A group of chemicals found in household plastics and medical supplies is linked to higher rates of diabetes in women--up to double the rate for women with the highest levels, according to new research led by Harvard scientists.
Blacks and Mexican Americans and women living in poverty are exposed to the highest levels of some of these compounds, called phthalates, the scientists reported.
Whether these chemicals actually cause diabetes in women, however, remains unclear.
"These findings are important clues, but it's only a first step," said Richard Stahlhut, an environmental health researcher at the University of Rochester Medical Center who co-authored the study. "It's extremely likely that phthalates and other chemical contaminants will turn out to be a big part of the obesity and diabetes epidemic, but at this point we really don't know how these chemicals are interacting with each other, or with the human body."
Phthalates make plastics such as polyvinyl chloride (PVC) more flexible, and they are added to some cosmetics, perfumes and other personal care products to stabilize colors and fragrances. A wide variety of household goods rely on phthalates, including vinyl flooring, adhesives and shower curtains. More than 75 percent of Americans have phthalates in their urine.
Until now, most phthalate research has focused on reproductive consequences because these compounds seem to disrupt male hormones. Boys exposed to phthalates in the womb had signs of feminized genitalia, which may lead to fertility problems. Researchers also have found neurological effects, including reduced IQs and attention problems in boys.
The new study examined diabetes and phthalate concentrations in 2,350 women who participated in a national survey by the Centers for Disease Control and Prevention from 2001 through 2008.
Diabetes, an endocrine disease marked by problems with insulin production or insulin resistance, affects nearly 26 million Americans, or 11 percent of the population older than 20, according to CDC data. Blacks have a 19 percent chance of developing diabetes--a rate 77 percent higher than that of whites--and Hispanics have a 66 percent higher rate than whites.
Although obesity is a risk factor for type 2 diabetes, nearly a quarter of normal-weight adults have diabetes or other metabolic disorders. Experts say chemical contaminants such as phthalates could play an important role in this disconnect between obesity and type 2 diabetes rates.
In the new research, certain phthalates--dibutyl phthalates (DBP), which are primarily used in adhesives and lacquer finishes, and benzyl butyl phthalate (BBP), a component of vinyl flooring, caulks and sealants--were linked to double the rate of diabetes in women with the highest levels of phthalate markers in their urine, according to the report published this month in Environmental Health Perspectives.
DBP and Di-2-ethylhexyl phthalate (DEHP), a plasticizer found in vinyl products including IV bags and tubing, were also linked to higher blood glucose levels and insulin resistance, two common precursors of type 2 diabetes, according to the study.
No relationship was found between diabetes and diethyl phthalate (DEP), according to the study, which was led by Tamarra James-Todd of Brigham and Women's Hospital and Harvard Medical School. That phthalate is found in high concentrations throughout the U.S. population and it is the phthalate most commonly associated with personal care products.
Other recent studies also have found similar links between phthalates and metabolic disorders.
Certain phthalates doubled the risk of diabetes in older Swedish adults, according to research published in April. And DEHP, the phthalate in flexible vinyl and medical supplies, was linked to higher rates of diabetes in a 2011 study of Mexican women. Higher levels of phthalates were also associated with greater waist circumference and insulin resistance, two major risk factors for type 2 diabetes, in a 2007 study of U.S. men.
Industry groups are skeptical of the significance of the new findings.
"The phthalate data are derived from a single (spot) sample. For substances like phthalates that are rapidly broken down and eliminated from the body, depending on a spot urine sample is a significant design flaw," said Steve Risotto, senior director of the American Chemistry Council, a trade association for chemical manufacturers.
A group representing cosmetics and fragrance manufacturers doubts personal care products have a role in diabetes.
"Diethyl phthalate, also known as DEP, is the only phthalate with significant use in cosmetics. The study found no association between DEP and diabetes," noted Linda Loretz, a director of the Personal Care Product Council.
Nail polish used to contain high levels, but most manufacturers voluntarily eliminated phthalates in recent years. The chemicals also have been banned in children's toys.
Black women in the study had more than double the concentrations of DEP, the phthalate in cosmetics, and DBP, the phthalate in adhesives and lacquers that was linked to a double rate of diabetes, when compared with white women. Mexican-American women had 75 percent higher concentrations of DEP. Poor women had up to 78 percent higher levels of BBP--the phthalate in vinyl flooring that was associated with a double rate of diabetes--than women living above poverty level.
The racial and economic trends were in line with another recent study. Published in April, it found that women ranking lowest in socioeconomic status (based on race, education, income and food security measurements) had up to 83 percent more BBP than women with the highest socioeconomic status.
Non-white women had significantly more DBP, the phthalate in adhesives and lacquers that was linked to diabetes in the new research, and DEP, the primary phthalate associated with cosmetics, than their white counterparts. Women with lower levels of education and income had more BBP, the vinyl flooring phthalate linked to diabetes in the study.
Consumer behavior patterns might explain these disparities, Stahlhut said. For example, if black women use more hair care products or cosmetics, they would likely have higher levels of DEP in their bodies. But it's impossible to distill trends like these from the current data, Stahlhut said.
"It's difficult to interpret these patterns," said Roni Kobrosly, an epidemiology researcher at the University of Rochester who led the socioeconomic study published in the journal Environmental Research. "They suggest that, on a large public health level, patterns of phthalate exposure vary with socioeconomic factors. But it's premature to talk about the implications on an individual or cultural level."
Because neither study included long-term follow-up with the women, the researchers cannot determine whether high phthalate concentrations actually led the women to develop diabetes or other diseases. Still, the findings are an important first step in sorting out the relationships between these chemicals and chronic diseases such as diabetes, experts say.
Several other pollutants have been linked to type 2 diabetes risks. Polychlorinated biphenyls (PCBs) and bisphenol A (BPA) are thought to disrupt the endocrine system by interfering with hormone signals. Studies suggest that phthalates may hinder glucose metabolism and stimulate fat cell production.
"With phthalates, the story is really still emerging," said Kristina Thayer, a researcher with the National Institute of Environmental Health Sciences and the National Toxicology Program. "Studies like these are considered exploratory, but they seem to be consistent."
"More needs to be done to really fill in this question of potential causality, and the roles that specific phthalates may play," she added.
Stahlhut noted that product formulations are often trade secrets, making it difficult for scientists and consumers to know which phthalates are in specific products.
"Figuring this out for sure either way will take a long time, unfortunately," he said. "So what's our best strategy in the meantime?"
To read more about health risks, read "The Poison Among Us."