The perceived energy content of foods is predictive of the self-reported appetite sensations elicited thereafter. Further, sensory experience, which may include the visual appeal of packaging, the aroma, or the associations drawn through advertising and branding, are known to affect food intake. Studies investigating the physiological effects of food often therefore contain these confounding variables, which would be difficult to eliminate. As more is understood about the importance of cognitive control during food selection, consumption and digestion, the need to control this aspect during observation of subjects involved in research grows. This is not an easy task, but researchers have attempted to achieve this in a number of ways. For example, one group of researchers attempted to investigate the cognitive contribution to the digestion of soft drinks by manipulating the subjects beliefs, in order to separate the cognitive from physiological aspects.
Subjects consumed a pre-load of food corresponding to about 10 % of their energy needs, and were then allowed to eat an ad libitum meal until they considered their appetite had become reasonably satisfied by the food. The first pre-load meal consisted of a liquid that was shown to the subjects and then fed into their stomach through a tube. The second pre-load meal consisted of the same tube fed liquid, but the subjects were shown a demonstration of a similar coloured liquid that was poured and became solid to make them believe the same would happen in their stomach. The third pre-load consisted of solid cubes designed to liquefy in the stomach and the subjects were shown the disintegration of the cubes before consumption. The fourth pre-load consisted of the same solid cubes, that would liquefy in the stomach, but the subjects were told they would remain solid.
Therefore the end result of all pre-loads was consumption of the same liquid meal, but the belief of the subjects differed between treatments, as in some cases they thought they had eaten solid food. The results showed that the pre-loads the subjects believed to be liquid, elicited greater post-prandial hunger and lower fullness sensations, increased gastric emptying speed, reduced orocaecal transit times, reduced insulin and glucagon-like peptide 1 (GLP-1) release, and lower ghrelin suppression compared to the pre-loads the subjects believed to be solid. The increase in gastric emptying speeds seen with the perceived liquid pre-loads were associated with greater energy intakes in the subsequent ad libitum meal. In addition, energy intakes were greater following perceived liquid pre-loads compared to perceived solid pre-loads (2311 versus 1897 kcal). Sub-group analysis of the 52 subjects revealed no findings of significance.
These results therefore suggest that sensory food cues play an important role in modifying the physiological responses to solid and liquid foods, and this should be taken into account in future research. Although, these results are interesting, care should however be taken in their interpretation because being fed through a tube in a laboratory setting is not a normal experience. Therefore it must be recognised that such research while useful, should always be considered in context. Having said this, the fact that the subject could have their physiological responses modified by the mere expectation of a particular type of food is worthy of serious consideration in a broader context. Just how far such an influence could extend into the psyche in unknown, but the prospect that the mind controls the physiology to a greater degree than previously credited is exciting.
RdB
