M.R.C. Greenwood

Sweet tooth reconsidered: Taste responsiveness in human obesity

Taste responses of normal-weight, obese, and formerly obese individuals for sucrose and fat containing stimuli were examined using a mathematical modelling technique known as the Response Surface Method. The subjects accurately rated intensities of sweetness, fatness, and creaminess of 20 different mixtures of milk, cream, and sugar, and no mixture phenomena or inter-group differences were observed. In contrast, hedonic taste responses varied across subject groups, and were affected differentially by the sucrose and lipid content of the stimuli. Normal-weight subjects optimally preferred stimuli containing 20% lipid and less than 10% sucrose. Obese subjects preferred high-fat stimuli (greater than 34% lipid) that contained less than 5% sucrose, while formerly obese subjects showed enhanced responsiveness to both sugar and fat. Hedonic responsiveness as measured by the optimal sugar/fat ratio was negatively correlated with the degree of overweight (body mass index: weight/height). We hypothesize that sensory preferences for dietary sugars and fats aren determined by body-weight status and may affect the patterns of food consumption.

Weight cycling in female rats increases dietary fat selection and adiposity

The effect of repeated food restriction-refeeding (weight cycling) on macronutrient selection and adiposity was investigated in female Sprague-Dawley rats. Rats were maintained on ad lib macronutrient self-selection and were put on one of two types of restriction. One group was reduced to 75% of their body weight on restricted amounts of chow and a second group was given ad lib chow during the concurrent period and were voluntarily hypophagic. During refeeding on macronutrient self-selection, animals previously restricted selected a higher percentage of dietary fat, had larger adipose depots and plasma insulin values, and had lower heart weights both expressed in grams and as a percentage of body weight than non-restricted groups. This suggests that both severe and moderate periods of restriction may have negative health consequences.

Infusion of cholecystokinin between meals into free-feeding rats fails to prolong the intermeal interval.

The sulfated octapeptide of cholecystokinin (CCK-8) was infused intraperitoneally into 7 free-feeding male Sprague Dawley rats over a 6-day period. Infusions were given near the end of each free-feeding meal (1.87 microgram/meal/rat), and also during the intermeal interval in gradually increasing doses (0.10-0.63 microgram/5 min/rat). Food intake was continuously monitored and the infusions were controlled by microcomputer. Meal patterns, total food intake, and body weights during drug infusion were compared with data collected during a baseline period when only saline was infused. Meal-contingent CCK-8 infusion produced a significant 29.9% decrease in meal size which persisted throughout the drug-infusion period. Intermeal infusion of CCK-8 failed to prolong the intermeal interval (IMI) but it did initially prevent the compensatory decrease in IMI and increased feeding frequency expected after meal size was reduced. By the last day of drug infusion, total daily food intake recovered to baseline levels due to increased feeding frequency. Body weight was only transiently reduced by CCK-8 infusion. These findings show that tolerance does not develop to the action of CCK-8 to suppress meal size, and the administration of exogenous CCK-8 to free-feeding rats does not persistently prolong the intermeal interval.

Site specific effects of acute exercise on muscle and adipose tissue metabolism in sedentary female rats

The effects of endurance exercise on muscle, and adipose tissue metabolism were investigated. Female lean Zucker rats swam for two hours at high intensity. Three groups were examined: pre-exercise control (C), exercised (E) and 24 hours post-exercise (E-24). Exercise increased fat cell lipolysis in the inguinal depot (p less than 0.05) while no effect was detected in the parametrial depot. In contrast, parametrial pad lipoprotein lipase (LPL) activity decreased after exercise with 24 hours post-exercise values being reduced below E and C rats (p less than 0.05). Gastrocnemius LPL activity remained unchanged during exercise while heart LPL increased, E having higher values than C and E-24 (p less than 0.05). Gastrocnemius, but not heart, citrate synthase activity increased with exercise, with E-24 values increased compared to E and C (p less than 0.05). These results demonstrate that adipose tissues response to exercise is site specific, and suggests a distinct physiological role for different adipose depots. Muscle LPL and citrate synthase activities were modified differently for gastrocnemius and heart, confirming the distinct metabolic response to exercise of these two muscles.