Mammals can maintain energy balance despite large fluctuations in physical activity and energy intakes because they have complex neuronal and hormonal systems that regulate appetite, satiety and metabolism. Short term appetite regulation is achieved by receptor mediated detection of food within the gastrointestinal tract, that results in hormonal and neuronal feedback signals to limit further intake. For example, fatty acids detected in the gastrointestinal tract are able to reduce gastric emptying (by stimulation of pyloric pressure waves (PWs) and suppression of antral and pyloric PWs), and stimulate release of cholecystokinin and peptide YY, which suppresses subsequent food intake. However, in obese individuals there is some evidence that this detection and response system is abnormal or insensitive to signals, which ultimately results in overeating and weight gain. The cause of these abnormalities are not understood, but may relate to a desensitisation of signals in response to certain dietary conditions.
Researchers1 have investigated the intraduodenal sensitivity to oleic acid (OA, C18:1 (n-9)) in 8 lean and 11 obese subjects. Either saline or OA (0.78 kcal/min) was infused into the duodenum of subjects for 90 minutes, while antral, pyloric and duodenal pressure waves; plasma cholecystokinin and peptide YY; and appetite were measured. Following the infusion, a buffet lunch was provided and energy intake was assessed. In response to the OA infusion, the lean subjects had increased pyloric PWs when compared to the saline, but there was no difference in PW number between treatments for the obese subjects. Obese subjects also showed a trend for reduced cholecystokinin and peptide YY response, but there was no difference in subsequent energy intake between groups at the buffet lunch. This suggests that obese subjects may have altered responses to OA in the duodenum, but that this may not affect short term energy intake.
When the researchers measured the oral sensitivity to OA they found that the obese subjects had a higher threshold for detection of the monounsaturated fat (7.9mmol/L in obese subjects compared to 4.1 mmol/L in lean subjects). Analysis of food diaries also suggests the that obese subjects had greater recent energy intakes and greater recent fat intake, when compared to lean subjects. When the data was analysed, it was found that there was an association between body mass index and detection threshold for OA, and an inverse association between isolated pyloric PWs with both body mass index and OA detection threshold. Taken as a whole, these results suggests that obese and overweight individuals have a blunted ability to detect OA orally and within the duodenum. However, this does not seem to affect short-term appetite regulation at a subsequent meal.
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