Selenium or Zinc: what is the difference
Selenium and zinc often stand side by side on the labels of “men’s” and “immune” complexes, so they are frequently perceived as interchangeable. In reality these are two entirely different trace elements with different chemistry, different “partner” enzymes and different safety margins. Our editors examined exactly what the difference between them is.
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 precisely through it that selenium works as part of the active site of enzymes.
Zinc is a transition metal present in the body in the form of 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 mainly a “part” 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. According to bioinformaticians’ estimates, zinc binds about a tenth of all proteins of 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, thousandths of a milligram. This single circumstance alone explains why an error in selenium dosing 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 of them are glutathione peroxidases, which neutralize peroxides, thioredoxin reductases, which maintain the cell’s redox balance, and deiodinases, which convert thyroxine (T4) into the active hormone triiodothyronine (T3). Selenoprotein P deserves separate mention — the main transporter of selenium in plasma.
Zinc participates in a much 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 depend on zinc.
- Shared:both elements are important for immunity, antioxidant defense and male reproductive function.
- Specific to selenium:metabolism of thyroid hormones, 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 related to spermatogenesis, but in different ways. The selenoprotein phospholipid hydroperoxide glutathione peroxidase (GPx4) is a structural component of the midpiece of the sperm, whereas zinc accumulates in high concentrations in the prostate and seminal fluid. Therefore a deficiency of each of them affects fertility through different mechanisms.
The manifestations of deficiency are also different. For severe selenium deficiency the classic example is Keshan disease — a cardiomyopathy described in regions of China with selenium-poor soils. Zinc deficiency manifests as growth retardation, dermatitis, hair loss, impaired wound healing and a tendency to infections.
Dietary sources and absorption
The selenium content in plant foods depends primarily on the soil on which they are grown. Therefore the same bread or cereal can contain substantially different amounts of selenium in different countries. More reliable sources are considered to be fish, seafood, offal, eggs. The Brazil nut is known for an extremely high but very variable selenium content.
Zinc is best obtained from animal products: oysters lead by a wide margin, followed by red meat, poultry, 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 intestine and reduce absorption.
| Parameter | Selenium | Zinc |
|---|---|---|
| Chemical nature | Non-metal, part of selenocysteine | Metal, Zn²⁺ ion |
| Order of daily requirement | Tens of micrograms | Several to more than ten milligrams |
| Main sources | Fish, offal, 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 |
Absorption of organic selenium in the form of selenomethionine is very high — over 90%. Sodium selenite is absorbed somewhat worse. With zinc the situation is more complex: the fraction of the absorbed mineral depends on the dose, the composition of the food and the body’s current status, and the intestine actively regulates intake through 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 limit is 400 mcg per day. The European Food Safety Authority (EFSA) set adequate intake at 70 mcg. The difference between the norm and the upper limit is about 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. Vegetarians, because of phytates, are advised to aim for a higher intake level, up to one and a half times the standard norm. The distance between the requirement and the upper limit here is only about fourfold.
An excess of selenium leads to selenosis: garlic breath, a metallic taste, brittleness and loss of nails and hair, rashes, irritability, and in severe cases damage to the nervous system. Cases of mass poisoning have been documented due to errors in supplement manufacturing, when the actual selenium content exceeded the declared one hundreds of times.
Chronic excess of zinc usually does not produce acute symptoms but gradually suppresses copper absorption. Zinc stimulates the synthesis of metallothionein in intestinal cells, which binds copper and does not let it pass further. The consequences can be anemia, neutropenia and neurological disorders. 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 better absorbed and create a store in tissues, inorganic ones are more quickly incorporated into selenoprotein synthesis but without accumulation. For a person with adequate status, the choice of form matters less than the mere fact of avoiding 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 familiar with the ZMA complex, in which zinc is combined with magnesium and vitamin B6. For lozenges against colds, researchers most often used zinc acetate and gluconate.
The largest long-term study of selenium — SELECT — showed that taking 200 mcg of selenomethionine a day does not reduce the risk of prostate cancer in men who already had adequate status. Moreover, in the selenium group a statistically non-significant trend toward more frequent type 2 diabetes was observed. This is an important argument against taking selenium “just in case.”
For zinc the best-studied effect is a reduction in the duration of a cold when taking lozenges in the first days of illness, confirmed by Hemila’s meta-analyses. The notion that zinc raises testosterone in healthy men is not confirmed by studies: in the work of Koehler and colleagues, taking ZMA did not change testosterone levels. An increase is possible only when correcting a real deficiency.
Editorial conclusions
Selenium and zinc are not analogs 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 of 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. An excess of selenium causes selenosis, an excess of zinc causes copper deficiency.
Supplements make sense primarily for correcting a confirmed or very probable deficiency. For people with a balanced diet, additional intake usually provides no benefit, and in some cases can be harmful.
If you are choosing a specific product, we recommend reading our practical material “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.
