New research suggests that exposure to mercury through seafood is one of the greatest risk factors for autoimmunity among women of childbearing age.

Researchers at the University of Michigan found that mercury -- even at low levels generally considered safe - was associated with autoimmunity. Autoimmune disorders, which cause the body's immune system to attack healthy cells by mistake, affects nearly 50 million Americans and predominately women.

"We don't have a very good sense of why people develop autoimmune disorders," Emily Somers, lead author of the study, said in a statement. "A large number of cases are not explained by genetics, so we believe studying environmental factors will help us understand why autoimmunity happens and how we may be able to intervene to improve health outcomes. In our study, exposure to mercury stood out as the main risk factor for autoimmunity."

For the study, researchers analyzed data among women between the ages of 16 and 49 from the National Health and Nutrition Examination Survey from 1999 to 2004.

They found that greater exposure to mercury was associated with a higher rate of autoantibodies, a precursor to autoimmune disease. Most autoimmune diseases are characterized by autoantibodies, proteins made by a person's immune system when it fails to distinguish between its own tissues and potentially harmful cells.

Many fish consumption recommendations are aimed at pregnant women, those who may become pregnant, nursing moms and young children. The Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) say pregnant women can safely eat up to 12 ounces (340 grams) of seafood a week. Fish such as swordfish, king mackerel and tilefish contain the highest levels of mercury while shrimp, canned light tuna and salmon have lower levels.

There many health benefits to seafood, a lean protein packed with vital nutrients. However, the findings provide further evidence that women of reproductive age should be mindful of the type of fish they're eating.

"The presence of autoantibodies doesn't necessarily mean they will lead to an autoimmune disease," Somers said. "However, we know that autoantibodies are significant predictors of future autoimmune disease, and may predate the symptoms and diagnosis of an autoimmune disease by years. For women of childbearing age, who are at particular risk of developing this type of disease, it may be especially important to keep track of seafood consumption."

The findings are detailed in the journal Environmental Health Perspectives.