The Danger of Mercury Poisoning From Fish

Jane Hightower, MD

On January 12, 2001, the FDA's press office released a statement that stated pregnant women and women of childbearing age who may become pregnant, nursing mothers and young children should not eat shark, swordfish, tilefish, and king mackerel. These individuals can safely eat up to 12 ounces of any other cooked fish per week. The EPA issued a statement for non-commercial fish on that same day for the above group to limit their consumption of fresh-water non-commercial fish to one meal a week and to check with their local health department to see if they recommend eating less.

If you missed this announcement, I am not surprised. There was no press conference and you certainly have not received a letter in the mail, or warning at the market or restaurant.

In the last 20 years, many of us have been reading about how good fish is for you. I recently discovered elevated mercury levels in many of my fish-eating patients. Many of these patients did not grow up eating fish, do not like a strong fishy flavor and do not like dealing with the bones. They are also not deterred by the price of their fish. Hence, they eat the large predator fish by the steak and often. This is a life-style which is unique to coastal communities. The fish most popular for them are swordfish, Ahi, halibut, seabass, snapper, sushi and canned tuna. Sixty-two of the patients had levels >10 mcg/L, 20 were greater than 20 mcg/L, and 4 were greater than 50 mcg/L. The most common symptoms for those affected were fatigue, hair loss, trouble thinking, memory loss, muscle aches and headache. A unique symptom was metallic taste. The levels also did not necessarily correlate to their level of symptoms.

In the late 1970s the FDA and one of the fish market owners went to court to settle how much mercury should be allowed for human consumption. Two categories of fish were agreed upon. Non-predator fish were to be less than 0.5 ppm (mcg/g). Predator fish were to be 0.5 to 1.0 ppm. Over 1.0 ppm was the FDA "action level," and was not to be sold on the market.

In 1998, the last known FDA survey, only 25 fish were tested by them for mercury in the entire United States and as of 1998, shark, swordfish and large tuna such as ahi have no longer been tested. The February, 2001 issue of Consumer Reports tested fish and found swordfish to be over 1ppm 50 percent of the time. I recently worked with local and national television networks on this subject. One of the swordfish steaks in a San Francisco market which they tested was >3 ppm.

Fish accumulate methyl mercury in their tissues which becomes strongly bound. It cannot be cooked out of the fish. This organic compound is nearly 100 percent absorbed when we eat it. It is lipophilic and can traverse every cell in the body. It is strongly bound to sulfhydril groups in our tissues which gives it a strong affinity for our brain and muscles. It binds to proteins involved with the production of microtubules and, in doing so, interferes with mitoses. For a developing brain, that can spell disaster. The half-life of consumed mercury for adults is approximately 54 days. It is excreted predominantly in the feces, but also in sweat and urine. Five percent will be excreted in breast milk. By the time of parturition, a baby's level can be 30 percent to nearly 200 percent that of the mothe's blood. This is because mercury readily passes the placental barrier and fetuses do not excrete and sweat in-utero to rid themselves of it.

Mercury cannot be created or destroyed. We just move it about our planet. The biggest polluters are the coal-burning power plants. The EPA studied the world's literature on mercury and their report was released in 1997 as the Mercury Study Report To Congress. Because of delays in its release and other new information Congress asked the National Academy of Sciences to evaluate the literature. Their report was released July, 2000. These two reports confirmed that the "no adverse affect level" for mercury is <5.0 mcg/L whole blood. The "benchmark dose calculation" (how much you can eat), was estimated to be 0.1 mcg/Kg body wt/ day. This is to protect fetuses, infants, children, sensitive individuals and exposure over a life-time. The "fudge factor" is still being debated and can be 3-10 fold, depending on which report you read.

To give an example, salmon has a mercury level on average of 0.035 mcg/g. A 60 Kg person could eat 42 mcg Hg/ week and stay within the current EPA guidelines. This would be about 2 lbs per week of salmon. For swordfish at 1 mcg/g this would be less than 2 ounces per week. For canned tuna at 0.206 mcg/g, a child should not eat more than 1 ounce per 20 lbs. body wt./week. Although the recent warning did not include large tuna, many researchers and advocates, including myself, feel that it should have. Canned tuna is also still being questioned as people eat so much of it.

The levels at which mercury can do harm is still being studied. As with most toxins, mercury poisoning is a continuum. It is recognized that a baby whose mother has a level of >24 mcg/L whole blood has a risk of learning difficulties, troubles with reading comprehension, fine motor skills, mild cerebral palsy, language skills. From 5-24 mcg/L there could be more subtle effects and there is considerable overlap with levels of the mother and degree of disability. What is known is that the damage sustained from the effects of mercury in a developing brain is permanent.

Mercury excess in adults can parallel natural aging; therefore it has always been a difficult problem to assess. Recent studies show that the brain lesion it produces in rats is similar to that in Alzheimer's patients. It can also stimulate auto-immune phenomena in those who are genetically susceptible and decrease the body's ability to fight infection. As for cancer, it is still category C, unknown risk. A recent Finnish study showed that men who ate fish and had very modest increases in their level of about 8 mcg/L had twice the myocardial infarction rate as fish-eating individuals without the elevation. My patients' symptoms are in line with other large population studies.

Treatment for my patients consisted of either no fish for six months or stopping the higher mercury-content fish. For these people, their levels have been falling to the normal <2 mcg/L, and symptoms have been slowly improving, but it is too early to tell if they will completely resolve. Also, the subjective nature of their symptoms make it difficult to analyze. When they return to eating fish, I advise them to stay well within the EPA guidelines so as not to accumulate mercury at all, until further research is concluded. I have not seen anywhere in the literature that chelation is necessary for these patients, therefore; I have not been advising patients to do so.

So, if the scientific community continues to tell us to eat fish, they should tell us what type and advise accordingly, so we can stay healthy with the benefits of those omega 3's and keep the heavy metals, PCB's, organochlorides, etc., to a minimal risk level.

Dr. Hightower is a board-certified internist, with a private practice in San Francisco since 1991. Her interest in the subject of fish and mercury was inspired by her patients and colleagues who have been questioning their symptoms and Hg levels. She is a member of the SFMS.

References

1. United States Environmental Protection Agency, Office of water. Mercury Update: Impact On Fish Advisories. http://www.epa.gov/ttnuatw1/112nmerc/mercury.html
2. Mahaffey KR, Rice GE, Environmental Protection Agency Office of Air Quality Planning and Standards. Mercury Study Report to Congress. Govt Reports Announcements and Index (GRA and I), Issue 09, 1998. Also Dec, 1999. http://www.epa.gov/ttnuatw1/112nmercury.html.
3. National Academy of Sciences. Toxicological Effects of Methylmercury, 2000; http://books.nap.edu/books/0309071402/html.
4. America's Fish Fair or Foul? Consumer Reports February, 2001.