Zinc Supplements: How Much is Too Much? Seahorse Study Reveals Cautionary Tale
Featured Image Caption: Seahorses are charismatic creatures with long snouts that resemble horses. Seahorses are slow swimmers and rely on camouflage to ambush their prey. Male seahorses are unique in that they carry the females eggs in a pouch and birth offspring. “Seahorse/Hippocampus” by TANAKA Juuyoh (田中十洋), CC BY 2.0 via Wikimedia Commons
Primary Source Article: Han, X., Li, C., Qin, G., Wang, X., Zhang, B., & Lin, Q. (2024). Growth increase and gonadal dysfunction of the lined seahorse triggered by zinc exposure. Aquatic Toxicology, 272, 106947–106947. https://doi.org/10.1016/j.aquatox.2024.106947
Secondary Source Article: Shi, Y., Hao, R., Ji, H., Gao, L., & Yang, J. (2024). Dietary zinc supplements: beneficial health effects and application in food, medicine and animals. Journal of the Science of Food and Agriculture, 104(10), 5660–5674. https://doi.org/10.1002/jsfa.13325
Zinc is an essential microelement that is important for the immune system, development, and reproduction. Generally speaking, our society characterizes taking vitamins as a good thing. Zinc is sold as a vitamin supplement, fortified in foods, and given to livestock. Zinc supplements are also used in aquaculture, the practice of raising aquatic organisms for commercial purposes. In aquaculture, zinc supplements have been given to various species of fish. For example, when catfish were given supplements for 12 weeks, they had improved their fat content, growth, and reproductive fitness. However, zinc supplementation should be moderated; there can be too much of a good thing. Zinc toxicity can cause neurotoxicity, skin conditions, anemia, low white blood cell count, and reduced fertility. This was the case in the zinc exposure experiments conducted by Dr. Han and colleagues. Although seahorses fed zinc supplements grew in size, it also negatively impacted their reproductive system.
Exposing Seahorses to Zinc
These images reveal changes in ovary and testis development after zinc exposure in comparison to seahorses not exposed to zinc. Estrogen and testosterone levels were also measured after zinc exposure. Image Source: Han, X., Li, C., Qin, G., Wang, X., Zhang, B., & Lin, Q. (2024). Growth increase and gonadal dysfunction of the lined seahorse triggered by zinc exposure. Aquatic Toxicology, 272, 106947–106947. https://doi.org/10.1016/j.aquatox.2024.106947
Safe levels of zinc in aquatic environments are generally less than 0.1 mg. Unfortunately, higher concentrations have been observed in the U.S. and China. Scientists used similar zinc levels observed in these aquatic environments to monitor the effect it had on seahorses. A total of 180 seahorses were randomly assigned to 6 treatments. After 2 to 4 weeks of exposure, seahorses brains and reproductive organs were tested. Some of these tests involved looking at the tissue under a microscope, performing RNA sequencing to determine what genes were turned on and off during exposure, and a blood test to determine testosterone and estrogen levels.
Seahorses are especially vulnerable to environmental pollutants because of their limited movement and shallow coastal habitats. Although seahorses were larger in size compared to seahorses that weren’t exposed to zinc, negative effects were observed. Female seahorses showed defects similar to those with individuals with polycystic ovarian syndrome (PCOS); the female seahorses had high testosterone levels, weight increase, and similar appearance of ovarian features to PCOS ovaries under the microscope. For males, cysts were observed in testes and increased levels of the hormone estradiol. This study accomplished 2 things: It helped identify missing pieces on how zinc is involved in PCOS in women and is a cautionary tale of what can happen if zinc intake is not regulated in humans and animals.
Using Environmental Studies to Build a Better Future
There are several reasons that zinc can enter waterways. It can be a combination of natural and human activity processes. Zinc compounds can enter waterways from the erosion of rocks underground, to mining, steel production, coal burning, and industrial waste. Mineral leaching from excess feed in aquaculture is another source of zinc pollution. China’s zinc pollution can partially be explained by being the country with the second largest zinc reserves and one of the top producers in industrial activities such as manufacturing, mining, and construction. Similarly, the US is a major manufacturing country, second to China. While these industrial activities can boost an economy, there are repercussions to the environment. Hopefully, environmental studies such as this one can help policymakers determine how to better develop policies that take serious consideration of the environment, wildlife, and people.
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