
Even after adjusting for such variables as obesity, race, sex, poverty, and diabetes, he says, "the results indicate that there may be a previously unexpected role for environmental pollution in the rising incidence of liver disease in the U.S. population. Clearly, more work needs to be done."
Although the link doesn't prove cause and event, Cave notes that previous animal studies have demonstrated the presence of liver disease in those exposed to many of the chemicals he is talking about.
Until more research is done, Cave says people could protect themselves by reducing or minimizing exposures to chemicals such as lead, found in old house paint, and to mercury, found in certain fish.
Environmental Toxins & Liver Disease: Second Opinion
The study results are "very important" and deserve attention, says Gina Solomon, MD, MPH, a senior scientist at the Natural Resources Defense Council in San Francisco, who reviewed the study for WebMD. "But it's hard to come to any solid conclusion based on this study alone."
"There is a chance they would find an association by statistical chance alone, because they looked at so many [toxins.]"
Some of the toxins associated with liver damage in the recent study, she says, have also been linked to liver damage in other studies, but at high levels of exposures. "The thing that is so dramatic about this study is they found associations at the levels that are in the general population. There's no surprise that these chemicals can cause liver disease, but previous research has always suggested that the doses needed to be much higher.
"The striking thing about this is that these are levels within the range that you or I might have in our bodies."
Although more study is needed, Solomon says that "If it holds up, it implies that some portion of the burden of liver disease out there in the U.S. may be due to chemical exposure outside the workplace."
Environmental Toxins & Liver Disease: Industry View
"The crop protection industry ... is one of the most highly regulated industries in the United States," says Susan E. Helmick, a spokeswoman for CropLife America, a trade organization representing makers of crop protection products and pesticides. "Safety to human health and any impact on the environment are always foremost in the development and use of crop protection products."
Further comment is not possible, she says, without seeing more findings than those in a brief abstract presented at the meeting.