Friday Favorites: Avoiding Fish for Five Years Before Pregnancy

Below is an approximation of this video’s audio content. To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video.

If you intentionally expose people to mercury by feeding them fish, like tuna, for 14 weeks and then stop, this is what happens to the level of mercury in their bloodstream: it goes up, up, up, and then as soon as you stop fish, it drops back down, such that you can detox down half in about 100 days. So, the half-life of total mercury in your blood is approximately 100 days. So, even if you eat a lot of fish, within a few months of stopping, you can clear much of it out of your blood. But what about out of your brain?

“Modelling studies” are all over the place, suggesting half-lives similar to blood at 69 days all the way up to 22 years. But when you put it to the test, autopsy studies suggest it may even be longer still. Once mercury gets in your brain, it can be decades before your body can get rid of even half of it. So, better than detoxing is to not “tox” in the first place.

That’s the problem with these fish advisories, where they tell pregnant women to cut down on fish. For pollutants “with long half-lives [like PCBs and dioxins,] temporary…decreases in [fish consumption] daily contaminant intake, will not necessarily translate to appreciable decreases in maternal [persistent organic pollutant] body burdens, [which is what helps determine the dose that the baby gets].”

For example, here’s how much exposure an infant gets to a tumor-promoting pollutant called PCB 153 if their mom ate fish. But if, for one year, mom ate only half the fish, or no fish at all, it wouldn’t budge levels much. Only if mom cut out all fish for five years before do you see a really substantial drop in infant levels. So, that’s the fish consumption caveat. “[T]he only scenarios that produced a significant impact on children’s exposures required mothers to eliminate fish from their diets [completely] for 5 years before their children were conceived.” “[S]ubstituting [plant foods] instead of fish would reduce prenatal and breastfeeding exposures by 37% each and subsequent childhood exposures by 23%.” So, “a complete ban on fish consumption may be preferable to targeted, life stage-based fish consumption advisories…”

But if you are going to eat fish, which is less polluted—wild-caught or farmed fish? In this recent study, researchers measured the levels of pesticides like DDT, PCBs, polycyclic aromatic hydrocarbons, and toxic elements like mercury and lead in a large sample of farmed and wild-caught seafood. And, in general, they found farmed was worse. Think of the suspect as “farmed and dangerous.” The measured levels of most organic and many inorganic pollutants were higher in the farmed seafood products, and consequently intake levels for the consumer, if such products were consumed. So, for example, this is for polycyclic hydrocarbons, persistent pesticides, and PCBs: significantly more contamination in all the farmed fish samples for all the contaminants; the salmon and sea bass, though it didn’t seem to matter for crayfish, and the wild-caught mussels were actually worse. And, if you split adult and child consumers into only eating farmed seafood or only eating wild-caught seafoods, the level of pollutant exposure would be significantly worse from the farmed seafood.

Overall, they investigated a total of 59 pollutants and toxic elements, and “[t]aking all these data as a whole, and based on the rates of consumption of fish and seafood of the…population [in Spain where the researchers hailed from], [the] results indicate that a theoretical consumer who chose to consume only [farmed fish] would be exposed to levels of pollutants [about] twice [as] high” than if they would have chosen instead wild-caught fish. So, you could eat twice the amount if you stuck to wild-caught. Easier said than done, though. Mislabeling rates for fish and other seafood in the U.S. are between 30 and 38 percent. So, the average fraud rate is like one in three.

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• Yaginuma-Sakurai K, Murata K, Iwai-Shimada M, et al. Hair-to-blood ratio and biological half-life of mercury: experimental study of methylmercury exposure through fish consumption in humans. J Toxicol Sci. 2012;37(1):123-30.
• Rooney JP. The retention time of inorganic mercury in the brain--a systematic review of the evidence. Toxicol Appl Pharmacol. 2014;274(3):425-35.
• Binnington MJ, Quinn CL, Mclachlan MS, Wania F. Evaluating the effectiveness of fish consumption advisories: modeling prenatal, postnatal, and childhood exposures to persistent organic pollutants. Environ Health Perspect. 2014;122(2):178-86.
• Betts KS. Fish consumption caveat: advisories may not help with long-lived contaminants. Environ Health Perspect. 2014;122(2):A57.
• Rodríguez-Hernández Á, Camacho M, Henríquez-Hernández LA, et al. Comparative study of the intake of toxic persistent and semi persistent pollutants through the consumption of fish and seafood from two modes of production (wild-caught and farmed). Sci Total Environ. 2017;575:919-931.
• Warner K, Mustain P, Lowell B, Geren S, Talmage S. Deceptive Dishes: Seafood Swaps Found Worldwide. Oceana Report. 2016.

Image credit: pauofficial via pixabay. Image has been modified.

Motion graphics by Avocado Video