Recent research suggests that kids may be more sensitive to mercury than adults and that toxic effects may occur at blood levels that are significantly lower than the conventional upper limit. But does that mean kids shouldn’t eat fish at all?
I pay special attention to any research that concerns health risks to children. As a parent and a clinician, there are few things more important to me than protecting the well-being of our future generations.
There’s no shortage of studies indicating that mercury is toxic to both adults and children. Thus far, however, virtually all environmental policy in the US regarding exposure to mercury (and other toxins) has been based on risk to the “average adult.” Policy makers have paid little attention to the unique risks that toxins pose to infants, children, and other vulnerable populations.
This is a serious mistake because there are at least four differences between children and adults that suggest children are far more vulnerable to the effects of toxic chemicals:
- Children have greater exposures to chemicals for a given body weight. Kids drink seven times more water, eat three to four times more calories, and breathe twice as much air per pound of body weight than adults. Studies have shown that both low weight and short stature increase the toxicity of mercury and other compounds.
- Children’s ability to metabolize and eliminate toxic compounds is significantly lower than that of adults, because they lack the enzymes needed to break down and remove these chemicals from the body.
- Children’s early developmental processes are easily disrupted. We now understand that children have “windows of vulnerability” where exposures to even minute doses of toxins can disrupt organ function and cause lifelong dysfunction.
- Children have more time to develop chronic disease. Some diseases, like cancer and neurodegenerative conditions, can take decades to develop. This makes causality less obvious and difficult to track in studies. For example, at Minamata Bay in Japan, impaired cognition and dysfunctions in mood and behavior were found in adults who did not exhibit any clinical effects as children following developmental, early-life exposures. (1, 2, 3)
Fish consumption and mercury toxicity in kids: a growing concern?
These differences may explain the findings in a study called “Fish consumption, low-level mercury, lipids, and inflammatory markers in children.” (4) Researchers studied 100 kids in Oswego County, New York, between the ages of nine and eleven. Not surprisingly, they found that the kids who ate fish had better lipid profiles, with higher levels of HDL cholesterol and lower levels of triglycerides. This is consistent with a large body of evidence linking fish consumption with improved cardiovascular health.
Less expected was the finding that the fish-eating children had a blunted cortisol response and higher levels of inflammatory markers. Cortisol is a hormone that plays a crucial role in our ability to respond to and tolerate stress. The researchers speculated that mercury from the fish increased the production of inflammatory proteins, which in turn eventually suppressed cortisol levels. This was the first study to document an association between blood mercury levels, systemic inflammation, and endocrine disruption in children.
Kids may experience toxic effects of mercury at doses lower than previously thought.
Disturbingly, these harmful effects were observed at blood mercury levels far below the current safe limit of 5.8 μg/dL established by the WHO and the US National Academy of Sciences and National Research Center. The kids in this study had an average mercury level in the blood of 0.77 μg/dL, and, with one exception, all affected kids had a mercury level below 3.27 μg/dL.
This is not the first study to show that mercury can have toxic effects in children at levels that are below the current safety threshold. Researchers in Granada, Spain, found declines in cognitive function, memory, and verbal processing in preschool age children at levels below the conventional limits. (5)
Not so fast! Other studies show that kids benefit from fish consumption
After reading that last section, you might be ready to pull fish and seafood out of your children’s diet entirely. But before you do that, consider the following:
- Fish consumption may only make up about 7 percent of the mercury levels in the body, according to a study published in the journal Environmental Health Perspectives. (6)
- A large body of evidence suggests that maternal consumption of seafood during pregnancy protects against mercury-associated impairments in cognitive function, attention, and behavior. (7, 8, 9, 10, 11, 12)
- Fish and seafood contain nutrients that are vital to proper brain development in children, including the long-chain omega-3 fats EPA and DHA, selenium, zinc, and highly absorbable protein. (13)
- Studies on the impact of public health advisories to limit fish consumption suggest that they may mask the nutritional value of fish and lead to a greater reduction in fish intake than is desirable. (14, 15)
There’s no one-size-fits-all approach when it comes to fish consumption and mercury!
So far we’ve reviewed studies suggesting that mercury may be toxic in children at levels much lower than the established cutoffs, as well as studies that consistently indicate benefits from consumption of seafood by pregnant mothers and during early childhood. How do we resolve this apparent contradiction?
There are two primary answers. First, mercury content is highly variable across different species of freshwater and ocean fish and shellfish. Not surprisingly, many studies suggest that consumption of seafood species that are lower in mercury (and higher in selenium) leads to better outcomes than consuming those that are higher in mercury. (16, 17, 18)
Second, we now know that there are a variety of genetic, epigenetic, and environmental factors that collectively determine susceptibility to mercury toxicity in a given individual. For example:
- Several genes have been identified that affect the toxicokinetics of mercury. These include genes that affect mercury uptake (LAT1, LAT2, OAT1, OAT2), biotransformation (particularly glutathione-related genes like GS, GCL, GR, GPx, GGT, GST), distribution (SEPP1, GPX1, GPX4, MT1A, MT1E), and elimination (MRP1, MRP2, MDR1). (19, 20) Our current understanding is that genes account for about 30 percent of the variability observed in mercury levels in individuals with similar exposure.
- Research indicates that polymorphisms in other genes, such as BDNF, COMT, and 5-HTTLPR can amplify the toxic effects of mercury. (21, 22, 23)
- Epigenetic markers such as urinary porphyrin excretion, MMP-9 and MMP-2 protein levels, nitric oxide production, and low birth weight have been shown to modify the effects of mercury exposure. (24)
- Males appear to be more adversely affected by mercury than females, possibly because of greater retention in tissues and organs and lower excretion via urine and stool. (25, 26, 27)
- Methyl mercury, the primary form found in fish, is eliminated in the bile as a glutathione conjugate. This means that anything that affects glutathione will affect mercury clearance. A wide variety of genetic and environmental factors influence glutathione levels, including polymorphisms (aka SNPs) in glutathione-related genes, diet (glutathione itself is present in foods, as are glutathione precursor nutrients like selenium, copper, zinc, magnesium, B6, B12, folate, and vitamin E), chronic stress, chronic disease, physical activity, advanced age, toxins, and oxidative stress—to name a few!
This is just a partial list; there are many other factors, both known and likely unknown, that influence mercury’s effect on both children and adults.
The important takeaway is that we can no longer simply designate a particular blood level of mercury as “harmful” or “safe.” That is far too simplistic. As the authors of a review of the genetic influences on mercury toxicity stated:
“These reports acknowledge that tremendous interindividual and interspecies variation exists in exposure and hazard and that complex gene-environment interactions may underlie such variation but as yet remain unresolved. With no resolution to such variability, decision-making is hampered and uncertain.” (28)
In other words, it’s hard for policy makers and clinicians to make recommendations about mercury exposure when there’s so much variability in how people respond to it.
What this means for your children
In the not-too-distant future, we’ll be able to run a battery of tests that quickly determine individual susceptibility to mercury (and other toxins) and make customized diet and lifestyle recommendations based on those results.
In fact, we’ve been working to create a diagnostic algorithm with exactly that purpose at California Center for Functional Medicine. Unfortunately, this approach is in its infancy and we still have a lot to learn. Another challenge is that the amount of testing that is required to identify all of the factors that influence how an individual responds to mercury is extensive—and expensive.
In the meantime, here are the recommendations that I can make based on my current understanding of the science:
- Give your children seafood at least twice a week, but restrict to species that are lowest in mercury. These include anchovies, sardines, clams, North Atlantic mackerel, herring, and salmon. Conveniently, many of these species are also highest in the beneficial omega-3 fats EPA and DHA. Another option is to avoid seafood but give your children cod liver oil or fish oil. However, although the marine oils do contain EPA and DHA (and vitamins A & D in the case of cod liver oil), they do not contain other beneficial nutrients found in seafood, such as selenium, zinc, and copper. This may not be an issue if your children are eating a diet that is nutrient-dense overall (see below).
- Have your kids avoid or significantly limit consumption of species that are higher in mercury. These include yellowfin and canned albacore tuna, Spanish and king mackerel, Chilean sea bass, marlin, orange roughy, shark, swordfish, and bigeye/ahi tuna.
- If your children develop cavities, take them to a dentist that uses composite resin fillings instead of mercury amalgams. Dental amalgams have consistently been shown to be a source of mercury exposure in both children and adults. (29) Mercury from amalgams readily crosses the blood brain and placental barriers, where it is oxidized to inorganic mercury. Many experts in mercury toxicity believe that inorganic mercury presents a greater challenge to the body than methyl mercury from fish.
- Feed your children a nutrient-dense diet with plenty of glutathione, selenium, copper, zinc, magnesium, B6, B12, folate, and vitamin E.
- Ensure that your children are doing other things that support glutathione pathways, such as getting adequate sleep, exercising regularly, and minimizing exposure to other environmental toxins.
If you’re working with a functional medicine provider, another step you can take is to have your children tested to determine how much mercury they are absorbing from fish consumption.
In our clinic, we prefer and use the Quicksilver Mercury Tri-Test. I recently interviewed Dr. Christopher Shade, the director of Quicksilver Scientific, about the various test methods available for mercury and the shortcomings of the “mercury challenge tests” that have been popular in the alternative medicine community. Check that out if you haven’t already.
I would also recommend reading a guest post entitled “Could Mercury Toxicity Be Causing Your Symptoms?” by my staff physician, Dr. Amy Nett. This may help you to recognize potential effects of mercury exposure in your children (and yourself).
I’ll continue to review new research on this topic as it is published and report back with any important developments.
Happy safe fish eating!