Search

Previous research has shown that moderate caffeine users develop a liking for the flavour of a novel caffeinated drink only if they experience this drink in a caffeine-deprived state. This study tested how sensitive these conditioned-flavour preferences are to subsequent changes in deprivation state and the continued presence or absence of caffeine. Thirty-six moderate caffeine consumers were given 4 training days during which they evaluated a novel flavoured caffeinated drink consumed mid-morning after 12 h caffeine deprivation. Subjects were then divided into four groups depending on whether or not they remained caffeine-deprived and whether the test drink continued to contain caffeine. They then re-evaluated the novel drink over a further 4 test days. As expected, liking for the test drink increased across the 4 training days, and this increased liking was maintained across the 4 test days in the group who continued to receive the caffeinated version of the drink in a caffeine-deprived state. Liking decreased in the test phase in the caffeine-deprived group who no longer received caffeine (extinction). It is surprising that both groups who were tested in a non-deprived state showed a marked decrease in liking on all 4 test days relative to the last training day. This implies that conditioned-flavour preferences may not be expressed in the absence of the relevant motivational state (caffeine deprivation). Together, these data suggest that flavour preferences conditioned by caffeine are very sensitive to changes in the contingent relationship between deprivation state and caffeine content of the drink., (Copyright 2000 Academic Press.)

Previously we have suggested that individual subjects interpret the phrase "palatable food" in different ways. To test the consistency of the use of this term, 50 male volunteers consumed a simple lunch on two occasions, once with a more palatable food and once with a more bland version. Ratings of hunger, fullness, pleasantness and palatability were completed at the start and end of each meal. Overall, subjects ate slightly more of the palatable food. The difference in intake between conditions correlated with differences in pleasantness and palatability but not hunger at the start of eating. Rated pleasantness of the food declined between the start and end of the meal in most subjects, but changes in palatability were more variable, with a significant minority showing no change across the meal. Detailed analysis confirmed the existence of two populations of responses, one where pleasantness and palatability were synonymous, and one where palatability was rated as a constant property of a food. The existence of two interpretations of the term palatability in common English usage cautions against the use of this term as a reliable means of evaluating hedonic responses to foods in appetite studies., (Copyright 1999 Academic Press.)

The effects of the introduction of timed pauses within meals and palatability on food intake and changes in rated appetite during a meal were assessed in three experiments in which volunteers ate a lunch of pasta with a tomato sauce. Eating was monitored using a disguised electronic balance attached to a micro-computer, which also allowed the introduction of timed pauses within meals. In the first experiment, 16 subjects were tested with both a bland and palatable food (with 0.27% oregano), with eating uninterrupted or with pauses after every 50 g consumed during which appetite ratings were completed. Both the addition of oregano and the introduction of regular within-meal pauses enhanced overall intake. Rated hunger increased in the early stages of eating the palatable food in the interrupted condition, and then declined, whereas hunger declined throughout with the bland food. Similarly, the linear function relating intake to time in the uninterrupted condition was greater with the palatable food. In the second experiment, nine subjects ate the palatable food with no pauses within meals, with 30-second pauses with appetite ratings or with 30-second pauses in a non-appetite task. Intake was greater in both pause conditions than when eating was uninterrupted. In Experiment 3, the effect of pause duration was investigated in a further 16 subjects, with either no pause or a pause of 5, 30 or 60 seconds. Subjects ate more in all pause conditions than with no pauses, while ratings of hunger and fullness suggested that subjects were less satisfied at the end of the meal with longer pauses. These data confirm previous work which suggests that palatability exerts its effect by stimulating appetite and eating rate, but also suggest that the introduction of pauses within meals enhances intake as well, contradicting the idea that pausing within meals should reduce intake by allowing more time for post-ingestive satiety to develop.

The effects of manipulated palatability on eating were assessed in 54 human volunteers eating pasta with a tomato sauce, with palatability adjusted by the addition of three levels of oregano. Meals were divided into 2 min eating episodes separated by brief pauses during which subjects rated aspects of appetite. Both intake and eating rate were greatest in the most palatable condition (0.27% oregano), whereas the addition of 0.54% reduced intake and eating rate. Hunger ratings increased during the initial stages of the meal with 0.27% oregano, but fell throughout the meal in the other conditions, whereas fullness ratings increased similarly in all three conditions. Rated food attractiveness was greatest with 0.27% oregano, and least with 0.54% oregano, but declined similarly through the meal with all three foods. Initial ratings of palatability were similar to those for food attractiveness, but changes in palatability across meals varied between subjects. Male subjects consistently ate more and faster than females, but similar effects of manipulated palatability on intake and subjective appetite were seen in both sexes. These data are consistent with the idea that palatability increases intake through a positive-feedback reward mechanism, and offers a novel method for measuring these effects.

It has been suggested that merely being exposed to a novel stimulus can enhance liking for it. Two experiments explored this hypothesis in relation to the development of liking for the chilli burn in a two-course meal. In the first experiment there was a linear increase in rated liking for the burning sensation produced by 2.5 and 5.0 ppm capsaicin between the first and the fifth exposure to an experimental meal, whereas no significant change was seen for the vehicle food or the preceding non-chilli flavoured food. In the second experiment a similar linear trend was observed with the 5.0 ppm stimulus. However, liking and sensory ratings for an ascending capsaicin concentration series in a tomato juice vehicle tested before and after the exposure period were unaffected by any change in liking for the chilli burn in food. This experiment also suggested that increases in liking were not dependent on initial unfamiliarity with the chilli burn or on changes in rated burn intensity or arousal. Potential explanations for the observed effects are considered.

Opioid peptides have been implicated in reward-related components of eating, especially with fatty and sweetened foods. This study examined the effect of pre-exposure of rats to milk on the subsequent anorexic effect of naloxone in a separate daily mash intake test. Rats were given constant access to either skimmed or whole milk for 15 days, in addition to normal diet. A third (control) group received only the normal diet. Subsequently, all groups received 0, 0.125, 0.5 and 1.0 mg/kg naloxone prior to their daily mash intake test. Naloxone caused dose-dependent reductions in mash intake in all groups, but this effect was significantly greater in the group with whole milk than in controls, with the group with skimmed milk intermediate. Daily intakes of milk were similar with skimmed and whole milk, and milk intake in separate two-bottle choice tests provided no evidence for overall preferences for either solution. These results suggest that prior exposure to milk enhances opioid involvement in feeding, and possible mechanisms for this are discussed.

The involvement of endogenous opioids in control of human food intake and appetite was investigated in a double-blind placebo-controlled study using a single, 2.5 mg oral dose of the opioid receptor antagonist, nalmefene. Ratings of the pleasantness of the smell and the taste, but not the appearance, of a number of foods was significantly lower in nalmefene-treated subjects. The magnitude of this effect was greater in food items independently rated as highly palatable. Caloric intake of a buffet-style meal was 20% lower in nalmefene-treated subjects, with the proportional reduction in intake of individual food items also depending on their palatability. These results lend further support to recent suggestions that opioids are involved in reward-related aspects of ingestion.