When the halibut on my hook breaks the surface, writhing in a splash of seawater off the coast of Bolinas, California, I am thinking less of this fish’s fate than of my own. Considering that I plan to kill and eat it, this might seem cruel. Yet inside the fat and muscle cells of this flat, odd-looking creature is a substance as poisonous to me as it is to him: methylmercury, the most common form of mercury that builds up inside people (and fish). At the right dose and duration of exposure, mercury can impair a person’s memory, ability to learn, and behavior; it can also damage the heart and immune system. Even in small quantities, this heavy metal can cause birth defects in fetuses exposed in the womb and in breast-fed newborns whose mothers’ milk is laced with it.
Scientists have assured me that one serving of halibut contains nowhere near a dosage that might cause harm. These are the same scientists, though, who admit that no one knows for sure what the threshold dose is that causes mercury to subtly poison cells in the brain and the liver, two organs where it tends to accumulate.
As frightening as that sounds, most of us were born with a defense against exposure to mercury, initiated by specific sequences of genetic code that cause most people to expel the metal in 30 to 40 days. Not everyone carries this natural resistance, however. A small minority of people carry a genetic mutation that apparently causes their cells to retain mercury for far longer—in rare cases up to 190 days—greatly increasing the chance for cellular damage.
Such genetic differences may explain why some people are more susceptible to mercury poisoning than others. This possibility is driving a nascent but growing effort among scientists to link the impact of mercury and other environmental factors (everything from pollutants and diet to the sun’s ultraviolet rays) to the individual genetic proclivities that each of us is born with. “Toxicologists say that ‘the dose makes the poison,’” says mercury expert Jane Hightower, who practices internal medicine in San Francisco, “but it’s clear that some people are more sensitive to even small exposures than others.”
For lack of a better term, I’ll call this new science human envirogenomics, the fusing of environmental toxicology and genetics, two fields that until recently didn’t interact much with each other. Yet researchers are finding that the interplay of the two makes us who we are and often determines whether we are healthy or sick. “Recent increases in chronic diseases like childhood asthma and autism cannot be due to major shifts in the human gene pool,” says physician and geneticist Francis Collins, former director of the National Human Genome Research Institute. While acknowledging that changes in diagnostic criteria and heightened awareness may play a role, Collins says that much of the increase “must be due to changes in the environment, which may produce disease in genetically predisposed persons.” One day, envirogenomics could provide clues to a person’s sensitivity to environmental toxins (such as mercury) and the potential for damage based on that person’s genes. Doctors might then better understand how to prevent such harm and how to treat patients exposed to deleterious chemicals.
Man versus mercury
The possible connection between mercury and my own DNA is why I’m now holding a quivering fishing rod on the bow of the Osprey, a weathered 24-foot trawler. I am conducting an investigation: testing my mercury levels before and after eating this fish—assuming I land him—and checking my personal genetic code to see if I am one of the lucky ones who seem to expel mercury quickly. At the same time, I can’t help but wonder if this self-experiment is a sign that I am indeed sensitive to mercury and that it has already addled my brain. My hope is that these tests, plus discussions with experts around the world and a visit to an envirogeneticist in Maine, will help guide my decision when choosing between a large fish and, say, a bowl of pasta the next time I’m in a restaurant.
This exploration is the opening salvo in an extensive project in which I am treating myself as a human guinea pig, exploring four major new areas of personal testing: genes, environment, brain, and body. In essence I am aiming to answer two big, personal questions: How healthy am I at the very deepest level? And what can the seemingly endless profusion of new high-tech tests for various diseases and traits tell me about my health now and in the future?
My fish trial began a few days earlier when I gave up nine milliliters of blood and a cupful of pee to test my normal level of methylmercury—that is, the background level that I typically have in my body from living in 21st-century San Francisco. I’ll give up another round of bodily fluids after eating today’s catch for lunch and some store-bought swordfish for dinner.
In my “before” test for methylmercury, I registered a level of less than 4 µg/l (micrograms per liter), safely below the EPA threshold of 5.8 µg/l. This is a relief. But will my “after” level be higher?
Big fish are by far the most prevalent source of human mercury exposure, although researchers are exploring a number of other potential contributors. In 2008 a study at Boston University tested traditional herbal products manufactured in India and the United States and found lead, mercury, or arsenic in about one-fifth of them. Last year the FDA cited another potential source of harm for children and, through their mothers, fetuses: mercury contained in dental amalgams (those silvery fillings many of us have in our teeth). But the FDA has reserved judgment on health impacts for those of us who are not in early development and who do not have a medical condition making us more sensitive to mercury.
Methylmercury got into my fish from the coal-burning power plants that rim the northern Pacific Ocean, from the United States and Mexico to Japan and China. Expelled from tall stacks, mercury stays in the upper atmosphere until rain carries it down over the eastern Pacific, where it joins mercury from other sources as bacteria and other microorganisms transform it into methylmercury. After being absorbed by plankton, the mercury moves up the food chain: The plankton is eaten by small fish, which are then gobbled up by larger predators, each bigger animal accumulating more mercury with every meal. This process extends to the halibut that was now tiring and allowing me to reel it in as the Osprey’s captain, Josh Churchman—a man in his fifties with a stubbly beard, graying hair, and a faded baseball cap—leaned far over a gunwale with a net.
Continue Reading over at Discover Magazine
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