Free fatty acids exert an inhibitory effect on adrenocorticotropin and cortisol secretion in humans

Free fatty acid (FFA) administration stimulates the hypothalamic-pituitary-adrenal (HPA) axis in rats, suggesting that the HPA axis and lipolysis may be linked by a positive-feedback loop. To clarify the influence of FFA on the HPA axis in humans, we studied the effect of lipid load on both basal and stimulated ACTH and cortisol secretion in normal subjects. In six young female volunteers (mean +/- SEM) age, 24.4 +/- 2.1 yr; body mass index, 23.1 +/- 1.2 kg/m(2)), ACTH, cortisol, FFA, glucose, and insulin levels were measured every 30 min for 330 min during the following procedures: 1) i.v. saline infusion (from 0 to 330 min); 2) i.v. FFA infusion (Intralipid 10%, from 0 to 210 min) followed by saline infusion (from 210 to 330 min); 3) human CRH (hCRH) administration (2 microg/kg i.v. at 90 min) during saline infusion (from 0 to 330 min); and 4) hCRH administration during FFA infusion (Intralipid 10%, from 0 to 210 min, followed by saline infusion from 210 to 330 min). During saline infusion, ACTH and cortisol levels progressively declined. Lipid-heparin emulsion (LHE) infusion strikingly increased circulating FFA levels and, simultaneously, amplified the ACTH and cortisol decrease (P < 0.05). After LHE withdrawal, FFA decrease was associated with an increase (P < 0.05) in ACTH and cortisol levels (restored to baseline values within 60 min). The ACTH and cortisol responses to hCRH, however, were unaffected by LHE that, concomitantly, induced an increase (P < 0.05) in glucose but not in insulin levels. This study shows that an LHE-induced increase in FFA levels has an inhibitory effect on spontaneous ACTH and cortisol secretion in humans. Lipid load, however, does not affect the ACTH and cortisol responses to hCRH; this evidence would indicate that the negative influence of FFA on the HPA axis in humans takes place at the suprapituitary level.