Selenium Analysis and Speciation in Dietary Supplements Based on Next-Generation Selenium Ingredients

Abstract

Selenium is essential for humans and the deficit of Se requires supplementation. In addition to traditional forms such as Se salts, amino acids, or selenium-enriched yeast supplements, next-generation selenium supplements, with lower risk for excess supplementation, are emerging. These are based on selenium forms with lower toxicity, higher bioavailability, and controlled release, such as zerovalent selenium nanoparticles (SeNPs) and selenized polysaccharides (SPs). This article aims to focus on the existing analytical systems for the next-generation Se dietary supplement, providing, at the same time, an overview of the analytical methods available for the traditional forms. The next-generation dietary supplements are evaluated in comparison with the conventional/traditional ones, as well as the analysis and speciation methods that are suitable to reveal which Se forms and species are present in a dietary supplement. Knowledge gaps and further research potential in this field are highlighted. The review indicates that the methods of analysis of next-generation selenium supplements should include a step related to chemical species separation. Such a step would allow a proper characterization of the selenium forms/species, including molecular mass/dimension, and substantiates the marketing claims related to the main advantages of these new selenium ingredients.

Keywords: analysis; next-generation supplements; selenium; selenium nanoparticles; selenium polysaccharides; speciation; zerovalent selenium.

Figure 1

Physiological window of selenium supplementation. Intakes lower than recommended dietary allowance (RDA) determine diseases related to selenium deficiency. Intakes higher than 200% could lead to disorders related to excess selenium. The U-shape relationship between selenium and disease status limits selenium supplementation to subjects with lower-than-optimal selenium status. The next-generation selenium supplements, due to reduced toxicity and controlled release, should present a lower risk of supplementation on subjects with (near) Se optimal status. Data were obtained from References [5,23,43,44,45,46,47].

Figure 2

Potential benefits of the next-generation dietary supplements based on nanoselenium and selenium polysaccharides. The new forms of selenium ingredients were proven, in several experimental models, to have lower toxicity and higher bioavailability. The biological activities of such putative slow, controlled-release selenium forms are not limited to the already known effects of selenium. Additional effects, such as in vitro inhibition of biofilm formation by the multi-antibiotic resistant pathogenic bacteria or stimulation of beneficial microbiome on chicken, were recently reported for these new forms of selenium. Data were obtained from References [14,33,37,41,74,75,76,77,78,79].

Figure 3

Next-generation Se supplements and their (potential) moderate epigenetic effects. In red are enzymes influenced by Se level. A low level of Se disturbs one-carbon metabolism and S-adenosyl methionine restoration, because betaine homocysteine methyltransferase (BMHT) needs higher levels of selenium for optimal activity. A high level of Se leads to inhibition of DNA methyltransferases (DNMTs) and to competition for methyl groups, needed to produce methylated selenium amino acids. Excess selenium influences histone deacetylation and further cross-talk with DNA methylation. The seleno-amino acids are converted via transamination to seleno-keto-α-acids, which inhibit histone deacetylase (HDACc). Zerovalent selenium and selenium polysaccharides slowly release active selenium species via an equilibrium reaction. The rate of such an equilibrium reaction depends on consumption of the reaction products. In tissues of subjects with a normal/optimal selenium status, the consumption of the reaction products is lower and the release of active selenium species over the optimal status is slower. At a deficit-to-optimal level of intake, the selenium species are rapidly used for the expression of the deficit amino acids and seleno-proteins, and the rate of release is higher. The slow and consumption-controlled release of the bioactive Se species should allow a more adapted response of supplementation to selenium status. Figure modified from Oancea et al. [186]. Data were obtained from References [31,37,186,187].

Figure 4

Schematic representation of the analytical procedures for determining and characterizing selenium in dietary supplements. The analysis of the next-generation selenium dietary supplements based on nanoselenium or on Se polysaccharides must include a step related to chemical species separation. Such a step would allow a proper characterization of the molecular mass/dimension of Se-based ingredients and it is mandatory to substantiate the marketing claims related to the main advantages of these new Se ingredients—lower toxicity, enhanced bioactivity, and slow and controlled release.