Selenium is an essential trace mineral obtained through the diet. It is highest in protein-rich foods, such as meat, fish, and legumes. Selenium is important for the function of natural antioxidants in the body. The level of selenium in the soil determines the level of selenium in the food grown in a particular region. North America has relatively high selenium levels, so deficiency is rare. Selenium deficiency is associated with a variety of adverse health outcomes, but in the absence of a deficiency, selenium supplementation is not associated with health benefits. Excessive intake of selenium is toxic.
Observational studies show an association between low selenium levels and dementia; however, selenium supplementation has largely failed to show benefit for preventing or treating dementia in clinical trials. Our search identified:
Epidemiological studies of nutritional status and cognition have found that older adults with the lowest dietary selenium intake levels showed reduced performance on some measures of cognition [1; 2]. While none of the participants were classified as deficient, individuals at the lower end of the normal range may have suboptimal activation of important selenium-containing antioxidants [3]. This places their brains at higher risk for oxidative stress damage. The effect of low selenium on cognitive function was more evident in men, which is consistent with sex differences in prioritization of selenium in the body [2]. Selenium is important in the male reproductive system, so there may be less available for the brain when levels are low. Insufficient selenium levels may promote processes, such as oxidative stress, which harm brain cells. However, there is no evidence to support a role for selenium supplementation for the prevention of dementia, in adults who get adequate levels of selenium from the diet [4]. A six-year study including over 3,000 men over age 60 found that selenium supplementation at 200 mcg per day had no effect on dementia rates [5].
Biomarker studies indicate that selenium levels are altered in dementia patients in the brain and blood, which may be reflective of both altered nutritional patterns and an inability to effectively utilize selenium in the body, due to disease-related processes [3]. Supplementation within the recommended dietary range is not associated with improvement in dementia patients [4]. A study in 40 patients showed stabilization on a measure of cognitive decline following a high-dose of selenium (10 mg, three times daily) which also increases the risk for side effects, but only in the small subset who showed an increase in selenium within the brain fluid with treatment [6]. Another small study found that the use of selenium in combination with a probiotic had a minor benefit on the same cognitive measure, but no effect was seen with selenium alone [7].
Excessive intake of selenium, defined as greater than 400 mcg per day for an adult, can lead to a form of toxicity called selenosis [8]. Symptoms include fatigue, hair loss, nail damage, nausea, muscle weakness, dizziness, burning or tingling feeling, and heart problems. Several studies have found that selenium may impact glycemic indices such that selenium supplementation is not recommended in those with or at high risk for diabetes [9]. Selenium interacts with some drugs used for chemotherapy, heavy metal chelation, and quinolone antibiotics.
NOTE: This is not a comprehensive safety evaluation or complete list of potentially harmful drug interactions. It is important to discuss safety issues with your physician before taking any new supplement or medication.
Selenium is primarily obtained through the diet through protein-rich foods including meat, chicken, fish, eggs, legumes, and cereals [8]. Brazil nuts contain the highest abundance of selenium, with a single Brazil nut containing more than the recommended daily intake level. The selenium content of food varies geographically according to the selenium soil content of a given region [10]. The recommended daily allowance of selenium is 55 mcg per day for adults [8]. Selenium is also available as supplements in organic and inorganic forms, but are not generally recommended unless adequate selenium intake cannot be obtained through food.
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