Selenium or Zinc: What's the Difference
Selenium and zinc often sit side by side on the labels of "men's" and "immune" formulas, so they are frequently perceived as interchangeable. In reality these are two entirely different trace elements with different chemistry, different partner enzymes and a different margin of safety. The editorial team has examined exactly what sets them apart.
Two trace elements — two different chemistries
Selenium is a non-metal, a chemical "relative" of sulfur. In the body it rarely exists in free form: almost all functional selenium is built into proteins as part of a special amino acid, selenocysteine, sometimes called the "21st amino acid". It is through this that selenium works as part of the active site of enzymes.
Zinc is a transition metal that is present in the body as a divalent ion. It is not part of amino acids but binds to proteins, held between cysteine and histidine residues. This forms structural motifs, for example the well-known "zinc fingers", which allow proteins to recognize DNA.
From this follows the main difference: selenium is chiefly a "component" of a small group of enzymes that govern redox reactions, whereas zinc is a universal structural and catalytic cofactor for hundreds of enzymes and thousands of regulatory proteins. By bioinformaticians' estimates, zinc binds about a tenth of all proteins in the human proteome.
Quantitatively these trace elements also differ by orders of magnitude. The daily requirement for zinc is measured in milligrams, and for selenium in micrograms, that is, in thousandths of a milligram. This single circumstance alone explains why a dosing error with selenium is far more dangerous than with zinc.
Biological functions: selenoproteins versus zinc enzymes
About 25 selenoprotein genes have been described in the human genome. The best known are the glutathione peroxidases, which neutralize peroxides, the thioredoxin reductases, which maintain the cell's redox balance, and the deiodinases, which convert thyroxine (T4) into the active hormone triiodothyronine (T3). Selenoprotein P, the main transporter of selenium in plasma, is also worth mentioning separately.
Zinc takes part in a far broader range of processes. It is needed by DNA and RNA polymerases, carbonic anhydrase, alcohol dehydrogenase, superoxide dismutase (together with copper), as well as hundreds of transcription factors. Cell division, wound healing, taste sensitivity, T-lymphocyte maturation and the synthesis of insulin and its storage in beta cells all depend on zinc.
- In common:both elements are important for immunity, antioxidant defense and male reproductive function.
- Specific to selenium:thyroid hormone metabolism, protection against lipid peroxidation, sperm motility.
- Specific to zinc:cell growth and division, regulation of gene expression, taste and smell, skin condition.
Both trace elements are involved in spermatogenesis, but in different ways. The selenoprotein phospholipid hydroperoxide glutathione peroxidase (GPx4) is a structural component of the sperm midpiece, whereas zinc accumulates in high concentrations in the prostate and seminal fluid. A deficiency of each therefore affects fertility through different mechanisms.
The manifestations of deficiency also differ. The classic example of severe selenium deficiency is Keshan disease — a cardiomyopathy described in regions of China with selenium-poor soils. Zinc deficiency manifests as stunted growth, dermatitis, hair loss, impaired wound healing and susceptibility to infections.
Dietary sources and absorption
The selenium content of plant foods depends primarily on the soil in which they were grown. The same bread or grain may therefore contain substantially different amounts of selenium in different countries. More reliable sources are considered to be fish, seafood, organ meats and eggs. The Brazil nut is known for an exceptionally high but highly variable selenium content.
Zinc is best supplied by animal products: oysters lead by a wide margin, followed by red meat, poultry and cheese. Plant sources — legumes, pumpkin seeds, whole grains — contain a decent amount of zinc, but at the same time phytates, which bind the mineral in the gut and reduce absorption.
| Parameter | Selenium | Zinc |
|---|---|---|
| Chemical nature | Non-metal, part of selenocysteine | Metal, Zn²⁺ ion |
| Order of the daily requirement | Tens of micrograms | A few to over ten milligrams |
| Main sources | Fish, organ meats, eggs, Brazil nut | Oysters, red meat, poultry, seeds |
| What hinders absorption | Few known dietary inhibitors | Phytates, high doses of iron |
| Main risk of excess | Selenosis | Secondary copper deficiency |
The absorption of organic selenium in the form of selenomethionine is very high — over 90%. Sodium selenite is absorbed somewhat less well. With zinc the situation is more complex: the fraction of mineral absorbed depends on the dose, the composition of the food and the body's current status, and the gut actively regulates intake via the ZIP and ZnT transporters.
Another difference is stores. Selenomethionine can be non-specifically incorporated into proteins in place of methionine, creating a kind of depot. For zinc there is practically no special reserve pool in the body, so it must be obtained from food regularly.
Intake norms and the safe limit
According to the US Institute of Medicine, the recommended daily allowance of selenium for adults is 55 mcg, and the tolerable upper intake level is 400 mcg per day. The European Food Safety Authority (EFSA) has set adequate intake at 70 mcg. The gap between the norm and the upper limit is roughly sevenfold.
For zinc the recommended allowance in the US is 11 mg for men and 8 mg for women, and the upper limit is 40 mg per day. Because of phytates, vegetarians are advised to aim for a higher intake, up to one and a half times the standard norm. The distance between the requirement and the upper limit here is only about fourfold.
Excess selenium leads to selenosis: garlic breath, a metallic taste, brittle and falling nails and hair, rashes, irritability, and in severe cases damage to the nervous system. Cases of mass poisoning have been documented due to manufacturing errors in supplements, when the actual selenium content exceeded the stated amount hundreds of times over.
Chronic excess of zinc usually produces no acute symptoms but gradually suppresses copper absorption. Zinc stimulates the synthesis of metallothionein in intestinal cells, which binds copper and prevents it from passing further. The consequences can be anemia, neutropenia and neurological disturbances. Acute large doses of zinc cause nausea and vomiting.
Forms in supplements and the evidence base
Selenium in supplements is represented mainly by selenomethionine, selenized yeast and sodium selenite or selenate. Organic forms are absorbed better and build up a reserve in tissues; inorganic ones are incorporated more quickly into selenoprotein synthesis but without accumulation. For a person with adequate status the choice of form matters less than the mere fact of avoiding an overdose.
Zinc is produced in the form of gluconate, citrate, picolinate, bisglycinate, acetate, sulfate and oxide. Zinc oxide is absorbed the worst. Athletes are well acquainted with the ZMA complex, in which zinc is combined with magnesium and vitamin B6. For cold lozenges, researchers most often used zinc acetate and gluconate.
The largest long-term study of selenium — SELECT — showed that taking 200 mcg of selenomethionine per day did not reduce the risk of prostate cancer in men who already had adequate status. Moreover, the selenium group showed a statistically non-significant trend toward more frequent type 2 diabetes. This is an important argument against taking selenium "just in case".
For zinc the best-studied effect is a shortening of the duration of a cold when lozenges are taken in the first days of illness, confirmed by Hemilä's meta-analyses. The notion that zinc raises testosterone in healthy men is not supported by research: in the work of Koehler and colleagues, taking ZMA did not change testosterone levels. An increase is possible only when correcting a genuine deficiency.
Editorial conclusions
Selenium and zinc are not analogues but two separate trace elements with different biochemistry. Selenium works as part of a small group of antioxidant enzymes and thyroid deiodinases; zinc is a structural and catalytic cofactor for hundreds of proteins that regulate growth, immunity and gene expression.
The requirement for selenium is measured in micrograms, for zinc in milligrams, and in both cases the range between the norm and a dangerous dose is relatively narrow. Excess selenium causes selenosis; excess zinc causes copper deficiency.
Supplements make sense primarily for correcting a confirmed or highly likely deficiency. For people with a balanced diet, additional intake usually provides no benefit and in some cases may do harm.
If you are choosing a specific product, we recommend reading our practical piece "Selenium vs Zinc: What to Choose and for Whom", as well as articles on magnesium and the ZMA complex and on tests for trace elements.
References
- Rayman MP. Selenium and human health. Lancet. 2012;379(9822):1256–1268.
- Institute of Medicine. Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. Washington, DC: National Academy Press; 2000.
- Institute of Medicine. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. Washington, DC: National Academy Press; 2001.
- Lippman SM, Klein EA, Goodman PJ, et al. Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA. 2009;301(1):39–51.
- Prasad AS. Zinc in human health: effect of zinc on immune cells. Mol Med. 2008;14(5–6):353–357.
- Hemilä H. Zinc lozenges and the common cold: a meta-analysis comparing zinc acetate and zinc gluconate, and the role of zinc dosage. JRSM Open. 2017;8(5).
- Koehler K, Parr MK, Geyer H, et al. Serum testosterone and urinary excretion of steroid hormone metabolites after administration of high-dose zinc supplementation. Eur J Clin Nutr. 2009;63(1):65–70.
Andriy Melnyk
A strength-sports coach and author of programs for beginner and intermediate levels. Writes about training planning.
