News
Aug 20, 2012
Mercury and Alaska fish: When do the benefits outweigh the risks?
Posted: Thursday, August 16, 2012 4:47 pm
Mary Lochner Anchorage Press
How much methylmercury exposure is too much and how much is acceptable when the source of the toxin also provides health benefits?
That’s the question that University of Alaska Fairbanks researcher Lawrence Duffy has been trying to answer.
“We’re trying to develop a mathematical formula for doing risk-benefit calculations,” Duffy said.
But calculating the risk of eating a lot of Alaska fish, versus the benefits, he said, is a complicated task.
The risks of exposure
Fish have historically contained small amounts of mercury from naturally-occurring sources, such as mercury leaching from rock into water systems. Today, thanks to human activity, mercury also leaches into water and spews into the air as a byproduct of industrial activities. Atmospheric mercury from air pollution stays in the air for one to two years, circling the globe until it falls back down to earth in rainwater.
Once mercury is in the ocean or a freshwater system, it gets transformed into methylmercury, a highly toxic form that harms human and animal health. The change happens thanks to microscopic creatures called sulfate-reducing organisms. These organisms provide an important biochemical service for the ecosystems in which they live, but in the process they produce sulfides, which bind to mercury in water to create methylmercury. That sulfide, as it turns out, is mercury’s ticket into the body: with a sulfide on board, mercury is able to easily cross cell membranes, getting into cells and wreaking havoc with cell function.
Methylmercury becomes more concentrated in animal tissues the higher up the food chain you go. The process is called biomagnification. It explains why wild-caught Alaska salmon is very low in metheylmercury, while tuna and farmed salmon are both higher. Both tuna and farmed salmon are higher on the food chain than wild Alaska salmon: tuna, because it eats more fish that have eaten other fish, and farmed salmon, because it consumes a variety of fish and other animals through its feed.
For children, infants and fetuses, the effects of methylmercury on the brain and nervous system could range from subtle developmental delays to serious neurological and physical impairments. Methylmercury poisoning in adults was historically referred to as “mad hatter’s disease.” People who were regularly exposed to mercury, such as hatters who breathed mercury fumes in the course of their work, exhibited the signs of acute mercury poisoning—impaired vision, movement, hearing and speech, hallucinations and dementia. According to the Environmental Protection Agency, a more subtle sign of mercury poisoning could be a sensation of pins and needles, especially “in the hands, feet and around the mouth.” Long-term health effects from methylmercury exposure (that don’t meet the criteria of acute mercury poisoning) include neurological and cardiovascular disease.
Research shows that, worldwide, there is twice as much man-made mercury deposited in water systems from air pollution as there is naturally-occurring mercury in water. Mercury that is naturally occurring in rock is sometimes transported into water more quickly from human activities, such as mining, that churn up the rock and expose it to air and water.
Most Americans have some methylmercury in their blood, typically at concentrations too low to cause health effects, according to the Centers for Disease Control. However, there are still occasional outbreaks of mercury poisoning, and some populations are more at risk.
In addition to blood sampling, one way of measuring mercury in the body is through hair samples.
When researchers analyzed hair samples from the 500-year-old mummified remains of Alaska Native Aluutiq Eskimos, they found levels of methylmercury that were lower or comparable to modern Eskimo populations worldwide, even though modern-day populations tend to consume fewer fish and marine mammals.
Benefits of seafood
But here’s the rub: fish and marine mammals contain nutrients that have been shown to enhance neurological and cardiovascular health, and may even counteract the toxic effects of mercury in the body.
Dietary and health research conducted in Western Alaska Yup’ik communities in the 1970s showed that people living on a traditional subsistence diet had dramatically lower incidences of cardiovascular disease, diabetes, and some cancers than most populations worldwide, even though the traditional diet contained a much higher percentage of calories from fats. Since then, researchers have studied a variety of nutrients in fish and sea mammals that could offer a protective health benefit for people who eat them. These include omega-3 fatty acids, which could reduce the risk of coronary heart disease and dementia, reduce inflammation, ease symptoms of depression, and support brain and nervous system health, according to a 2009 review of scientific studies on omega-3s published in the journal Nutrition and Food Science.
But some other, less well-known nutrients in seafood could provide important health benefits as well. Alaska salmon is high in magnesium, an essential mineral for regulating mood. It is one of the very few food sources of Vitamin D, a nutrient vital for bone health that is normally created by the human body in response to sunlight, but often appears in inadequate levels among people living in the arctic who don’t eat much salmon. A diet reliant on sea mammals and fish is also naturally high in antioxidants, such selenium and vitamin E.
According to a 2004 report from the State of Alaska Department of Epidemiology on mercury in traditional foods, fish oil has been shown to reduce methylmercury absorption in animal studies.
Finding the right balance
The many health benefits of eating fish, along with the cultural importance of fish to Alaskans, has led the State of Alaska to take what it calls a “risk and benefit approach” to what it advises when it comes to eating Alaska fish.
“We recognize there are many benefits to eating fish. It’s an important source of nutrients and protein. You have to consider if people don’t eat fish, what are they going to eat?” said Nim Ha, a health educator with the Alaska Department of Health and Human Services.
In contrast, she said, the Environmental Protection Agency uses a risk-based approach that is designed to protect public health from the detrimental effects of mercury exposure, but doesn’t take into account the health benefits of eating seafood.
Using tissue samples measuring methylmercury in Alaska fish species, and applying a calculation used by the EPA to determine how much of a species of fish is safe to eat, researchers from the University of Alaska Fairbanks derived fish consumption recommendations for children, pregnant women and adults. For example, they found that, based on the EPA formula and levels of methylmercury found in Alaska sockeye salmon, it would be safe for an adult to eat 31 meals of sockeye a month, or 16 king salmon meals per month, but only one pike meal per month. For children, the consumption limit was calculated to be three king salmon meals, six sockeye salmon meals, or one-fifth of a pike meal per month.
But that risk-based approach is a very conservative one that sets acceptable methylmercury exposure levels at a lower level than would typically cause health effects in a person, said UAF researcher Lawrence Duffy, who worked on the 2007 study. The approach is protective of the broader public health, taking into account that individuals in a population vary in how likely or unlikely they are to be negatively affected by methylmercury exposure, Duffy said. And, he said, the EPA-set acceptable level for exposure includes a buffer, or “safety factor” between the recommended upper intake limit, and the amount that would be unsafe to ingest. Duffy said the method’s abundance of caution doesn’t make sense for Alaska.
“The EPA level is a good level, but it has to be tempered based upon place location, and based upon what the benefit of fish is,” Duffy said.
In urban Alaska communities, he said, people have the option of going to the store and purchasing alternative sources of protein if they choose, but in rural Alaska communities where store-bought food is prohibitively expensive or unavailable, the alternative to eating less fish might be malnourishment.
He advocates for what he calls a “place-based approach” to making recommendations about how much Alaska fish people should eat. Such an approach would take into consideration the importance of fish species to the local community, he said, as well as variations in the amount of methylmercury in the same species of fish.
All five species of salmon are low in methylmercury, he said, whereas pike have higher levels. But even in pike, the methylmercury amounts found in samples vary greatly.
“Your pike might not have a lot of mercury in it,” Duffy said, “But 10 miles up river, the pike might have a lot of mercury in it.”
Right now the Alaska DHHS recommends Alaskans eat as much Alaska fish as they want to—with a few exceptions, such as pike, in which case the department issues fish advisories recommending moderate restrictions.
The state’s fish consumption guidelines are based on the Alaska Department of Environmental Conservation’s fish monitoring program, Ha said, but haven’t been updated since 2007 and don’t reflect levels of contaminants found in fish since that time.
“They have thousands more fish samples that we’re planning to analyze, and we will address this guidance accordingly,” Ha said. “It’s not static, it’s an ongoing process where ideally every one to two years you look at the most current data and then revise your fish consumption guidances to reflect the current situation.”
The Alaska DHHS hasn’t had the staff or resources to update the fish consumption guidelines, Ha said, but it recently hired a toxicologist who has the expertise to conduct the work necessary for creating guidelines based on fish tissue samples.
In the meantime, Duffy is hoping the data-based mathematical models he and his colleagues are trying to develop will give the state a more precise means of determining how much of each species of Alaska fish people could eat in order to maximize the health benefits, and minimize the health risks.
“The consequences of not eating the fish, and not having the omega-3s, might be worse than having a little bit of mercury,” Duffy said.