Effect of the Duration of a Postprandial Fast on the Acquisition of Appetites in the White Rat (first published in French in 1957)☆

Food choice and eating behavior is influenced by a complex array of physiological and psychological processes. Culture, social surroundings, environmental factors, mood and emotion, and economics have strong influences over our dietary habits. The food choices we make also have a major impact on our health. Not all calories are equal, and our food choices have a direct influence on subsequent appetite and energy balance. For example, diets with a high- energy density can lead to passive overconsumption with little or no mechanism for defense. Moreover, in an increasingly obesogenic environment, it is extremely difficult to maintain energy balance. Further research is required to develop an integrated approach across health and technological disciplines to advance our understanding of the complex issue of food choice and eating behavior.

The control of food intake based on the sensory properties of foods, such as flavors or textures, has been tested. It has been demonstrated that choices of foods and drinks are achieved by learning relative preferences/aversions for foods and drinks and appetite/satiety for nutrients, regardless of the source of energy. Learning is a change in the organization of an individual's behavior so that performance represents an adaptation to the external and internal environments. When sensory cues from foods (conditioned stimuli) are paired with prompt nutritional after-effects of eating (unconditioned stimuli), conditioned responses of food choice and intake can be induced. Classical conditioning and instrumental conditioning are two types of associative control of eating, in which presented relations between responses and/or stimuli result in a persisting change in behavior. In classical conditioning, first identified by Ivan Petrovich Pavlov (1927), a relatively neutral stimulus (CS) is paired with a stimulus of significant biological and behavioral change, such as food (US). Classical conditioning occurs when associations are synthesized between two neutral stimuli, meaning that the predictive relation between the CS and the US leads to a conditioned response (CR) to the CS. Most of our likes and dislikes are learned through classical conditioning.

The control of food intake based on the sensory properties of foods, such as flavors or textures, has been tested. It has been demonstrated that choices of foods and drinks are achieved by learning relative preferences/aversions for foods and drinks and appetite/satiety for nutrients, regardless of the source of energy. Learning is a change in the organization of an individual's behavior so that performance represents an adaptation to the external and internal environments. When sensory cues from foods (conditioned stimuli) are paired with prompt nutritional after-effects of eating (unconditioned stimuli), conditioned responses of food choice and intake can be induced. Classical conditioning and instrumental conditioning are two types of associative control of eating, in which presented relations between responses and/or stimuli result in a persisting change in behavior. In classical conditioning, first identified by Ivan Petrovich Pavlov (1927), a relatively neutral stimulus (CS) is paired with a stimulus of significant biological and behavioral change, such as food (US). Classical conditioning occurs when associations are synthesized between two neutral stimuli, meaning that the predictive relation between the CS and the US leads to a conditioned response (CR) to the CS. Most of our likes and dislikes are learned through classical conditioning.

Recent evidence indicates that rats can learn to avoid aversive consequences of several hours of food deprivation by eating more of food having orosensory characteristics that predict a protracted fast. Two new studies tested if macronutrient composition of a flavoured meal before the reinforcing fast influences this acquisition of anticipatory hunger/satiety in a smooth-brained mammal. In one study, female Sprague-Dawley rats were trained for 11 cycles of 4 days with experimental meals of either carbohydrate or protein flavoured with either grape or cherry, with one odour followed by a 3-h fast and the other by 10 h of food deprivation. Both nutrient groups acquired anticipatory hunger. Then that learnt increase in meal size started to extinguish the hunger-avoidance response to the longer fast. The other study tested if this finding extended to experimental meals that combined carbohydrate and protein, and had vanilla or chicken flavours, to fasts of 4 and 12 h and to male rats as well as females within a training period of 6 cycles of 4 days. Evidence of anticipatory hunger early in training was clear only in the males. The combined results from the two experiments indicate that either carbohydrate or protein is sufficient for negative reinforcement of flavour-specific anticipatory hunger when the shorter fast lasts for 3 h. This food-discriminative anticipatory adjustment of meal size could be an automatically learnt part of hunger management for socially scheduled eating in human beings.

To determine if an increase in intake at a meal before a long fast can be conditioned to food texture cues, male Sprague-Dawley rats were given a high- or low-fat diet in one texture (powder or pellet) for 1 h prior to a 12·5-h fast and in the other texture before a 3-h fast. Each group (N = 9) went through a pseudorandom sequence of four duplicates of each texture-fast pairing over 4 experimental days in each of three training trials, followed by one 4-day trial under extinction, i.e. without the difference in fast lengths between textures. Neither the high-fat group nor the low-fat group as a whole gave a clear indication of a learnt texture-cued increase in meal size before the longer fast relative to the shorter fast. However, the rats trained on the high-fat diet that had the highest intakes on the first 4 days of training showed a relative increase in the amount eaten of the texture predicting the longer fast during the third training trial, and this effect also approached statistical significance in the extinction test. These results provide some support for the conclusion that anticipatory hunger/satiety can be differentially conditioned to dietary texture cues, but only if sufficient food is eaten before a short fast to prevent the rise in hunger during longer fasts that reinforces the discriminative increase in meal size.