Diurnal pattern of plasma cortisol in preruminant calves fasted or fed different milk proteins.

The objective was to clarify the possible role of meals in the nycthemeral evolution of cortisol and the influence of intestine absorption kinetic of nutrients. Preruminant calves were fed two kinds of diets, a conventional curdled milk and a milk that did not curdle in the abomasum. These calves were also fasted for 24 h. Blood was sampled regularly during a 24-h period under three different dietary situations. Daily mean plasma cortisol was lower in calves fed uncurdled milk than in those fed curdled milk or fasted. Diurnal changes in plasma cortisol were characterized in fed animals by sharp postprandial decreases; the morning meal induced a more intense decrease than the evening one, which was followed by a regular increase of cortisol concentrations overnight. Between meals, several peak values were detected at the same times in all animals with both kinds of meals. The postprandial decreases did not occur in fasted animals, and peak values could not be linked to those occurring when animals were fed. Thus, cortisol secretion by adrenals seemed to be influenced by nutrient intestinal absorption, whereas during fasting different mechanisms were involved.