Fat emulsions behave differently in the digestive tract depending on their initial physicochemical properties. A lower initial fat droplet size facilitates fat digestion by gastric lipase in the stomach and duodenal lipolysis. Overall fat assimilation in healthy subjects is not affected by differences in initial droplet size because of efficient fat digestion by pancreatic lipase in the small intestine. Nevertheless, these new observations could be of interest in the enteral nutrition of subjects suffering from pancreatic insufficiency.

Seven fasting subjects were fitted with nasogastric and nasoduodenal tubes and received intragastrically a coarsely emulsified test meal. Gastric and duodenal aspirates were collected after 1, 2, 3, and 4 h. In the duodenum, most lipids (> 90%) were present as emulsified droplets 1-100 microns in size. Large droplets and unemulsified material present in the test meal (> 100 micron) disappeared, whereas smaller droplets (1-50 microns) were generated after 1 h of digestion. Thus the median lipid droplet diameter significantly decreased (19.6 vs. 56.5 microns in the test meal) and the droplet surface area significantly increased (1.58 vs. 0.70 micron2/g fat). Intermediate droplet diameters were 34.3, 46.3, and 27.6 microns after 2, 3, and 4 h, respectively. In the stomach, a comparable emulsion particle size pattern was observed, with median droplet diameters of 17.2, 37.9, 52.4, and 41.6 microns after 1, 2, 3, and 4 h, respectively. However, the extent of triglyceride hydrolysis was much lower in the stomach (6-16%) than in the duodenum (42-45%), where small droplets were enriched in lipolytic products, cholesterol, and phospholipids. The present findings show for the first time that most dietary lipids are present in the human duodenum as emulsified droplets 1-50 microns in size and that no further marked emulsification of dietary fat occurs in the duodenum compared with the stomach.