Writing in the British Journal of Nutrition, researchers from Flinders University of South Australia report that selenium-enriched yeast did not have the same effect as the selenium-dairy on certain selenium-dependent proteins reported to have anti-cancer activity.
Led by Dr Ying Hu, the researchers report that the source and form of the selenium impacts on the potential gene regulatory activity of the mineral. According to the new data, only the selenium-enriched dairy proteins had an effect on selenium-dependent proteins, such as selenoprotein P (SeP), cytosolic glutathione peroxidase-1 (GPx-1), and gastrointestinal GPx-2 (GPx-2).
“The present study indicates for the first time that rectal selenoprotein gene expression (i.e. SeP, GPx-1 and GPx-2) is significantly regulated by dietary selenium supplementation independent of plasma levels of selenium and GPx activity; furthermore, regulation depends on the source/form of dietary selenium supplementation; dairy-selenium had a more sustained effect,” report the researchers.
“Up-regulation of rectal SeP, GPx-1 and GPx-2 raises the potential for selenium supplementation to directly influence the risk for colorectal disease.
“Therefore, a novel form of Se such as this dairy-sourced Se warrants further investigation to test this possibility, especially in relation to the possibility of reducing the risk of colorectal cancer,” they added.
There are an estimated 945,000 new cases of colorectal every year globally, with about 492,000 deaths from the cancer each year. Only about five percent of colorectal adenomas are thought to become malignant, and this process could take between five and ten years.
Selenium and cancer
Selenium is a trace element that occurs naturally in the soil and is absorbed by plants and crops, from where it enters the human food chain - either directly or through consumption of meat and other products from grazing animals.
The mineral is included in between 50 and 100 different proteins in the body, with multifarious roles including building heart muscles and healthy sperm. However, cancer prevention remains one of the major benefits of selenium, and it is the only mineral that qualifies for a Food and Drug Administration (FDA)-approved qualified health claim for general cancer reduction incidence.
Dr Hu and co-workers obtained selenium-enriched dairy from Milk Industries Limited (Australia) and then prepared a protein isolate for human use containing this bioavailable form of selenium by Ozscientific Limited (Australia). It was provided as a powder, said the researchers. The selenium-rich yeast was provided by Alltech Biotech Private Limited (Australia).
Twenty-three healthy people with low selenium levels were recruited to participate in the study. Volunteers were randomly assigned to receive 150 micrograms per day of selenium in one of the two forms for six weeks. This was followed by six weeks of observation but not selenium intervention.
Results showed that both selenium forms produced rapid increases in blood selenium levels, but activity of GPx in the blood was not affected. When the researchers looked at the expression of various markers in the rectum, they observed that only the selenium-enriched dairy produced a “sustained elevation of SeP after the washout period”.
Furthermore, expression of GPx-1 and GPx-2 was higher following dairy-selenium supplementation, compared with the yeast selenium.
“Currently, we are unable to explain why the two selenium-rich products are significantly different in their influence,” said the researchers. “According to the selenium speciation data available, both products were similar with respect to their predominant components of selenomethionine (83 percent) and selenocysteine (5 percent), albeit there was a significant component of dairy-Se unidentified.
“Given the nature of its preparation, dairy-selenium did not contain selenium compounds smaller than 10 kDa. The present results support the concept that the chemical form of selenium and not selenium per se is a critical determinant of selenium influences on gene expression,” they added.
Europe versus America
The study is of added importance in Europe where selenium levels have been falling since the EU imposed levies on wheat imports from the US, where soil selenium levels are high.
As a result, average intake of selenium in the UK has fallen from 60 to 34 micrograms per day, leading to calls from some to enrich soil and fertilizers with selenium to boost public consumption. Selenium-enriched fertilizers are used in Finland.
The European recommended daily intake (RDI) is 65 micrograms. The recommended EC Tolerable Upper Intake Level for selenium is 300 micrograms per day.
In an email to NutraIngredients-USA.com, Dr Graeme McIntosh from Flinders University of South Australia and co-author of the study, said: "A highlight of this study is that increases in rectal glutathione peroxidases [GPX] 1 and 2 are seen with intakes above current RDA [55 Se micrograms per day], nutritional adequacy which was based on intake for maximum plateaued plasma PGX levels.
"This builds a case for why higher than RDA levels may have cancer preventing potential in an Australian population," added Dr McIntosh.
Source: British Journal of Nutrition
Published online ahead of print, doi: 10.1017/S0007114511000420
“The influence of selenium-enriched milk proteins and selenium yeast on plasma selenium levels and rectal selenoprotein gene expression in human subjects”
Authors: Y. Hu, G.H. McIntosh, R.K. Le Leu, J.M. Upton, R.J. Woodman, G.P. Young
Correction: This article has been corrected from the original, which cited the dose used in the study as 150 milligrams, whereas the actual dose used was 150 micrograms. Such a dose in milligrams would potentially be toxic. We apologize for the error.