Frontiers in Integrative Neuroscience (Jul 2014)
Taste: The Bedrock of Flavor
Abstract
The significance of taste for human health:Throughout most of human evolution, the daily decisions of what to put into ones mouth and swallow and what to reject presented challenges fraught with danger. Energy-rich foods were often difficult to find; protein was in short supply; sodium was scarce. Moreover, many plants that did contain nutrients were also equipped with defensive compounds that were poisonous. Now many humans over consume exactly the foods that they evolved to find particularly attractive when they were scarce such as foods high in sweet sugars, fats, and added salt. Consequences of this overconsumption may include obesity, diabetes, hypertension which is a risk factor for cardiovascular disease, and other diseases of “excess.” Helping to drive overconsumption of some nutrients and under consumption of others is the evolved taste for sweetness and saltiness and the distaste for bitter compounds that are often found in foods such as green vegetables. Thus, understanding of the biology and psychology of the taste system is central to enhancing human health. Basic tastes: Evolutionary significance and basic receptor mechanisms. Most investigators recognize 5 independent basic taste qualities: Sweet, bitter, sour, umami and salty and recent evidence indicates that each quality is recognized by specialized taste cells in taste buds in the oral cavity and elsewhere. There is also evidence that compounds that do not elicit such distinct qualities (e.g. fat and fatty acids, calcium, complex carbohydrates) may also impact the taste system in ways that are not fully understood [taste receptors reviewed in 1]. Sweet taste is elicited by simple sugars (e.g. sucrose, glucose, fructose, lactose) and many other compounds of diverse chemical structure, many of which are not found in nature (e.g. saccharin, sucralose, aspartame and the protein monellin). All of these compounds interact with a dimerized pair of proteins forming G-protein coupled taste receptors (GPCRs). This receptor system is able to recognize so many diverse compounds because they can interact with various portions of the heterodimer in addition to the traditional binding pocket. More recently, at least one other mechanism for detection of some sweeteners by sweet taste cells has been identified [2]. Humans and most other species that consume plants innately like sugars and some other sweeteners. Presumably, a liking for natural sweeteners evolved to insure intake of energy-rich plant material. Consistent with this idea are recent findings [3] that several obligate carnivores such as cats have lost sweet taste receptor function through fixation of disabling mutations. Bitter taste is elicited by a huge array of chemicals of greatly different structures. Presumably as a consequence, there are multiple structurally-related GPCR receptors (numbering approximately 25 in humans) that are differentially tuned to different classes of compounds that are innately avoided. It is believed that bitter taste evolved as a protective mechanism to avoid these toxic compounds although in some cases they may be selected by a sick animal because they have medicinal properties. Sour taste is elicited by acids. It is believed that one or more ion channel (as distinct from a GPCR) is responsible for sour taste but the identity of these channels is still unknown. The precise adaptive function for detecting sourness is also not clear although it is thought to involve protection against ingesting potential foods with very low pH as well as a way to judge ripeness of fruits. Umami taste is the most recently identified of the 5 basic tastes. The English word that most accurately expresses the umami quality is “savory” or chicken broth-like although the Japanese word roughly translates into delicious taste. In humans it is elicited by the amino acid glutamate (most commonly experienced as the sodium salt: MSG) and is enhanced synergistically by some 5’ ribonucleotides. One umami receptor is closely related to the sweet receptor although there may be other receptors involved. It is widely believed that this pleasant quality (at least when combined with other tastes and flavors in foods) is a signal for amino acids and hence protein. Finally, salty taste is stimulated almost exclusively by sodium-containing compounds, most potently by NaCl (salt). Recent studies have identified an epithelial sodium channel (“ENaC”) as at least one mechanism underlying salty taste. However there is at least one other mechanism underlying detection of salt that remains to be identified. Salt taste liking may have an innate component but recent work also suggests that experiences with salt in human infants and children may modulate liking [4]. Salt taste is critically involved in obtaining sodium, a mineral necessary for life and one that is often in short supply in nature. However, now there is a clear excess in both availability and intake which impacts human health. Salt taste and human health. Consumption of high-salt diets, which is common in virtually all human societies, leads to increases in blood pressure and can cause premature death [5]. As a consequence most government and many non-government health organizations around the world recommend that salt intake in the general population be reduced. However this is difficult in large part because of the taste-based attractiveness of salt in foods. Humans prefer many foods with salt for a variety of reasons. First, they like the taste of salt itself, although this is usually only in the context of other food flavors. Additionally, the sodium ion acts as a powerful inhibitor of bitterness and thus addition of salt to bitter foods such as vegetables increases their palatability by reducing some of the unpleasant bitter notes. Finally salt, by mechanisms not yet understood, increases palatability of foods by enhancing perception of mouth feel. Thus removal of salt in foods is not easy and many low salt foods are unpalatable [5, Chapter 4]. There are two general approaches to reducing dietary sodium. First, there is considerable interest in developing salt substitutes and salt enhancers. Potassium chloride is widely used (usually in combination with NaCl) as a substitute but it is not ideal since many find it has an unpleasant off-taste. There is considerable academic and industry research to identify new substitutes but to date there are none for salty as there are for sweet taste. A second approach to lowering sodium intake on a population-wide level in the United States, where more than 80% of the average person’s salt intake comes from food purchased and not from being added during cooking or at the table, is for food manufacturers and restaurants to gradually reduce the amount of salt in prepared foods. Experimental studies have demonstrated that if one reduces salt intake preferences for salt are similarly reduced. Based on this, the Institute of Medicine (IOM) recommended that the Food and Drug Administration require gradual reduction by food manufacturers and large restaurant chains (IOM). The FDA has not acted on this recommendation. Conclusion. As illustrated by the difficulties in reducing salt in spite of the health benefits (a similar set of arguments for reducing excess consumption of carbohydrate sugars could be made), the sense of taste is a powerful driver of food intake. A deeper understanding of this important but neglected sensory system is required if we are to adequately address critical health problems in modern society that are often driven by excess consumption of tasty nutrients.
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