Sensory Influences on Food Intake

Human sensory systems evolved, in part, to aid in the identification, procurement, and ingestion of healthful foodstuffs. Each system contributes different information, but because the chemical senses serve as the final checkpoint before ingestion, research has focused primarily on these senses.

Whether taste is composed of four primary qualities (sweet, sour, salty, bitter) that in different combinations yield all gustatory sensations or whether there are multiple unique tastes remains controversial. Most researchers support the four-quality view and it has been proposed that sensitivity to each of the qualities has conveyed an adaptive protective benefit. Sweet taste may have aided in the identification of energy sources since many carbohydrates are sweet.

Salt sensation could have helped ensure intake of adequate electrolytes as many possess a salty note. Sour sensitivity could have been used to avoid substances at pH extremes and it was useful to recognize and avoid bitter substances since a relatively high proportion of such compounds are toxic. The taste of monosodium glutamate (Umami in Japanese) may represent a fifth basic taste and could signal the presence of protein sources.

The prevailing view with regard to olfaction is that there are no olfactory primaries. This sensory system is more like the immune system in its ability to recognize a seemingly endless array of unique compounds. Whereas teleological arguments have been made with respect to “gustatory wisdom” and food intake (see earlier), no such speculation has been offered for the olfactory system. It should be noted that the oral cavity also receives trigeminal innervation (adding mouth feel, temperature, and pain). This system probably mediates the perception of fats in the diet as well as acids, bases, astringent compounds, and the textural properties of foods.

In part because of the currently greater interest in the role of other physiological systems (e.g., neural, endocrine) in the control of ingestive behavior, the contribution of sensory input under present living conditions is not well characterized. Perhaps the most compelling evidence stems from assessments of individuals with nutrient-related health disorders. There are case reports of individuals with salt-wasting pathologies who crave salt. Protein-calorie malnourished children have also shown a greater preference for stimuli containing an amino acid or protein source than nutritionally replete peers.

There are few additional examples of such gustatory wisdom, although subtle sensory-based adjustments may occur without notice since the potential problems are mitigated. In contrast, the historical failure of individuals with scurvy, beriberi, and pellagra to correct their problem with available foods argues against a functional role of gustatory wisdom. Moreover, appropriate dietary responses to many chronic diseases, such as diabetes, hypertension, obesity, and dental caries, are not only absent, but efforts to promote more healthful eating are often met with strong opposition due to the perceived lower palatability of foods comprising the recommended diets.

Food cravings and food aversions, which are often based on the sensory qualities of foods, also hold nutritional implications. For example, cravings expressed by pregnant women, bulimic patients, and depressed patients treated with tricyclic antidepressants have generally involved sweet items. Such cravings can influence nutritional status if the sought after foods are ingested to the exclusion of items supplying needed nutrients, or if the craved items contain compounds that alter the bioavailability of nutrients provided by other foods.

An estimated one-third to two-thirds of the U.S. population have formed a food aversion at some point in their life. These aversions are marked decreases in the acceptability of particular foods after their ingestion has been associated with illness such as nausea or emesis. In clinical populations (e.g., cancer patients), aversions are typically specific (i.e., targeted to a few foods) and transient (i.e., durations of weeks to months). Aversions are specific, but often of longer duration among the general population. Because of the specificity, aversions rarely compromise diet quality and nutritional status, although they can be problematic in individual cases. High-protein items (e.g., red meat) are the most commonly affected foods, with sweets (especially chocolate), caffeinated beverages (e.g., coffee), and high-fat items being implicated often. The taste or smell of aversive foods is sufficient to elicit revulsion responses.

There are estimates that up to 20 million Americans suffer from taste and/or smell disorders. These range from slight diminutions of sensitivity to complete loss of function. Persistent tastes or smells and distorted sensations are also reported. These sensory abnormalities lead to decreased food enjoyment in a majority of sufferers, but alter dietary behavior to a clinically significant degree (i.e., changes of body weight exceeding 10% of predisorder weight) in only about 15-20% of affected individuals (based on findings from taste and smell research centers). Some patients report increasing intake in an attempt to achieve a sought after sensory experience or to mask an unpleasant sensation, whereas others reduce consumption in frustration with the lack of enjoyment with food or because food elicits an unpleasant sensation.

Diverse experimental evidence supports a view that sensory factors influence ingestive behavior in healthy individuals. Studies focusing on individual meals have shown that the palatability of foods influences eating behaviors such as rate of chewing and number of chews per bite. It is inferred that such effects impact on actual food intake. Other work has shown that sensory variety in a meal promotes greater total intake relative to when a single food is available. When chemosensory receptors are bypassed by delivering food directly into the stomach via a tube (intragastric feedings), patients often express dissatisfaction due to the lack of oral stimulation. Such patients can maintain adequate caloric intake via this intragastric feeding regimen. However, when oral intake is permitted, intragastric feeding does not induce a compensatory reduction in voluntary oral intake that offsets the extra calories derived from the intragastric feed. Thus, the desire for oral stimulation may override shortterm satiety cues.

Common experience supports a role for sensory factors in the selection and ingestion of foods in a longer time frame. The sensory attributes of diets from different cultures are widely discrepant and are resistant to change, as evidenced by the reluctance of immigrants to modify their dietary practices. Efforts to supplement the diets of developing nations with various nutrients by introducing them into staple foods have frequently been unsuccessful due to some change they impart in a sensory property of the food. For example, efforts to add iron to sugar have not succeeded, ip part because the sugar becomes discolored, rendering it unacceptable to consumers. Weak sales of “natural” or modified (e.g., low sodium, low fat) products that are purportedly more healthful, owing to their lower palatability, also attest to the importance of the sensory factors in food selection and ingestion.

There are numerous mechanisms by which sensory factors can influence dietary behavior. First, it has been argued that humans possess certain innate sensory preferences. The most commonly cited example is for the sweet taste. This quality is able to promote fetal drinking and increase sucking responses in neonates. Changes in mimetic reflexes as well as certain autonomic responses (e.g., heart rate) have also been recorded. In adults, there is evidence that 40-50% of calories are derived from primarily sweet-tasting foods. Preference for the salty taste may also be innately determined, but shows a maturational lag. It is manifest at approximately 6 months of age. Adults consume approximately 40% of calories from primarily salty-tasting foods.

Food intake may also be negatively influenced by sensory qualities. Deviations in any sensory attribute (e.g., color) will likely lead to rejection of a product with which an individual is familiar. In addition, some sensory properties may be innately undesirable and, in the absence of repeated exposures or cultural norms that promote acceptability, are avoided. Foods with predominantly sour and bitter tastes may reflect this since they constitute only about 10% of total energy intake. Often times when an unpreferred quality are consumed, but only after the undesirable sensory properties are modified (e.g., adding cream and sugar to a bitter beverage).

A third mechanism stems from the view that humans have an innate preference for sensory variety. One clear example of this mechanism is the premium placed on novel foods by populations forced to subsist on diets with restricted variety. Monotonous diets have actually been advocated as a means to achieve weight reduction and can be effective over a short period. However, frustration with this approach builds over time and long-term compliance is universally poor.

One final mechanism (though this list is not intended to be exhaustive) involves the role of sensory cues as predictors of the metabolic consequences of foods. Through experience, the satiety value of foods is learned and freely selected portion sizes are adjusted accordingly. Sensory cues, which have consistently been associated with each food, can supply the needed information. In studies where the energy values of meals have been surreptitiously manipulated, 25- 50% of study participants were found to behave as though they were attending to the previously learned sensory cues rather than the actual metabolic implication of the meal. That is, they ingested less of a meal previously associated with a higher energy content and more of a meal previously associated with a low energy content. Humans are able to maintain adequate levels of energy intake in the absence of sensory input; however, when sensory information is available, it may be prepotent at least in some individuals over the short term.