By Willow Lawson, published on March 18, 2003 - last reviewed on June 9, 2016
Seafood is often touted as the best source of omega-3 fatty acids, key nutrients that keep hearts and brains healthy. But consumers need to fish for more information when choosing whether to have salmon or swordfish for dinner, warn nutrition experts.
The risk comes from toxins like PCBs (Polychlorinated biphenyls) and mercury that are present in a variety of fish, even popular types served in restaurants and sold in supermarkets. Fish can take in toxins by living in polluted water. But many large fish, like tuna and swordfish, accumulate toxins over their lifetime by eating smaller fish and ingesting their toxins as well as their own.
Knowing which fish to eat is especially important for women who are pregnant, nursing or of child-bearing age, because the small bodies of children are easily overwhelmed by toxins, which can slow brain development. The Food and Drug Administration recommends that pregnant women not eat any swordfish, king mackerel, shark or tilefish.
"It takes six years to rid the body of PCBs and one year for mercury," says Charles Santerre, Ph.D., a foods and nutrition professor at Purdue University who specializes in chemical contaminants in food.
Mercury and PCBs have many similar effects on the human body, many of which are subtle, but can be documented by comparing groups of people who have and have not been exposed to the toxins. Mercury kills nerve cells because it is easily transported across the blood-brain barrier that keeps many contaminants out of brain.
Slower reflexes, poorer vision and coordination are sometimes seen in people who have ingested small amounts of mercury. More severe symptoms include fatigue, memory loss and nausea. Children with high levels of mercury may have mood changes, withdraw socially and have memory difficulties.
Santerre says there is still much debate about how harmful toxins like mercury are and whether the brain delays they cause in children linger over a lifetime. Few studies have tracked the effects of mercury long-term.
However, one recent study showed there is cause for concern among people who eat fish regularly. In a study published in last November's Environmental Health Perspectives, a San Francisco doctor surveyed 720 of her patients for a year and tested the blood of 116 patients who reported eating fish more than twice a week. Of that group, 63 patients had twice the Environmental Protection Agency's recommended levels of mercury in their blood. Nineteen patients were quadruple the limit and four patients had 10 times more mercury than was considered safe. One 7-year-old boy who ate canned tuna, mackerel and fresh tuna regularly had 15 times the EPA's level.
Patients with elevated mercury levels in the study often had symptoms of depression, irritability, headaches, tremors, numbness and tingling in hands and feet.
So how should consumers protect themselves from pollution? Studies show the safest seafoods are farmed and wild salmon, oysters, shrimp, farm-raised channel catfish, farm-raised rainbow trout, flounder, perch, tilapia, scallop, clams, and red swamp crayfish. These fish can be eaten more than once a week, according to Santerre.
Canned tuna, crab, cod, mahi-mahi, haddock, whitefish, herring and spiny lobster have slightly higher levels of mercury and should be limited to once a week.
Some seafood should be eaten only once a month: tuna steaks, red snapper, orange roughy, Pollack, halibut, northern lobster, marlin, moonfish, saltwater bass, wild trout, bluefish, grouper, croaker and sablefish. If a pregnant or nursing woman eats halibut, she should wait another month before eating another fish in this group.