Decreasing the reinforcing effect of caloric intake is an important characteristic of diet-induced weight loss strategies that depend on caloric restriction. The research team examined the effect of a high and low glycemic index all-carbohydrate meal on food reinforcement in lean and obese adults. After completing two four-hour experimental sessions separated by at least two days in which a high or low GI meal was offered, participants performed an operant task to earn additional food for consumption. The timing of the availability of the earned food was also varied and samples were collected several times over the four hours. The researchers hypothesized that food reinforcement would be dependent upon a decrease in ghrelin in lean individuals and an increase in insulin in obese individuals.
· When food availability was delayed till the end of the session, subjects did not work for subsequent food.
· When food availability was early (two hours post preload), three of the seven did not work for additional food.
· The calories consumed after the preloads did not differ between the preloads
· Percent decrease in ghrelin at 30 minutes was significantly lower in those earning food in the operant task, independent of preload Glycemic Index.
· Glycemic Index of the earned food correlated positively with the Glycemic index of the preload.
· Thus, food reinforcement is dependent upon ghrelin response to the preload and not of the GI of the preload; low and high GI foods are equally reinforcing.
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Nasser JA, Evans SM, Geliebter A, Pi-Sunyer FX, Foltin RW. Use of an operant task to estimate food reinforcement in adult humans with and without BED. Obesity (Silver Spring). 2008 Aug;16(8):1816-20. Epub 2008 May 29.