E.L. Gibson

Protein appetite demonstrated: Learned specificity of protein-cue preference to protein need in adult rats

After food had been withheld for 4 hours, casein was infused into the stomach of rats while they drank a distinctively flavored non-nutritive fluid. On testing under the same food-deprivation, the rats showed conditioned preference for the flavor. When protein was infused during the deprivation period before the test, they did not prefer the flavor, although they did when equicaloric carbohydrate had been pre-infused. These results demonstrate for the first time a learned hunger specifically for protein.

Disguised protein in lunch after low-protein breakfast conditions food-flavor preferences dependent on recent lack of protein intake ☆

As in the conditioning of appetite for protein in the rat, human preference for and intake of a food at lunch was increased when the flavor of that food was paired with an adequate supply of protein, following a breakfast lacking in protein. Men and women rated their preferences for two flavors in tasted foods (soup and cornflour dessert) on test days before and after a day when one flavor was eaten in very low protein food and another day with a different flavor eaten in food containing protein, but with minimal sensory differences between these foods. Subjects given a low-protein drink preload preferred the protein-paired flavor, while those receiving a high-protein drink did not. In a second experiment, preferences were measured by intake as well as ratings, and the difference in amount of protein between high- and low-protein lunches was increased. By both measures, relative preference for high-protein-paired dessert flavors increased from before to after pairing. The increase in intake preference ratio for the protein-paired flavor was abolished by a high-protein preload. Thus, people have a learning mechanism whereby a lack in protein intake comes to cue the selection of protein-rich foods that are not known to be such, and/or loading with protein might trigger avoidance specifically of a high-protein diet.

Acquired protein appetite in rats: dependence on a protein specific need state

Rats are shown to acquire a preference for protein-predictive olfactory cues which depends on a state of mild deficit in protein intake—i.e. a learned protein-specific appetite.

Dependence of carbohydrate-conditioned flavor preference on internal state in rats ☆

Abstract Rats rapidly acquired preferences for a flavor incorporated in meals of a dilute carbohydrate diet, whether or not they were trained following consumption of a substantial volume of nonnutritive fluid. However, this carbohydrate-conditioned flavor preference was elicited only when the rats were tested in the same gastrointestinal distension condition in which they had been trained, i.e., either with or without the nonnutritive preload. This is evidence that ingestion can be controlled by associatively conditioned combinations of dietary cues with internal cues or that preferences can be conditioned to depend on physiological states. This compound conditioning of physiological and dietary stimuli provides a mechanism for tuning the motivation to eat to nutritional needs and supplies.

Gastromotor mechanism of fenfluramine anorexia

A gastric slowing effect of fenfluramine accounts for most of the drugs suppressant effect on food intake in freely feeding rats. It is conceivable on the evidence to date that this gastromotor action of fenfluramine explains all its effects on appetite and metabolism, but additional peripheral and central effects--such as motor inhibition--are likely. Rate of gastric emptying is quantitatively the dominant physiological control of appetite: it determines the duration for which absorption of a meal sustains metabolic satiety; it also influences gastric distension, which can be a source of innate satiation and of learned carbohydrate-specific satiation. Since most of the neurotransmitter serotonin (5HT) resides in the gastrointestinal wall, not the brain, gastromotor suppression of appetite should be the first working hypothesis for a serotoninergic drug such as fenfluramine. The largest effect on food intake that arises from gastric slowing by fenfluramine and active metabolites is a lengthening of the period of satiety after a meal of a given size. The residue of this extended satiety could reduce appetite at a subsequent fixed mealtime and hence the size of such a meal. Fenfluramine appears not to intensify satiation processes generated by a meal. Rather, it affects eating processes from the start. Also, fenfluramine disrupts learned carbohydrate-specific satiation operative within a meal. This negates the claim that fenfluramine reduces carbohydrate-specific appetite--which in any case (like other claims that drugs modulate nutrient selection) is not based on adequately designed dietary selection tests.

Fenfluramine and amphetamine suppress dietary intake without affecting learned preferences for protein or carbohydrate cues

The effects of pharmacological modulation of monoamine transmitter activity on genuine nutrient selection were assessed: that is, drug-induced changes in nutrient-specific dietary choice behaviour were measured, using rats that had learned to select an odour cueing protein content or carbohydrate content of the diet. Anorexigenic doses of DL-fenfluramine-HCl (1.25 or 2.5 mg/kg) and D-amphetamine2 SO4 (0.5 or 1.0 mg/kg) did not affect the selection of protein-paired or carbohydrate-paired odours by trained rats. This weighs against the claim that 5-hydroxytryptamine or catecholamine transmitters are involved in the selection of macronutrients, as distinct from selection between diets in response to sensory differences functionally unrelated to nutrient composition.

Norepinephrine-facilitated eating: Reduction in saccharin preference and conditioned flavor preferences with increase in quinine aversion

Paraventricular (PVN) hypothalamic norepinephrine (NE) injections which facilitated feeding were nonetheless found to reduce both the unconditioned preference for saccharin and starch-conditioned preferences for almond odor and lemon taste, as well as enhancing aversion to quinine. These results add to the evidence that PVN NE elicits eating by attenuating a satiety signal.

Control of Eating Behaviour by Amino Acid Supply

There are two distinct ways in which amino acid supply could influence behaviour. Amino acid supply might control eating behaviour specifically, or eating and other behaviour might be modulated by effects of amino acid supply on neurotransmitters in the pathways organising such behaviour. Given that behaviour is organised by multisynaptic networks, it is extremely unlikely that an effect of precursors on transmitters would have a specific consequence for eating behaviour. If we allow realistically for the nature of behaviour and for the way the brain works, we must expect any control of eating behaviour itself by amino acid supply to be via some specific receptor system. If monoamine precursor metabolism were critical to such a sensor, then these widespread metabolic processes would have to be set within a specialisation that makes the operation of the sensor specific to eating behaviour, preventing it from affecting all the other physiology and behaviour that would be modulated by those same neurotransmitter effects elswhere in the brain or indeed the rest of the body.