Changes Of Microstructure Of Human Milk Fat Globules With Lactation Progress And Its Effects On Lipid Digestion

Human milk exists in fat globules structure that the triglyceride core is surrounded by a membrane of tri-layer phospholipids.The special microstructure of human milk fat globules(HMFGs)is one of the key factors affecting fat absorption and metabolism.At present,the conclusions of structure related physical and chemical properties of HMFGs(such as particle size and lipid composition,etc.)during lactation are quite different.The microstructure of HMFGs and its effects on milk fat digestion are still unclear.Therefore,this study investigated the changes of structure related properties of HMFGs during lactation and the effects of microstructure on the lipid digestion characteristics in simulating in vitro digestion of infants.The main research contents are as follows.For the first,human milk fat was labeled by different fluorescent probes and the three-dimensional microstructure of HMFGs was observed by confocal laser scanning microscopy.The results confirmed the spatial structure of HMFGs that triglycerides were encapsulated by milk fat globule membrane.Phospholipids were heterogeneously distributed on the milk fat globule membrane and there were sphingomyelin-riched liquid ordered phase(lipid rafts)and liquid disordered phase.A certain proportion of cytoplasmic remnents on HMFGs were also observed,also known as cytoplasmic crescents.The results above supported the viewpoint of existence of “lipid rafts” and “cytoplasmic crescents”.Secondly,the changes on physical and chemical properties of HMFGs during lactation were investigated.A total of 134 human milk samples including colostrum,transitional milk and mature milk were analyzed of particle size,potential value and fatty acids composition,etc.The results showed that the particle size of HMFGs increased from 3.77±0.95 μm to 5.09±0.88 μm,with Zeta potential value decreased from-6.23±1.14 mV to-7.32±0.63 mV.Colostrum has lower fat content(3.07±1.03%)and higher content of medium-chain fatty acids(12.21±3.87%),very long-chain fatty acids(4.09±1.07%)and saturated fatty acids(39.36±3.65%).After that,12 fore milk(first 5 mL of single suckling)and hind milk(5 mL after suckling)samples were analyzed of.their particle size and lipid composition,etc.Results showed a significant increase in fat content(from 3.13±0.95% to 4.73±0.38%)and a relative increase in particle size(from 4.00±0.59% to 4.25±0.56%).The results above reflected some regular changes of structure related physical and chemical properties of HMFGs.Finally,the effects of particle size and membrane of HMFGs on lipid digestion in the stomach and small intestine were investigated using the optimized in vitro fat digestion system of infants.The results showed that: HMFGs structure in fresh human milk maintained in gastric phase and gradually broke in intestinal phase.High pressure homogenized human milk with smaller particle size(0.77±0.12 μm)and incompleted membrane was in higher lipolysis degree(15.77±1.66% and 77.04±2.70% in gastric and intestinal phase respectively)than fresh human milk(9.84±3.00% and 65.64±4.76%,respectively)in the same time,and the fat globules were easier to aggregate.The lipolysis degree(62.74±2.51%),size distribution in gastric phase and lipid morphology of simulated infant formula emulsion were closer to fresh human milk,which was mainly due to its similar structure with HMFGs.The standard infant formula has similar fatty acids composition to fresh human milk,but the hydrolysis characteristics were still different.The lipolysis degree of standard infant formula was lower than others(61.98±3.11%),and the particle size distribution was also unstable.The results above demonstrated that the changes on HMFGs microstructure had an effect on the size distribution,lipolysis degree and lipid morphology.In this study,HMFGs were characterized by intuitive three-dimensional structure.The changes on structure related physical and chemical properties of HMFGs during lactation were studied.The effects of particle size and membrane of HMFGs on lipid digestion were also investigated,which provided important theoretical basis for the research and development of new infant formula.