Zinc, Selenium, and Iodine: The Trace Minerals Often Left Off the Radar

Zinc participates in more than 300 enzymatic reactions in the human body. It is essential for immune cell development, protein synthesis, wound healing, DNA replication, and the senses of taste and smell. Without adequate zinc, immune responses slow, wounds take longer to close, and growth can be impaired — making it a particularly critical nutrient during childhood, pregnancy, and older adulthood.

The Recommended Dietary Allowance (RDA) for zinc is 11 mg per day for adult men and 8 mg for adult women, according to the National Institutes of Health Office of Dietary Supplements. Oysters are by far the richest single source, but red meat, poultry, beans, nuts, whole grains, and fortified breakfast cereals all contribute meaningfully. Plant-based sources contain phytates — naturally occurring compounds that can reduce zinc absorption — so people who eat little or no animal protein may need somewhat higher intakes to compensate.

Selenium is incorporated into a family of proteins called selenoproteins, which play a central role in protecting cells from oxidative damage and supporting thyroid hormone metabolism. Glutathione peroxidase, one of the body's key antioxidant enzymes, depends on selenium to function. Research also suggests selenium is involved in DNA repair and immune regulation, though its precise role in preventing chronic disease is still an active area of study.

The RDA for selenium is 55 micrograms per day for adults. Brazil nuts are famously concentrated in selenium — sometimes supplying the entire daily requirement in a single nut — though the selenium content varies widely depending on where the nuts were grown. Seafood, organ meats, chicken, eggs, and whole grains are more consistent everyday sources. One important caution: the tolerable upper intake level for selenium is 400 micrograms per day for adults, and chronic high intake from aggressive supplementation has been associated with toxicity, including hair loss and nerve damage. More is not better here.

Iodine's primary role in the body is straightforward: it is a required building block for thyroid hormones, specifically thyroxine (T4) and triiodothyronine (T3). These hormones regulate metabolism, growth, and development throughout life, and during fetal development they are critical for brain formation. Iodine deficiency remains a leading preventable cause of intellectual disability worldwide, and even mild deficiency in pregnancy can affect infant cognitive development.

In the United States, iodized salt and dairy products have made outright iodine deficiency relatively uncommon, though certain populations — including pregnant women and people who avoid dairy and iodized salt — remain at risk. The RDA is 150 micrograms per day for most adults, rising to 220 micrograms during pregnancy and 290 micrograms during lactation. Seaweed, seafood, dairy, and eggs are reliable food sources. Sea salt and kosher salt are typically not iodized, a detail worth noting for households that have shifted away from standard table salt.

The mineral content of plant foods is directly tied to the mineral content of the soil in which they are grown. Selenium levels in soil, for example, vary dramatically across different U.S. regions and countries, which means the same crop grown in different locations can have very different nutrient profiles. This is one reason why food-sourcing geography matters more for selenium than for many other nutrients.

Processing also plays a role. Refining whole grains into white flour removes a significant portion of their naturally occurring zinc and selenium. Fortification can partially restore what is lost, though the degree of replacement and the bioavailability of added minerals varies. For a deeper look at these trade-offs, see our piece on micronutrients from whole foods versus fortified products.

Trace mineral deficiencies often develop gradually and produce symptoms that are easy to attribute to other causes — fatigue, poor concentration, slow wound healing, or unusual hair thinning. This overlap with common complaints makes self-diagnosis unreliable. If you follow a strict vegan diet, avoid iodized salt, live in a selenium-depleted region, or have a gastrointestinal condition that impairs nutrient absorption, those are meaningful risk factors worth discussing with a healthcare provider.

Blood and urine tests can measure zinc and iodine status, and serum selenium is a standard marker for selenium assessment, though interpretation requires clinical context. The guide to reading your body's signals for vitamin deficiencies offers additional context on how nutrient shortfalls can manifest — and when it is time to seek professional evaluation rather than guessing.