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Pre-ingestive and Post-ingestive Influences on Dietary Fat Intake
Virginia Clyburn
Spring, 2003
A Critical Literature Review submitted in partial fulfillment of the requirements for the
Senior Thesis.
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Pre-Ingestive and Post-Ingestive Influences on Dietary Fat Intake | PSY 451, Papers of Psychology
Material Type: Paper; Professor: Pittman; Class: Senior Thesis I; Subject: Psychology; University: Wofford College; Term: Spring 2003;
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Pre-ingestive and Post-ingestive Influences on Dietary Fat Intake
Virginia Clyburn Spring, 2003
A Critical Literature Review submitted in partial fulfillment of the requirements for the Senior Thesis.
Abstract There are various pre-ingestive and post-ingestive factors that can modulate dietary intake and specifically dietary fat consumption. This review on dietary fat consumption will examine pre-ingestive influences such as orosensory cues (texture, taste and perception of dietary fat), PROP sensitivity, age, gender, experience and post- ingestive, which consists of triacylglyceral (TAG) and satiety responses. Evidence reveals that orosensory cues modulate what is consumed by the texture, taste, and perception of the individual. Genetically determined PROP sensitivity has also been found to have a relationship with consumption. PROP tasters have been found to prefer or avoid certain foods. Differences in the perception of PROP tasters and nontasters create variations in their diets as well. Age differences were detected among the age groups of children, adolescents and adults, but no differences were found within these groups. The differences found could account for the discrepancy in the increased consumption of foods high in sugar of children in comparison to adults. Many differences also exist between male and female perception and neurological activity, which causes differences in their diets. Past experience with certain foods is key in determining what is presently consumed. Evidence reveals that experiences perceived as pleasant will likely be repeated while unpleasant experiences will be avoided. The post- ingestive factors that were observed involve humoral signals such as triacylglyceral (TAG) responses and satiety impact food assessment. Specific fatty acids have been found to enhance the postprandial rise of TAG. Satiety is active in helping individuals control their dietary fat intake. Certain foods are found to produce stronger satiation affects, and some of these were foods containing high amounts of fat.
Introduction As obesity develops into a prevailing epidemic in our country today, much of the research has been geared towards establishing determinates of fat consumption. Fat found in foods is designated as dietary fat. There are three types of natural dietary fat— monounsaturated, polyunsaturated, and saturated fats (Rago, 2003). The majority of consumed foods have a combination of these three fats. Dietary fat has an essential role in physiological functioning, but only moderate amounts are necessary. Too much of the same types of fat can be harmful. Rago acknowledges that healthy diets consist of more unsaturated fat than saturated fat. Fats and oils are composed of lipids, whose basic units are fatty acids. Each type of fat or oil has a different combination of fatty acids. Monounsaturated and polyunsaturated fatty acids are two types of unsaturated fatty acids, and should be the primary constituent of fat in one’s diet (Rago, 2003). Monounsaturated fatty acids are found in olive oil, canola oil, and peanut oil (NIN, 2002). Food sources that contain polyunsaturated fats are safflower, sunflower and corn oils, soybeans, many nuts and seeds, and seafood (Rago, 2003). Saturated fatty acids are the most detrimental fats and should be consumed sparingly. Saturated fats are found in animal foods, coconut oil, and palm oil (NIN, 2000). Trans fat is another type that is concentrated in shortening, margarine, crackers and cookies (NIN, 2000). This review examines the factors that influence dietary fat intake. Pre-ingestive as well as post-ingestive influences are included. Pre-ingestive factors include orosensory cues (texture, taste and perception of dietary fat), PROP sensitivity, age, gender, and experience, and post-ingestive factors consist of triacylglyceral (TAG) and satiety responses. One major pre-ingestive influence is taste. Therefore, the reader must be
provided with background information for comprehension of this review. The basic structures and mechanisms of the taste system are key concepts to comprehend. They primarily focus on taste buds, papillae, and transduction. Taste signaling begins when receptors are stimulated on the tongue. On the tongue there are ridges and valleys called papillae, which contain taste buds (Goldstein, 2002). Fungiform papillae are shaped like mushrooms and found at the anterior two- thirds of the tongue. These papillae contain taste buds, which are made up of taste cells. These taste cells have receptor sites where transduction occurs when a chemical stimulates receptors in the membrane of the taste cells (Goldstein, 2002). There are four basic taste categories: sweet, bitter, sour and salty. Each of these categories has a different transduction mechanism. Molecules of bitter and sweet substances bind to the receptor site and stimulate the activation of g-proteins in the cell. In comparison, sour substances contain H+^ ions that block K+^ channels in the membrane. Sodium (Na +) of salty substances flows through the membrane channels directly into the cell. Each transduction mechanism affects the cell’s electrical charge, impacting the flow of ions into the cell (Goldstein, 2002). Each mechanism results in the depolarization of the cell, which facilitates the neurotransmitter release. In addition to the many pre-ingestive influences, such as taste, there are many post-ingestive influences that modulate the consumption of dietary fat. An example of post-ingestive influences is humoral signals released after food consumption. Triacylglycerol (TAG) and cholecystokinin are two important post- ingestive signals of dietary fat. Triacylglycerols are stored lipids that are found in adipose (fat) cells and tissues. The TAGs are highly concentrated forms of metabolic
Texture and viscosity have been found to have a relationship with satiety and the consumption of various foods. It was found that viscosity might affect the amounts of beverages consumed and play a role in the detection of fat. There were also differences discovered in the tactile sensitivity on areas of the tongue. Also, neurons in the cortex of primates have been discovered that response to the texture of fat. A study that focused on the effects of beverage viscosity discovered that viscosity affects an individual’s eating habits (Mattes and Rothacker, 2001). Though both thick and thin shakes resulted in a decrease in hunger and prompted fullness, the more viscous shake resulted in a significant decrease in hunger, which remained lower than the baseline up to four hours after the ingestion of the shake. A more viscous shake produced a lower hunger rating, a decrease in the desire to eat, and promoted fullness in the subjects. In contrast, significant associations were not observed in relation to the changes of hunger and other appetitive ratings following the ingestion of the less viscous shake. This research suggests that larger viscosity in beverages may produce a greater and longer decrease in hunger than less viscous beverages. Mattes suggests that since caloric beverage consumption is high, these beverages may be to blame for the increasing numbers of overweight and obese individuals (Mattes and Rothacker, 2001). This assumption depends on the caloric content of the beverage for if an individual was to consume beverages high in viscosity and low in calories, then weight loss may be a possible result. As the high viscosity beverage creates fullness and lessens the desire to eat, then it seems safe to assume that an individual would consume less energy in response to the perceived fullness.
In addition to psychophysical experiments, physiological evidence supports the interaction between texture and taste. Neurons in the primate cortex have found to be bimodally responsive to the texture and taste of fat in the mouth (Rolls et al., 1999). These neurons responded to pure fats based on their particular texture. More specifically, the neurons responded more intensely to fat in a liquefied and emulsified form rather than fat in a solid form. Neurons in the orbitofrontal region of the monkey cortex that were found to respond not only to fat, but also to substances that were not fat but had a similar texture. Neurons showed an increased firing rate to cream and the fat-like texture (Rolls et al., 1999). Based on the similarities between human gustatory cortexes, it is likely that humans also have these ‘texture specific’ neurons. If humans do possess these neurons then they may account for the satiety effect of a more viscous beverage (Mattes and Rothacker, 2001), for the neurons may receive a stronger stimulation from the more viscous liquefied form of the substance. The existence of ‘texture specific’ neurons would also support the following study’s findings, which suggests that humans can perceive oiliness in salad dressing (a liquefied substance). Two studies show a possible link between PROP sensitivity and sensitivity to texture and tactile cues. To be a PROP taster is to possess the ability to taste the bitter compound 6-n-propylthiouracil. The ability to taste PROP is a genetically inherited trait. The number of fungiform papillae also appears to correlate with PROP sensitivity. PROP tasters have a greater density of fungiform taste papillae than nontasters, with supertasters possessing the highest density (Tepper and Nurse, 1997). One study reveals that PROP medium and supertasters rated a 40% fat sample of salad dressing higher in perceived oiliness than the 10% fat sample, but nontasters could not (Tepper and Nurse, 1997).
substance or object, then perhaps this area of the tongue is the location of the receptors that become activated in response to texture. The ability to perceive different textures is not always clear. One study presents evidence that subjects reported no variance in the perceived creaminess of butter, margarine, and jelly (Tittelbach and Mattes, 2001). The lack of variance reported could be due in part to the fact that the perception of texture was not a main focus of this study. In comparison to this study, another investigation reported that the subject’s perception of the creaminess of butter was similar to fat replacers such as Olestra, Simplesse, and Passelli. When subjects tasted butter and fat replacers there was no significant difference observed in how they rated creaminess (Mattes, 2001b). These results are not in accordance with the studies previously noted in this review. Studies by Mattes and Rothacker (2001), Rolls et al. (1999), Tepper and Nurse (1997), and Yackinous and Guinard, (2002) all suggest that humans may possess the ability to detect the texture of foods and possibly fats; this ability may effect what they consume. Therefore, we must examine the studies that do not support this and see if for any reason the discrepancy can be determined. A possible explanation for this established difference could be that in the studies conducted by Mattes (2001b) and Tittelbach and Mattes (2001) the term ‘creaminess’ was used. These studies used the word ‘creaminess’ when asking the subjects if a difference in texture existed. This word could have had adverse affects on the data, primarily because it requires subjective interpretation. The subjects had to define for themselves what they considered the ‘creaminess’ of the samples. Neither study by Mattes (2001b) or Tittelbach and Mattes (2001) placed a great emphasis on the subject’s sensitivity to the texture of the samples in the experiments. The perception of creaminess
of the samples did not and would not impact the primary results of the investigation. These could all be explanations for the conflicting evidence among the studies. The evidence indicates that texture can affect the amount of food and dietary fat consumed; moreover, texture may assist in the perception of fat in existing foods. Studies found that viscosity effects satiety and food consumption. Viscosity appears to play a role in the detection of fat concentrations in beverages (Mattes and Rothacker, 2001). Differences were discovered in the tactile sensitivity on areas of the tongue, accounting for further differences of texture perception (Yackinous and Guinard, 2002). Physiological evidence was also discovered in that a neuron in the cortex of primates responds to the texture of fat (Rolls et al., 1999). Some conflicting reports may be explained by their lack of rigor in investigating the role of texture. These conflicting findings are derived from one or two ancillary questions regarding the subject’s perception of creaminess. The role of texture was not the main focus and the term creaminess is a subjective term that requires personal interpretation. Perhaps if the questions were re-worded and more thorough, then results would have been consistent with other studies reflecting the overall precept of a role or texture in fat detection and consumption. Influence of Taste on Dietary Fat Intake In addition to orosensory cues of texture influences, there is also evidence that the orosensory cue of taste may play a role in fat detection as well. Some of the research discovered a potential dependence on PROP sensitivity for the ability to detect fat that may suggest a taste component of lipids (Tepper and Nurse, 1997). Moreover, studies on primates have found possible biological evidence supporting the existence of fat taste
The perceptions of the subjects were accurate in determining the sample that contained the highest amount of fat; this sample was butter. This accurate detection of fat composition is proof that humans can accurately detect high-fatty substances (Mattes, 2001b). There is further biological evidence to support the existence of fat perception based on taste in the oral cavity. In primates, neurons in the orbitofrontal cortex have been discovered that respond to fat in the oral cavity. This research discovered several patterns of responsiveness for the “fat detection” neurons. A bimodal neuron was found that responded to the texture and taste of fat in the mouth. Ten out of 1145 neurons in the orbitofrontal region of primates responded to fat stimuli. The neurons that responded to the texture and taste fat had specific stimuli, which they responded to stronger; these include: glucose, NaCl, MSG, water and cream, and complex food like an apple or banana (Rolls, et al., 1999). One study uncovered a possible association between fat perception based on taste and PROP sensitivity (Tepper and Nurse, 1997). Medium and supertasters possessed the ability to discriminate between 40% fat and 10% fat salad dressing, while the nontasters could not (Tepper and Nurse, 1997). If supertasters and medium tasters obtain a keener perception of fat based on taste, perhaps a correlation could be detected between obesity and PROP tasters. If individuals can truly perceive fat by taste, maybe more research in the future could produce a mechanism that would block the transduction mechanism of the fat receptors as a treatment for obesity. Another study did not support these recorded results. This study found no differences in fattiness ratings among the PROP taster groups, except for a sample of
mashed potatoes. The samples were potato chips, chocolate drink, mashed potatoes, and vanilla pudding. The author notes that this finding could be due to the small difference in fattiness rating observed between low- and high fat samples (one of two points on a sixteen point scale) (Yackinous and Guinard, 2002). The reports indicate the possible existence of a taste receptor for fat. Research found that subjects could accurately discriminate between butter and fat replacers (Mattes, 2001b). Biological evidence supports Mattes’ findings in discovering that primates have cortical neurons that respond to the taste of fat (Rolls, et al., 1999). It was also discovered that PROP sensitivity might be a key factor in the ability to detect fat by taste (Tepper and Nurse, 1997). If humans possess the ability to taste fat, this evidence may transform attitudes about the obese. For some obese individuals might have more fat receptors in the oral cavity, which promotes their consumption amounts. New treatment mechanisms would probably be created as well, and perhaps slow down this raging epidemic facing our country today. The taste as well as the perception of fat reveals crucial information in research of what modulates fat intake. Influence of Perception on Dietary Fat Intake Various findings of individual’s perceptions have revealed that perception is a key component in why people consume what they do. Many different variables effect an individual’s perception of foods; some of these include PROP sensitivity and age. We will examine studies that discovered PROP sensitivity and age may influence the perception of individuals. Studies also examined the perception of sweetness and fat. Moreover, reports reveal that subject’s accurately perceived the fat content of butter and
sucrose levels. This study by Degraaf and Zandstra obviously could account for the discrepancy between the diets of children and adults. Many of the snacks and drinks produced for children, tend to be high in sugar, such as cookies and kool-aid. Children appear to consume foods high in sugar on a regular basis in comparison to adults. Perhaps, the manufacturers of these high sugar products target children, because they know that children will continue to consume these foods high in sucrose. The fact that children readily consume sugar and will likely continue doing so may be due to the evidence found in the present study by Degraaf and Zandstra, which found children to be less sensitive to sucrose. The perception of sucrose seems to be the key factor in the difference between the diets of adults and children. If differences exist between adults and children in their perception of sucrose, perhaps differences exist in their perception fatty acids. Individuals in one study perceived greater fat composition of given samples that contained higher concentrations of fatty acids. The subjects that were given a cracker as a vehicle for various stimuli such as jelly, margarine, and butter, reported that the stimulation of jelly and a cracker alone as having the lowest levels of fat in comparison to butter, a saturated fatty acid (SFA), margarine, an unsaturated fatty acid (UFA), and the margarine and jelly combination (Tittelbach and Mattes, 2001). This suggests that the subjects correctly identified the samples with the higher fat content. Perceived sweetness was greater for jelly, a carbohydrate (CHO) and jelly, and a margarine (UFA) combination in comparison to a group that included butter (SFA) and margarine (UFA) alone on the cracker. Perceived sweetness did not differ between the two jellies. This may suggest that fatty acids did not affect sweetness perception. (Tittelbach and Mattes,
2001). Perception was pertinent in correctly identifying the fat content of the samples in this study. Another study, in concordance with the previous findings, reported that subject’s ratings for butter and other fat replacers were similar in response to most perceptions except fat (Mattes, 2001b). Subjects rated all samples, which included butter, Passelli, Simplesse, and Olestra, similarly in respects to sweetness, saltiness, sourness, and bitterness (Mattes, 2001b). Also, all samples were rated equally in palatability. Palatability was the subject’s perceived pleasantness of the sample. However, perception diverged in the subject’s estimation of fat content. Individuals perceived various levels of fat in butter, Passelli, Simplesse, and Olestra. Olestra and butter were rated as containing significantly higher amounts of fat concentrations (Mattes, 2001b). The perceptions of the subjects were correct in identifying butter as having the highest fat content, suggesting that individuals may indeed be able to perceive fat. However, this perception of fat content apparently did not affect the palatability of the samples, for they were all rated equally palatable. An earlier study supporting this concept found that a prior knowledge of the fat content of chips did not affect the subject’s intake (Miller, 2000). This may lead to the conclusion that subjects consume what they perceive as pleasant regardless of fat content. Therefore, individual’s perception of palatability may be a primary determinant of what they choose to consume. Individuals consume what they consider to be appetizing, and this seems to be an intuitive fact. One study determined the percent of palatable foods consumed on a daily basis. Results revealed most self-selected meals consumed are palatable with only 9.3% of the food rated as unpalatable (DeCastro et al., 2000). Therefore, if people consume
postprandial reactions between men and women. Moreover, their brain activity in response to consumption differs. Past experience appears have a large impact on what people will consume. A palatability rating of food that has been consumed determines if that food will be consumed in the future. All four factors influence the dietary intake as well as fat consumption. Studies reveal that PROP tasters are more sensitive to selective bitter and sweet substances, sharp tasting foods, and to the trigeminal irritant capsaicin (Tepper and Nurse, 1997). Medium tasters and supertasters perceived more oral burn from capsaicin. This may lead to the likely assumption that if a supertaster were presented with a food that contained capsaicin, the individual would perceive a stronger oral burning sensation and would most likely refrain from consuming that product. Medium and supertasters appeared to possess the ability to discriminate between 40% fat salad dressing and 10% fat salad dressing. (Tepper and Nurse, 1997). Therefore, PROP may be a determinate in what individuals prefer to consume, and this preference may be dependant on the perceived fat content. Moreover, if supertasters and medium tasters have a keener perception of fat, perhaps a correlation could be established between obesity and PROP tasters. In contrast, one study found no differences in fattiness ratings among the PROP taster groups except with a sample of mashed potatoes. The author notes that this finding could be due in part to the small difference in fattiness rating observed between low and high fat samples (one of two points on a sixteen point scale) (Yackinous and Guinard, 2002). Perhaps the indistinguishable difference in the detection of fat was due to the fact that the increasing flavor levels in the potato chips and mashed potatoes led to an increase the perception of fattiness.
The numbers of fungiform papillae appear to correlate with PROP sensitivity as well. PROP tasters have a greater density of fungiform taste papillae than nontasters, with supertasters possessing the highest density (Tepper and Nurse, 1997). A subsequent study by Yackinous and Guinard (2002) found comparable results to support the notion that supertasters have the highest papillae density. Therefore, greater numbers of fungiform papillae may lead to more transduction in the oral cavity; the result would be a greater perceived intensity, which would effect food consumption. So, PROP sensitivity would then impact food choice and consumption. Differences in tactile sensitivity were observed among the supertasters, medium tasters, and nontasters when the median section of the tongue was stimulated with the no. 2.36 Von Frey filament. The supertasters had the highest tactile sensitivity, followed by the nontasters and tasters (Yackinous and Guinard, 2002). If part of the tongue is more sensitive to touch, then the texture of certain food may result in different effects on various tasters. This study identifies another gustatory cue that could produce differences among dietary fat consumption related to PROP sensitivity. Another attribute that may contribute to dietary intake is age. DeGraad and Zandstra (1999) discovered that age has an impact of the perception of sucrose. Children, age’s nine to ten, were found to have a lower slope of psychophysical function (less sensitive), of the perceived sweetness intensity of sucrose than adolescents. Therefore, it takes larger amounts of sucrose for children to perceive the same amount of sucrose perceived by adults. Adolescents appeared to be less sensitive to sucrose than adults. So, children and adolescents in comparison to adults require more sugar to reach a preferred level of sweetness. Children were less able to distinguish between different sucrose