In Vitro Characterization of the Impact of Different Substrates on Metabolite Production, Energy Extraction and Composition of Gut Microbiota from Lean and Obese Subjects

The aim of this study was to investigate the effect of galacto-oligosaccharides, lactulose, apple fiber and sugar beet pectin on the composition and activity of human colonic microbiota of lean and obese healthy subjects using an in vitro model of the proximal colon: TIM-2. Substrate fermentation was assessed by measuring the production of short-chain and branched-chain fatty acids, lactate and ammonia and by studying the composition of the bacterial communities over time. The results suggest that energy harvest (in terms of metabolites) of lean and obese microbiotas is different and may depend on the fermentable substrate. For galacto-oligosaccharides and lactulose, the cumulative amount of short-chain fatty acids plus lactate produced in TIM-2 was lower in the fermentation experiments with the lean microbiota (123 and 155 mmol, respectively) compared to the obese (162 and 173 mmol, respectively). This was reversed for the pectin and the fiber. The absolute amount produced of short-chain fatty acids including lactate was higher after 72 h in the fermentation experiments with apple fiber-L (108 mmol) than with apple fiber-O (92 mmol). Sugar beet-L was also higher (130 mmol) compared to sugar beet-O (103 mmol). Galacto-oligosaccharides and lactulose boosted the balance of health-promoting over toxic metabolites produced by the microbiota from obese subjects. Firmicutes were more predominant in the inoculum prepared from feces of obese subjects compared to lean subjects. The average abundance at time zero was 92% and 74%, respectively. On the other hand, Bacteroidetes were more dominant in the microbiota prepared with homogenates from lean subjects with an average abundance of 22% compared with the microbiota prepared with homogenates from obese subjects (3.6%). This study brings evidence that different fermentable carbohydrates are fermented differently by lean and obese microbiotas, which contributes to the understanding of the role of diet and the microbiota in tackling obesity.本研究旨在利用近端结肠体外模型TIM-2，探究低聚半乳糖(galacto-oligosaccharides)、乳果糖(lactulose)、苹果纤维及甜菜果胶(sugar beet pectin)对健康瘦体型与肥胖受试者人体结肠微生物群组成及活性的影响。研究通过检测短链脂肪酸、支链脂肪酸、乳酸及氨的生成量，并随时间动态监测细菌群落组成，以评估各底物的发酵情况。研究结果显示，瘦体型与肥胖受试者的肠道微生物群的能量获取能力（以代谢物水平表征）存在显著差异，且该差异可能依赖于所使用的可发酵底物。针对低聚半乳糖与乳果糖，在TIM-2发酵实验中，瘦体型微生物群产生的短链脂肪酸与乳酸总累积量（分别为123 mmol与155 mmol）低于肥胖微生物群（分别为162 mmol与173 mmol）；而对于果胶与纤维则呈现相反趋势。以苹果纤维-L组为底物时，72小时后生成的含乳酸的短链脂肪酸绝对总量为108 mmol，高于苹果纤维-O组的92 mmol；甜菜纤维-L组的生成量（130 mmol）亦高于甜菜纤维-O组的103 mmol。低聚半乳糖与乳果糖可提升肥胖受试者微生物群产生的有益代谢物相对于毒性代谢物的占比，优化其代谢平衡。与瘦体型受试者相比，肥胖受试者粪便制备的接种物中厚壁菌门(Firmicutes)占比更高，初始时刻（t=0）的平均丰度分别为92%与74%。反之，拟杆菌门(Bacteroidetes)在瘦体型受试者粪便匀浆制备的微生物群中占比更高，平均丰度达22%；而肥胖受试者粪便匀浆制备的微生物群中拟杆菌门平均丰度仅为3.6%。本研究证实，不同可发酵碳水化合物可被瘦体型与肥胖肠道微生物群以差异化方式发酵，该结果有助于理解饮食与肠道微生物群在肥胖防控中的作用。