Effects of modified sham feeding on ghrelin levels in healthy human subjects

The mechanisms involved in the preprandial rise and postprandial fall of circulating ghrelin levels are as yet unknown. Many hormonal and metabolic responses to nutrient intake begin during the cephalic or preabsorptive phase and are mostly mediated by the autonomous nervous system. The aim of the present study was to investigate the effects of the cephalic phase on ghrelin response to feeding in human subjects. The modified sham feeding (MSF), a well established technique in which nutrients are smelled, chewed, and tasted, but not swallowed, was used. Sixteen healthy volunteers ( seven men and nine women; mean age +/- SD: 31 +/- 8 yr; body mass index, 22 +/- 3 kg/m(2)) were studied after overnight fasting. Seven of them received a standardized mixed meal, and nine underwent MSF. Blood samples for ghrelin, insulin, and glucose were taken at time - 30, 0, 15, 30, 45, 60, 120 min during both tests. Pancreatic polypeptide determinations were evaluated at all times as markers of vagal activity only during MSF. Ghrelin levels significantly increased from time - 30 to 0 min before the two tests, then significantly decreased: after the real feeding from 933 +/- 479 pg/ml ( 277 +/- 142 pmol/liter) to 455 +/- 185 pg/ml ( 135 +/- 55 pmol/liter; P < 0.05), and after the sham feeding from 917 +/- 313 pg/ml ( 272 +/- 93 pmol/liter) to 519 +/- 261 pg/ml ( 154 +/- 77 pmol/liter; P < 0.05). There were no significant differences between the patterns of the responses as evaluated by ANOVA ( P = 0.863). As expected after MSF, plasma pancreatic polypeptide concentrations promptly increased from 58 +/- 29 pg/ml ( 14 +/- 7 pmol/liter) to 113 +/- 38 pg/ml ( 27 +/- 9 pmol/liter) at 15 min ( P< 0.01). Both insulin and glucose levels increased during the actual mixed meal, whereas they were not significantly modified by MSF. In conclusion, circulating ghrelin concentrations are decreased by sham feeding as they are by real feeding in humans. These findings underline the importance of the cephalic response to nutrient intake, i.e. the role of vagal activity, in the control of ghrelin secretion.