Pathogens 2021 , 10 , 235
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In order to exert their health-related properties, β -glucans need to be released from the cell wall of the oat groats, a process which is highly affected by the applied process- ing techniques. Indeed, hydrothermal processing has been linked with reduced release of oat β -glucans [5]. Mechanical processing, on the other hand, results in increased ex- tractability of β -glucans from the oat groats, probably by increasing the exposed surface area [6]. Recently, Van den Abbeele et al. [7] reported an association between the β -glucan content and the prebiotic potential of different oat ingredients that were produced by dif- ferent mechanical processing techniques, suggesting that different processing techniques might impact the potential functional properties of the final oat ingredients by affecting β -glucan extractability. Several human studies have explored the prebiotic activity of oat products [8–11]. Connolly et al. [11], for instance, investigated the prebiotic activity of whole-grain oat granola, containing 1.3g β -glucan per dose in a human population with mild to moderate hypercholesterolemia. It was shown that intake of whole-grain oat granola selectively enhanced the abundance of Bifidobacterium and Lactobacillus species and total bacterial pop- ulation compared to baseline levels, accompanied by reduced systemic levels of total and LDL cholesterol. Indeed, numerous studies have linked the consumption of oat products with hypocholesterolaemic effects [12–14]. While their cholesterol-lowering activity has been associated with increased excretion of bile acids and cholesterol in faeces due to the enhancement of viscosity of the intestinal content [15], recent research suggests a potential role of the gut microbial community [16]. Indeed, modulation of the intestinal microbiota might impact bile acid metabolism by stimulating bacterial bile salt hydrolase activity of certain bacterial groups, thereby affecting cholesterol excretion [17,18]. Furthermore, microbial production of propionate has been associated with a reduction of cholesterol levels [19,20]. As several in vitro and in vivo studies have reported modulation of the gut microbiota upon oat supplementation [11,21,22], a potential mechanism of action of oat products in controlling cholesterol homeostasis might be related to its prebiotic properties. Assessments of the prebiotic properties of dietary fibres are often performed during in vitro studies. In vitro approaches offer an appropriate alternative to clinical trials because interactions between the gut microbial community and dietary fibres can be investigated at the site of action by strict control of environmental parameters [23,24]. As the prebiotic properties of oat ingredients might be involved in the lowering of cholesterol levels, the initial aim of this study was to compare the prebiotic activity of a commercially available oat product and a novel oat ingredient, standardised to provide similar levels of β -glucan, in a model of the human gastrointestinal tract of different healthy individuals with elevated cholesterol levels, making use of the validated Mucosal Simulator of the Human Intestinal Microbial Ecosystem (M-SHIME ® , ProDigest and Ghent University, Ghent, Belgium) [25], in order to analyse potential similarity in prebiotic response of the microbial community following oat consumption. Moreover, it was assessed if the potential prebiotic properties of the novel oat product remained at lower β -glucan levels in terms of effects on microbial metabolic activity and community composition of the luminal and mucosal gut microbiome in vitro. Finally, a clinical trial, with a corresponding human population, was carried out with the commercially available oat product in order to determine if in vitro observations translated in vivo to provide evidence that the use of in vitro gut models can be used to predict dietary outcomes following supplementation of novel ingredients in vivo.
2. Results 2.1. Altered Microbial Metabolic Activity in Response to Oat Treatment in vitro
The short-chain fatty acids (SCFAs) detected consisted mainly of acetate, propionate, and butyrate (Figure 1) and trace amounts of branched-chain fatty acid (bCFA) (Figure 1). First, all test conditions significantly increased acetate ( p < 0.0001), propionate ( p <0.0001), and butyrate ( p < 0.0001) levels in both colon regions as compared to the control period. The only exception was noted in the proximal (PC) upon supplementation of pre-cooked oat
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