Selenium and mercury are naturally occurring elements found throughout the environment. Selenium is a trace element required in small amounts for human health, playing a role in various biological processes. It functions as a component of certain proteins and enzymes that help protect cells from damage. Mercury is a heavy metal that exists in different forms and is known for its toxic properties, even at low concentrations.
How Mercury Harms the Body
Mercury can exist in several forms, but methylmercury and inorganic mercury compounds are the most relevant for human exposure. Methylmercury is an organic form, produced when microorganisms in water and soil convert inorganic mercury into this more toxic compound. Inorganic mercury compounds are formed when mercury combines with other elements like chlorine or sulfur.
Mercury enters the human body primarily through the consumption of contaminated fish and shellfish, particularly in the form of methylmercury. Ingested methylmercury is readily absorbed from the gastrointestinal tract and can then distribute throughout the body. Inorganic mercury can also be absorbed through ingestion or inhalation of mercury vapor.
Once inside the body, mercury exerts its toxicity by binding to sulfhydryl groups on proteins. These groups are found in amino acids, components of proteins and enzymes. This binding disrupts the normal function of these proteins, interfering with cellular processes. The nervous system is particularly susceptible to mercury’s effects, leading to neurological damage. Mercury can also harm the kidneys and compromise the immune system.
Selenium’s Antidote Effect
Selenium counteracts mercury’s harmful effects, primarily through selenoproteins. Selenoproteins contain selenocysteine, which incorporates selenium into their structure. These proteins are central to selenium’s protective actions.
Selenium protects against mercury toxicity through its strong chemical affinity for mercury. Selenium and mercury form an inert complex within the body. This complex effectively sequesters mercury, preventing it from binding to and damaging other biological molecules like enzymes and proteins. By binding to mercury, selenium diverts mercury away from vulnerable sites.
Selenoproteins, such as glutathione peroxidase (GPx), are important in mitigating oxidative stress induced by mercury. Mercury exposure can lead to the production of reactive oxygen species, which cause cellular damage. Glutathione peroxidase, a selenium-dependent enzyme, acts as an antioxidant, neutralizing these harmful free radicals and reducing oxidative damage to cells and tissues. This protective action helps preserve cellular integrity and function in the presence of mercury.
Dietary Sources and Safe Consumption
Selenium is widely available in various dietary sources, with Brazil nuts being a rich source. Other good sources include seafood, lean meats, poultry, eggs, and certain grains and legumes. The selenium content in plant-based foods can vary depending on the selenium levels in the soil where they are grown.
Mercury exposure, particularly methylmercury, primarily occurs through the consumption of fish, especially large, predatory species that accumulate mercury over their lifespan. Examples of fish high in mercury include shark, swordfish, king mackerel, and tilefish. Smaller fish and shellfish generally have lower mercury levels.
The safety of consuming certain fish species is influenced by the molar ratio of selenium to mercury. If a fish contains more selenium than mercury, the selenium can offer a protective effect, making the mercury less bioavailable or harmful. This concept implies that the presence of selenium in fish can modify the toxicological impact of mercury.
To ensure adequate selenium intake while minimizing mercury exposure, incorporate selenium-rich foods into the diet, such as Brazil nuts, various types of seafood low in mercury, and lean protein sources. Moderation is recommended for the consumption of fish known to have high mercury levels. Choosing a variety of fish species, especially those lower in mercury, can help balance nutritional benefits with the need to limit mercury intake.