Sensory Signals

Sensory signals affecting feeding behavior fall into two categories: orosensory and postingestive. When one thinks of orosensory stimuli, the concept of flavor immediately comes to mind; however, olfaction and perception of physical attributes of a food (e.g., temperature, texture, consistency) contribute to the relative appeal of that food. Sense of smell is more highly developed in dogs and cats than in humans; consequently, it is likely to have a greater role in influencing food intake in these species. Warming food often enhances its aroma, and this may explain why dogs and cats prefer foods that are warmed to approximately their body temperature. Food acceptance drops off sharply, however, when the food is perceptibly warmer than normal body temperature.¹

Although olfaction is involved in initiation of food intake and contributes to what is perceived as the flavor of food, the way foods taste is important for maintaining food consumption. Dogs and cats are believed to be able to taste salty, bitter, acidic, and meaty (umami) flavors. Dogs, but not cats, can also taste sweet flavors.² Increasing fat and protein content of foods generally improves palatability for dogs and cats, as will adding natural sweeteners for dogs. Cats, on the other hand, seem to have a preference for slightly acidic flavors. Texture and consistency of a food can be an important aspect of palatability for companion animals. Dogs and cats generally dislike sticky or powdery foods, while increasing moisture content correlates with enhanced appeal. However, individuals can develop strong preferences for a particulate texture or kibble shape and refuse foods of higher moisture content.

Postingestive sensory signals come from the gastrointestinal tract and prompt termination of a meal. Mechanoreceptors located in the stomach, for example, detect gastric distention and provide feedback signals to the brain via vagal stimulation.

Metabolic Signals

There are several orexigenic factors that are produced in the CNS; however, neuropeptide Y (NPY) plays the central role in regulating appetite in the hypothalamus. A variety of signals that influence NPY production will increase or decrease food intake.³ Decreased circulating insulin concentration can increase NPY levels while postprandial elevations in serum insulin concentration decreases NPY. Leptin, a peptide secreted by adipocytes, plays a key role in maintaining energy balance. Leptin levels increase as fat reserves increase, which reduces NPY production. The polyphagic effect of glucocorticoids is believed to be due to these substances increasing NPY while there is evidence that certain cytokines have the opposite effect on NPY production.

Digestion and absorption of nutrients from a meal also trigger release of factors that feedback to the CNS. Cholecystokinin release by the gut in response to arrival of fat and certain amino acids in the duodenum suppresses food consumption. Peptide YY is another factor believed to inhibit food intake; it is released by the gut after food ingestion.

Environmental Signals and Learned Behavior

Although food itself and the physiologic response to ingestion of a meal are major factors influencing food intake, the environment in which food is offered may override any preexisting positive or negative sensory and metabolic signals. Companion animals usually become accustomed to some kind of routine associated with meals. Timing and location of meals, ambient noise level, type of feeding bowls, the person offering the food, and positive or negative associations of these environmental cues can influence patient appetite. Past experiences have some degree of impact on what types of food an animal find acceptable. Although cats in particular tend to be neophilic in their food preferences, individual animals may develop fixed food preferences with partiality to particular flavors or textures.⁴