The weight and energy density of foods play a crucial role with respect to the impact of food components on satiety. From a large number of short-term studies it is clear that humans primarily regulate their food intake on the basis of the weight of foods, and not the energy content (Poppitt and Prentice, 1996). For example, when subjects have ad libitum access to foods (e.g. yoghurts) with varying energy densities (e.g. by manipulating the fat content), these subjects will generally ingest equal weights of the different foods. The energy intake is then positively related to the energy density. The constant weight intake can be conceived of as a learned response, based on the association between the sensory properties of foods and their post-ingestive hunger and satiety consequences. For example, after a number of exposures we learn that we need to eat a certain amount of certain foods for breakfast (e.g. two sandwiches with cheese) in order to stay satiated until lunch. This learning must be based on the association between the sensory properties of bread and cheese, and the associated post-ingestive consequences.
The idea that we gradually learn this association between sensory properties and post-ingestive consequences, explains why in many short-term studies, subjects do not respond very sensitively to covert manipulations of the energy content of foods (Stubbs et al., 2000). These learned associations enable us to know how much to eat of various foods, e.g. for breakfast or other meals. The regulation on the basis of weight may also explain the weak satiating efficiency of fat that is found in many studies. Foods/diets with a high fat content generally have a high energy density, and consequently a low satiating efficiency.
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