Selenium is an essential trace element present in all cells of all plants and animals, and particularly abundant in ocean fish such as tuna. Humans have 25 enzyme types that need selenium, including several that prevent and reverse oxidative damage to the brain. Brain tissues use a lot of oxygen and produce reactive oxygen byproducts that would cause considerable damage if not for the protection provided by selenoenzymes. To ensure production of these selenoenzyme activities is never interrupted, the brain is preferentially supplied with selenium. But mercury irreversibly binds with selenium, and high exposures can decrease how much is available to make selenoenzymes. If too much selenium becomes bound to mercury, brain selenoenzyme production will be reduced, and the protection they provide against oxidative damage will be diminished.
Because pilot whale meat contains far more mercury than selenium, eating too much at once could prevent the brain from getting enough selenium to produce the selenoenzymes it needs to protect itself from harm. However, eating selenium-rich ocean fish ensures the brain maintains the production of selenoenzymes, even when large amounts of pilot whale meat are eaten.
The Health Benefit Values of Ocean Fish
To differentiate hazardous seafoods such as pilot whale meat from ocean fish that improve the selenium status of mothers and their children, the US Environmental Protection Agency (EPA) funded the development of the Health Benefit Value (HBV), the most reliably accurate seafood safety criterion. Seafoods that contain more mercury than selenium have negative HBVs, which indicate that eating them will tend to deplete the consumer’s selenium status. For example, the HBVs of pilot whale meat that were eaten at the time of the Faroes study averaged -82. In addition to having the most negative HBVs ever measured, pilot whale meats also contain very high concentrations of PCBs, dioxins, cadmium, and many other toxic substances, which make them particularly hazardous to consume.
In contrast, ocean fish such as tuna contain far more selenium than mercury and have positive HBVs, indicating that eating them will improve the selenium status of mothers and their children. A recent US EPA study measured the HBVs of over 14,000 ocean and freshwater fish, and aside from certain types of shark and other large, long-lived predators, nearly all ocean fish have positive HBVs. Skipjack, yellowfin, bigeye, and albacore HBVs were notably high at 20, 16, 11, and 10, respectively. This appears to explain why dozens of studies involving over 200,000 mother-child pairs have clearly established that mothers eating more ocean fish during pregnancy are associated with their children having higher IQ, social, and scholastic aptitude scores.
Scientific Understanding of the Seafood Mercury Issue has Advanced
Earlier, toxicologists were unaware of the importance of selenium in brain metabolism and did not know mercury inhibited selenoenzymes and sequestered selenium. This explains many aspects of mercury toxicity that were previously impossible to understand. For example, latency periods lasting months were noted between receiving poisonously high mercury exposures and the onset of toxic effects. Now that it is obvious that mercury needs to bind most of the selenium in the body before it can begin to impair production of brain selenoenzymes, the latency effect fits with expectation and serves to confirm the biochemical mechanisms as they are currently understood. It was also well known that unborn children were more vulnerable to mercury than their mothers, but the reasons were not known. Since maternal selenium must cross placental and fetal brain barriers and each transition provides mercury with greater opportunities to bind selenium, the higher vulnerability of the child’s developing brain is understandable.
Finally, toxicologists did not know that the meat of pilot whales contained far more mercury than selenium, while nearly all ocean fish contain far more selenium than mercury. The studies performed in the Faroe Islands were designed to find out if pregnant mothers with high mercury exposures from seafood might harm the development of their child’s brain. Subtle effects were associated with increasing mercury exposures, but it is important to note that no clinical harms were observed. While over 90 percent of the total mercury exposure among mothers in the Faroes Study came from eating the meat, organs, and blubber of pilot whales, and less than 10 percent came from ocean fish, ocean fish ended up being blamed for causing the effects. This is particularly ironic since the selenium provided by ocean fish prevented nearly all of the effects of the mercury-induced selenium deficiency caused by eating pilot whale meat. This is like blaming firefighters for causing a fire when their only fault was not being able to stop it from causing damage before they arrived.
Conclusion
The simplicity and clarity of the HBV will enable consumers and regulatory agencies to reliably distinguish hazardous seafoods like pilot whale and shark meats from healthy choices such as tuna and other types of ocean fish. This will enable public health agencies and physicians to update the guidance provided to mothers during pregnancy and better protect and improve the health of future generations.
This information was provided by Dr. Nick Ralston, a biomedical research scientist at the University of North Dakota (USA) who has worked on omega-3 fatty acids, selenium, and mercury biochemistry in relation to seafood safety issues for the past 30 years. He studied molecular mechanisms of disease at the Mayo Clinic Graduate School of Biomedical Sciences and Wake Forest University before beginning work on selenium physiology and its pivotal role in mercury toxicology.