Here’s the conundrum. Seafood contains omega-3 fatty acids, which are good for you. But seafood also contains mercury, which is not (think the Mad Hatter). So given the tradeoff, should you have that tasty swordfish steak? Will it improve your health, or undermine it? A new study answers the first of the two questions, and perhaps the second.
Let me restate the issue, seafood contains healthful nutrients, specifically omega-3 fatty acids, which are thought to reduce the burden of cardiovascular disease. The dietary guidelines recommend eating 8 ounces of seafood a week – two portions. But seafood has a bad rap when it comes to containing mercury. Studies among those populations that consume a lot of fish and have a great deal of mercury exposure demonstrated increased cardiovascular disease. Of course, we in the US eat less seafood in general than those populations, about a tenth as much, so how risky is that lobster roll, fish stick, or tuna tartare?
The researchers made use of our old friend, the National Health and Nutrition Examination Survey (NHANES), which conveniently enough had both food questionnaires and laboratory data on individuals’ levels of mercury. The participants between 2003 and 2016 had completed two 24-hour dietary recalls (yes, I know this makes for uncertainty), along with a blood test for mercury. Our ability to detect mercury had improved during the entire period, so earlier “undetectable” values were assigned a low value, further adding a bit of statistical uncertainty. The outcomes, dead or alive, were more certain, as were the cause of death attributed to their death certificates.
The researchers also had access to the usual demographics and risks, including categorizing physical activity and overall “healthy” eating as defined by the Healthy Eating Index (HEI), a practical tool for this type of assessment. The cohort under review, roughly 17,300 participants, age 46, and slightly more female (53.3%), had mean blood mercury concentrations of 1.62 μg/L (Those previously mentioned populations eating a lot of seafood had an average level of 10.0 μg/L)
- Mercury concentrations were “weakly” correlated with fish consumption
- Those with higher concentrations were more often older, with greater education and income (seafood is not cheap!), drinkers, smokers, with a higher dietary quality (that HEI), lower BMIs, and exercised more. They were less likely to have diabetes or a family history of cardiovascular disease, “although they had higher total cholesterol levels.
- Higher mercury concentrations were associated with higher cadmium and lead blood levels.
So these individuals, from a health perspective, were a mixed bag with good and bad habits – just like you and me. Overall there were slightly more than 1000 deaths, 181 due to cardiovascular disease leading the researchers to conclude there was:
“no association between an increase in seafood consumption of 1 oz equivalent per day and all-cause and CVD-related mortality."
Actually, the adjusted risk was 0.84 overall, 0.89 for CVD-related mortality, so the enthusiastic might claim a seafood benefit. The researchers tried to quench that enthusiasm by pointing out that there is no bioplausible explanation of why methylmercury (the form ingested from seafood) would provide a healthful benefit. They were also quick to point out that methylmercury is definitely a bad choice for “pregnant women” because of the detrimental neurotoxicity to their children.
Predator fish are the most concerning because they accumulate mercury from all the other fish they eat. That said, perhaps the EWG might want to correct this chart
And the NRDC may also want to reconsider that advice. For those not pregnant, seafood can be part of a balanced diet, and there should be no fear of any detrimental effects except perhaps to your wallet; prices have risen nearly 19% in the last few months.
Source: Association of Seafood Consumption and Mercury Exposure With Cardiovascular and All-Cause Mortality Among US Adults JAMA Network Open DOI: 10.1001/jamanetworkopen.2021.36367