Feeding influences plasma cortisol levels during preweaning life in baboons

The effect of feeding on plasma cortisol concentrations was examined in neonate (preweaning), infant (postweaning), and adult baboons (Papio cynocephalus). Neonates were bottle fed four times daily, infants twice daily, and adults once daily. In neonates, plasma cortisol declined within 30 min after feeding. This decline in plasma cortisol was dependent on ingestion of formula and was independent of the time of feeding. During the day, cortisol fluctuations were synchronized with feeding times: peak levels occurred before feeding and trough levels occurred 30-60 min after feeding. Plasma adrenocorticotropic hormone (ACTH) declined within 30 min after feeding but returned to prefeeding levels at 60 min after feeding and did not exhibit significant variation associated with feeding times as did cortisol. The decline in plasma cortisol after feeding in neonates was delayed with increasing age. By 18 wk, the feed-related decline in plasma cortisol was delayed until 60 min after feeding compared with 30 min at age of 8 wk. After weaning, the decline in plasma cortisol after feeding was delayed until 90 min, and there was no effect of feeding in the afternoon on plasma cortisol. Finally, there was no significant decline at all in adult baboons greater than 6 yr of age. These results demonstrate plasma cortisol concentration is associated with feeding in neonates fed formula. The cortisol-feeding rhythm changes before weaning, and after weaning it begins to disappear, until it is completely absent in adults.