Data on thousands of health professionals put to rest the fear that seafood-eaters face a higher risk of cardiovascular disease because of the mercury in some species.
The researchers analysed toenail clippings and diet questionnaires to assess previous exposure to mercury. They found that people with the greatest exposure were no more likely to experience a heart attack, stroke or cardiovascular death than those with the least exposure.
The results from 6,854 participants in the Nurses’ Health Study and the Health Professionals Follow-Up Study appear in the March 24 New England Journal of Medicine.
"I think this shows as definitively as possible from an observational study that there's no signal for harm for the general population," Dr Dariush Mozaffarian of the Harvard School of Public Health and leader of the study said.
The study should have no effect on the recommendation that pregnant women, who can have healthier babies if they eat fish, nonetheless avoid species that aggregate mercury, which can cause subtle neurodevelopmental delays in their babies.
Mercury-laden fish are swordfish, shark, king mackerel from the Gulf of Mexico, and tilefish, also known as blanquillo. Doctors also urge women to limit consumption of albacore white tuna to once a week.
Fish lowers heart disease
Among other adults, fish consumption in general has long been linked to a lower risk of cardiovascular disease because fish is rich in omega-3 fatty acids.
But when it comes to assessing the risk of heart disease from methyl mercury exposure, "the previous studies had been quite mixed and relatively small compared to this study," Dr Mozaffarian said.
From the 51,529 men in the Health Professionals Follow-Up Study and the 121,700 women in the Nurses' Health Study, the researchers identified 1,532 individuals who had a nonfatal heart attack, 831 who had died from coronary heart disease, and 1,064 who had experienced a stroke. Those people were matched by age, race, sex and smoking status to 3,427 from the databases who had not experienced those problems.
When their toenail clippings, collected in the 1980s, were measured for mercury using neutron-activation analysis, the researchers found median concentrations of 0.23 mcg per gram for the cardiovascular patients versus 0.25 mcg per gram for the control group.
Mercury exposure actually dropped the risk for cardiovascular disease by about 15%, but the confidence intervals were too broad to be significant.
"Further adjustment for consumption of saturated fat, monounsaturated fat, polyunsaturated fat, trans fat, dietary cholesterol, and total energy had little effect on the results," the researchers conclude.
Some doctors have suggested that selenium intake may affect the risk, but the Mozaffarian team, which measured levels of that element in the clippings as well, found no link there either.
Why toenail clippings?
"Mercury tightly binds to proteins, so hair and toenails provide an excellent source of disposable tissue to measure exposure over months, in the case of hair, and a year in the case of toenails," said Dr Mozaffarian.
"The nail in your small toe has been growing for 2 or 3 months and the nail in your big toe has been growing for almost a year by the time you clip it." he said. "It gives you a time-integrated measure of all exposures over the last year." (Reuters Health/ March 2011)
Mercury in fish not toxic