Selenium is essential to your body in very small amounts, which is why it’s often referred to as a trace mineral. In larger doses, selenium is toxic to virtually all tissues, especially your liver. Selenium poisoning also harms your kidneys, but more as a secondary complication related to liver damage and dysfunction. When mineral-poor soil is used to grow vegetables, the risk of selenium deficiency increases. Be cautious if you are supplementing with selenium and consult your doc about safe levels.
Within the soil, selenium almost always combines with other elements and forms selenium salts. In plant and animal foods, selenium is usually incorporated into amino acids, which are readily absorbed in your digestive tract. When you eat selenium-rich foods, your body isolates the mineral and either uses it immediately or stores the excess in your liver and kidneys. Selenium is needed for cellular function because it’s a co-factor for many enzymes. For example, selenium is needed to manufacture glutathione peroxidase and thioredoxin reductase, which are powerful antioxidant enzymes that destroy free radicals and protect your tissues from oxidative damage. The total amount of selenium in a healthy body ranges from about 13 to 20 milligrams.
The recommended daily amounts of selenium for women range from 55 to 70 micrograms depending on age, pregnancy and lactation. Sometimes people mistake the word micrograms for milligrams -- representing a dosage a thousand-times larger -- which can easily lead to overdose when taking selenium supplements. As such, eating foods containing selenium is a safer way to get what you need. Good sources of selenium include most animal products, especially fish, beef and chicken. Brazil nuts are by far the richest source of selenium, as they contain about 540 micrograms per ounce. High sulfur intake tends to interfere with selenium absorption, whereas a diet rich in calcium enhances its uptake.
Selenium toxicity is rare in people, but more of a concern for farm animals that live in areas where the soil is alkaline and rich in the mineral. The amount of selenium needed to cause toxicity symptoms in the average-sized person is debated and can range anywhere from 500 to 2,000 micrograms daily, although the Institute of Medicine of the National Academy of Sciences has set a tolerable upper intake level at 400 micrograms daily to be on the safe side. The initial symptoms of selenium toxicity -- also called selenosis -- include garlic-like breath, hair loss, skin rash, fingernail loss, nausea, vomiting, diarrhea, irritability and depression. Severe selenosis usually leads to liver damage, stomach ulcers, extreme weight loss and eventually heart and kidney failure.
Selenosis affects the kidneys in a few different ways. Perhaps the most damaging aspect is a dysfunctional liver. More specifically, when the liver is damaged it doesn’t filter the toxins out of your blood, which tends to injure the tissues of the kidneys and other organs such as the heart. Furthermore, some selenium is stored in the kidneys as a type of protein called selenoprotein. Too much of this protein clogs the kidneys, which reduces their filtering ability and increases the risk of kidney failure. Elevated levels of selenoproteins secreted in the urine is a sign of selenosis. Other symptoms of kidney failure include fluid retention, high blood pressure, fatigue, back pain and nausea.
- Encyclopedia of Human Nutrition; Benjamin Caballero et al.
- PDR for Nutritional Supplements; Sheldon Hendler and David Rorvik
- Contemporary Nutrition: Functional Approach; Gordon M. Wardlaw et al.
- Textbook of Functional Medicine; David S. Jones
Sirah Dubois is currently a PhD student in food science after having completed her master's degree in nutrition at the University of Alberta. She has worked in private practice as a dietitian in Edmonton, Canada and her nutrition-related articles have appeared in The Edmonton Journal newspaper.