Some suggestions on dealing with palatability--response to Ramirez

Food sensory characteristics: their unconsidered roles in the feeding behaviour of domestic ruminants

When domestic ruminants are faced with food diversity, they can use pre-ingestive information (i.e. food sensory characteristics perceived by the animal before swallowing the food) and post-ingestive information (i.e. digestive and metabolic consequences, experienced by the animal after swallowing the food) to evaluate the food and make decisions to select a suitable diet. The concept of palatability is essential to understand how pre- and post-ingestive information are interrelated. It refers to the hedonic value of the food without any immediate effect of post-ingestive consequences and environmental factors, but with the influence of individual characteristics, such as animal's genetic background, internal state and previous experiences. In the literature, the post-ingestive consequences are commonly considered as the main force that influences feeding behaviour whereas food sensory characteristics are only used as discriminatory agents. This discriminatory role is indeed important for animals to be aware of their feeding environment, and ruminants are able to use their different senses either singly or in combination to discriminate between different foods. However, numerous studies on ruminants' feeding behaviour demonstrate that the role of food sensory characteristics has been underestimated or simplified; they could play at least two other roles. First, some sensory characteristics also possess a hedonic value which influences ruminants' intake, preferences and food learning independently of any immediate post-ingestive consequences. Further, diversity of food sensory characteristics has a hedonic value, as animals prefer an absence of monotony in food sensory characteristics at similar post-ingestive consequences. Second, some of these food sensory characteristics become an indicator of post-ingestive consequences after their initial hedonic value has acquired a positive or a negative value via previous individual food learning or evolutionary processes. These food sensory characteristics thus represent cues that could help ruminants to anticipate the post-ingestive consequences of a food and to improve their learning efficiency, especially in complex environments. This review then suggests that food sensory characteristics could be of importance to provide pleasure to animals, to increase palatability of a food and to help them learn in complex feeding situations which could improve animal welfare and productivity.

Not just a matter of taste: palatability of bait markers is influenced by the need to search for alternative food

Context Bait palatability is a key issue influencing the uptake of toxic baits or non-toxic bait markers. Animals often reject baits with high concentrations of the active compound (whether it is a toxin, vaccine or marker) because of poor palatability, thus reducing the efficacy of baiting. Foraging theory predicts that palatability will be affected not only by the taste of active ingredients in bait but also by an animal’s ability to access alternative foods. Yet few studies of bait palatability are measured in the context of an animal’s need to search and forage for other food types. Aims The present study examined whether the palatability of Rhodamine B (RB) baits for black rats (Rattus rattus) was affected when foraging constraints were placed on access to alternative food compared with when alternative food was freely accessible. Rhodamine B is a bait marker and was used as a surrogate for other active ingredients likely to be used in pest control management. Methods Each day, RB bait at one of four concentrations was provided to an individual rat along with an alternative food that was either freely available (spatially clumped with foraging constraints absent) or hidden within a matrix of tubes (spatially scattered, thus with foraging constraints present). Key results Black rats exhibited a gradient in how palatable they found RB and preferred baits that contained the lowest concentrations of RB. Importantly, RB baits were more palatable when access to alternative food was made more difficult by applying a foraging constraint. In particular, a 0.2% RB concentration appeared to represent a threshold in palatability where intake at or above this concentration was significantly affected by a rat’s ability to freely access alternative foods. The ingestion of RB dye (mg kg–1) was highest in rats that consumed the highest concentrations, even though food intake was reduced. Conclusions The consumption of baits at high RB concentrations was greatly affected by the ease of access to other foods. We suggest the willingness of the animal to consume the bait can be influenced by the effort needed to find alternative foods. Implications A higher incidence of marking in the whiskers or hair of target individuals in the field will only be achieved with the use of the most palatable concentrations of RB and environments providing low alternative food access and abundance. A trade-off between reliable marking and palatability of RB at varying concentrations must be achieved if actual bait uptake in the field is to be more accurately represented. A re-evaluation of palatability experiments may be required as access to alternative foods can have profound impacts on bait uptake.

Eating rate and preference of different concentrate components for cattle

Different feeds or combination of feeds were studied in 11 short-term experiments with the objective to identify concentrates that were especially desirable for cattle. Eating rate of different feeds was studied in 6 experiments using 10 heifers in a Latin square design with 2 blocks of 5 animals, 5 treatments (feeds), and 5 periods (days). Preference as shown by feed choice was studied in 5 experiments with 12 heifers. Paired comparisons of 4 different feeds (1 through 4) in the 6 possible combinations of 1 and 2, 1 and 3, 1 and 4, 2 and 3, 2 and 4, and 3 and 4 were performed. A control feed of ground barley was included in all experiments. In the eating rate and preference experiments, a total of 25 and 16 feeds, respectively, were studied. The categories of feeds studied were basic feeds, such as cereals, soybean meal, and rapeseed products, and feed mixtures based on ground barley with sweet additives or additives based on fat products. Pelleted concentrate mixtures were also evaluated. From the results obtained, the following feeds were identified as being among the most preferred feeds: pelleted feeds, heat-treated rapeseed meal, barley with 10% rapeseed fatty acid, barley with 10% palm oil, and barley with 10% glycerol, whereas ground palm kernel expeller was undesirable. A clear preference for pellets over ground barley was demonstrated, but no difference in preference was observed for the 3 different pellets that were compared.

Palatability: response to nutritional need or need-free stimulation of appetite?

The traditional view of palatability was that it reflected some underlying nutritional deficit and was part of a homeostatically driven motivational system. However, this idea does not fit with the common observation that palatability can lead to short-term overconsumption. Here, we attempt to re-evaluate the basis of palatability, first by reviewing the role of salt-need both in the expression of liking for salty tastes, and paradoxically, in dissociating need from palatability, and second by examining the role of palatability in short-term control of appetite. Despite the clarity of this system in animals, however, most salt (NaCl) intake in man occurs in a need-free state. Similar conclusions can be drawn in relation to the palatability of food in general. Importantly, the neural systems underlying the hedonic system relating to palatability and homeostatic controls of eating are separate, involving distinct brain structures and neurochemicals. If palatability was a component of homeostatic control, reducing need-state should reduce palatability. However, this is not so, and if anything palatability exerts a stronger stimulatory effect on eating when sated, and over-consumption induced by palatability may contribute to obesity. Differential responsivity to palatability may be a component of the obese phenotype, perhaps through sensitisation of the neural structures related to hedonic aspects of eating. Together, these disparate data clearly indicate that palatability is not a simple reflection of need state, but acts to promote intake through a distinct hedonic system, which has inputs from a variety of other systems, including those regulating need. This conclusion leads to the possibility of novel therapies for obesity based on modulation of hedonic rather than homeostatic controls. Potential developments are discussed.

Time course and pattern of compensatory ingestive behavioral adjustments to lysine deficiency in rats

We and others have demonstrated that rats deficient in an essential amino acid (EAA) will consume sufficient quantities of the lacking nutrient to produce repletion when it is made available in solution. In the current series of experiments, we made rats deficient in lysine (LYS) by limiting the level of this EAA in the diet. We then examined licking behavior during approximately 23-h two-bottle intake tests over 4 consecutive days. In three separate experiments, rats were presented with the following: 1) 0.1 mol/L LYS and water, 2) 0.2 mol/L threonine (THR) and water and 3) 0.1 mol/L LYS and 0.2 mol/L THR. Lysine-deficient (LYS-DEF) rats drink significantly more LYS than did nondepleted controls (CON) when this amino acid was available. Meal pattern analysis revealed that the enhanced intake of LYS occurred as a function of a greater number of ingestive bouts, not changes in bout size. A cumulative analysis of LYS intake between CON and LYS-DEF rats revealed that a potentiation of intake developed within 30 min of sampling the solution when LYS and water were available and within 90 min when LYS and THR were the contrasting choices. In conclusion, increased LYS intake in the deficient rats occurs relatively rapidly and appears to be at least somewhat specific. Moreover, LYS deficiency does not seem to enhance the palatability of the limiting amino acid as judged by behaviors such as lick rate and bout size. Instead, LYS-DEF rats relieve the deficiency by increasing the number of drinking episodes initiated.

Palatability and intake relationships in free-living humans: measurement and characterization in the French

To investigate palatability influences on the ad lib eating behavior of free-living humans, 54 French participants were paid to maintain food intake diaries for four 7-day periods. They recorded their intake along with palatability ratings, on a seven-point scale, of each individual item eaten and also a global rating of the palatability of the entire meal. Higher levels of palatability were found to be related to larger meal sizes, durations, and deprivation ratios, smaller satiety ratios, greater hunger, and lower depression and anxiety. The global palatability rating was found to be superior to individual item palatability ratings as a measure of the palatability of the meal. Although palatability was found to have fairly large effects on intake, it accounted for less than 2% of the variance. It was concluded that, in the natural environment, there are a large number of other powerful variables present that add variance. In addition, people tend to self-select only a restricted range of highly palatable foods. As a result, in the natural environment, the influence of palatability on intake is limited.

Food reward: brain substrates of wanting and liking

What are the neural substrates of food reward? Are reward and pleasure identical? Can taste pleasure be assessed in animals? Is reward necessarily conscious? These questions have re-emerged in recent years, and there is now sufficient evidence to prompt re-examination of many preconceptions concerning reward and its relation to brain systems. This paper reviews evidence from many sources regarding both the psychological structure of food reward and the neural systems that mediate it. Special attention is paid to recent evidence from "tasty reactivity" studies of affective reactions to food. I argue that this evidence suggests the following surprising possibilities regarding the functional components and brain substrates of food reward. (1) Reward contains distinguishable psychological or functional components--"liking" (pleasure/palatability) and "wanting" (appetite/incentive motivation). These can be manipulated and measured separately. (2) Liking and wanting have separable neural substrates. Mediation of liking related to food reward involves neurotransmitter systems such as opioid and GABA/benzodiazepine systems, and anatomical structures such as ventral pallidum and brainstem primary gustatory relays. Mediation of wanting related to food reward involves mesotelencephalic dopamine systems, and divisions of nucleus accumbens and amygdala. Both liking and wanting arise from vastly distributed neural systems, but the two systems are separable. (3) Neural processing of food reward is not confined to the limbic forebrain. Aspects of food reward begin to be processed in the brainstem. A neural manipulation can enhance reward or produce aversion but no single lesion or transection is likely abolish all properties of food reward. (4) Both wanting and liking can exist without subjective awareness. Conscious experience can distort or blur the underlying reward process that gave rise to it. Subjective reports may contain false assessments of underlying processes, or even fail at all to register important reward processes. The core processes of liking and wanting that constitute reward are distinct from the subjective report or conscious awareness of those processes.

The influence of food on food intake: methodological problems and mechanisms of action

Emphasis has been placed on the understanding of the regulation of food intake in the hope of aiding the battle against obesity and of helping to ameliorate the anorexia of cancer and eating disorders. Available data suggest that the regulatory system is multifaceted and complex. This review focuses on current research on the regulation of appetite and satiety by carbohydrates, fats, and proteins as well as by artificial sweeteners. Some methodological problems and potential mechanisms of action at the biochemical level are discussed. Evidence suggests that organisms are more successful in defending against calorie dilution than in adjusting to increases in calories. The implications of that defense relative to the use of ersatz nutrients are explored.