Post-prandial changes of osmolality and electrolyte concentration in the upper jejunum of normal man

The effect of an ordinary hyperosmolar solid/liquid meal (milk and doughnuts) on the osmolality, sodium and potassium concentrations in the upper jejunum was studied in 5 normal subjects. Though the meal was diluted three times when it reached the upper jejunum, the jejunal contents were consistently hyperosmolar (greater than 300 mosm/kg) for more than 2 h. There was a linear correlation between gastric and jejunal osmolality (r = 0.89, p less than 0.001), and gastric and jejunal potassium concentrations (r = 0.76, p less than 0.001) but not between gastric and jejunal sodium concentrations (r = 0.25). The latter decreased after 30 min to 93 +/- 10 mmol/l and returned to fasting levels (127 +/- 12 mmol/l) after 3 h. These results show that the composition of upper jejunal contents after a meal reflects that of the gastric contents. Despite mixing with upper gastrointestinal secretions and transepithelial movement of fluid and electrolytes, osmotic and electrolytic equilibration of intestinal contents with plasma is not produced at this level up to 2 h following a meal.