Selenium

Selenium is a trace mineral incorporated into selenoproteins — most importantly glutathione peroxidase (antioxidant defence), iodothyronine deiodinases (thyroid hormone activation), and thioredoxin reductases. The gap between adequate and toxic intake is one of the narrowest of any nutrient.

Daily requirements

• Adults: 55 µg/day.
• Pregnancy: 60 µg/day.
• Lactation: 70 µg/day.
• Tolerable Upper Intake Level: 400 µg/day — this is roughly 7× the RDA, the tightest safety margin in the fat- and water-soluble vitamin/mineral list.

Thyroid — Hashimoto's and Graves'

The thyroid gland has the highest selenium concentration of any organ in the body. Multiple RCTs of selenomethionine 200 µg/day for 3–12 months in Hashimoto's thyroiditis show a modest but significant reduction in TPO antibody titres. Whether that translates to reduced hypothyroidism progression or clinical benefit is less clear. In Graves' orbitopathy, selenium 200 µg/day BID for 6 months improved eye symptoms and quality of life in a multicentre European trial.

Cancer — the SELECT surprise

The 35,000-man SELECT trial (2011) tested selenium (200 µg) and/or vitamin E for prostate cancer prevention. Not only did selenium not prevent cancer, it may have increased type 2 diabetes risk in men with high baseline selenium. This overturned a decade of enthusiasm from earlier smaller trials and is a good example of why single positive trials don't warrant supplementation across the general population.

Toxicity — selenosis

Chronic intake > 400 µg/day causes selenosis: garlic breath, brittle/lost hair and nails, GI upset, and peripheral neuropathy. High-dose supplement mishaps have caused acute poisonings. Because content varies wildly, brazil nuts can push you into excess: a single large nut ranges from 68 to 91 µg, and one poorly sourced batch analysed at 500+ µg per nut. Two brazil nuts a day is a sensible ceiling.

Food sources

• Brazil nuts (1 oz, 6–8 nuts): 544 µg — highest of any food; wildly variable
• Tuna, yellowfin (3 oz): 92 µg
• Halibut (3 oz): 47 µg
• Sardines (3 oz): 45 µg
• Turkey (3 oz): 31 µg
• Beef (3 oz): 33 µg
• Egg (1 large): 15 µg
• Brown rice (1 cup cooked): 19 µg