A multi-university team reports that mercury from fish does not raise the risk of heart disease or stroke.
Researchers led by doctors at the Harvard School of Public Health compared mercury levels in toenail clippings from 173,229 men and women to their heart health histories.
As the authors wrote,” We found no evidence of any clinically relevant adverse effects of mercury exposure on coronary heart disease, stroke, or total cardiovascular disease in U.S. adults at the exposure levels seen in this study.” (Mozaffarian D et al. 2011)
In fact, the participants with higher mercury levels actually had slightly lower heart disease rates … an outcome the team attributed to the beneficial omega-3s, vitamin D, and selenium in ocean fish.
No heart harm seen at highest mercury levels
The observed lack of harm held true even for people with the highest mercury levels – 1 microgram per gram – which are about twice the recommended limit for pregnant women.
Average toenail mercury levels measured 0.7 micrograms per gram among study participants in the top 20 percent of mercury levels.
Current U.S. advisories for children and pregnant/nursing women advise keeping mercury exposure below a level that would produce toenail levels of 0.4 micrograms per gram.
The authors noted that their research should not change advisories for eating fish with higher mercury levels among women who are or may become pregnant.
“There is no strong or moderate evidence that mercury has any effects in adults," Mozaffarian told HealthDay News. “So it's important and helpful that people can feel comfortable with fish as a normal part of their diet.”
“It doesn't mean we can stop worrying about mercury in the environment,” he added. “But for the individual consumer making a decision about eating fish, they can take this worry off the table.” (Salamon M 2011)
The study was funded by the National Institutes of Health, and had no support from seafood interests.
Details of the study affirm seafood as good for heart health
The new study used information from male physicians participating in the Health Professionals Follow-up Study (begun in 1986) and from women enrolled in the Nurses’ Health Study (begun in 1976).
The analysis looked at mercury levels in toenail clippings submitted by participants when the work started decades ago.
Because toenails grow slowly, they provide a more accurate assessment of long-term mercury exposure, compared with hair samples.
The researchers recorded the mercury levels in toenail samples from 3,427 participants who had suffered strokes or developed heart disease about 11 years after enrolling in the original studies.
They then compared those levels to the mercury levels in toenail samples from an equal number of participants who did not develop heart problems.
After adjusting for factors such as age, gender, and smoking, there was no link between mercury exposure and risks for heart disease or stroke.
This applied to people with an average toenail mercury level of 1 microgram per gram, which is about twice the recommended limit for pregnant women.
Because fish provide heart-healthy omega-3 fatty acids, the American Heart Association recommends that adults eat fish at least twice a week.
The U.S. EPA and FDA currently advise pregnant/nursing women and young children to avoid fish with the highest mercury concentrations and limit themselves to 12 ounces a week of fish such as shrimp, salmon and catfish that contain lower mercury levels.
Outcome ratifies overall seafood safety
The new analysis, led by Harvard cardiologist Dariush Mozaffarian, M.D., strengthens the case for seafood safety, which rests in large part on two facts.
First, the most popular seafood species – shrimp, and fish like salmon, cod, pollock, and tuna – are low in mercury.
(Farmed catfish and tilapia have even less mercury, but are low in omega-3s and high in pro-inflammatory omega-6 fats, due to their grain- and soy-oriented feed regimens.)
The second reason for the well-documented safety of seafood is the long overlooked – but scientifically undisputed – relationship between selenium and mercury in the body.
Almost all seafood is very rich in selenium, which is essential to life and health, but is bound up by and rendered useless by mercury.
However, when a food delivers more selenium than mercury, that ratio renders the selenium-binding activity of mercury virtually irrelevant.
This characteristic of seafood explains the remarkable lack of evidence linking fish-heavy diets to mercury-related harm.
This lack of harm was noted in a review paper published last year by Harvard-based researchers who wrote, “Net health benefits of overall fish consumption in adults are clear.” (Park K, Mozaffarian D 2010).
And it was echoed by leading experts in selenium-mercury biology: “… studies of populations exposed to MeHg [mercury] by eating Se [selenium]-rich ocean fish observe improved child IQs instead of harm.” (Ralston NV, Raymond LJ 2010)
The only known exceptions are marine mammals (seals, whales, and porpoises) and a handful of less-popular species like shark, which contain substantially more mercury than selenium.
Bates CJ, Prentice A, Birch MC, Delves HT. Dependence of blood indices of selenium and mercury on estimated fish intake in a national survey of British adults. Public Health Nutr. 2007 May;10(5):508-17.
Mozaffarian D et al. Mercury Exposure and Risk of Cardiovascular Disease in Two U.S. Cohorts. N Engl J Med 2011; 364:1116-1125March 24, 2011
Mozaffarian D. Fish, mercury, selenium and cardiovascular risk: current evidence and unanswered questions. Int J Environ Res Public Health. 2009 Jun;6(6):1894-916. Epub 2009 Jun 23. Review.
Park K, Mozaffarian D. Omega-3 fatty acids, mercury, and selenium in fish and the risk of cardiovascular diseases. Curr Atheroscler Rep. 2010 Nov;12(6):414-22.
Peterson SA, Ralston NV, Peck DV, Van Sickle J, Robertson JD, Spate VL, Morris JS. How might selenium moderate the toxic effects of mercury in stream fish of the western U.S.? Environ Sci Technol. 2009 May 15;43(10):3919-25.
Ralston NV, Raymond LJ. Dietary selenium's protective effects against methylmercury toxicity. Toxicology. 2010 Nov 28;278(1):112-23. Epub 2010 Jun 16. Review.
Salamon M. Americans' Exposure to Mercury From Fish Won't Harm Hearts. HealthDay News. March 23, 2011. Accessed at http://health.usnews.com/health-news/family-health/heart/articles/2011/03/23/americans-exposure-to-mercury-from-fish-wont-harm-hearts-study