Selenium is an essential trace mineral that the human body requires in only very small quantities. Despite the minimal amounts needed, this element plays a fundamental role in maintaining overall health and supporting numerous bodily processes. Since the body cannot produce selenium, it must be obtained entirely through diet or supplementation.
Essential Physiological Functions
Selenium performs its biological functions primarily through selenoproteins. These proteins incorporate selenium as the amino acid selenocysteine, sometimes called the 21st amino acid, and are involved in diverse metabolic pathways throughout the body.
One recognized function is the powerful antioxidant activity provided by selenoproteins like glutathione peroxidase. This enzyme system protects cells and DNA from damage caused by free radicals and oxidative stress, helping maintain cellular integrity.
The thyroid gland contains the highest concentration of selenium compared to any other organ. Selenoproteins, specifically iodothyronine deiodinases, regulate thyroid hormone metabolism. These enzymes facilitate the conversion of inactive thyroxine (T4) into the active form, triiodothyronine (T3), which controls metabolism and growth.
Food Sources of Selenium
The amount of selenium in plant-based foods varies significantly because it depends directly on the selenium content of the soil where the food was grown. Regions with high selenium soil, like parts of the United States and Canada, produce crops with higher concentrations. Conversely, areas with low soil content, such as parts of China or Europe, can lead to lower selenium intake.
Brazil nuts are famously the most concentrated natural source of selenium, but their content can be highly unpredictable. A single Brazil nut can provide the entire Recommended Dietary Allowance (RDA) or vastly exceed it, depending on its specific geographic origin. Other reliable dietary sources include seafood, such as tuna and sardines, which absorb the mineral from the water. Meats, poultry, and whole grains also contribute meaningful amounts of selenium to the diet.
Supplementation Guidelines and Safety Limits
For most healthy adults, the Recommended Dietary Allowance (RDA) for selenium is 55 micrograms (mcg) per day. This intake level is established to maximize the activity of selenoproteins like glutathione peroxidase in the blood plasma. Pregnant and lactating women require slightly higher amounts, with the RDA typically set between 60 mcg and 70 mcg daily.
It is important to be aware of the Tolerable Upper Intake Level (UL), which is set at 400 mcg per day for adults, representing the maximum daily intake unlikely to cause adverse health effects. Consuming selenium above this UL over a prolonged period can lead to a condition known as selenosis, or selenium toxicity. Early indicators of selenosis include a metallic taste in the mouth and a characteristic garlic odor on the breath. More advanced symptoms involve hair loss, nail brittleness or loss, fatigue, irritability, and in severe cases, nervous system abnormalities.
Selenium supplements are available in both organic and inorganic forms. The organic forms, such as selenomethionine, are derived from yeast or plants and are generally absorbed more efficiently by the body than inorganic forms like sodium selenite or sodium selenate. Selenomethionine is often preferred in supplements due to its high bioavailability and lower risk of acute toxicity.
Where to Purchase Selenium Supplements
Selenium supplements are widely available across various retail environments, making them easy to find both in person and online. Traditional brick-and-mortar locations like major pharmacies and drugstores typically carry a selection of single-mineral selenium products and multivitamin formulas containing the trace element. Large grocery chains with dedicated health and wellness sections or supplement aisles also stock various brands.
Online supplement retailers and the direct websites of reputable manufacturers often provide the largest selection of products, including specific formulations like selenomethionine or selenium-enriched yeast. When purchasing, look for quality markers to ensure the product contains what the label promises. Third-party certifications, such as the USP Verified mark, indicate that the supplement has been tested for ingredient potency and purity.
Checking for these verification seals provides assurance that the supplement meets established quality standards. While selecting a form like selenomethionine may be beneficial due to its superior absorption profile, the most appropriate supplement and dosage should always be discussed with a healthcare professional.