Enshi was an enigma during China's coronavirus outbreak.
The city in western Hubei province recorded six Covid-19 cases per 100,000 residents. Elsewhere in Hubei, infection rates were between two and 20 times higher. Scientists had no clue why the coronavirus spared Enshi until speculation formed last year that the answer might be in its soil.
A new study by Chinese scientists published this week supported the theory and found "human selenium levels may contribute to antioxidant, anti-inflammatory and immune effects in Covid-19".
Enshi has the world's largest deposit of selenium, a non-metal trace element sitting next to deadly arsenic on the periodic table. The concentration of selenium in Enshi's natural environment was so high that overexposure caused some local residents to lose their hair or fingernails.
But "relatively high dietary selenium intake in selenium-rich areas can enhance human immunity ... (and) contribute to resisting Sars-CoV-2", said a team led by Professor Ma Jin of the State Key Laboratory of Environmental Criteria and Risk Assessment at the Chinese Research Academy of Environmental Sciences in Beijing. The team's paper was published in Environmental Research journal on Sunday.
As selenium levels dropped, the infection rate soared, Ma and colleagues found in data collected from cities across Hubei. Suizhou and Xiaogan, for instance, had the worst selenium deficiency and their positive case rates were the highest outside Wuhan, the provincial capital and home of the first-reported outbreak in China.
The researchers believed the element might have played a "key role" in the human immune response to Sars-CoV-2 via various mechanisms, such as cutting the production of harmful reactive oxygen that could trigger excessive inflammation.
Similar phenomena have occurred elsewhere.
A study published in June and led by Margaret Rayman, professor of nutritional medicine, found the Covid-19 cure rate had a positive correlation with the concentration of selenium in human hair.
In Germany, a study by the Heidelberg University Hospital found Covid-19 patients with lower selenium levels had a higher risk of death.
Heilongjiang, a province in northeast China with the country's lowest selenium level, also recorded the highest death rate at 2.6 per cent, more than four times the average of other provinces outside Hubei, according to Chinese government data.
Li Jianke, professor of nutritional science with Shaanxi Normal University, said these discoveries had prompted some "lively discussions" but the mainstream research community still remained cautious about the results.
"We need more solid evidence. So far, there is no data from a controlled experiment to prove the link really exists," said Li, who was not involved in the study.
In pandemic-hit areas, for instance, volunteers should be recruited to take selenium supplements and compared with those who do not.
"Otherwise, some other factors might be attributed to selenium by mistake," Li said.
One such factor could be demography.
Enshi has one of the longest-living populations in China. Life expectancy there is over 80 years, four years higher than the national average.
Some studies suggest that selenium could have a positive effect on the prevention or treatment of some viruses - including HIV, Ebola, and influenza - but the mechanism remains largely unknown.
A wide range of non-viral health issues, ranging from heart disease and cancer to the effects of ageing have also been linked to selenium. Patients suffering from several types of cancer had less selenium than healthy people, according to some studies.
But several population-wide studies monitoring people's selenium intake failed to find a definitive link.
Molecular-level investigations found the element was involved in various types of critical life processes in cells, such as DNA repair, but excessive selenium could also lead to severe health problems, including brain damage.
"Taking selenium supplements to prevent Covid-19 is not recommended without doctors' supervision," Li said.
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