Monday, 22 October 2012

Learning to want

The sensory properties of foods – their tastes, odours, textures – are crucial to determining what we eat.  This is because these qualities, together and apart, evoke pleasure. So, when we talk about motivations to consume foods, it is often taken for granted that food acceptability and preferences underlie our behaviours. Of course, this is not to ignore a variety of other motivations – nutrition, convenience, and so on – but foods that are not liked are generally not eaten. And if we find a food is especially palatable, we will eat more of it.

Consider though if you were very hungry and your food choices were limited. A plate of something that we would otherwise regard as unpalatable might still be gratefully eaten if that was all that was available. Our motivation here is driven not by liking, but by wanting.

Although this distinction between liking and wanting was first seen in the drug addiction literature, it is increasingly seen as important in helping to explain motivations to consume foods. On the majority of eating occasions, we want what we like, and vice versa. One major reason for this is that foods that are high in energy, either from fats or carbohydrates, are those foods that are both highly liked and stimulate wanting.

It is possible to distinguish between liking and wanting, and in some studies this has been done by contrasting ratings of liking for a food with ratings of desire to eat it. One other way is to observe facial expressions. A study by Julie Mennella [1] in which infants were fed a novel food, green beans, demonstrated how infants’ facial expressions clearly indicated dislike for this food. Following repeated exposure to eating the beans, the infants were willing to eat increasing amounts of the beans – a clear sign of wanting. However, their facial expressions did not change with repeated exposure to the beans – unless the infant had been fed peaches after the beans, in which case the facial expressions were much more positive. Pairing the sweet peaches had conditioned a liking for the beans, resulting in a changed facial expression.

This finding, and the dissociation between indicators of liking (facial expressions) and wanting (consumption), can be understood in terms of the everyday processes of flavor-flavour learning and flavor-calorie learning. As shown in humans by Yeomans [2] ingestion of energy or other wanted nutrients, especially while hungry, conditions a liking for a food flavor. In addition, however, experiencing the conditioned food odour/flavor can also elicit increased appetite and consumption. This contrasts with the pairing of an odour/flavour with just a sweet taste (which may or may not be associated with calories), which only reliably conditions liking for that odour. While, as mentioned above, we seldom want to eat what we do not find palatable, it is highly likely that it is not this preference that pushes us to eat, but rather the engagement of wanting. In studies by Yeomans and others, odours have been conditioned through pairing with nutrients. In everyday situations, not only odours but also sights, sounds and contexts can become associated with foods.

The relevance of conditioned wanting is evident when we try to understand why we eat particular foods at particular times. Most of our eating is not done because we are severely depleted of energy or other nutrients. It is done in response to a particular amount of time having passed, or the presence of cues that remind us of food. If your stomach rumbles when you enter a kitchen where something delicious is being cooked, or when passing a bakery from which the aroma of fresh bread wafts, it is a signal that your gastric juices have been conditioned to the food odours by prior pairing of those odours with the calories that followed them.

One very plausible reason why people in affluent societies are nowadays eating so much is that our worlds are filled with a multitude of such cues to wanting that occur without our being consciously aware of them: odours, flavours, sights, sounds associated with eating.  There is a good reason, for example, why television advertising of snack foods and confectionary is effective, and this is because highly realistic cues for foods can elicit wanting. In a study of these effects, Ferriday & Brunstrom [3] showed that exposure to the sight and smell of pizza in a laboratory setting increased consumption of freely available pizza after participants had already consumed a fixed amount.

Another construct – hedonic hunger – has also recently been discussed as a major motivation for eating. Hedonic hunger is seen as a drive towards food pleasure-seeking that coexists with hunger driven by energy needs. It is a very similar idea to both food craving and conditioned wanting in that it is elicited by food sensory cues. It is, by definition, satisfied only by highly preferred foods. Of course, as noted above these are the foods that are most of the time both liked and wanted.

The idea of hedonic hunger appears to be useful in helping to explain the drive to consume highly palatable foods when we are trying to eat a ‘healthy’ diet or one that leads to weight-loss. Dietary restriction reduces both energy intake and food pleasure, and so if we are genuinely motivated by pleasure-seeking in our eating, then this helps to explain why diets so often fail.

On the face of it, this problem ought to be addressed by good tasting, low calorie foods, and of course the food industry is working hard to provide these. However, widespread use of low calorie foods may be a problem in itself. Because wanting is driven by conditioned associations between energy and flavours, we may find that we start to selectively want only high calorie versions of foods. Just such a finding was suggested recently by O’Sullivan [4] who showed that a low calorie, but familiar, version of a pasta dish became less and less liked relative to the regular version over repeated eating occasions. Whether this would have led to reduced amount consumed or desire to consume was not measured. Another inadvertent consequence of proliferation of low-calorie foods may be a reduced ability to estimate energy intake. At present, sensory properties provide important information about the calories in what we consume. So, thick, sweet, rich foods tend to be higher in calories; these same, palatable qualities uncoupled from their calorie consequences may limit our ability to implicitly monitor our energy intake. Since most of us – but especially those trying to restrict intake – rely heavily on such cues, we may be losing an important part of our ability to monitor what we eat and, for example, compensate for high energy intake at one meal a with lower intake at another.