It is generally thought that macronutrients stimulate intake when sensed in the mouth (e.g., sweet taste) but as food enters the GI tract its effects become inhibitory, triggering satiation processes leading to meal termination. Here we report experiments extending recent work (see Zukerman et al., 2011 ) showing that under some circumstances nutrients sensed in the gut produce a positive feedback effect, immediately promoting continued intake. In one experiment, rats with intragastric (IG) catheters were accustomed to consuming novel flavors in saccharin daily while receiving water infused IG (5ml/15min). The very first time glucose (16% w/w) was infused IG instead of water, intake accelerated within 6min of infusion onset and total intake increased 29% over baseline. Experiment 2 replicated this stimulatory effect with glucose infusion but not fructose nor maltodextrin. Experiment 3 showed that the immediate intake stimulation is specific to the flavor accompanying the glucose infusion. Rats were accustomed to flavored saccharin being removed and replaced with the same or a different flavor. When glucose infusion accompanied the first bottle, intake from the second bottle was stimulated only when it contained the same flavor, not when the flavor switched. Thus we confirm not only that glucose sensed postingestively can have a rapid, positive feedback effect ('appetition' as opposed to 'satiation') but that it is sensory-specific, promoting continued intake of a recently encountered flavor. This sensory-specific motivation may represent an additional psychobiological influence on meal size, and further, has implications for the mechanisms of learned flavor-nutrient associations.
Keywords: Appetite; Flavors; Food preference; Glucose; Meal size; Motivation; Nutrient sensing; Satiation.
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