Palatability

Palatable is defined by Webster's Dictionary as “agreeable to the taste, hence, acceptable, pleasing.” Palatability is then the attribute of being palatable. This entry will consider various definitions and operations for measuring palatability, the neural mechanisms that underlie it, and its potential contribution to obesity.

Arguments among specialists abound in whether palatability inheres in the food or beverage being tasted, the reaction of the subject, or an interaction of the two. It has been stated that hunger widens the range of acceptability, thereby making foods more palatable. However, hunger may not change the actual pleasure from consuming the ingesta, but only the subject's motivation to consume. Technically, there may be more than one kind of palatability and several modulators of the word have been proposed to distinguish between the mechanisms by which changes in the subject or food result in changing intake. At least three modulators of palatability should be considered. First is intrinsic palatability, which attributes the responsiveness of subjects under constant conditions to changes inhering in the food or beverage. The property of tasting sweet is likely to be such an attribute because newborns show immediate acceptability responses to sweet tastes, although arguably, this could have been acquired in utero, because it is known that amnionic fluid is imbibed by the fetus. Organismic palatability is a label that has been appended to the change in acceptability that comes with changes in the state of the organism being tested. For example, reduction in blood glucose can make sweet-tasting foods taste sweeter. Palatability has also been described as either absolute or relative. Absolute refers to measures of palatability of a single item at a time, whereas relative palatability refers to a hierarchy among two or more items. Finally, palatability is probably largely learned as the result of pairing the stimuli in food with gastrointestinal, metabolic, or neurotransmitter system after-effects of consumption.

Palatability of foods and fluids is a quality which makes them particularly useful in reward paradigms in which effort to obtain food is used to assess the rewarding properties and is particularly useful in studying the neural basis of palatability. In the paradigms, an animal or person must perform some task for which it periodically receives a small piece of food or drop of fluid. In such a situation under unchanged deprivation or physiological state, reward value or palatability is greater for more effort expended, faster response rate, or toleration of impediment.

Two major neurotransmitter systems, opioid and dopamine participate in the control of palatability and it has been proposed that they control separate aspects of reward. “Liking” indexed by the opioid system is the label for the hedonic or pleasure-inducing effects of rewards, whereas “wanting,” indexed by the [Page 580]dopamine system is the label for “incentive salience” or the quality of the reward that impels the individual to pursue it. Alternatively, dopamine may function as neurotransmitter that stamps in the memory of the association of an action with the stimulus.

Palatability is measured by differences in intake, differences in response rates or facial expressions, or differences in ratings made with appropriate scaling metrics between foods or individuals. Measurement of intake leads to circularity when it is argued that what is palatable is what is eaten in greater quantity, so that the confounding of various controls of intake with palatability cannot be ruled out. Hence, measures that are independent of intake would be preferred. Such measures as rate of eating, running down alleys, choices in T-mazes, lick bursts and interlick intervals, pressing levers on various schedules of reinforcement have all been used as alternatives to amount consumed in animals. However, no measure is without its own problems. In animal studies, drugs presumed to affect the rewarding value may also affect motor responses, which could interfere with performance.

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