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 Dr. 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.

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