When it comes to the meats we eat, fish are at the top of the nutritional value ladder. They’re high in protein, heart-healthy omega-3 fatty acids, and other vitamins and minerals, while also being low in saturated fat. But because they also tend to be higher in mercury, it’s advised everyone eat fish in moderation, and even more so for pregnant women. Now, a new study shows why women capable of getting pregnant need to pay special attention to their fish consumption, as it found they could be at risk for autoimmune disease.

“We don’t have a very good sense of why people develop autoimmune disorders,” said lead author Dr. Emily Somers, an associate professor at the University of Michigan Medical and Public Health Schools, in a press release. “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.”

Autoimmune diseases develop when a person’s immune system turns against their body. White blood cells, unable to tell the difference between healthy tissue and antigens — viruses, toxins, bacteria — attacks, destroying tissue and organs, sometimes even changing organ function. Common autoimmune diseases include celiac disease, multiple sclerosis, lupus, rheumatoid arthritis, and Addison’s disease.

For the study, Somers and her team looked at data on women aged 16 to 49 who had participated in the National Health and Nutrition Examination Survey between 1999 and 2004. They found that women who exposed themselves to higher levels of mercury were more likely to develop precursors to autoimmune diseases called autoantibodies. These proteins, which can’t tell healthy organs from antigens, can appear years before the onset of an autoimmune disease.

Women of reproductive age should pay special attention to this because not only do autoimmune diseases tend to develop more often in women, but also because methylmercury, the form commonly found in fish, is secreted from the body slower than it is absorbed. If women consume high levels of mercury and aren’t yet aware they’re pregnant, it can cause problems in babies’ immune systems, genetic and enzyme systems, and the nervous system — including problems with coordination and the five senses.

Last year, the Food and Drug Administration began revising its fish consumption recommendations for pregnant women to include 8 to 12 ounces of a variety of low-mercury fish, such as salmon, shrimp, Pollock, tuna (light canned), tilapia, catfish, and cod. Fish higher in mercury, which should be avoided, include tilefish from the Gulf of Mexico, shark, swordfish, and king mackerel — white albacore tuna should be limited to 6 ounces.

A recent study, however, found mercury levels in fish may actually be rising. Published last week, it found climate change is warming the oceans, forcing smaller fish to eat more, and thus passing higher quantities of mercury through the food chain to bigger fish. If more evidence of this emerges, those recommendations may need to be revised again. Nevertheless, all women of reproductive age should be mindful of the amounts they’re eating.

Source: Somers E, Ganser M, Warren J, et al. Mercury Exposure and Antinuclear Antibodies among Females of Reproductive Age in the United States: NHANES. Environmental Health Perspectives. 2015.