Eating fish may be good for you, or it may poison you. (Pick one)

In the 1970s, researchers reported that native people living in Greenland (Inuits) had very low rates of heart disease compared with counterparts living in Denmark. Scientists attributed these health benefits to the consumption of fish and sea mammals containing high levels of long-chain polyunsaturated fatty acids. Recently, however, research has questioned the accuracy of these early studies, as more recent research shows that the rate of heart disease and heart attack among the Inuit are similar to those in non-Inuit populations. Thus, there has been some question regarding how strong the association is between reduced risk of cardiovascular disease and fish consumption. The situation reminds me of one of my favorite sayings: It ain’t what we don’t know that’s gonna hurt us, it’s what we do know that just ain’t so.

Over the years, thousands of research studies have been conducted, with the result that the consumption of fish is included in most dietary guidelines. The benefits are primarily considered to be the previously mentioned reduction in the risk of coronary heart disease in adults, but also an improvement in cognitive development in infants and young children.

The current dietary guidelines in the USA have moved away from the concept of the minimum daily requirement. Instead they describe recommended patterns of healthy eating. The recommendation for seafood has not changed, however: 8 oz. of seafood per week. (Dietary Guidelines, p. 18)

It is generally recognized, however, that some fish species contain significant levels of contaminants. These contaminants include a number of really nasty poisons, including chlordane, polychlorinated biphenyls (PCBs), lead, and methylmercury. These compounds can be toxic even in very small amounts, and they are bioaccumlative.

Bioaccumulation is an important concept in understanding environmental toxins. The basic idea is that even tiny amounts of toxin can build up in the body. Here’s how: at any given feeding, a toxin may be eaten in such tiny amounts that there is no immediate effect on the animal that consumes it. However, it is absorbed by the body, and it is not readily eliminated by natural processes. Thus, over time, the amount in the body builds up each time the animal eats a little more.

Imagine a lake. Mercury emitted by coal-burning power plants falls into the lake, where microbes convert it to methylmercury. Algae living in the lake take in some of that methylmercury. Along comes a tiny fish fry, and it eats some of that algae. Now with each mouthful of algae, the fish fry ingests a dose of methylmercury. And it starts to build up. How many mouthfuls of algae does a fish fry eat? I don’t know, but it is quite a lot. Now, along comes a medium-sized fish, and it eats the fish fry. With one bite, it has ingested not just a tiny amount of methylmercury, but all the methylmercury that built up in the body of the fish fry during its lifetime. How many fish fry does a medium-sized fish eat? I don’t know, but it is quite a few, and the medium-sized fish ingests all of the methylmercury built up in the bodies of each fish it eats. Now, along comes a large fish, and it eats the medium-sized fish. With one bite, it has ingested not just a tiny amount of mercury, but all the methylmercury built up in the body of the medium-sized fish. How many medium-sized fish does a large fish eat? I don’t know, but it is quite a few, and the large fish ingests all of the methylmercury built up in the bodies of each fish it eats.

Now, let’s imagine that our fish are living in a Missouri lake. Along comes a fisherman, and he catches one fish per week and eats it. That will be 52 fish per year. Now, I don’t know what the actual numbers are, but let us assume that a fish fry eats 1,000 individual alga, while a medium-sized fish eats 100 fry, and a large fish eats 100 medium-sized fish. These estimates may be wildly wrong, but the point is to illustrate the principle of bioaccumulation, and they will allow us to do so.

Using the estimates above, each fish fry will ingest the methlymercury contained in 1,000 algae; each medium-sized fish will ingest the amount contained in 100,000 algae; each large fish will ingest the amount contained in 10 million algae, and in a year, our fisherman will ingest the amount contained in 520 million algae. If he continues for 10 years, he will consume the amount contained in 5.2 billion algae.

Over time, the amount of methylmercury in our fisherman’s body will build up, perhaps eventually reaching the point where it starts to poison him.

Now, my presentation is over-simplified; in real life bioaccumulation is much more complex. Further, the numbers I chose for my progression were totally arbitrary. Nonetheless, they illustrate the basic idea of bioaccumulation. And the principle applies not only to methylmercury, but also to lead, PCBs, and dioxins.

The result is that, however good for you eating fish may be in theory, there are limits due to environmental contaminants. The next post will look at what those limits are in Missouri.

Sources:

Committee on a Framework for Assessing the Health, Environmental, and Social Effects of the Food System; Food and Nutrition Board; Board on Agriculture and Natural Resources; Institute of Medicine; National Research Council; Nesheim MC, Oria M, Yih PT, editors. A Framework for Assessing Effects of the Food System. Washington (DC): National Academies Press (US); 2015 Jun 17. ANNEX 1, DIETARY RECOMMENDATIONS FOR FISH CONSUMPTION. Available from: https://www.ncbi.nlm.nih.gov/books/NBK305180.

Missouri Department of Health and Senior Services. 2017. 2017 Missouri Fish Advisory: A Guide to Eating Missouri Fish. Downloaded 3/9/17 from www.health.mo.gov/fishadvisory.

U.S. Department of Health and Human Services and U.S. Department of Agriculture.
2015–2020 Dietary Guidelines for Americans. 8th Edition. December 2015. Available at http://health.gov/dietaryguidelines/2015/guidelines.