Selenium is a nutrient that all animals and humans need, but it is poisonous if we take too much of it. We therefore need to be a little careful with selenium. However, it has been shown that extra selenium can protect us from the effects of methylmercury, an organic form of mercury that is toxin to the nervous system.
In a study that has just been published in the journal Aquatic Toxicology, NIFES scientists studied interactions between selenium and methylmercury in zebrafish. They found that high levels of selenium had negative effects, but only when fish were allowed to breed. It turned out that the level of selenium that at first protected the fish against the negative effects of methylmercury had a destructive effect on the production of the next generation, and methylmercury amplified the negative effects.
“This was a surprising finding,” says Samuel Penglase, a PhD student at NIFES.
The study is among the first to look at the effects of selenium and methylmercury on both the growth phase and reproduction in fish. It may help to improve our understanding of how selenium and methylmercury affect each other in human reproduction too.
Previous studies, including some from NIFES, have shown that selenium can reduce the negative effects of methylmercury, and this was confirmed by the scientists in the new study. The selenium helped to reduce mortality and prevent the reduced growth that is among the effects of increased exposure to methylmercury.
In the experiment, zebra fish were fed different diets. Two groups were given food with a high content of methylmercury, and the diet of one of these groups also had a high content of selenium. A third group was given the same high level of selenium, while a fourth acted as a control group, and was not fed extra selenium or methylmercury.
The fish in the high-selenium groups had fewer viable offspring than those in the control groups. In the group that was fed high levels of both selenium and methylmercury, the effect was more pronounced, while fish that were fed a high level of methylmercury alone produced more offspring than those in the group that were also given extra selenium.
“We expected that methylmercury on its own would have a negative effect on reproduction, but instead, we found that the diet which contained methylmercury had little effect on reproduction in zebrafish, and that a larger dose of selenium had a greater negative effect,” says Penglase.
The levels of selenium and methylmercury used in the experiment were 10 and 12 mg/kg body weight respectively. These levels are similar to what can be found in fish from extremely polluted waters. It is not permitted to sell food that contains more than 0.5 mg/kg mercury. The tolerable daily intake (TDI) of selenium for humans is 0.4 mg. Selenium is an essential mineral, and most animals need it in their diet. Human beings need about 0.05 mg per day.
Penglase emphasises that the study does not say anything about the effect of these substances on human reproduction, but it is possible that the effects of methylmercury and selenium we observed in zebrafish will also apply to human. For that reason, the study ought to trigger new investigations of how selenium and methylmercury affect reproduction in mammals,” says Penglase.