A more thorough evaluation of selenium supplementation is needed, to better understand its benefits to our immune system, and the risks.
This is the finding of a new study from the Institute of Food Research, and funded by the Food Standards Agency, which has found that selenium supplementation can have positive and negative effects on our immune system, and that these changes depend on how we receive the selenium and the dose.
Selenium deficiency has for a long time been associated with an impaired immune system. People with low levels of selenium have been shown to have lower levels of key components of the immune system, and a reduced ability to respond to viral infections. Studies with mice have shown selenium supplementation increases immunity to flu, but similar evidence in humans is missing.
In the UK, the amount of selenium in the diet has been dropping over recent years, in part due to a switch away from flour imported from North America, which is naturally higher in selenium than European wheats. The generally low selenium status in the UK has led to calls for a supplementation programme, but before this can happen we need a better idea of the overall benefits, and potential risks or side-effects.
To address this, a study on how selenium supplementation affected immunity to flu was carried out by Dr Kamal Ivory and Professor Claudio Nicoletti of the Institute of Food Research, which receives strategic funding from the Biotechnology and Biological Sciences Research Council.
Groups of people with marginal selenium status, aged 50-64, were given selenium supplements, at different doses, to include in their diet. Others were given meals containing onions enriched with selenium. Other participants were given placebos, not containing selenium, and neither the participants nor the scientists knew which were supplemented until after the study. After 10 weeks of the dietary intervention, the participants received a flu vaccine, and important components of their immune system were closely measured over the next two weeks.
The study, published in the journal Clinical Nutrition, found that T-cells, white blood cells that circulate looking for possible infections, were increased at intermediate, but not at high or low selenium doses of selenium. In parallel, the levels of molecules that T-cells use to kill infected cells were reduced by selenium supplementation.
"Selenium supplementation can increase some of the weapons at our disposal to fight off viral infections, but at the same time it seems the amount of ammunition available tends to decline" said Professor Claudio Nicoletti.
The study also showed that different forms of selenium have different effects on the production of cytokines, which are molecules that play an important role in coordinating the immune system.
Also, how we get the selenium is important. Usually these nutrients are delivered using certain carriers that favour consumption. In some cases the carrier used can have immunoregulatory properties of its own and this might also have an effect on the outcome of the selenium supplementation
"This shows that when used as a supplement, selenium has a narrow effective range and might even be detrimental. It would be prudent to thoroughly evaluate the risks and benefits associated with selenium supplementation, especially in segments of the population particularly vulnerable to influenza infection, such as the elderly and chronically ill" said Professor Nicoletti.