Researchers found the higher the levels of methylmercury - a compound often found in fish - in reproductive-age women, the higher the levels of autoantibodies, a characteristic of autoimmune diseases.
The research team, led by Dr. Emily Somers - an associate professor in the departments of Internal Medicine, Environmental Health Sciences and Obstetrics and Gynecology at the University of Michigan Medical and Public Health Schools - publishes its findings in the journal Environmental Health Perspectives.
Mercury is a naturally occurring metal that comes in various forms. When mercury enters the environment, it combines with other elements to create inorganic or organic compounds that, if ingested at high levels, may be harmful to human health.
When mercury enters water or soil, for example, microscopic organisms convert it to an organic compound called methylmercury. This compound can accumulate in the tissue of fish or shellfish. According to the Environmental Protection Agency (EPA), almost all methylmercury exposures in the US stem from consuming fish or shellfish that contain high levels of the compound.
Ingesting high levels of mercury is associated with permanent brain damage and kidney damage. During pregnancy, the harmful effects of mercury may also be passed to the fetus, causing developmental problems.
As such, the Food and Drug Administration (FDA) and the EPA say pregnant women should eat no more than 340 g of low-mercury fish (such as salmon, shrimp and light canned tuna) a week - the equivalent of two to three portions a week. Fish containing high levels of mercury - such as swordfish, king mackerel and tilefish - should be avoided, according to the recommendations.
But in this latest study, Dr. Somers and her team say that for women of reproductive age, consuming fish even at low levels recommended by the FDA and the EPA may pose another harm to health; it could raise the risk of autoimmune disorders, such as multiple sclerosis (MS), lupus and rheumatoid arthritis, in which the immune system attacks and damages the body's own tissues.
Exposure to mercury 'stood out as main risk factor for autoimmunity'
The researchers assessed data of 1,352 women aged 16-49 years who were part of the 1999-2004 National Health and Nutrition Examination Survey (NHANES).
The researchers assessed the levels of mercury present among participants by analyzing hair and blood samples.
They found that the higher the levels of mercury among the women, the higher the levels of autoantibodies - proteins that are a characteristic of autoimmune diseases. Autoantibodies are made when an individual's immune system is unable to distinguish between healthy tissues and potentially harmful cells.
"The presence of autoantibodies doesn't necessarily mean they will lead to an autoimmune disease," Dr. Somers notes. "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."
The researchers found that methylmercury was the strongest driver for autoantibodies. "Notably," they add, "a dose-response relationship was observed for low methylmercury exposure levels in the range generally considered safe for women of childbearing potential by regulatory agencies."
According to the American Autoimmune Related Disease Association, around 50 million people in the US are living with an autoimmune disease. Of these, more than 75% are women. Women are three times more likely to develop an autoimmune disease than men.
With these statistics and their findings in mind, the researchers say women of reproductive age should consider the amount of seafood they are consuming. Dr. Somers adds:
"In our study, exposure to mercury stood out as the main risk factor for autoimmunity. 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."
Last month, Medical News Today reported on a study claiming that the nutritional benefits of fish consumption during pregnancy may outweigh the risks associated with mercury exposure.