Study finds little evidence linking their bodies’ mercury levels to mental deficits
by Craig Weatherby
Dietary mercury poses risks to the human nervous system, and intake should be minimized, especially when it comes to protecting growing brains. This is why experts advise children and pregnant or nursing mothers to avoid high-mercury fish.
However, a new study from Johns Hopkins University’s School of Public Health holds reassuring news for older adults.
The researchers tested the blood mercury levels of 474 Baltimore residents aged 50 to 70, and tested the participants’ performance on 12 brain-function measures, including memory, manual dexterity, intelligence, behavior, and verbal skills.
As the investigators concluded, “Overall, the data do not provide strong evidence that blood mercury levels are associated with worse neurobehavioral performance in this population of older urban adults.”
Results offer reassurance to younger fish-lovers, too
The study’s encouraging results indicate that diets that include frequent enjoyment of fish present no serious risk of increased cognitive decline to middle-aged and older adults.
Better yet, the study’s findings show that the median amount of mercury found in the participants’ blood (2.1 mcg per liter) falls well within the Environmental Protection Agency's "recommended" range (5.8 micrograms per liter or less) for those cautioned most strongly against dietary mercury: that is, pregnant or nursing women and women of childbearing age. (Mercury intake standards are a subject of controversy: click here for more information.)
The study also support findings by the Centers for Disease Control and Prevention (CDC), which show that even the upper range of mercury levels found in Americans’ bodies don’t come near the levels known to cause health problems.
Study bolsters seafood benefits
Thanks to an extensive body of research, it’s clear that the omega-3 fatty acids in fish reduce the risk of both sudden death and death from coronary heart disease in adults, reduce the risk of blood clots and stroke, protect against certain cancers, exert therapeutic effects on autoimmune diseases, and may help prevent and relieve depression.
Accordingly, the USDA’s 2005 Dietary Guidelines recommend that Americans eat two eight-ounce servings a week of foods rich in the omega-3 fatty acids EPA and DHA... and the only such foods are fatty fish.
Seafood and mercury: which species are safest?
The following lists reflect from the uniquely comprehensive data analysis contained in "Brain Food": a joint report by the Environmental Working Group (EWG) and the U.S. Public Interest Research Group (U.S. PIRG), and the results of our own independent lab tests (species marked with an asterisk, and colored crimson).
Wild Pacific salmon, Pacific albacore tuna* (young, low-weight only*), Alaskan halibut*, farmed trout or catfish, shrimp, fish sticks, flounder, croaker, haddock, shrimp, pollock, catfish, and mid-Atlantic blue crab.
Note: Drs. Michael Roizen and Mehmet Oz told Oprah Winfrey—see “Oprah and Expert Guests,” in this issue of Vital Choices—that they rank tilapia, flounder, cod, mahi-mahi among the safer species.
As the Environmental Working Group has said, “"The risk of mercury in salmon appears to be minimal. In fact, the FDA states that limiting consumption is unnecessary for salmon.”
Pregnant women, nursing mothers and women of childbearing age
Not recommended for any consumers
- Not Recommended: Tuna steaks*, sea bass, oysters from the Gulf Coast, marlin, halibut*, pike, walleye, white croaker, and largemouth bass.
- No more than one meal per month, combined: Canned tuna*, mahi-mahi, blue mussel, Eastern oyster, cod, pollock, salmon from the Great Lakes, blue crab from the Gulf of Mexico, wild channel catfish and lake whitefish.
Shark, swordfish, king mackerel, and tilefish
Vital Choice tuna, sablefish, and halibut are safer
Tests performed on Pacific halibut, sablefish, and young, low-weight albacore tuna from Vital Choice indicate that they contain substantially less mercury than the older, larger fish of the same species that dominate the mass market. (All salmon are relatively low in mercury, but wild salmon have far lower PCB levels than farmed.)
A summary of comparative test results is shown below. The mercury levels in Vital Choice fish are shown by the blue bars, while the maroon bars show the far higher average levels found in typical fish of the same species sold in standard food and fish markets (Click on the chart to see a larger version).
While the uniquely comprehensive EWG/PIRG mercury report included more data than any academic or government analysis to date, it did not include mercury data for all possible geographical sources and ages of all seafood species.
Why didn’t the EWG/PIRG report include these distinct within-species differences? After all, it’s well known that younger, smaller members of a species contain less mercury, and that members of the same species from different parts of the oceans (e.g., mackerel) can have very different mercury levels.
One reason for this omission may be that the data was simply unavailable. Or, the authors knew that most consumers cannot distinguish the age or origin of fish on display. By the time a fish steak or fillet appears on the ice at the supermarket, it is nearly impossible to know these details and select accordingly. Hence, mercury information covering fish of the same species, but of different ages and origins, would have been of no practical use to most consumers.
- Weil M, Bressler J, Parsons P, Bolla K, Glass T, Schwartz B. Blood mercury levels and neurobehavioral function. JAMA. 2005 Apr 20;293(15):1875-82.