Our modern, industrial diets can be full of calories but paradoxically low in some important minerals. Having enough of these minerals, such as magnesium, is vital to good physical and mental health. Selenium is another extremely important micronutrient that may be low in the diets of some modern populations, leading to chronic health problems.
Selenium is found in the soil and in marine bivalves (such as oysters). Levels in the soil are quite high in certain parts of China, Turkestan, and the Western United States. Levels are very low in Finland, Scotland, New Zealand, and other parts of China, so that livestock must be supplemented, or they will fall ill (1). According to some gardening message boards, soils where I live here in New England aren't great, and Texas where I grew up is rather middling. And according to various sources, it is somewhat difficult to figure out how much you do get from the soil, as areas vary widely and many forms of selenium are more bioavailable than others.
What does selenium do? Well, selenium is key to one of the body's master antioxidants, glutathione peroxidase. This complex keeps the delicate polyunsaturated acids in our cell membranes from getting oxidized (rancid). Polyunsaturated fats, such as fish and a certain percentage of poultry oils, are particularly vulnerable to becoming rancid, so having lots of glutathione peroxidase around to protect our cell membranes is helpful when we consume these essential fats. We really don't want to run low on the master antioxidants - doing so leaves us vulnerable to a lot of toxic gunk building up in our systems, with long-term effects.
Selenium deficiency has been described in China as Keshan Disease, a type of heart problem (2). It is also thought to be a factor in gastrointestinal cancer, liver cancer, and prostate cancer. In addition, selenium in concert with iodine seems to be important to thyroid hormone production (specifically in the conversion of inactive T4 to active thyroid hormone, T3). Selenium deficiency seems to be one of the major dangers of a carelessly designed ketogenic diet for treatment of epilepsy in children, leading to death via an enlarged heart and abnormal heart rhythm. Selenium deficiency was also deemed the cause of death (via cardiomyopathy) of adult patients on selenium-deficient TPN (total parenteral nutrition -- given to patients in desperate circumstances who, for some reason, cannot tolerate any food given via mouth or tubes in the gut).
Selenium is a trace mineral, so it is appropriate for humans in trace amounts. As with nearly all vitamins and minerals, just because we need some doesn't mean more is better. Humans need micrograms rather than milligrams. People have died (from disastrously low blood pressure and heart failure) via the ingestion of gram amounts of selenium, and toxicity occurs at milligram amounts. The first symptoms of toxicity are hair and nail brittleness and garlic smell on the breath. Gun bluing is 2% selenious acid, and ingestion can lead to death via selenium poisoning. I wouldn't recommend drinking gun bluing, as severe selenium toxicity seems like one of the more horrible ways to die.
Selenium is available in grains, nuts, fruits, vegetables, animals, and bivalves. Fruits and vegetables, however, don't seem to have that much in the US, and the best food source is actually brazil nuts (one ounce will get you over 500 micrograms, so take it easy and have just a few at a time). Brazil nuts are also a bit high in omega 6 fatty acids and radium, as it happens, which might be a reason not to make them a major staple of your diet. Organ meats are also high in selenium. 3 ounces or so of tuna, beef, cod, and chicken breast will get you around 30 mcg, whereas a typical grain meal (pasta, oatmeal) will net you around 10-15 mcg. Though phytic acids in grains do interfere with mineral absorption, studies of women eating wheat from selenium-rich soils showed a nice elevation in plasma levels of selenium.
The adult US RDA is 55 micrograms (though a bit higher for pregnant or breastfeeding women) (3). The doctors Jaminet recommend 200 mcg daily, and the US upper tolerable limit is 400 mcg (the level above which those nails start to get brittle, you get irritable and tired, and that breath begins to be garlicky.). On the other hand, Chinese studies of daily doses up to 1500 micrograms showed no adverse effects, nor did another study of 600 mcgs daily. Cool it on the supplements if your breath gets garlicky or your nails start to look like this picture. (Actually, go see a doctor pronto if your hair falls out and your nails look like that.)
Now we have a general idea about what selenium is, where it comes from, and what is too little and too much. But what does it have to do with the brain? Turns out the most selenium and mental health data is associated with pregnancy, aging and/or thyroid gland function.
Let's talk pregnancy and depression for a minute. Little known fact that slightly more women are depressed during pregnancy than after it (4). And if you combine ante and post-natal depression statistics, this is what you get for the moms and kiddos: poor maternal self-care, increase in alcohol and drug use during pregnancy, decrease in seeking medical care during pregnancy, more pre-eclampsia, birth difficulties, preterm delivery, reduced breastfeeding, lower APGAR scores, poor sleep, failure to thrive, developmental delays, greater risk of illness in the baby, more behavioral problems, and at 16 years, offspring of depressed mothers are almost five times more likely to suffer depression themselves.
There are any number of social and medical factors that are linked to perinatal depression, but let's focus a bit on the nutritional ones - links have been found with folate status, vitamin B12, calcium, iron, selenium, zinc, and omega 3s. Kaplan and colleagues, in a great literature review, found potential beneficial effects from B vitamins, vitamin C, D, and E, calcium, chromium, iron, magnesium, zinc, selenium, and choline on mood symptoms. (Real Food = Best Fetal Dinner). A recent study showed a significantly decreased Edinburgh Postnatal Depression Scale score (that's good) in pregnant women randomized to receive 100 mcg selenium daily from the first trimester until delivery.
Selenium is required for the synthesis and metabolism of thyroid hormones. And way back in 1991, Benton and Cook did a randomized controlled crossover trial of 100 mcg of selenium vs placebo in 50 people for 5 weeks, followed by a 6 month washout, then the crossover arm of the trial. Selenium supplementation was associated with increases in self-reported mood. This same paper tells us that when push comes to shove and selenium is deficient, the brain is the last place that selenium levels drop, suggesting that in the brain, selenium is important.
More recently, Gosney et al reviewed the effects of micronutrient supplementation on mood in nursing home residents, finding that no residents started out with insufficient serum levels of selenium, yet 8 weeks of 60 mcg selenium supplementation (included in a multivitamin/multimineral with 150 mcg iodine) was directly correlated with decreases in depression scores and increases in serum levels. Supplementation with selenium resulted in reduced serum thyroid hormone T4 and increased serum thyroid hormone T3, suggesting that the additional selenium helped the rather boring T4 become the metabolically active T3.
So selenium is certainly something we don't want to be deficient in, particularly if we have hypothyroidism, histories of depressive illness or are worried about cognitive decline. However, with an average US intake of 60-220 micrograms a day, many of us will have more than adequate intake. We should worry about pregnant women, children, the elderly, and those who eat almost entirely processed junk food getting sufficient amounts. The best way to get vitamins and minerals is almost always from whole, unprocessed foods. Good for your thyroid, your heart, and your brain.
Copyright Emily Deans, MD