Contribution of gustation to the palatability of linoleic acid

Perceived Intensity and Palatability of Fatty Culinary Preparations is Associated with Individual Fatty Acid Detection Threshold and the Fatty Acid Profile of Oils Used as Ingredients

The term oleogustus was recently proposed to describe a sixth basic taste that could guide preference for fatty foods and dishes to an extent. However, experimental data on food preference based on fatty acid (FA) content is scarce. Our aim was to examine the role of FA profile of oils and preparations as well as FA sensory thresholds on the palatability of salty and sweet culinary preparations representative of traditional Spanish Mediterranean cooking. In this study, we used three oils with similar texture and odor profile but different in their FA composition (saturated, monounsaturated, and polyunsaturated) and compared subjects in regard to their FA detection threshold and perceived pleasantness and intensity. Our results indicate that whereas saturated FAs cannot be detected at physiological concentrations, individuals can be categorized as tasters and nontasters, according to their sensory threshold to linoleic acid, which is negatively associated with perceived intensity (r = -0.393, P < 0.001) but positively with palatability (r = 0.246, P = 0.018). These differences may be due to a possible response to a fat taste. This sixth taste, or oleogustus. would allow establishing differences in taste intensity/palatability considering the FA profile of the culinary preparations. Given that tasters can detect linoleic and oleic acid at lower concentrations than nontasters, a greater amount of unsaturated FAs in culinary preparations could provoke an unpleasant experience. This finding could be relevant in the context of the culinary sector and to further our understanding of food preference and eating behavior.

Does eating good-tasting food influence body weight?

Does eating good-tasting food influence body weight? To investigate, we first established some concentrations of sucralose and mineral oil in chow that mice strongly preferred. Then, in Experiment 1, we compared groups of 16 mice fed plain chow (i.e., chow with no additives) to groups fed chow with added (a) sucralose, (b) mineral oil, (c) sucralose and mineral oil, or (d) sucralose on odd days and mineral oil on even days. During a 6-week test, the body weights and body compositions of the five groups never differed. In Experiment 2, we compared groups of 18 mice fed plain chow or plain high-fat diet to groups fed these diets with added sucralose. During a 9-week test, the high-fat diet caused weight gain, but the body weights of mice fed the sucralose-sweetened diets did not differ from those fed the corresponding plain versions. Two-cup choice tests conducted at the end of each experiment showed persisting strong preferences for the diets with added sucralose and/or mineral oil. In concert with earlier work, our results challenge the hypothesis that the orosensory properties of a food influence body weight gain. A good taste can stimulate food intake acutely, and guide selection toward nutrient-dense foods that cause weight gain, but it does not determine how much is eaten chronically.

Mechanisms Involved in Guiding the Preference for Fat Emulsion Differ Depending on the Concentration

High-fat foods tend to be palatable and can cause addiction in mice via a reinforcing effect. However, mice showed preference for low fat concentrations that do not elicit a reinforcing effect in a two-bottle choice test with water as the alternative. This behavior indicates the possibility that the mechanism underlying fat palatability may differ depending on the dietary fat content. To address this issue, we examined the influences of the opioid system and olfactory and gustatory transductions on the intake and reinforcing effects of various concentrations of a dietary fat emulsion (Intralipid). We found that the intake and reinforcing effects of fat emulsion were reduced by the administration of an opioid receptor antagonist (naltrexone). Furthermore, the action of naltrexone was only observed at higher concentrations of fat emulsion. The intake and the reinforcing effects of fat emulsion were also reduced by olfactory and glossopharyngeal nerve transections (designated ONX and GLX, respectively). In contrast to naltrexone, the effects of ONX and GLX were mainly observed at lower concentrations of fat emulsion. These results imply that the opioid system seems to have a greater role in determining the palatability of high-fat foods unlike the contribution of olfactory and glossopharyngeal nerves.

Behavioral palatability of dietary fatty acids correlates with the intracellular calcium ion levels induced by the fatty acids in GPR120-expressing cells

We recently reported that G-protein-coupled receptor 120 (GPR120) is expressed on taste buds, and that rodents showed preference for long-chain fatty acids (LCFA) at a low concentration. We also showed that the LCFA (1% linoleic acid) increased the extracellular dopamine (DA) level in the nucleus accumbens (NAc), which participates in reward behavior. However, the mechanism underlying the involvement of the GPR120-agonistic activity of LCFA in the palatability of dietary fat remains elusive. Therefore, we examined the association between the GPR120-agonistic activity and palatability of LCFA. First, we measured Ca(2+) signaling in HEK293 cells stably expressing GPR120 under stimulation by various LCFAs. We then assessed the palatability of the various LCFAs by testing the licking behavior in mice and measured the changes in the NAc-DA level by in vivo microdialysis. Consequently, 14- to 22-carbon unsaturated LCFAs showed strong GPR120-agonistic activity. Additionally, mice displayed high licking response to unsaturated 16- and 18-carbon LCFAs, and unsaturated 18-carbon LCFA significantly increased the DA level. The licking rate and the LCFA-dependent increase in DA level also correlated well with the GPR120- agonistic activity. These findings demonstrate that chemoreception of LCFA by GPR120 is involved in the recognition and palatability of dietary fat.

Concentration and state dependent reductions in corn oil intakes after glossopharyngeal nerve transections in rats

Previous studies indicate a role for the glossopharyngeal nerve (GL) in the detection of dietary fats. The present experiments examined the effects of bilateral glossopharyngeal nerve transections (GLx) on the intake of low (4.8%), moderate (16%), and full-fat (100%) corn oil in non-deprived, food-deprived, and water-deprived rats. The rats had access to oils, 0.3 M sucrose, and water in a gustometer that measured number of licks and latency to the first lick during brief access trials. The behavioral measures were used as indices of the amount consumed and the motivation to ingest, respectively. After baseline intakes had stabilized, the rats received GLx or sham transections (Sham) and were then re-tested. Pre and post-surgery responses were compared to determine the impact of GLx on intake and the motivation to ingest. In non-deprived rats, GLx reduced the intake of 4.8% and 16% oils and decreased the motivation to ingest these oils. In food-deprived rats, GLx prevented increases in the ingestion of 4.8% and 16% oils and in the motivation to ingest these oils. In water-deprived rats, GLx reduced the intake of 100% oil and produced a general decrease in the motivation to consume low, moderate, and full-fat emulsions. These results indicate that GL is partially involved in corn oil intake and suggest an interactive effect of oil concentration with homeostatic state.

Endocannabinoid signaling in the gut mediates preference for dietary unsaturated fats

Dietary fat exerts a potent stimulatory effect on feeding. This effect is mediated, at least in part, by a cephalic mechanism that involves recruitment of the vagus nerve and subsequent activation of endocannabinoid signaling in the gut. Here, we used a sham-feeding protocol in rats to identify fatty-acid constituents of dietary fat that might be responsible for triggering small-intestinal endocannabinoid signaling. Sham feeding rats with a corn oil emulsion increased endocannabinoid levels in jejunum, relative to animals that received either mineral oil (which contains no fatty acids) or no oil. Sham-feeding emulsions containing oleic acid (18:1) or linoleic acid (18:2) caused, on average, a nearly 2-fold accumulation of jejunal endocannabinoids, whereas emulsions containing stearic acid (18:0) or linolenic acid (18:3) had no such effect. In a 2-bottle-choice sham-feeding test, rats displayed strong preference for emulsions containing 18:2, which was blocked by pretreatment with the peripherally restricted CB1 cannabinoid receptor antagonists, AM6546 and URB447. Our results suggest that oral exposure to the monoenoic and dienoic fatty acid component of dietary fat selectively initiates endocannabinoid mobilization in the gut, and that this local signaling event is essential for fat preference.

Role of gut nutrient sensing in stimulating appetite and conditioning food preferences

The discovery of taste and nutrient receptors (chemosensors) in the gut has led to intensive research on their functions. Whereas oral sugar, fat, and umami taste receptors stimulate nutrient appetite, these and other chemosensors in the gut have been linked to digestive, metabolic, and satiating effects that influence nutrient utilization and inhibit appetite. Gut chemosensors may have an additional function as well: to provide positive feedback signals that condition food preferences and stimulate appetite. The postoral stimulatory actions of nutrients are documented by flavor preference conditioning and appetite stimulation produced by gastric and intestinal infusions of carbohydrate, fat, and protein. Recent findings suggest an upper intestinal site of action, although postabsorptive nutrient actions may contribute to flavor preference learning. The gut chemosensors that generate nutrient conditioning signals remain to be identified; some have been excluded, including sweet (T1R3) and fatty acid (CD36) sensors. The gut-brain signaling pathways (neural, hormonal) are incompletely understood, although vagal afferents are implicated in glutamate conditioning but not carbohydrate or fat conditioning. Brain dopamine reward systems are involved in postoral carbohydrate and fat conditioning but less is known about the reward systems mediating protein/glutamate conditioning. Continued research on the postoral stimulatory actions of nutrients may enhance our understanding of human food preference learning.

Fatty-acid preference changes during development in Drosophila melanogaster

Fatty-acids (FAs) are required in the diet of many animals throughout their life. However, the mechanisms involved in the perception of and preferences for dietary saturated and unsaturated FAs (SFAs and UFAs, respectively) remain poorly explored, especially in insects. Using the model species Drosophila melanogaster, we measured the responses of wild-type larvae and adults to pure SFAs (14, 16, and 18 carbons) and UFAs (C18 with 1, 2, or 3 double-bonds). Individual and group behavioral tests revealed different preferences in larvae and adults. Larvae preferred UFAs whereas SFAs tended to induce both a strong aversion and a persistent aggregation behavior. Adults generally preferred SFAs, and laid more eggs and had a longer life span when ingesting these substances as compared to UFAs. Our data suggest that insects can discriminate long-chain dietary FAs. The developmental change in preference shown by this species might reflect functional variation in use of FAs or stage-specific nutritional requirements, and may be fundamental for insect use of these major dietary components.

Decreased expression of CD36 in circumvallate taste buds of high-fat diet induced obese rats

Mammals spontaneously prefer lipid rich foods. Overconsumption of high-fat diet leads to obesity and related diseases. Recent findings indicate that taste may participate in the orosensory perception of dietary lipids and the fatty taste may contribute to a preference for and excessive consumption of dietary fat. CD36, a trans-membrane glycoprotein, which is located in the taste buds of circumvallate papillae of rodents, appears to be a plausible receptor for this fatty taste. Obese subjects present a stronger preference for fatty foods, though the mechanisms involved are complex and are not fully investigated. Our data from immunofluorescence and real-time RT-PCR showed that the expression levels of CD36 in circumvallate taste buds were significantly lower in high-fat diet induced obese rats as compared with that of control rats fed a normal diet. These results suggest that decreased expression of CD36 in circumvallate taste buds of high-fat diet induced obese rats may be associated with diminished fatty taste sensitivity and in order to compensate the preference for dietary fat, rats consume more fatty foods. Therapeutic strategies designed to alter or manipulate CD36 expression or function in taste buds may have important implications in treating obesity and related diseases.

Necessity of the glossopharyngeal nerve in the maintenance of normal intake and ingestive bout size of corn oil by rats

Recent evidence in the literature suggests that signals carried by the glossopharyngeal nerve (GL), which supplies sensory and parasympathetic innervation of the posterior tongue, might be essential in the maintenance of normal gustatory responses to fat stimuli. Here, we report that GL transection (GLX) significantly decreased corn oil intake and preference in 23-h two-bottle tests relative to sham-operated controls (Sham). Drinking-pattern analysis of corn oil licking revealed that bout size, rather than the number of bouts initiated, was smaller in GLX than Sham rats. We also tested a range of glucose concentrations and found that total licks over daily 23-h sessions significantly decreased in GLX compared with Sham rats, but this difference failed to reach significance when intake or any bout parameter was measured. These results show that the signals in the GL normally contribute to processes involved with corn oil bout termination as opposed to bout initiation. GL-derived signals could potentially provide input to "reward" circuits in the ventral forebrain that could serve to maintain ingestion during a meal or, alternatively, could act at the level of the brain stem to attenuate the inhibitory potency of vagal signals, thus delaying the onset of satiation, or perhaps contribute to a cephalic phase reflex modulation of the gut. Parasympathetic efferents in the GL innervating the von Ebner's glands, which secrete lingual lipase, which is thought to break down corn oil into detectable ligands, could also be playing a role in driving corn oil intake. Whatever the mechanism, an intact GL is clearly necessary in maintaining normal intake of corn oil.

Intragastric infusion of glucose enhances the rewarding effect of sorbitol fatty acid ester ingestion as measured by conditioned place preference in mice

We investigated substances that induce a rewarding effect during the postingestive process using the conditioned place preference (CPP) test. Although mice showed high affinity for a low-energy fat substitute--sorbitol fatty acid esters and low-concentration linoleic acid solution--they did not exhibit a place preference toward a voluntary intake of fat substitute in the CPP test. However, during a conditioning session of CPP that involved intragastric administration of corn oil immediately before the intake of the fat substitute, the test mice displayed a place preference. Similarly, intragastric administration of glucose, galactose, and dextrin also induced CPP; however, fructose, mannose, and a nonmetabolized carbohydrate did not. These results suggest that administration of corn oil and glucose has the same postingestive effect with regard to inducing CPP and that the structural specificity of carbohydrates influences the postingestive effect.

Linking peripheral taste processes to behavior

The act of eating and drinking brings food-related chemicals into contact with taste cells. Activation of these taste cells, in turn, engages neural circuits in the central nervous system that help animals identify foods and fluids, determine what and how much to eat, and prepare the body for digestion and assimilation. Analytically speaking, these neural processes can be divided into at least three categories: stimulus identification, ingestive motivation, and digestive preparation. This review will discuss recent advances in peripheral gustatory mechanisms, primarily from rodent models, in the context of these three major categories of taste function.

Assessing palatability of long-chain fatty acids from the licking behavior of BALB/c mice

Dietary oils such as corn oil, olive oil, and canola oil, which primarily contain triacylglycerol and small quantities of fatty acids, are highly palatable to animals. In a previous study, we examined the short-term (60 s) licking behavior of mice and observed that they exhibited a high licking response to a low concentration of fatty acid (linoleic acid), which is comparable to that observed for pure corn oil. This finding suggests that fatty acids contribute to the palatability of dietary oils. In order to supplement our knowledge of the fundamental features of fatty acid palatability in the oral cavity, we assessed the licking behavior of BALB/c mice to investigate the palatability of various types of long-chain fatty acids. The mice showed high licking responses to 1% unsaturated 16- and 18-carbon fatty acids (palmitoleic acid, 16:1; oleic acid, 18:1; linoleic acid, 18:2; and linolenic acid, 18:3), low licking responses to 16- and 20-carbon fatty acids (palmitic acid, 16:0 and arachidonic acid, 20:4), and no significant response to saturated fatty acids (stearic acid, 18:0 and arachidic acid, 20:0) or fatty acid derivatives (methyl linoleate and linole alcohol). Additionally, there were differences in the palatability of 18-carbon unsaturated fatty acids at very low concentrations. At fatty acid concentrations of 0.04% and 0.0625%, the mice showed significant preference for linoleic acid and linolenic acid, but not oleic acid, when compared with mineral oil. These results suggest that mice show high licking responses to 16- and 18-carbon unsaturated long-chain fatty acids at low concentrations. Further, we suggest that sensitivity to fatty acids is affected by the saturated state of the fatty acid, carbon chain length, and terminal carboxyl group.