The Career of Dr. Nick Ralston: Discovering the Connection Between Mercury and Selenium

The Career of Dr. Nick Ralston: Discovering the Connection Between Mercury and Selenium

Many people in the seafood industry recognize Dr. Nick Ralston as an accomplished scientist whose research has changed the way we understand the relationship between seafood and health, but how did he get there? SIRF interviewed Dr. Ralston to get more insight into his career and discoveries.

Q: Your career researching mercury and selenium has spanned decades. But, going back to the start, how did your journey begin in studying the effects of mercury on human health? How did this become your passion?

Dr. Ralston: My training in disease pathology, but my work has become so involved with ocean and freshwater fish that I am now more of an ecotoxicologist. This transition may have occurred because I love the ocean and have been a scuba diver since I was young. One of my all-time favorite summers was spent in the Florida Keys diving two to three times a day as part of a marine biology course. Although I eventually went into biomedical research and was at the Mayo Clinic for a number of years, my real love is marine biology. One of my mentors at the Mayo Clinic always said that you had to understand a disease at the molecular level in order to properly diagnose and treat it. He also felt that it was our duty to take on the problems that had the worst effects on the most people and I agreed. When I first became aware of the mercury issue, I was researching the influence of dietary selenium on inflammation. Selenium is a mineral in soil that is taken up by plants, but it has key roles in animal health. It is required to make an amino acid which is essential in proteins made by all forms of life that have complex brains. One day I came across a research paper describing a protein in fish that was unusually rich in that selenium-containing amino acid. Knowing that mercury binds extremely well with sulfur and that it binds even better with selenium, I thought, “Wow, I bet that would bind a lot of mercury. I wonder if that’s why it has so much selenium”. Recognizing how bad it would be for the brain to have its selenium become bound by mercury, my interest was aroused. Within a year I began investigating the seafood mercury issue. I initially thought ocean fish must contain more mercury than selenium, but that wasn’t the case. It took a while to learn that ocean fish hadn’t caused the mercury-related problems we all heard about, but they were being blamed. Two major studies of the problem had been done when I started looking into the issue. One examined children whose mothers ate ocean fish during pregnancy but found there were no harmful effects. Another reported mercury-related problems, but in that study the majority of the mercury the mothers had been exposed to had come from eating whale meat! Why anyone would base fish consumption advice on a study of the effects of eating whale meat was a mystery, and it is my job to study mysteries.

Q: You’ve been at the University of North Dakota for nearly two decades and currently lead research programs involving human and environmental health. With over 70 publications under your belt, can you share the vital role selenium plays in human health and where we can find it in foods?

Dr. Ralston: Selenium is a vitally important nutrient that we can’t live without. My research team started examining mercury’s effects on selenium-dependent enzymes (selenoenzymes) just over 20 years ago. However, we were not the first to notice these interactions. In 1967, selenium was shown to be extremely effective in protecting against otherwise uniformly lethal effects of mercury toxicity in rats. Since selenium’s importance was largely unknown at that time, no one understood how it could provide such complete protection. We now know that all forms of life that have complex brains express selenoenzymes in every cell of their bodies. Of the 25 selenoenzyme genes expressed in humans, over half are involved in preventing and/or reversing oxidative damage, while others perform essential functions like controlling thyroid hormones and regulating calcium levels in cells. The brain cannot survive without selenoenzyme protection against oxidative damage. Antioxidants such as vitamin C are also important, but for them to provide protection, they need selenoenzymes to continually restore them to their functional forms. Without the protection provided by selenoenzymes, brain damage would happen quickly, and death would soon follow. This is why we need selenium, and ocean fish are among the richest selenium sources in our diets. Eating seafood is beneficial because it provides selenium and other nutrients like the omega-3 fatty acids which are uniquely abundant in ocean fish.

My group and a team in Hawaii enrolled a group of expectant mothers and asked about their recent seafood consumption. Shortly after delivery, samples of placenta and umbilical cord blood were taken for mercury and selenium analysis. Although maternal ocean fish consumption increased mercury levels in these samples, their selenium concentrations were far higher and increased ~10 times faster. While the miniscule amount of mercury present could bind a small amount of selenium, the children were enriched with additional selenium when their mothers ate ocean fish. It was clear that the more ocean fish their mothers ate, the better protected their children were against mercury-induced selenium deficiency.

Q. And we know that nearly all fish contain traces of mercury, but we don’t see harm from mercury in people who eat even very large amounts of seafood. How does selenium keep ocean fish from causing harm?

Dr. Ralston: It was formerly imagined that mercury directly caused oxidative damage, but that idea was disproven decades ago. Mercury steals selenium from the body and that appears to be the only way it causes harm. Mercury will bind to selenium like a magnet and make it unavailable to make new selenoenzymes. There is usually plenty of selenium available in tissues, so losing a small amount to mercury binding is no problem. However, exposures to large amounts of mercury can bind selenium faster than dietary sources can make up for the loss. It may take months for the mercury to steal enough to keep brain selenoenzymes from working, but once that happens the brain is no longer protected, and oxidative damage will occur. As oxidative damage accumulates, neurological signs and symptoms will begin to be observed. What matters most is how much selenium is available in relation to mercury exposures. To establish which types of fish might be risky and which were safe to eat during pregnancy, the U.S. EPA funded my project to compile mercury and selenium data from over 14,000 ocean and freshwater fish. The good news is that hardly any ocean fish contain more mercury than selenium, so only people that eat top predators like pilot whales and great white sharks encounter mercury-dependent risks. But the great news is that virtually all ocean fish are so rich in selenium that eating them prevents and reverses mercury-dependent selenium deficiencies.

Q: What guidance would you give to someone who believes seafood is bad for pregnant women to consume because of a fear of mercury?

Dr. Ralston: As scientific understanding has advanced, public health guidance has been steadily improving. Our earlier concerns regarding whether or not ocean fish are safe to eat arose because subtle effects were noted in a study of children whose mothers had been eating pilot whale meat. This study was done in the Faroe Islands where nearly 90% of their total mercury exposure came from eating pilot whale meat, organs, and blubber. These “seafoods” contained high levels of mercury, but also had higher levels of many other toxic metals and organic contaminants than any food consumed by humans. Perhaps the real surprise is that children whose mothers ate these foods were only subtly affected. It is clear to me that if the mothers of those children had not been eating enough selenium-rich ocean fish to offset their high mercury exposures from eating pilot whale meats, their children could have been severely harmed. The researchers in that study recognized that “something” in the fish was protecting against the effects of mercury but since they were unaware of selenium physiology, they missed making the proper connection. The findings of the Faroes study are important but have been largely misunderstood. Studies including a total of 200,000+ mother-child pairs have found increasing maternal seafood intakes are accompanied by substantial benefits instead of harm. Compared with children whose mothers avoided eating ocean fish, mothers that eat at least 2 ocean fish meals per week during pregnancy have children with IQs that are improved by 4-7 points and demonstrate better scholastic and social performance. Since the reasons why eating pilot whale meats caused subtle harms in the Faroes and why eating ocean fish has substantial beneficial effects on child outcomes are now known, it would be wrong to continue to advise women against eating ocean fish. Therefore, the National Academy of Science and other groups will soon be recommending updates to current seafood consumption advisories. Unfortunately, since mothers have been warned to limit ocean fish consumption for so long, it is likely that misunderstandings of the seafood mercury issue will persist. Protecting and improving public health is everyone’s goal and that is exactly why so much caution was applied in the past. That is also why we advise pregnant women to improve their seafood consumption during pregnancy. Now that mercury toxicity is well understood and ocean fish consumption has proven beneficial, improved outcomes can be achieved for future generations.


Q: Misinformation regarding mercury and selenium is a recurring issue for the seafood industry. How should we talk about the science to policymakers and healthcare providers?


A: Two decades ago, there were many questions about how mercury caused toxicity, why it took months for its toxic effects to show, and why it was so harmful to the brain, especially the brains of developing children. With so many unknowns, it was quite appropriate for earlier scientists to proceed with extreme caution and create a stringent seafood safety advisory to protect the public. However, the mechanisms of mercury toxicity have transitioned from being mostly mysterious to now being among the best defined of all toxic agents. Today, the only ones that are still worried about maternal mercury exposures from ocean fish consumption are those that haven’t kept up with recent research. After hundreds of discussions with scientists, policy makers, and health professionals, none of those that I have spent time with has failed to recognize the importance of mercury’s effects on selenium physiology in the brain. Since there is no way to properly understand the mercury issue without first understanding selenium physiology, the first step in a discussion about this subject is to ask about their familiarity with recent research. Once they learn how eating ocean fish prevents mercury from inducing selenium deficiency, it should be easy for them to figure out the rest. After learning that, any that continue to push the idea that mercury from ocean fish consumption poses health risks may have their reasons, but they are not based on the current science.

Q: Furthermore, research and federal policy are two things that should go hand in hand, but often don’t. Over the course of your career, can you share any key lessons you’ve learned on how to bring a science-based approach more effectively to the policy process?

Dr. Ralston: Advice and information for the public must be clear and simple. Policymakers and the general public won’t easily understand and don’t want to hear about molecular interactions. When an expectant mother is wondering whether or not she should eat fish, she only wants to know if it is safe for her child.  It is our duty to provide the information she needs. This is exactly why the U.S. EPA funded our project to develop the Health Benefit Value (HBV), a food safety criterion that is the most reliable index of risks or benefits associated with fish consumption. Because pilot whale or great white shark meats contain far more mercury than selenium, they have severely negative HBVs ( -80 and -110 respectively) that indicate they should not be eaten during pregnancy. Fortunately, virtually all ocean fish and the majority of freshwater fish from most North American lakes have highly positive HBVs. Eating them will improve the selenium status of the mother and her child and enhance their omega-3 fatty acid levels. Since selenium and omega-3 fatty acids are both required for healthy brain development, it is not at all surprising that increasing ocean fish consumption during pregnancy results in substantial improvements in child IQs and enhances their scholastic and social abilities. Working with policy makers is critical, but getting the word out to physicians, nurses, nutritionists, and especially the academic institutions that educate them is essential.

Q: Looking ahead, what do you think is on the horizon for seafood, especially in terms of increasing seafood consumption levels?

Dr. Ralston: “Pregnant women are already being advised to eat more seafood during pregnancy instead of less. The advice used to be to eat no more than two fish meals a week. The advice moving forward is to eat no less than two fish meals a week. We all want brighter and healthier children, and now that we know the benefits of ocean fish consumption are so substantial, we are encouraging pregnant women to eat more seafood.”

Q: Where have your studies taken you that you never thought you would go? 

Dr. Ralston: “I never imagined that I would become involved in regulatory policy, but it is important to help decision makers to protect and improve the health of future generations, so I am happy to help.”

For more information specific to Dr. Ralston’s selenium-mercury research, check out this article: Dr. Nick Ralston’s Selenium Mercury Research