Selenium is an essential trace mineral required in small amounts to maintain numerous biological processes. A deficiency occurs when the body does not receive or absorb enough of this element to support its necessary functions. While insufficient intake is rare in many developed nations, it remains a significant health concern in regions with low soil selenium content. A lack of selenium can compromise cellular defense mechanisms and affect the function of major organ systems.
The Essential Roles of Selenium in the Body
Selenium is integrated into specific proteins known as selenoproteins, which are the main biological form of the mineral. These selenoproteins act as enzymes that perform diverse, protective functions throughout the body. One well-known selenoprotein is glutathione peroxidase, a powerful antioxidant enzyme. This enzyme neutralizes harmful reactive oxygen species, protecting cell membranes and DNA from oxidative damage.
The thyroid gland holds a higher concentration of selenium than any other organ. Selenoproteins, such as the iodothyronine deiodinases (DIOs), regulate thyroid hormone metabolism by converting inactive thyroxine (T4) into the active form, triiodothyronine (T3). A lack of selenium impairs the production of active thyroid hormones and compromises the gland’s antioxidant defense.
Selenium also supports a healthy immune system response. It influences the activity and proliferation of white blood cells, including T-cells, which defend against pathogens. Insufficient selenium status weakens immune function, potentially leading to increased susceptibility to viral infections.
Primary Indicators of Low Selenium
The observable signs of low selenium status often begin with myopathy, or general muscle weakness and fatigue. This is thought to be a result of increased oxidative stress damaging the muscle cells, particularly the skeletal muscles. Individuals may notice difficulty performing physical tasks or experience unexplained muscle pain.
Chronic deficiency can manifest in outward physical changes that affect fast-growing tissues like hair and nails. Hair loss and changes in nail appearance, such as brittleness or discoloration, are sometimes noted in long-term cases. Low selenium has also been associated with neurological and psychiatric symptoms, including increased anxiety, depressed mood, and general cognitive confusion.
In severe, chronic cases linked to extremely low dietary intake, a condition called Keshan disease can occur. Keshan disease is a serious type of cardiomyopathy, characterized by an enlarged heart, weakened cardiac function, and heart failure. This condition was historically prevalent in areas of China with very low soil selenium levels. While rare globally, this outcome demonstrates the potential for profound cardiovascular dysfunction when selenium levels are low.
Factors Leading to Deficiency
The primary global determinant of selenium status is the geographical content of the mineral in the soil. Since plants absorb selenium from the soil, the selenium content of crops and the animals that consume them varies widely depending on the region where they are grown. Areas with low soil concentrations, such as parts of China and certain European countries, put the local population at a higher risk of deficiency.
Dietary choices can also contribute to inadequate intake, especially a reliance on locally grown food in selenium-poor regions. Individuals following strict vegetarian or vegan diets may also be at risk if they do not plan their meals carefully.
Certain medical conditions can impair the body’s ability to absorb or retain selenium. Gastrointestinal disorders causing malabsorption, such as Crohn’s disease or Celiac disease, prevent sufficient selenium from entering the bloodstream. Patients on long-term total parenteral nutrition (TPN) without supplementation, or those with conditions increasing nutrient loss like severe burns or kidney failure requiring dialysis, are also vulnerable.
Addressing and Preventing Deficiency
A medical diagnosis of selenium deficiency is typically confirmed through a blood test that measures plasma or serum selenium levels. A healthcare provider may also evaluate the activity of selenoproteins like glutathione peroxidase, which can serve as a functional marker of selenium status. Accurate diagnosis is important because the symptoms of deficiency can overlap with those of other conditions.
The most direct way to prevent and address low selenium is through dietary intake of selenium-rich foods. Brazil nuts are an exceptionally concentrated source, with just a few nuts potentially providing more than the recommended daily allowance. Other good sources include seafood, organ meats, eggs, and certain grains, though the selenium content of these foods is variable.
For adults, the Recommended Dietary Allowance (RDA) is 55 micrograms (µg) per day, an amount easily met with a balanced diet. Supplementation may be necessary in cases of diagnosed deficiency or malabsorption, but it requires caution and medical guidance. Selenium has a narrow therapeutic window; excessive intake, typically above the tolerable upper intake level (UL) of 400 µg per day, can lead to toxicity (selenosis). Symptoms of selenosis include hair loss, brittle nails, and gastrointestinal upset, emphasizing the importance of careful dosing.