For some time, researchers have been wondering about the risks of selenium overdose, particularly in relation to the onset of type 2 diabetes. An important contribution comes from an analysis of the Molisani study, which has been monitoring a large population cohort in the Molise region of Italy for some time. This research is helping to expand the scientific evidence for an increasingly adequate and up-to-date definition of safe doses of selenium intake.
The research originated for two reasons: firstly, the great interest that nowadays gravitates around a metalloid and trace element, selenium, of which epidemiological studies have highlighted both the nutritional importance and the toxicological properties, thus suggesting the importance of an accurate identification of the safe intake dose, to avoid risks of both deficiency and overdose, the latter being associated by several studies with an increased risk of diabetes. The second motivation stems from the availability of increasingly accurate and time-extensive data from this cohort study. To date, the only evidence available on the association between selenium intake and diabetes in a longitudinal cohort design is that provided by the Molisani study.
The study was able to count on a period of more than 8 years of follow-up, i.e. continuous health surveillance, of a cohort composed of more than 21 thousand residents in the Molise region and not affected by diabetes at the time of recruitment in the study. It was possible to estimate the average dietary intake of selenium at the time of recruitment into the study, i.e. between 2005 and 2010, and subsequently follow these participants to assess, through hospital admissions, whether the incidence of diabetes was associated with previous selenium intake and, if so, how. In order to analyse the results, statistical procedures suitable for the flexible, i.e. also curvilinear, “modelling” of the association between selenium and diabetes were used, based on the so-called spline regression analysis. This was done in order to answer not only the question that prompted the study based on the results of the US NPC trial 15 years ago, i.e. whether an excessive intake of selenium increases the risk of diabetes, but also to identify precisely the intake levels at which this association might possibly arise.
In the population-wide analysis, it was observed that selenium intake up to the average value for this population, i.e. 60 micrograms per day, was not associated with an increased risk of type 2 diabetes. However, this association was seen at daily intakes of 80-100 micrograms or more, which is higher than the value recommended as nutritionally adequate by food safety agencies and public health bodies. The study results also suggest possible differences on the basis of age and gender, but subgroup analyses were not statistically accurate enough to provide reliable indications of such possible interactions.
The study did not include diabetics, in whom we do not know exactly whether and to what extent selenium may have specific effects, this being left to ad hoc experimental studies. Instead, the study adds to the large number of studies specifically looking at the effect of selenium intake on the future risk of diabetes in non-diabetics, which have generally suggested that the risk is greater in people who have consumed relatively large amounts of selenium over a long period of time.
The recommendations to be followed are those that should characterise, in a broad sense, all issues, nutritional and otherwise, of interest to health: carry out further observational and preferably prospective investigations into the association between selenium and diabetes, and refer only to the evidence provided by the biomedical literature and the most up-to-date publications and opinions issued by food safety agencies, such as Efsa for us Europeans, and by public health and human nutrition bodies and scientific societies. Selenium, like many other nutritionally essential trace elements, has a safe recommended dietary intake range that must be respected, even taking into account age and special conditions such as pregnancy and breastfeeding, in order to avoid risks of both deficiency and toxicity.
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