Background: Minerals, often referred to as micronutrients, are one of the 5 fundamental groups of nutrients needed to sustain life. Micronutrient malnutrition affects >50% of the worldwide population. In particular, zinc (Zn) deficiency is considered an emerging public health problem in India and in other developing countries. Selenium (Se) is another trace mineral essential for humans and animals. Dietary Se exists primarily as selenomethionine and selenocysteine. In addition, Se may be present in its inorganic form (selenite) in some vegetables. To increase the daily intake of these minerals, numerous food supplements containing different inorganic and organic forms of Zn or Se are commercially available. At any rate, it is quite well known that inorganic salts have a very low bioavailability. Organic salts, commonly based on gluconate, orotate, citrate, or other molecules, are characterized by a higher systemic effect. The innovative opportunity of using certain species of probiotics enriched with the 2 minerals could represent an interesting alternative to these preparations. Diet integration with bacteria able to internalize Zn and Se may embody a new application of probiotics.
Methods: To overcome the difficulties of in vivo animal or human trials, in this work a cell culture model using Caco-2 cells in bicameral chambers (Transwell system) was developed and validated to quantify the bioavailability of some commercial forms of Se and Zn compared with the organic forms accumulated intracellularly by Lactobacillus buchneri Lb26 (DSM 16341) and Bifidobacterium lactis Bb1 (DSM 17850), respectively.
Results: The experimental data collected demonstrated a significantly higher bioavailability of Se and Zn internalized by L. buchneri Lb26 (DSM 16341) and B. lactis Bb1 (DSM 17850), respectively, compared with the inorganic and even organic forms tested. In particular, the Se accumulated at the intracellular level by L. buchneri Lb26 proved to be 5.9, 9.4, and 65 times more absorbable than sodium selenite, seleno-L-methionine, and seleno-L-cysteine, respectively. In contrast, Zn internalized by B. lactis Bb1 showed an absorption that was >16 times higher by Caco-2 cells compared with zinc gluconate and a 31.5 times higher absorption compared with zinc sulfate. Most notably, Se and Zn internalized by the 2 probiotics studied are the only forms able to reach the Transwell basolateral compartment at a concentration higher than the concentration found in the apical compartment, therefore suggesting a considerably higher in vivo ability to be absorbed into the bloodstream. Both organic and inorganic forms of Se and Zn were predominantly found in the apical compartment, thus demonstrating their poor ability to diffuse into the cell and become bioavailable in all subcellular areas.
Conclusions: The opportunity of delivering minerals in a highly bioavailable form by means of a probiotic bacterium has not been deeply investigated to date. This is the first study reporting quantitative data on the bioavailability and percentage of absorption of minerals internalized by specific probiotics. The most noticeable aspect is the significantly higher absorption of both probiotic Se and Zn compared with their organic forms, with particular reference to seleno-L-methionine, seleno-L-cysteine, and zinc gluconate.