Selenium

Why selenium matters for longevity

Selenium is a trace mineral your body needs in small amounts but can't do without. It gets built into roughly 25 selenoproteins that handle antioxidant defense, thyroid hormone conversion, DNA synthesis, and immune regulation. Unlike calcium or iron, selenium doesn't become part of your bones or blood cells. Instead, it works as a functional component inside enzymes. When selenium runs low, these enzymes stop working properly, and the effects ripple across multiple systems ^[1].

What makes selenium especially interesting from an aging perspective: a 2023 umbrella review of 76 meta-analyses found that higher selenium intake is linked to lower all-cause mortality, reduced risk of digestive cancers, and decreased rates of depression ^[2]. A separate 2024 meta-analysis of cohort studies confirmed a clear inverse relationship between blood selenium levels and mortality risk ^[3]. The window between too little and too much is narrow, though. The recommended daily intake sits at 55 micrograms for adults, and the upper safe limit is 400 micrograms. Most of the benefits show up when correcting deficiency, not from megadosing.

Selenium and thyroid function

The thyroid gland concentrates selenium at levels roughly 100 times higher than most other tissues. Three families of selenium-dependent enzymes keep it running: deiodinases convert inactive T4 into active T3, glutathione peroxidases protect thyroid tissue from hydrogen peroxide generated during hormone production, and thioredoxin reductases regulate redox balance ^[1].

A 2024 systematic review and meta-analysis of 35 randomized controlled trials found that selenium supplementation in Hashimoto's thyroiditis patients reduced thyroid peroxidase antibodies (TPOAb) after 3 and 6 months, lowered thyroglobulin antibodies at 3 months, and decreased TSH levels after 6 months ^[4]. These results are clinically meaningful because Hashimoto's is the most common cause of hypothyroidism in developed countries. If you're supplementing iodine for thyroid health, adequate selenium is equally important, since the two minerals work in tandem and one can't compensate for the other.

Antioxidant defense and immune function

Glutathione peroxidase (GPx) is the body's primary intracellular antioxidant enzyme system, and it requires selenium to function. GPx breaks down hydrogen peroxide and lipid peroxides before they can damage DNA, mitochondria, and cell membranes. No other antioxidant system can fully replace what GPx does ^[1].

On the immune side, selenium affects both innate and adaptive immunity. T-cell proliferation, natural killer cell activity, and antibody production all depend on adequate selenoprotein levels. Deficiency weakens interferon-gamma production and makes respiratory infections more severe. The KiSel-10 trial, a Swedish double-blind placebo-controlled study of 443 elderly participants, found that combined supplementation with 200 micrograms of selenium and 200 mg of CoQ10 daily for four years reduced cardiovascular mortality by 49%, with the protective effect still measurable 12 years later ^[5].

Selenium and brain aging

Emerging research connects selenium status to cognitive health in older adults. A 2024 NHANES analysis found that higher dietary selenium intake was associated with better scores on cognitive tests including memory recall and verbal fluency ^[6]. In the laboratory, selenium deficiency impairs hippocampal neurogenesis, the process of forming new neurons in the brain region responsible for memory. The mechanism appears to involve BDNF signaling and protection of neural progenitor cells from oxidative damage ^[7].

That said, a six-year trial of over 3,000 men above age 60 found no effect of 200 mcg/day selenium on dementia rates. The discrepancy likely reflects baseline selenium status: people who are already getting enough won't benefit from taking more. This pattern repeats across selenium research. Benefits concentrate in populations with low or marginal status.

Food sources and supplementation

Brazil nuts are by far the richest selenium source, but their content varies wildly depending on where they were grown. A single nut can contain anywhere from 10 to 90 micrograms. Two Brazil nuts per day typically covers the RDA, but the unpredictability makes them unreliable as a sole source ^[1]. Fish, shellfish, organ meats, and eggs provide consistent amounts of 20-60 micrograms per serving. Whole grains and legumes contribute smaller amounts, with bioavailability that depends on soil selenium content.

This is where geography comes in. Soils across much of Europe, particularly in Germany, Denmark, Finland, and parts of the Balkans, are low in selenium ^[8]. People eating locally grown food in these regions may not reach the RDA through diet alone. If you supplement, selenomethionine is the best-absorbed form. It uses the same transport mechanism as the amino acid methionine and is roughly 40-70% more bioavailable than sodium selenite. Selenium-enriched yeast is another good option. Avoid chronic doses above 200 micrograms without testing, since excess selenium causes selenosis: hair loss, brittle nails, garlic-like breath, gastrointestinal problems, and at high levels, neurological damage ^[1].