This is a tale of fish, its demonstrable health benefits, and its less demonstrable health risks.

The issue is not so much whether there are beneficial heart effects from most fish, a clearly shown phenomenon, but whether they are outweighed by the public’s perception of risks of mercury poisoning in some fish, an issue that is less well defined.

In particular, researchers looked at fish consumption among women of childbearing age.

It turns out, say researchers, that the heart benefits are extremely important and the mercury risks are less clear and easy to avoid.

But it also turns out that this is a difficult and complex public health message, according to a series of articles in the November issue of The American Journal of Preventive Medicine.

Researchers at the Harvard Center for Risk Analysis and other centers weighed the evidence for a protective effect of fish consumption on heart disease and stroke risk versus possible negative consequences on cognitive function of methyl mercury in fish.

They looked askance at the unintended impact of federal health warnings suggesting that women of childbearing age limit fish consumption. Women who shun fish out of fear of mercury-induced harm to fetuses may lose the cardiovascular benefits of fish consumption, when they could instead substitute fish with lower levels of mercury.

On a broader scale, the papers explored the promises and pitfalls of communicating health messages to the public. They looked into whether the narrow mercury-risk message might have been absorbed by the population at large to the detriment of the broader, more important heart-healthy message.

Fish are an important dietary source of n-3 polyunsaturated fatty acids (PUFAs), which have been demonstrated to offer cardiovascular and cognitive benefits, but some species also contain comparatively high levels of methyl mercury (MeHg), a neurotoxin.

“Because fish consumption confers both benefits and risks, the advisories issued by the U.S. federal government raise the possibility of a classic risk-risk trade-off,” wrote Harvard’s Joshua T. Cohen, Ph.D., and colleagues in the main paper. “By avoiding one risk (exposure to MeHg), consumers who follow these advisories may be incurring another (adverse health consequences associated with lower n-3 PUFA intake). Likewise, individuals who increase their consumption of fish because of this food’s nutritional benefits may incur risks associated with MeHg exposure.”

To try to get a handle on whether possible shifts in fish consumption based on public health recommendations could have unintended consequences, the Center for Risk Analysis convened an expert panel to look at the effects of fish consumption on prenatal cognitive development, coronary heat disease mortality, and stroke.

On the plus side, the panel found that consuming even small quantities of fish was associated with a 17% reduction in coronary heart disease mortality risk, and that each additional serving per week was associated with a further reduction in this risk of 3.9%.

In addition, modest fish consumption was associated with a 27% reduction in risk of nonfatal myocardial infarction, but additional fish consumption per week did not appear to offer any additional benefits.

Similarly, the investigators found that any fish consumption lowered the relative risk of stroke by about 12%, and that each additional serving per week appeared to offer an additional 2% risk reduction.

However, when they looked at whether PUFAs such as docosahexaenoic acid (DHA) could boost brain power prenatally, they found that a pregnant woman would have to take in 1 gram of DHA per day to increase her child’s IQ by 0.8 to 1.8 points.

“Because typical DHA intake associated with fish consumption is well under 1 g/day, changes in fish consumption will result in IQ effects amounting to a fraction of a point,” the authors wrote. “These differences are not clinically detectable.”

They noted, however, that “as with changes associated with exposure to neurotoxins like lead, which are also typically undetectable at the level of the individual, these changes can result in important impacts when aggregated over a population.”

On the negative side, they reviewed evidence from three major prospective epidemiology studies, and found that prenatal MeHg exposure sufficient to increase the concentration of mercury in the mother’s hair at the time of birth by 1 microgram/gram would decrease the infant’s IQ by 0.7 points, although uncertainties about the estimate mean that the actual effects could range from 0 to 1.5 IQ points, the investigators wrote.

The risk of MeHg exposure to children’s cognitive abilities appears to be significantly lower than the risk of lead exposure, they noted, but added that the risk is not negligible, and could be cumulative.

What to make of all of this? In their final analysis, the authors concluded that “substitution of fish with high MeHg concentrations with fish containing less MeHg among women of childbearing age yields substantial developmental benefits and few negative impacts.”

If, however, women of childbearing age and other adults were to cut down on fish eating altogether out of concern for theoretical mercury exposure, there would be an overall detrimental effect on public health, they noted.

“Although high compliance with recommended fish consumption patterns can improve public health, unintended shifts in consumption can lead to public health losses,” the investigators wrote. “Risk managers should investigate and carefully consider how populations will respond to interventions, how those responses will influence nutrient intake and contaminant exposure, and how these changes will affect aggregate public health.”

In an accompanying editorial, Walter C. Willett, M.D., Dr.Ph., of the Harvard School of Public Health wrote, “This analysis supports current guidelines that focus on changes in the type of fish eaten by women in the reproductive age, but also highlights concerns that educational messages and the implementation of policies must be carefully crafted to avoid unintended consequences.”