| Dietary lipids play a vital role in maintaining human health.For Chinese dietary structure,liquid oils are mostly consumed,however,the intake of dietary lipids with high melting points(semi-solid or solid at 37 ℃),such as palm stearin and beef tallow,is increasing,nevertheless,impacts of these high melting point lipids on human health remain unclear.The in vivo metabolic processes of dietary lipids include digestion,absorption,and extraintestinal metabolism.In addition,unabsorbed lipid digesta interacts with gut microbiota.Therefore,it is necessary to comprehensively evaluate impacts of high melting point lipids on human health from the above-mentioned aspects.In view of this,palm stearin(PS),a semisolid high-melting-point fraction from palm oil,was selected as the main research object,and palm oil(PO),a low melting point faction from palm oil,was selected as the control,meanwhile,4%(w/w)and 20%(w/w)lipid contents were selected to simulate the oil amount in low-fat and high-fat foods,respectively,to systematically study the metabolism of PS in vitro and in vivo and the intake of PS on gut microbiota,based on this,the gastrointestinal digestion mechanism of PS and the potential mechanisms of PS affecting the extraintestinal lipid metabolism and gut microbiota were revealed.The main contents and results were as follows:(1)In vitro digestion behavior of a high melting point palm oil fractionDietary lipids must be emulsified to oil-in-water emulsions before being digested in the gastrointestinal tract(GIT),therefore,the digestion behavior of PS-in-water emulsions were studied using an in vitro gastrointestinal digestion model.After gastric digestion,PS-and PObased emulsions underwent flocculation/coalescence and/or partial coalescence,resulting in their significantly increased particle sizes(d4,3),besides,high-fat emulsions showed significantly larger particle sizes than low-fat emulsions;after intestinal digestion,particle sizes of all samples tended to decrease except for the emulsion containing 4 wt.% PS.The FFA-released assay revealed that during intestinal digestion,the FFA release rate and level decreased as the lipid content increased;in the low-fat emulsion system,PS had a close maximum digestion extent with PO,however,in the high-fat emulsion system,PS had a significantly higher maximum digestion extent than PO.(2)Study on the in vitro gastrointestinal digestion mechanism of a high melting point palm oil fractionThe digestion mechanism of PS in the GIT was revealed by analyzing the fat crystal structure of PS,the properties of oil-water interface and the rheological behavior of emulsions during in vitro digestion.After gastric digestion,compared with low-fat emulsions,the apparent viscosities and particle sizes(d4,3)of high-fat emulsions were apparently higher and significantly larger,respectively,suggesting that lipase in high-fat emulsions had a lower mobility and there were smaller oil-water interfacial areas available for it to adsorb,in addition to a lower lipase concentration per lipid droplet in high-fat emulsions,thus high-fat emulsions showed lower intestinal digestion rates and extents.DSC and XRD results indicated that after gastric digestion,PS-based emulsions mainly contained β fat crystals.Compared with liquid PO,these β fat crystals with large sizes,dense structures,and high melting points(the onset melting point is 48.7 °C)greatly suppressed intestinal enzymatic hydrolysis,therefore,the in vitro digestion rate of PS was lower than PO.The in vitro digestion extent of PS-based emulsions was strongly dependent on their SFC.The low SFC of the low-fat emulsion did not significantly affect the hydrolysis of PS by lipase,whereas for the high-fat emulsion,PLM images showed that fat crystals in PS promoted the stability of lipid droplets by adsorption at the oil-water interface,thereby increasing the adsorption area of lipase and the removal rate of interfacial lipolysis products,thus PS showed a significantly higher in vitro maximum digestion extent than PO.(3)In vitro fermentation of the unabsorbed digesta of a high melting point palm oil fractionAfter in vitro gastrointestinal digestion,the unabsorbed FA of all samples is mainly composed of LCFA(C16:0,C18:1 and C18:0),based on this,different concentrations of LCFA mixtures with a ratio similar with the unabsorbed FA were prepared to explore their in vitro fermentation behavior.16 S r RNA results showed that after in vitro fermentation for 24 h,both low(50 μM)and medium concentration(100 μM)of LCFA mixtures did not significantly affect the microbial structure,however,compared with the blank group(0 μM),a high concentration(200 μM)LCFA mixture significantly elevated the F/B ratio and the relative abundance of Lactobacillus(43.2%),resulting in significantly decreased relative abundances of Bifidobacterium and Blautia as well as the α diversity.Thus,a high concentration of unabsorbed digesta of PS may disturb the intestinal homeostasis.(4)Mechanisms of a high melting point palm oil fraction affecting the extraintestinal lipid metabolismBy feeding C57BL/6J mice with PS-based low-fat and high-fat diets for 8 weeks,mechanisms of PS affecting the extraintestinal lipid metabolism were revealed combining hepatic lipidomic and proteomic analysis of abdominal adipose tissue.q PCR results showed that increased PS intake significantly upregulated the expression of some key genes in hepatic fat synthesis pathways,leading to significantly increased total fat,TAG and bile acids contents in mice liver;specifically,quantitative lipidomic analysis with internal standards revealed that excessive intake of PS significantly increased hepatic contents of some longchain polyunsaturated phospholipids/glycerolipids and monounsaturated sphingolipids,which might cause lipid metabolism disorders via disrupting glycerophospholipid metabolism,linoleic acid metabolism,and α-linolenic acid metabolism etc.Excessive intake of PS also significantly increased the adipocyte size of abdominal adipose tissue,proteomic analysis revealed that this might be due to the significantly up-regulated pathways related to fatty acid synthesis.The extraintestinal lipid metabolism disorders in mice liver and abdominal adipose tissue caused by increased PS intake then contributed to liver steatosis,dyslipidemia,and overweight of C57BL/6J mice.At a low-fat level,PS induced similar changes in the extraintestinal lipid metabolism with PO,while at a high-fat level,PS caused more significant changes in liver lipid profiles than PO.Overall,PS-based high-fat diet(HFPS)showed the greatest adverse effect on extraintestinal lipid metabolism.(5)Effect of a high melting point palm oil fraction on gut microbiota in vivoEffects of 8-week PS-based low-fat and high-fat diets on cecal and fecal gut microbiota of C57BL/6J mice were investigated.16 S r RNA analysis showed that compared to the other diets,HFPS significantly increased cecal and fecal F/B ratios and relative abundances of bacteria which could produce bile acid hydrolase,such as Lactobacillaceae and its genus Blautia and Oscillibacter,and Ruminiclostridials and its genus Intestinimona,besides,HFPS stimulated the cecal colonization of Turicibacter which was relevant to lipid metabolism.Therefore,HFPS significantly enriched the gut microbiota which could use bile acids and lipid digesta as their metabolic substrates,consequently,HFPS exerted the strongest disturbing effect on gut microbiota in vivo. |
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