Response to Methylmercury Levels in Seafood

The recent Chicago Tribune article (The Mercury Menace – December 11, 2005) has raised concerns regarding the safety and efficacy of seafood in a healthy diet. As we at Entertaining Tuna share these concerns, we felt the need to address this issue.

The following information comes from a web site created by the University of Maryland (http://www.realmercuryfacts.org) and includes information gathered from studies sponsored by government regulatory and public health agencies based in the United States, United Kingdom, World Health Organization as well as several university studies.

Attitudes and Beliefs About Eating Fish:

Fish is part of a healthy diet. There is a growing body of research linking the omega-3 fatty acids found in fish with optimal brain function and cognition and improved eye and skin health. Moreover, studies show that these omega-3 fatty acids are protective against certain cancers and may have a therapeutic effect on depression and specific autoimmune diseases including lupus, psoriasis and arthritis. Because of these significant health benefits, the 2005 Dietary Guidelines for Americans issued by the Department of Health and Human Services (HHS) and the U.S. Department of Agriculture (USDA) and guidelines from the American Heart Association and the American Dietetic Association recommend consuming two eight-ounce servings of fish a week for optimal health.

But while the health benefits of fish are well known, almost all ocean fish and seafood naturally contain minute amounts of methylmercury, an organic form of mercury that can be toxic to humans at very high exposure levels. Of key concern is that methylmercury may harm an unborn baby’s or young child’s developing nervous system if pregnant and nursing women consume large amounts of fish containing high levels of methylmercury. To safeguard the public, the two federal agencies responsible for regulating mercury levels in fish -- the U.S. Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) -- have put in place very stringent restrictions, including issuing a joint seafood advisory for pregnant and nursing women, women who may become pregnant and young children about the best ways to add fish to their diets.

According to the joint FDA/EPA advisory, these individuals can safely consume up to 12 ounces a week of fish that are low in mercury, such as shrimp, salmon, pollock, catfish and canned light tuna. The advisory also tells these special groups that they can safely eat up to 6 ounces a week of canned albacore tuna. In general, there are 2 ounces of albacore in a typical serving. However, FDA and EPA have also identified those types of fish that have higher levels of methylmercury and should be avoided by pregnant and nursing women, women who might become pregnant and young children. These fish are shark, swordfish, tilefish and king mackerel.

In the United States, the basis for regulatory policy is the “reference dose” developed by the U.S. Environmental Protection Agency (EPA). According to EPA, the reference dose (RfD) is an estimate of the highest daily dose of a chemical that the most sensitive in the population can be exposed to over a lifetime of exposure without experiencing an adverse effect. EPA’s reference dose for methylmercury in fish is 0.1 microgram per kilogram of body weight per day (µg/ kg-day) or 0.7 µg/kg bw/week, which builds in a ten-fold factor of safety for the most vulnerable populations.

EPA’s reference dose is also applied in developing seafood consumption advice for the U.S. population. The most recent advice was issued jointly by FDA and EPA in March 2004 and updates previous advisories issued separately by the two agencies. Focusing specifically on pregnant and nursing women, women who might become pregnant and young children, the advisory offers these major recommendations:

1. Do not eat shark, swordfish, king mackerel, or tilefish because they contain high levels of mercury.
2. Eat up to 12 ounces (2 average meals) a week of a variety of fish and shellfish that are lower in mercury. Five of the most commonly eaten fish that are low in mercury are shrimp, canned light tuna, salmon, pollock, and catfish.
3. Another commonly eaten fish, albacore ("white") tuna has more mercury than canned light tuna. So, when choosing your two meals of fish and shellfish, you may eat up to 6 ounces of albacore tuna per week.
4. Check local advisories about the safety of fish caught by family and friends in your local lakes, rivers, and coastal areas. If no advice is available, eat up to 6 ounces (one average meal) per week of fish you catch from local waters, but don't consume any other fish during that week.

By following this advice, pregnant and nursing women, women who may become pregnant and young children will consume the concentrations of methylmercury in fish that are at least ten times lower than the lowest level for any known risk. However, these special populations will also receive the many health benefits of eating fish.

Understanding the EPA’s Reference Dose

What is the “reference dose” set by the Environmental Protection Agency (EPA) to regulate the public’s exposure to mercury and other chemicals in the environment? The following is a layman’s explanation.

The reference dose (RfD) is an estimate of the highest daily dose of a chemical that the most sensitive in the population can be exposed to over a lifetime of exposure without experiencing an adverse effect. Used by EPA to determine amounts of a chemical that are not likely to be associated with any health risks, the reference dose is intended as a gauge, not an absolute number above which a problem is likely to occur. That is because EPA builds in a margin of safety every time it develops a reference dose for a chemical. Thus, EPA’s reference dose is intended to guide public policy by gauging the potential effects of exposure to a chemical over a lifetime.

As a result, EPA cautions the public not to confuse the “reference dose” with the term “safety factor.” In its paper “Reference Dose: Description and Use in Health Risk Assessments,” EPA states: “The term ‘safety factor’ suggests, perhaps inadvertently, the notion of absolute safety (i.e. absence of risk). While there is a conceptual basis for believing in the existence of a threshold and ‘absolute safety’ associated with certain chemicals, in the majority of cases a firm experimental basis for this notion does not exist.”

Regarding mercury in the environment, EPA has calculated the RfD at 0.1 microgram per kilogram of body weight per day (µg/ kg-day) or 0.7 µg/kg bw/week. To develop these levels, EPA scientists follow a standardized process involving these steps:

First, EPA determined the lowest dose of mercury at which a subclinical (meaning an actual effect was not observed) adverse effect was thought to have occurred. In the case of mercury and seafood, EPA did not use one of the landmark studies conducted in the Seychelles Islands -- where the population consumes an average of 12 fish meals a week -- because that study found no adverse effect from dietary exposure to mercury through regular fish consumption. Instead, EPA relied on a study conducted in the Faroe Islands, where the population’s exposure to mercury resulted primarily from the consumption of whale meat. This study found a small effect, which served as the basis for EPA to calculate the “benchmark dose lower level” (BMDL) for mercury. The BMDL is the lowest amount of a chemical at which an effect may occur.

According to EPA’s analysis, the BMDL is 58 parts per billion (ppb) of mercury in the blood or 12 parts per million (ppm) in hair. To put these amounts into perspective, during the 1950s when 111 people from Minamata City, Japan were poisoned by consuming fish contaminated with very high levels of mercury from a chemical plant, the lowest concentration associated with neurological problems was 50 ppm in hair.

Having established the BMDL for mercury, EPA then built in a ten-fold safety factor to calculate the reference dose. This meant dividing the BMDL by 10 and coming up with 0.1 microgram per kilogram of body weight per day (µg/ kg-day) or 0.7 µg/kg bw/week. Therefore, if women have this level of mercury in their blood, they actually have an amount that is ten times lower than the BMDL.

Calculating the reference dose by this very conservative process results in the most stringent regulatory standard for mercury in the world. For example, the United Kingdom’s Food Standards Agency (FSA) uses the safety standard applied by the World Health Organization (WHO) -- called the Provisional Tolerable Weekly Intake (PTWI). For the general population, the FSA uses a PTWI of 3.3 micrograms of methylmercury per kilogram of body weight a week (µg/kg bw/week). This contrasts with the 0.7 µg/kg bw/week standard established by EPA.

For pregnant women, the FSA has determined that a PTWI of 1.6 µg/kg bw/week will not affect the neurological development of the fetus. Further, the FSA stated that the PTWI of 3.3 ug/kg bw/week is appropriate for breastfeeding mothers because new data find that the level of mercury in breast milk is insignificant.

Besides being the most stringent regulatory standard, EPA’s reference dose is based solely on calculating possible risks, and does not factor in potential health benefits. This is especially important regarding fish consumption, which is an important part of a healthy diet. Accordingly, the Dietary Guidelines for Americans 2005 recommend that consumers eat two eight-ounce servings a week of foods rich in omega-3 fatty acids, such as fatty fish. At the same time, in 2004, EPA and the Food and Drug Administration (FDA) issued a joint advisory intended only for pregnant women and nursing women, women who might become pregnant and young children. The advisory states: “women and young children in particular should include fish and shellfish in their diets due to the many nutritional benefits.”

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