Selenium is an essential trace mineral required for proper function that the body cannot produce internally. It must be obtained consistently through diet to support numerous biological processes. Selenium is integrated into specific proteins called selenoproteins, which are foundational to maintaining systemic health. These selenoproteins allow the body to carry out functions ranging from cellular protection to hormone regulation.
Selenium’s Role as an Essential Antioxidant
The most fundamental function of selenium involves protecting cells from damage caused by metabolic byproducts. Cellular metabolism and environmental exposures constantly generate reactive oxygen species, or free radicals, which are unstable molecules that can harm cell structures, including DNA and cell membranes. Selenium acts as a structural component of antioxidant enzymes that neutralize these species.
One of the most important of these enzymes is glutathione peroxidase (GPx), which incorporates selenium into its active site as the amino acid selenocysteine. GPx uses this selenium component to catalyze a reaction that converts harmful hydrogen peroxide into harmless water. This action stops the free radical chain reaction before it can cause widespread cellular damage.
The activity of GPx is directly dependent on selenium availability; low intake decreases GPx activity, compromising the body’s defense against oxidative stress. This protective mechanism preserves the integrity of tissues throughout the body, including the fatty membranes of neurons. By buffering the internal environment against oxidative damage, selenium ensures that cells can function and replicate correctly.
Supporting Thyroid Function and Immunity
Beyond its general antioxidant role, selenium is uniquely concentrated in the thyroid gland, the organ responsible for producing hormones that regulate metabolism and growth. The synthesis of thyroid hormones naturally generates hydrogen peroxide, making the thyroid vulnerable to oxidative stress. Selenoproteins like glutathione peroxidase are present in high concentrations here to prevent this damage.
Selenium is also an integral part of enzymes called iodothyronine deiodinases (DIOs). These DIOs convert the inactive thyroid hormone T4 into the biologically active hormone T3, which is required for cellular function. Without sufficient selenium, this conversion process is impaired, disrupting hormone regulation.
The mineral also supports the immune system’s ability to respond effectively to pathogens. Selenoproteins help modulate the adaptive immune response by influencing the proliferation and differentiation of T-cells, which are white blood cells central to fighting infection. Selenium helps regulate the internal environment of these immune cells, ensuring they can activate and multiply appropriately when facing a viral or bacterial challenge.
Getting the Right Amount of Selenium
Intake must come from food sources, but the content of the mineral in plant-based foods is highly variable. The amount of selenium in grains, vegetables, and other crops depends almost entirely on the concentration of the mineral present in the soil where they were grown. For this reason, selenium levels can differ significantly even between the same foods grown in different geographical regions.
Animal products tend to be more reliable sources, as the animals consume the mineral from plants in their feed. Seafood, organ meats, and eggs are recognized as good sources of selenium. Brazil nuts are an especially dense source, with just one nut potentially providing more than the Recommended Dietary Allowance (RDA) for an entire day.
For most adults, the RDA is set at 55 micrograms (mcg) per day, an amount achievable through a varied diet. Consistent intake is important because, although selenium is stored in muscle tissue, its functional role demands a steady supply as a component of rapidly cycling enzymes.
Signs of Imbalance
Maintaining a balance of selenium intake is important because both too little and too much can lead to adverse health effects. A deficiency can weaken the immune system, making the body less effective at fighting off infections. Historically, severe selenium deficiency was linked to Keshan disease, a condition involving damage to the heart muscle.
Conversely, excessive intake of the mineral can lead to a toxic condition called selenosis. The tolerable upper intake level for adults is 400 micrograms per day, and symptoms of toxicity often appear when this limit is regularly exceeded. Signs of selenosis include hair loss, brittle or discolored nails, fatigue, and digestive issues like nausea and diarrhea. A distinctive feature of severe overconsumption is a breath odor resembling garlic.