Orosensory factors in the ingestion of corn oil/sucrose mixtures by the rat

Rats learn to prefer the late-consumed flavor over the early-consumed flavor in a multi-flavored meal paired with oral glucose and corn oil

Conditioned flavor preference (CFP) is established by association: where a neutral flavor (conditioned stimulus, CS) is paired with orosensory and post-ingestive components of nutrients, including sugar and fat (unconditioned stimulus, US). A previous study reported that rats can learn to prefer flavors that they consumed earlier and later in a multi-flavored solution paired with an intragastric infusion of glucose, but they expressed only a preference for a late-consumed flavor when they were tested after feeding (Myers and Whitney, 2011). This paradigm can be a suitable rodent model to explain how humans acquire a selective preference for routinely late-served "dessert" foods and why these foods remain attractive even in the absence of hunger. Here, we examined whether oral glucose (Experiment 1) or fat (Experiment 2) acts as a US for flavor preference learning processes in this paradigm. In Experiment 1, adult female rats under food restriction were trained in 16 daily sessions with two distinct flavor CSs in succession per session; eight CS(+) sessions in which two distinct flavor CSs (early(+), late(+)) were sequentially presented for 8 min each with oral glucose (12%) as a US, and eight CS(-) sessions in which different CSs (early(-), late(-)) were unpaired with the US. In the 30-minute two-bottle choice test, rats preferred late(+) over late(-) only when tested 90 min after consumption of normal chow (fed test) but not after overnight deprivation (hungry test). Early(+) was not preferred over early(-) in both tests. Moreover, a significant preference for late(+) over early(+) was observed only in the fed test, which is a unique feature of oral glucose-CFP. These results indicate that taste sensations of oral glucose promote a rewarding effect of late-onset glucose nutrients. In Experiment 2, separate rats were trained with the same conditioning paradigm, but used a caloric matched fat solution (5.3% corn oil) for a US. The results showed that they expressed stronger preferences for early(+) and late(+) relative to their respective CS(-) flavors in both tests. Similar to Experiment 1, it was observed in the fed test that there was a preference for late(+) over early(+) in oral fat-CFP. Taken together, the present results suggest that routine timing arrangements can cause qualitative differences in conditioned preferences between multiple flavors within a sugar or fat-containing meal in rats, and that rats prefer the late-consumed flavor over the early-consumed flavor in the absence of hunger.

Cross-Generalization Profile to Orosensory Stimuli of Rats Conditioned to Avoid a High Fat/High Sugar Diet

The orosensory characteristics of a diet play a role in its acceptance and rejection. The current study was designed to investigate the gustatory components that contribute to the intake of a palatable, high-energy diet (HE; 45% calories from fat, 17% calories from sucrose). Here, rats were conditioned to avoid HE diet by pairings with i.p. injections of LiCl to induce visceral malaise. Subsequently, the degree of generalization was tested to an array of taste compounds using a brief-access lick procedure (10-s trials, 30-min sessions). Compared to NaCl-injected controls, LiCl-injected rats suppressed licking response to 100% linoleic acid and 20% intralipid, and to a lesser extent 17% sucrose. There was more variability in the lick responses to sucrose among the LiCl-injected rats. Rats that tended to suppress licking responses to sucrose generalized this response to glucose, fructose and Na-saccharin but not to Polycose. In contrast, LiCl-injected rats did not significantly suppress lick responses to water, NaCl, citric acid, or quinine compared to controls rats. The brief access feature of this procedure, allows for behavioral measures when postingestive factors are minimized. These findings support a role for gustatory cues in the detection of high fat/high sugar diets. Furthermore, it appears that the fat component is a more salient orosensory feature of the HE diet.

Human Tongue Electrophysiological Response to Oleic Acid and Its Associations with PROP Taster Status and the CD36 Polymorphism (rs1761667)

The perception of fat varies among individuals and has also been associated with CD36 rs1761667 polymorphism and genetic ability to perceive oral marker 6-n-propylthiouracil (PROP). Nevertheless, data in the literature are controversial. We present direct measures for the activation of the peripheral taste system in response to oleic acid by electrophysiological recordings from the tongue of 35 volunteers classified for PROP taster status and genotyped for CD36. The waveform of biopotentials was analyzed and values of amplitude and rate of potential variation were measured. Oleic acid stimulations evoked positive monophasic potentials, which represent the summated voltage change consequent to the response of the stimulated taste cells. Bio-electrical measurements were fully consistent with the perceived intensity during stimulation, which was verbally reported by the volunteers. ANOVA revealed that the amplitude of signals was directly associated, mostly in the last part of the response, with the CD36 genotypes and PROP taster status (which was directly associated with the density of papillae). The rate of potential variation was associated only with CD36, primarily in the first part of the response. In conclusion, our results provide direct evidence of the relationship between fat perception and rs1761667 polymorphism of the CD36 gene and PROP phenotype.

Cd36 is a candidate lipid sensor involved in the sensory detection of fatty acid in zebrafish

Recently more and more evidences raise the possibility for the taste system in the role of the perception of lipids in mammals, and the fatty acid receptor CD36 has been proved to be as an important candidate receptor of fat taste. Fish has different taste modality with mammals. No information was known about the function of cd36 in fish taste till now. Here, using in situ hybridization and immunofluorescence technologies, we showed that fish cd36/Cd36 localized in taste buds. Real-time PCR technology demonstrated that, in zebrafish cd36 (zcd36)-transfected cells, linoleic acid (LA) increased the expression level of tryptophan hydroxylase-1 (TPH-1), which encodes the enzyme involved in the biosynthesis of monoamine neurotransmitter of 5-HT. Moreover, the LA-induced up-regulation expression of TPH-1 was significantly curtailed by SSO, a specific inhibitor of LCFA binding to CD36, suggesting zCd36 is implicated in the LA-induced release of neurotransmitter. Importantly, we observed that zcd36 gene knockout zebrafish reduced the preference for LA contrast to wild-type zebrafish. Together, our findings indicate that Cd36 is a candidate lipid sensor involved in the sensory detection of fatty acid in zebrafish.

Associations between orosensory perception of oleic acid, the common single nucleotide polymorphisms (rs1761667 and rs1527483) in the CD36 gene, and 6-n-propylthiouracil (PROP) tasting

Orosensory perception of dietary fat varies in individuals, thus influencing nutritional status. Several studies associated fat detection and preference with CD36 or 6-n-propylthiouracil (PROP) sensitivity. Other studies have not confirmed the latter association. We analyzed the relationship between orosensory perception of oleic acid, two CD36 variants, and PROP tasting. Thresholds of oleic acid perception were assessed in 64 subjects using a modification of the three-alternative forced-choice procedure. Subjects were classified for PROP taster status and genotyped for TAS2R38 and CD36 (SNPs: rs1761667 and rs1527483). Subjects homozygous for GG of the rs1761667 polymorphism showed higher sensitivity to oleic acid than AA subjects. The capability to detect oleic acid was directly associated with TAS2R38 or PROP responsiveness. PROP non-tasters had a lower papilla density than tasters, and those with genotype GG of the rs1761667 polymorphism had lower oleic acid thresholds than PROP non-tasters with genotype AA. In conclusion, results showed a direct association between orosensory perception of oleic acid and PROP tasting or rs1761667 polymorphism of CD36, which play a significant role in PROP non-tasters, given their low number of taste papillae. Characterization of individual capability to detect fatty acids may have important nutritional implications by explaining variations in human fat preferences.

Maternal high-fat diet during pregnancy and lactation reduces the appetitive behavioral component in female offspring tested in a brief-access taste procedure

Maternal high-fat diet appears to disrupt several energy balance mechanisms in offspring. Here, female offspring from dams fed a high-fat diet (HF) did not significantly differ in body weight compared with those fed chow (CHOW), when weaned onto chow diet. Yet when presented with both a chow and a high-fat diet, high-fat intake was significantly higher in HF compared with CHOW offspring. To assess taste-based responsiveness, offspring (12 wk old) were tested in 30-min sessions (10-s trials) to a sucrose concentration series in a brief-access taste test. Compared with CHOW, the HF offspring initiated significantly fewer trials but did not significantly differ in the amount of concentration-dependent licking. Thus, rather than affect lick response (consummatory), maternal diet affects spout approach (appetitive), which may be attributed to motivation-related mechanisms. Consistent with this possibility, naltrexone, an opioid receptor antagonist, further reduced trial initiation, but not licking in both groups. With naltrexone administration, the group difference in trial initiation was no longer evident, suggesting differences in endogenous opioid activity between the two groups. Relative expression of μ-opioid receptor in the ventral tegmental area was significantly lower in HF rats. When trial initiation was not required in one-bottle intake tests, no main effect of maternal diet on the intake of sucrose and corn oil emulsions was observed. Thus, the maternal high-fat diet-induced difference in diet preference is not likely due to changes in the sensory orosensory component of the taste stimulus but may depend on alterations in satiety signals or absorptive mechanisms.

Are free fatty acids effective taste stimuli in humans? Presented at the symposium "The Taste for Fat: New Discoveries on the Role of Fat in Sensory Perception, Metabolism, Sensory Pleasure and Beyond" held at the iNstitute of Food Technologists 2011 Annual Meeting, New Orleans, LA, June 12, 2011

The popularity of low- and reduced-fat foods has increased as consumers seek to decrease their energy consumption. Fat replacers may be used in fat-reduced products to maintain their sensory properties. However, these ingredients have been largely formulated to replicate nongustatory properties of fats to foods and have only achieved moderate success. There is increasing evidence that fats also activate the taste system and uniquely evoke responses that may influence product acceptance. Work supporting a taste component of fat has prompted questions about whether fat constitutes an additional "primary" or "basic" taste quality. This review briefly summarizes this evidence, focusing on human studies, when possible. Effective stimuli, possible receptors, and physiological changes due to oral fat exposure are discussed. Some studies suggest that there are fatty acid tasters and nontasters and if verified could have implications for targeted product development.

Role of CD36 in oral and postoral sensing of lipids

Obesity and associated plethora of diseases constitute a major public health challenge worldwide. The conjunction of profound changes in our lifestyle and a thrifty genetic that evolved in an environment of food scarcity largely explains this epidemic situation. Food abundance promotes our specific appetite for the more palatable food generally rich in lipids. It is noteworthy that this attraction for fatty food is not specific to humans. Rats and mice also spontaneously prefer lipid-rich food in a free-choice situation. Detection of lipids in food requires the presence of specific sensors located in strategic places (e.g., oral cavity, small intestine, brain) whose activation results in a modulation of the eating behavior. Recent data strongly suggest that the glycoprotein CD36 plays a significant role in this sensing system.

Accumulating evidence supports a taste component for free fatty acids in humans

The requisite criteria for what constitutes a taste primary have not been established. Recent advances in understanding of the mechanisms and functions of taste have prompted suggestions for an expanded list of unique taste sensations, including fat, or more specifically, free fatty acids (FFA). A set of criteria are proposed here and the data related to FFA are reviewed on each point. It is concluded that the data are moderate to strong that there are: A) adaptive advantages to FFA detection in the oral cavity; B) adequate concentrations of FFA to serve as taste stimuli; C) multiple complimentary putative FFA receptors on taste cells; D) signals generated by FFA that are conveyed by gustatory nerves; E) sensations generated by FFA that can be detected and scaled by psychophysical methods in humans when non-gustatory cues are masked; and F) physiological responses to oral fat/FFA exposure. On no point is there strong evidence challenging these observations. The reviewed findings are suggestive, albeit not definitive, that there is a taste component for FFA.

G protein-coupled receptors in human fat taste perception

In contrast to carbohydrates and proteins, which are detected by specialized taste receptors in the forms of their respective building blocks, sugars, and L-amino acids, the third macronutrient, lipids, has until now not been associated with gustatory receptors. Instead, the recognition of fat stimuli was believed to rely mostly on textural, olfactory, and postingestive cues. During the recent years, however, research done mainly in rodent models revealed an additional gustatory component for the detection of long-chain fatty acids (LCFAs), the main taste-activating component of lipids. Concomitantly, a number of candidate fat taste receptors were proposed to be involved in rodent's gustatory fatty acid perception. Compared with rodent models, much less is known about human fat taste. In order to investigate the ability of the human gustatory system to respond to fat components, we performed sensory experiments with fatty acids of different chain lengths and derivatives thereof. We found that our panelists discriminated a "fatty" and an irritant "scratchy" taste component, with the "fatty" percept restricted to LCFAs. Using functional calcium-imaging experiments with the human orthologs of mouse candidate fat receptors belonging to the G protein-coupled receptor family, we correlated human sensory data with receptor properties characterized in vitro. We demonstrated that the pharmacological activation profile of human GPR40 and GPR120, 2 LCFA-specific receptors associated with gustatory fat perception in rodents, is inconsistent with the "scratchy" sensation of human subjects and more consistent with the percept described as "fatty." Expression analysis of GPR40 and GPR120 in human gustatory tissues revealed that, while the GPR40 gene is not expressed, GPR120 is detected in gustatory and nongustatory epithelia. On a cellular level, we found GPR120 mRNA and protein in taste buds as well as in the surrounding epithelial cells. We conclude that GPR120 may indeed participate in human gustatory fatty acid perception.

Pontine and thalamic influences on fluid rewards: I. Operant responding for sucrose and corn oil

The reward strength of orosensory sucrose and corn oil was measured using fixed and progressive ratio operant schedules. Because the orosensory effects of the stimuli were of interest, Experiment 1 compared operant responses for sucrose in sham and real feeding rats. The results demonstrated that rats would work for sucrose solutions without the accompanying postingestive effects. Furthermore, the break points for high concentrations of sucrose (1.0 M or 2.0 M) were significantly higher in sham feeding rats than in real feeding controls. Experiment 2 investigated the role of the parabrachial nucleus (PBN) and of the thalamic orosensory area (TOA) in sucrose and corn oil reward. During free access, rats with PBN lesions (PBNx) licked significantly less sucrose solution than their controls, but both groups ingested a similar volume of corn oil emulsion. When an operant was imposed, these same PBNx rats failed to respond for sucrose and continued only modestly for corn oil. In contrast, the TOA lesioned rats (TOAx) showed no impairment in responding for sucrose or corn oil during either the free access or operant sessions. Furthermore, rats with TOA lesions demonstrated significantly higher break points for sucrose than did their controls. Together, the data imply that the PBN but not the TOA is critical for the perception of, or responding to the reward value of sucrose and corn oil.

Gut T1R3 sweet taste receptors do not mediate sucrose-conditioned flavor preferences in mice

Most mammals prefer the sweet taste of sugars, which is mediated by the heterodimeric T1R2+T1R3 taste receptor. Sugar appetite is also enhanced by the post-oral reinforcing actions of the nutrient in the gut. Here, we examined the contribution of gut T1R3 (either alone or as part of the T1R3+T1R3 receptor) to post-oral sugar reinforcement using a flavor-conditioning paradigm. We trained mice to associate consumption of a flavored solution (CS+) with intragastric (IG) infusions of a sweetener, and a different flavored solution (CS-) with IG infusions of water (23 h/day); then, we measured preference in a CS+ vs. CS- choice test. In experiment 1, we predicted that if activation of gut T1R3 mediates sugar reinforcement, then IG infusions of a nutritive (sucrose) or nonnutritive (sucralose) ligand for this receptor should condition a preference for the CS+ in B6 wild-type (WT) mice. While the mice that received IG sucrose infusions developed a strong preference for the CS+, those that received IG sucralose infusions developed a weak avoidance of the CS+. In experiment 2, we used T1R3 knockout (KO) mice to examine the necessity of gut T1R2+T1R3 receptors for conditioned flavor preferences. If intact gut T1R3 (or T1R2+T1R3) receptors are necessary for flavor-sugar conditioning, then T1R3 KO mice should not develop a sugar-conditioned flavor preference. We found that T1R3 KO mice, like WT mice, acquired a strong preference for the CS+ paired with IG sucrose infusions. The KO mice were also like WT mice in avoiding a CS+ flavor paired with IG sucralose infusions These findings provide clear evidence that gut T1R3 receptors are not necessary for sugar-conditioned flavor preferences or sucralose-induced flavor avoidance in mice.

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.

Interactions of temperature and taste in conditioned aversions

The influence of temperature on taste cues and the ability to discriminate and learn about different temperatures of foods are important factors regulating ingestion. The goal of this research was to demonstrate that thermal orosensory input can serve as a salient stimulus to guide ingestive behavior in the rat, and also that it interacts with gustatory input during choice and conditioned aversion experiments. A novel apparatus with Peltier refrigerators was used to control the temperature of solutions in 10-min, 2-bottle tests. It was determined that naive rats preferred cold water (10 degrees C) to warm water (40 degrees ). When cold water was paired with a toxic LiCl injection, rats avoided cold water and drank warm water, thus demonstrating that cold water could serve as the conditioned stimulus in a conditioned temperature aversion. Rats conditioned against cold water could discriminate 10 degrees C water from 16 degrees C water, but not from 13 degrees C water, thus showing an ability to discriminate orosensory thermal cues to within 3-6 degrees C. Rats also generalized conditioned aversions from cold water to cold saccharin and cold sucrose solutions. However, if rats were conditioned against a compound taste and thermal stimulus (10 degrees C, 0.125% saccharin), the rats could distinguish and avoid each component individually, i.e., by avoiding cold water or warm saccharin. Finally, daily 2-bottle extinction tests were used to assess the strength of aversions conditioned against a taste cue (0.25 M sucrose), a thermal cue (10 degrees C water), or the combination. Aversions to taste or temperature alone persisted for 7-14 days of extinction testing, but the combined taste-temperature aversion was stronger and did not extinguish after 20 days of extinction testing. These results demonstrate that temperature can serve as a salient cue in conditioned aversions that affect ingestion independent of taste cues or by potentiating taste cues.

Is there a fatty acid taste?

Taste is a chemical sense that aids in the detection of nutrients and guides food choice. A limited number of primary qualities comprise taste. Accumulating evidence has raised a question about whether fat should be among them. Most evidence indicates triacylglycerol is not an effective taste stimulus, though it clearly contributes sensory properties to foods by carrying flavor compounds and altering texture. However, there is increasing anatomical, electrophysiological, animal behavior, imaging, metabolic, and psychophysical evidence that free fatty acids are detectable when non-taste cues are minimized. Free fatty acids varying in saturation and chain length are detectable, suggesting the presence of multiple transduction mechanisms and/or a nonspecific mechanism in the oral cavity. However, confirmation of "fatty" as a taste primary will require additional studies that verify these observations are taste specific. Oral exposure to free fatty acids likely serves as a warning signal to discourage intake and influences lipid metabolism.

Orosensory perception of dietary lipids in mammals

Obesity constitutes a major public health problem for the twenty-first century, with its epidemic spread worldwide, particularly in children. The overconsumption of fatty foods greatly contributes to this phenomenon. Rodents and humans display a spontaneous preference for lipid-rich foods. However, the molecular mechanisms underlying this pattern of eating behaviour in mammals remain unclear. The orosensory perception of dietary lipids was long thought to involve only textural and olfactory cues. Recent findings challenge this limited viewpoint, strongly suggesting that the sense of taste also plays a significant role in dietary lipid perception and might therefore be involved in the preference for fatty foods and obesity. This minireview analyses recent data relating to the molecular mechanisms and physiological consequences of this means of orosensory lipid perception.

Molecular mechanisms of fat preference and overeating

Obesity is recognized as a worldwide health problem. Overconsumption of fatty foods contributes significantly to this phenomenon. Rodents, like humans, display preferences for lipid-rich foods. Rodents thus provide useful models to explore the mechanisms responsible for this complex feeding behavior resulting from the integration of multiple oral and postoral signals. Over the last decades, the lipid-mediated regulation of food intake has received considerable attention. By contrast, orosensory lipid perception was long thought to involve only textural and olfactory cues. Recent findings have challenged this limited viewpoint. These recent data strongly suggest that the sense of taste also plays significant roles in the spontaneous preference for fatty foods. This paper provides a brief overview of postoral regulation of food intake by lipids and then highlights recent data suggesting the existence of a "fatty taste" which might contribute to lipid overeating and hence to the risk of obesity.

CD36 gene deletion reduces fat preference and intake but not post-oral fat conditioning in mice

Several findings suggest the existence of a “fatty” taste, and the CD36 fatty acid translocase is a candidate taste receptor. The present study compared fat preference and acceptance in CD36 knockout (KO) and wild-type (WT) mice using nutritive (triglyceride and fatty acid) and nonnutritive (Sefa Soyate oil) emulsions. In two-bottle tests (24 h/day) naive KO mice, unlike WT mice, displayed little or no preference for dilute soybean oil, linoleic acid, or Sefa Soyate emulsions. At high concentrations (2.5–20%), KO mice developed significant soybean oil preferences, although they consumed less oil than WT mice. The postoral actions of fat likely conditioned these preferences. KO mice, like WT mice, learned to prefer a flavored solution paired with intragastric soybean oil infusions. These findings support CD36 mediation of a gustatory component to fat preference but demonstrate that it is not essential for fat-conditioned flavor preferences. The finding that oil-naive KO mice failed to prefer a nonnutritive oil, assumed to provide texture rather than taste cues, requires explanation. Finally, CD36 deletion decreased fat consumption and enhanced the ability of the mice to compensate for the calories provided by their optional fat intake.

Chorda tympani nerve transection impairs the gustatory detection of free fatty acids in male and female rats

Sprague-Dawley rats with intact (SHAM) and bilaterally transected chorda tympani nerves (CTX) received conditioned taste aversions (CTAs) to the free fatty acids (FFAs), linoleic and oleic acid, at micromolar quantities. Two-bottle preference tests showed that CTX eliminated avoidance of 88 muM linoleic acid but did not affect CTA avoidance of corn oil or 250 mM sucrose. Short-duration stimulus tests following single-pairing CTAs revealed that 8-s stimulus durations resulted in higher detection thresholds for linoleic acid than 30-s trials. In these short-duration tests, CTX rats showed 2-fold elevations in threshold for linoleic acid compared to the SHAM rats. A single-pairing CTA did not produce avoidance of oleic acid during the short-duration tests; however, 3 consecutive days of CTA pairings did produce avoidance of oleic acid in both male and female rats. Finally, both male and female rats received SHAM or CTX surgery after demonstrating successful CTAs to either 100 microM linoleic or oleic acid. The ability to detect and avoid linoleic and oleic acid was eliminated by CTX for both sexes. Differences in the ability of rats to form CTAs to linoleic and oleic acid suggest that linoleic acid is a more salient stimulus than oleic acid. Our results suggest that FFAs stimulate afferent taste signals in the chorda tympani nerve of male and female rats and that these signals play an important role in the gustatory behavior of accepting or avoiding taste stimuli following a conditioned taste aversion.

Multiple routes of chemosensitivity to free fatty acids in humans

Selected free fatty acids (FFAs) are documented effective somatosensory and olfactory stimuli whereas gustatory effects are less well established. This study examined orthonasal olfactory, retronasal olfactory, nasal irritancy, oral irritancy, gustatory, and multimodal threshold sensitivity to linoleic, oleic, and stearic acids. Sensitivity to oxidized linoleic acid was also determined. Detection thresholds were obtained using a three-alternative, forced-choice, ascending concentration presentation procedure. Participants included 22 healthy, physically fit adults sensitive to 6-n-propylthiouracil. Measurable thresholds were obtained for all FFAs tested and in 96% of the trials. Ceiling effects were observed in the remaining trials. Greater sensitivity was observed for multimodal stimulation and lower sensitivity for retronasal stimulation. There were no statistically significant correlations for linoleic acid thresholds between different modalities, suggesting that each route of stimulation contributes independently to fat perception. In summary, 18-carbon FFAs of varying saturation are detected by multiple sensory systems in humans.

Evidence for Human Orosensory (Taste?) Sensitivity to Free Fatty Acids

Accumulating evidence suggests dietary fatty acids (FAs) may be sensed in the oral cavity. However, the effective cues have not been characterized. In particular, influences from other sensory cues have hampered identification of an independent gustatory contribution. Experiment 1 examined techniques to minimize the formation of FA oxidation products and improve the homogeneity of water/lipid emulsions to be used as stimuli in Experiment 2, a psychophysical study to determine FA detection thresholds in humans. Through sonication of chilled samples held in polypropylene labware and the addition of 0.01% ethylenediaminetetraacetic acid, calcium disodium salt, homogenous emulsions of unoxidized linoleic and oleic FAs were obtained. Spectrophotometric analysis revealed no oxidation product formation over a 24-h period. Coupled with these techniques, a masking approach was used to minimize other sensory cues imparted from linoleic, oleic, and stearic FAs. Concentration ranges from 0.00028% to 5% (w/v) were prepared in mixtures with 5% mineral oil (w/v) and 5% gum acacia (w/v) to mask lubricity and viscosity effects, respectively. Testing was conducted under red light with nares blocked to eliminate visual and olfactory cues. Oral rinses with 20 ppm capsaicin were administered to desensitize participants to selected irritation effects prior to remeasuring linoleic acid detection thresholds. To determine if the effective stimulus was an oxidation product, oxidized linoleic acid was included among the test stimuli. Detection thresholds were obtained using a 3-alternative, forced-choice ascending-concentration presentation procedure. The mean detection threshold for linoleic acid pre-desensitization was 0.034 +/- 0.008%, for linoleic acid post-desensitization was 0.032 +/- 0.007%, for oleic 0.022 +/- 0.003%, for stearic 0.032 +/- 0.005%, and oxidized linoleic 0.025 +/- 0.005%. The results suggest that linoleic, oleic, stearic, and oxidized linoleic acids are detectable in the oral cavity of humans with minimal input from the olfactory, capsaicin, and viscosity-assessing tactile systems.

Do we taste fat?

Sense of taste informs the body about the quality of ingested foods. Five sub-modalities allowing the perception of sweet, salty, sour, bitter, and umami stimuli are classically depicted. However, the inborn attraction of mammals for fatty foods raises the possibility of an additional orosensory modality devoted to fat perception. For a long time, dietary lipids were thought to be detected only by trigeminal (texture perception), retronasal olfactory, and post-ingestive cues. This minireview analyses recent findings showing that gustation also plays a significant role in dietary lipid perception.

Detection of free fatty acids following a conditioned taste aversion in rats

A gustatory transduction mechanism for free fatty acids (FFAs) has been described in isolated rat taste receptor cells; however, the ability of behaving rats to detect FFAs has not been characterized. Through conditioned taste aversion (CTA) methodology, this study defines the ability of rats to detect and avoid the two principal FFA components of corn oil, linoleic and oleic acid. Following taste aversion conditioning, rats avoided both linoleic and oleic acid at greater than or equal to 66 muM and failed to avoid either 44 muM linoleic or oleic acid. Rats demonstrated generalized avoidances between 88 muM linoleic and oleic acid irrespective of presenting the FFAs as either unesterified acids dissolved in 5 mM ethanol or aqueous sodium salts, sodium linoleate and sodium oleate. Following a CTA to linoleic acid, rats did not show generalized avoidance of NaCl or ethanol, two potentially concomitant tastants in the oral cavity. A CTA to linoleic or oleic acid did produce a generalized avoidance to the other FFA. These results support the ability of rats to detect linoleic and oleic acid (>44 muM) and suggest that the two FFAs share common orosensory properties. Furthermore, it is unlikely that the detection of the FFAs is due to an enhancement of other concomitant tastants such as saliva or the delivery solution.

Taste, salience, and increased NaCl ingestion after repeated sodium depletions

Previous studies have shown that repeated sodium depletions using the natriuretic-diuretic furosemide induce progressive increases in NaCl ingestion. We investigated the role of taste in this behavioral sensitization in Sprague-Dawley rats using short-term lickometer testing along with 2-h stimulated intake tests. Our results show maximal licking across a range of NaCl concentrations after each of the three depletions, regardless of whether the solutions contained sucrose or were presented alone. Similarly, the presence of sucrose did not affect stimulated NaCl intake in long-term tests, although ingestion of NaCl solutions increased progressively with successive depletions. Finally, both licking and ingestion returned to baseline levels during need-free conditions. These results suggest that sodium imbalance acutely increases the salience of sodium taste and thereby the likelihood of NaCl ingestion, which may, in turn, contribute to progressive increases in NaCl intake that occur with multiple furosemide-induced sodium depletions.

Fat intake and energy-balance effects

This paper focuses on the effects of dietary fats or fatty acids on key targets of metabolic intermediates for body-weight control, i.e. satiety, thermogenesis, fat oxidation and body composition. With respect to sensory satiety, it appeared, e.g. that linoleic acid tasters showed a different mechanism for meal termination than non-tasters did. They stopped eating linoleic acid containing food based upon satiety, whereas the non-tasters stopped eating based upon the change in pleasantness of taste. Moreover, in the normal range of body mass index, an inverse relationship was shown between % 'tasters' and BMI. In a high fat diet vs. a low fat high protein high carbohydrate diet, metabolic satiety appeared to be continuously lower and correlated positively to diet-induced energy expenditure. However, with respect to the intermeal interval, satiety appeared to be more sustained following a high fat vs. a high CHO preload, resulting in a lower meal frequency. Covert fat replacement during breakfast by sucrose polyester was successful in combination with dietary restraint, yet overt fat replacement in snacks was successful in the dietary-unrestrained subjects, i.e. those who habitually ate snacks. With respect to fat oxidation, from a respiration-chamber experiment on the effects of diacylglycerol compared (DG) to triacylglycerol (TG) intake, it was concluded that consumption of DG increased fat oxidation and beta-hydroxy-butyrate levels, but did not affect energy metabolism or triacylglycerol level. Parameters of appetite were all lowered by DG compared to TG. With respect to body composition, the effects of 13 weeks CLA supplementation in overweight subjects during weight regain were assessed. Although CLA did not affect %body-weight regain, the regain of fat-free mass was increased by CLA, independently of %body-weight regain and physical activity, and as a consequence resting metabolic rate was increased. At the same time, appetite was reduced and satiety and fullness were increased. In conclusion, we also determined bright sports with respect to fat intake.

Is the control of fat ingestion sexually differentiated?

Sexual differentiation is a fundamental aspect of human physiology [Wizemann TM, Pardue M-L, editors. Exploring the biological contributions to human health: does sex matter? Washington DC, National Academy Press, 2001]. Therefore, this review considers whether the physiological control of eating, as related to dietary fat, is sexually differentiated. The effects of dietary fat are considered from the perspective of stimuli controlling eating that arise from oral, gastric, intestinal, hepatic, and adipose sites. The data reviewed provide substantial support for hypothesis that many such controls of fat ingestion are sexually differentiated in both humans and laboratory animals. Because as yet little is established definitively, however, the apparently most promising questions and methodologies for future work are emphasized.

Is there a parotid-salivary reflex response to fat stimulation in humans?

The perception of fats in foods may involve gustatory, olfactory or textural cues. There is contradictory evidence as to whether the orosensory perception of fat is as a basic quality of taste or related to the physical characteristics of fat. A dose-response reflex parotid-salivary secretion has, however, been shown for the accepted basic taste qualities. The aim of this study was to establish whether varying fat concentration in two food types causes an associated dose-response reflex parotid secretion in humans. Parotid salivary flow was recorded using Lashley cups and cannulae connected to an instantaneous flow meter. Gustatory stimuli were achieved using 3 ml of skimmed (0.1% fat), semi-skimmed (1.7% fat) or full (3.6% fat) milk (Sainsbury) or 5 g of extra-light (5% fat), light (16% fat) or original (24% fat) cream cheese (Kraft). No significant differences in salivary flow rate were shown within the milk group (n=10, P=.93) or within the cream-cheese group (n=11, P=.82). Furthermore, no correlation was observed between increasing fat concentration and flow within either the milk (P=.98) or the cream-cheese group (P=.69; Pearson Product Moment Correlation). These results do not support the hypothesis that there is a fat-specific dose-response parotid reflex.

Gustation as a factor in the ingestion of sweet and fat emulsions by the rat

This paper was written to pay honor to Professor Gerard P. Smith because of his strong influence on me to study the ingestion of sweet and fat mixtures. Three experiments are reported here, in which the laboratory rat was given an emulsion of a glucose+saccharin mixture with corn oil. In the first experiment, a two-bottle, 24-h test was given comparing the emulsion with water. Over 6 weeks, the concentration of the corn oil was gradually increased. When given only food and water, or the glucose/saccharin solution, the rats regulated their caloric intake and grew at a normal rate. In contrast, when the corn oil was present, the rats significantly increased their caloric intake, resulting in a marked increase in body weight. In the second experiment, a detailed analysis of the ingestion revealed that the rate of licking the emulsion during drinking bouts increased in a linear manner as the concentration of the corn oil was increased. In the third experiment, a conditioned taste aversion to the sweet/fat emulsion generalized to the fat more than to the sweet solutions. The implications for a gustatory input are discussed.

Taste, food intake and obesity

Research in human eating behaviour prior to 1990 has shown that taste impacts the palatability and selection of food for intake; sensory-specific satiety; satiation; and thermic effect of food. Research in the last decade has added information to these areas; expanded the field to comparisons of the impact of 'wanting' vs. 'liking' food on intake, and provided insight into the relationship of food intake and brain functioning through new imaging techniques. This article will review literature from the last decade on research in the area of taste and its impact on food intake. Emphasis will be placed on differences seen between lean and obese humans and how these may contribute to the development of human obesity. Suggestions for future research directions will also be discussed.

The taste of fat elevates postprandial triacylglycerol

To explore the contributions of taste and smell stimulation by dietary fat on the postprandial rise of serum triacylglycerol (TAG), 19 healthy, overnight-fasted adults received lipid loads (50 g safflower oil in capsules) followed by oral (i.e., taste and smell) or odor-only stimulation with cream cheese on crackers or no load with odor or no stimulation. In eight participants, lipid loading was followed by taste-only stimulation. Stimulation was provided (where applicable) at 3-min intervals for 60 min and 15-min intervals for an additional 60 min. Blood was drawn at stipulated times. Fat loading with oral stimulation led to serum TAG concentrations that were significantly elevated over baseline at 2, 4, and 6 h. Fat loading with odor stimulation led to a significant rise only at 4 h. No significant elevation was observed with either no-load treatment. Taste-only treatment led to a significantly higher TAG response than all other treatments except loading with oral stimulation. These data provide additional evidence supporting a taste component for human fat perception.

Increased flavor preference and lick activity for sucrose and corn oil in SWR/J vs. AKR/J mice

Nutrient preferences and orosensory responses were characterized in two mouse inbred strains. In two-bottle solution tests (tastant vs. vehicle; ascending concentrations), the effects of strain and chow type (12 or 26% fat) on preference thresholds for sucrose and corn oil were compared in AKR/J and SWR/J mice. SWR/J mice displayed lower preference thresholds and ingested more sucrose than AKR/J mice did. SWR/J mice also showed lower preference thresholds and consumed more corn oil than AKR/J mice did; corn oil preference was suppressed 3.5-fold in AKR/J mice compared with SWR/J mice when fed 26% fat chow. Next, licking was recorded during 30-s access to sucrose or corn oil across a range of concentrations. SWR/J mice licked the tastants more than AKR/J mice did. Analysis of modal interlick intervals during lick training revealed that SWR/J mice licked water faster than AKR/J mice when water deprived, suggesting that motor as well as sensory factors may determine lick responses to tastants in brief-access tests. Finally, in two-bottle tests pitting maximally preferred concentrations of sucrose (8 or 16%) against corn oil (20%), SWR/J mice highly preferred sucrose over corn oil at either sucrose concentration. AKR/J mice preferred corn oil over 8% sucrose but reversed their preference when 16% sucrose was offered. These results support a primary role of flavor in the nutrient preferences of SWR/J mice. In AKR/J mice, the low lick activity for sucrose and corn oil and greater suppression of corn oil preference by the high-fat chow suggest that their preferences depend more on postingestive factors than on flavor.

The evolution of neural coding ideas in the chemical senses

A brief review of the evolution of hypotheses about neural coding in the chemical senses provides some perspective on the current status of these fields, and implications for further development.