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protection against cancer development [48]. The stimulation of SCFA production by the different oat ingredients therefore suggests their prebiotic potential. In addition, some differences were observed between the fermentation of OFO and POF. For instance, supplementation of POF resulted in the strongest stimulation of Lac- tobacillus levels in the PC and DC, while OFO administration more strongly enhanced butyrate production and Bifidobacterium concentrations in the PC. Previous in vitro research by Van den Abbeele et al. [7] revealed that product-specific microbial pathways were boosted upon administration of structurally different oat ingredients. For instance, while all products resulted in a significant increase in Bifidobacterium levels, oat bran showed the strongest bifidogenic effect of the six oat ingredients investigated, indicating that slight differences in prebiotic response exist among oat-derived products, as observed in the current study. Moreover, in vitro investigations were carried out to determine if the potential prebi- otic properties of the novel oat product POF persisted at lower test doses. It was observed that acetate, propionate, and butyrate levels increased in a dose-dependent manner in the DC with the highest final SCFA levels observed upon supplementation of the highest concentration of POF. However, even at the lowest concentration tested, POF stimulated SCFA production and Lactobacillus and Bifidobacterium levels, indicating that the prebiotic activity of POF remained at lower β -glucan levels. Overall, the obtained results demon- strate the prebiotic potential of the novel oat product POF, even when administered at lower concentrations. In order to further investigate the impact on the luminal microbial community com- position upon supplementation of POF and OFO at similar β -glucan load, 16S-targeted Illumina sequencing was performed. Treatment with both test products increased Prevotel- laceae levels, mainly in the PC. Saccharolytic fermentation by members of this bacterial family results in the production of acetate and succinate [49], with the latter being a sub- strate for succinate-converting, propionate producing micro-organisms such as Bacteroides and Veillonella species [50]. In the current study, an increased abundance of Veillonellaceae was observed upon treatment with POF, but not upon OFO supplementation. However, it might be that specific members of the Veillonellaceae family were stimulated upon OFO administration, while others reduced, resulting in the absence of effects at the family level. Finally, both test products increased the abundance of Enterobacteriaceae . This bacterial family contains several opportunistic pathogenic species, however, also many commensals able to ferment proteins are members of this bacterial group [51]. The strongest effects were observed upon OFO supplementation, which correlated with the more strongly increased levels of branched SCFA and ammonium, which are both markers of proteolysis that were observed upon fermentation of OFO as compared to POF. Overall, these results indicate that in vitro gut models combined with accurate molecular techniques have the potential to highlight specific microbial taxonomic changes and functional activities pathways upon dietary intervention. From this study, it can be concluded that the novel oat product POF and the com- mercially available OFO, when administered at similar β -glucan load, exerted equivalent prebiotic activity in the human gastrointestinal tract in vitro, with profound effects being observed on Lactobacillus and Bifidobacterium levels. The stimulation of Lactobacillus spp. observed in vitro was confirmed during an in vivo trial investigating the effect of OFO in human subjects with mild hypercholesterolemia. Moreover, the in vitro prebiotic activity of POF remained at lower β -glucan levels, demonstrating the potent prebiotic potential of the novel oat product even when administered at lower concentrations.
4. Materials and Methods 4.1. Chemicals and Test Product
Unless otherwise stated, all chemicals were obtained from Sigma-Aldrich (Overijse, Belgium). PepsiCo, Inc. (Barrington, IL, USA) provided the different oat ingredients, including Quaker whole grain pre-cooked oat flour (POF) and old-fashioned oats (OFOs).
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