11.1. INTRODUCTION

Fat plays a unique role in the human diet. In addition to being the most concentrated source of dietary energy, fat contributes to the texture, flavor, and aroma of a wide variety of foods. In general, the most palatable foods are those that are both energy-dense and high in fat content (Drewnowski, 1997a,b). The taste, smell, mouthfeel, and hedonic properties of fat all contribute to the popular concept of fat “taste” (Drewnowski, 1997a).

Fat is one reason why palatability and energy density of foods are closely intertwined (Drewnowski, 1997a,b). Energy density of foods is largely determined by their water and fat content (Drewnowski, 1998). By providing weight without energy, water has more impact on energy density of foods than does any macronutrient, even fat (Drewnowski, 1997a). Together, water and fat account for over 95% of the variance in the energy density of foods in the food supply.

Energy-dense foods and diets have been associated with higher energy intakes (Andrieu et al., 2006) and with higher prevalence of obesity and the metabolic syndrome (Mendoza et al., 2007). By all reports, the energy density of the Western diet is on the rise, as is the consumption of dietary fats (Drewnowski and Popkin, 1997; Popkin et al., 2001; Drewnowski, 2005). In the United Kingdom, dietary fat accounted for an average of 36% of dietary energy for men and 35% for women (Henderson et al., 2003). In France, dietary fat accounted for 37%–42% of dietary energy, with saturated fat providing 16% (Drewnowski et al., 1996; Perrin et al., 2002; Astorg et al., 2004). In the United States, dietary fat accounted for 33%–36% of dietary energy (Allred, 1995; CDC, 2004). Despite public health efforts to lower fat consumption, global consumption of both animal and vegetable fats continues to rise (CDC, 2004; Amuna and Zotor, 2008). Innate preferences for dietary fats seem to be tempered only by incomes.

As incomes rise, developing nations typically replace plant-based diets with more animal fats, vegetable oils, and caloric sweeteners, a phenomenon known as the “nutrition transition” (Drewnowski and Popkin, 1997). Paradoxically, developed nations recommend replacing fat-rich foods with water-laden grains, vegetables, and fruit in order to increase bulk and so reduce the energy density of the diet. Recommendations to reduce dietary energy density are one way to reduce energy intakes in an effort to address the global obesity epidemic (WHO Report, 2003).

The problem is that the palatability and enjoyment of foods are often tied to their energy density and therefore fat content. Energy-dense foods that are rich in fat are more palatable than are many low-energy-density vegetables and fruit (Drewnowski, 1998). High-fat foods, many containing sugar or salt, have an undeniable sensory appeal and are difficult to resist (Folkenberg and Martens, 2003). Energy-dense high-fat diets are consumed in preference to plant-based diets of grains, pulses, and legumes.

There are many explanations for why humans like fat (Drewnowski, 1997a,b). Several physiological mechanisms have been proposed, many of which are based on the strong links found between fat content, palatability, satiety, and energy density. The orosensory properties of fat or fat “taste” seem to be perceived through a combination of taste, texture, and olfaction (Drewnowski, 1997a; Schiffman et al., 1998). Fat is a concentrated source of energy with rewarding postingestive effects (Drewnowski, 1995). The learning of food preferences may be based on associating sensory attributes with the physiologic consequences of ingestion, such as satiety and well-being (Birch, 1999).

Parallels have been drawn between sugar and fat preferences. Sensory preference for sweet taste is present at birth, and the “sweet tooth” of early childhood helps to introduce new foods into the children’s diet (Birch, 1999). Children learn quickly to prefer flavors associated with high-energy content and begin to select high-fat foods early in life (Johnson et al., 1991; Birch, 1992). Genetic predisposition, metabolic needs, and behavioral or emotional factors can influence the liking for fats. Human preferences for fat-rich foods may also be influenced by economic factors and sociocultural values (Drewnowski, 1995; Drewnowski, 1997a; Tuorila and Pangborn, 1988).

This chapter will cover recent advances in the understanding of the genetic, physiological and behavioral factors related to the perception of and preference for fat in humans. As will be discussed further, human liking for fats may be a consequence of evolutionary pressures to select energy-dense foods to assure nutrition and survival (Drewnowski and Rock, 1995).

11.2. PERCEPTION OF FAT-RICH FOODS: TASTE, OLFACTION, AND TEXTURE

Sensory perception of foods involves the initial stage of chemosensation, which includes the detection of tastants, odorants, and textural attributes of foods, followed by the integration of sensory signals by the brain. Sensory processes begin with the placement of food in the mouth, the fracturing of the food by the teeth and its dilution with saliva, oral perception of temperature and texture, and the binding of taste and flavor molecules to receptors in the oral and nasal cavities. Activated receptors act on secondary messengers or other systems, sending a signal through the sensory nerves to the brain. Upon integration of sensory input, we become aware of the taste, aroma, and texture of foods (Chandrashekar et al., 2006; Engelen and Van der Bilt, 2008). The hedonic component of sensation, known as palatability or the “pleasantness” of food is an integral part of a complete sensory profile for a given food (Yeomans, 1998).

Fats are perceived by a variety of sensory mechanisms (Drewnowski 1989a; Mattes, 2005). The concept of food “taste” as understood by consumers includes the chemical sense of taste (gustation); smell/aroma (olfaction), and the perception of food texture in the mouth. The first sensory response tends to be the olfactory perception through the nose or mouth of fat-soluble volatile flavor molecules (Drewnowski et al., 1989; Drewnowski, 1990a). Texture, defined as the overall measure of oral sensations associated with placing food in the mouth (Drewnowski, 1997a), adds to the sensation of fatty taste (Drewnowski et al., 1989). The physical form that fat takes at different temperatures can also influence mouthfeel and contribute to the enjoyment of high-fat foods.

Traditionally, studies in sensory psychophysics have examined human taste acuity (detection and recognition thresholds) or taste sensitivity (intensity scaling), using aqueous solutions of sweet, salty, sour, or bitter compounds. However, such techniques have not been applied to the study of fats in foods. One reason is that the chemical structure of triglycerides makes them insoluble in saliva such that the delivery of fat molecules to receptors in aqueous solutions is not possible. Therefore, many early studies have focused on the texture of fat-containing foods in relation to food quality and food preference.

11.2.2. Perception and Preference for Fat Texture

Fats endow foods with some key textural properties, including viscosity (thickness) and lubricity (slipperiness or oiliness) (Drewnowski and Greenwood, 1983; Mela, 1988; Schiffman et al., 1998). Food texture studies focused on the viscosity, elasticity, and orientation and elongation of food particles, often in relation to food acceptance. Texture studies showed, for example, that consumer preferences for yogurts and other dairy products were driven by perception of smooth or creamy textures that depended on fat content (Folkenberg and Martens, 2003).

The classic definition of food texture (Szczesniak, 1963) held that the perception of product texture arises from the dynamic interaction between the food and the consumer, occurring along the dimension of time. Multiple senses were involved. Whereas oral sensation or mouthfeel refer to the sense of touch, the perception of texture could also involve both sight and hearing, and the manner in which the food responds to the applied forces (Engelen and Van der Bilt, 2008). Another definition held texture to be a dynamic interaction with time between the food’s physical properties and the senses of touch, sight, and hearing (Engelen and Van der Bilt, 2008).

Mastication and swallowing played an essential role in the dynamic perception of texture in semisolid foods (Engelen and Van der Bilt, 2008). Semisolid is a texture represented by such foods as sauces, mayonnaise, cream, spread cheeses, ice creams, and margarines. The presence of particles, the size of particles, and their orientation are important, but only when combined with physiological processes taking place in the oral cavity, such as salivation (saliva flow, rate, and composition), tongue movements, and temperature exchanges between food and oral cavity (Engelen and Van der Bilt, 2008).

Because fats endow foods with a wide range of taste and texture properties, it is difficult to distinguish which particular oral sensations contribute to perception of fat content. Sensory evaluation of fats in solid foods is further complicated by the fact that the fat may be an integral part of the food (such as marbling in meat) or fats may be added during preparation. Fat globules in dairy products cannot be individually perceived in the oral cavity, but the substance may feel creamy (Cooper, 1987). Fat in frozen dairy products contributes to creaminess and smoothness by preventing the formation of large ice crystals. The principal sensory cue for fat content in milk or cream is stimulus thickness, smoothness, or viscosity (Drewnowski, 1987). The fat content of dairy products (Mela, 1988), peanut butter, margarine, and mayonnaise were also associated with thickness, smoothness, and viscosity (Kokini et al., 1977; Cussler et al., 1979). Temperature of the food, or temperature of the oral cavity does not seem to influence perception of fat content of oil-and-water emulsions (Mela et al., 1994a); but particle size (degree of emulsification) does, with smaller particle size slightly enhancing perception of fat content (Mela et al., 1994b).

Some of the terms used to characterize the perception of texture in the oral cavity, and particularly fat texture, and their definitions are summarized in Table 11.1. Table 11.2 shows how descriptive terms, classified into different categories may apply to characterizing the mouthfeel of beverages.

TABLE 11.1

Terms Used to Characterize the Oral Sensation of Dietary Fats

TABLE 11.2

Terms Used to Characterize the Mouthfeel of Beverages

The general foods (GF) Texture Profile was created to standardize evaluation criteria for product texture (Brandt et al., 1963; Szczesniak et al., 1963). This model evaluated sensory properties of foods over time, from placement in the mouth to complete mastication. It identified primary mechanical characteristics of food such as hardness, cohesiveness, adhesiveness, and viscosity; and secondary characteristics such as brittleness, chewiness, and gumminess. Further, geometrical characteristics were defined based on the orientation, shape and size of the food particles (gritty, grainy, and coarse), and mouthfeel characteristics were defined based on perception of moisture or fat (wet, oily, and greasy). The use of the texture profile entailed the introduction of a number of anchored rating scales (Szczesniak et al., 1963), such as for hardness, fracturability, chewiness, gumminess, adhesiveness, and viscosity. Each scale represented a wide range of textures, so that many different foods could be evaluated (reviewed in Drewnowski, 1991a). It is important to note that some of these descriptors are not hedonically neutral, i.e., while “creaminess” may be a desirable property, “greasiness” generally is not. Table 11.3 summarizes the GF Texture Profile scales for two attributes, viscosity, and adhesiveness, together with product anchors.

TABLE 11.3

Descriptors for Oral Sensations Associated with Fat and the Viscosity Scale from the General Foods Texture Profile

Different attribute scales reflect different aspects of fat texture perception. In one study, 25 normal weight men and women rated seven liquid dairy products along multiple 9-point attribute scales. Attribute scales included the standard texture and mouthfeel terms above as well as more abstract evaluations of the energy density of the sample (rich, fattening). There was a log-linear relation between attribute ratings and stimulus fat content, best seen when the graphs were plotted on a logarithmic scale. These data are summarized in Figure 11.1.

FIGURE 11.1

Relation between texture attributes and fat content of fluid dairy products. Sensory evaluation was carried out using 9-point category scales for each attribute, where 1, “not at all –” and 9, “extremely –”. Subjects (more...)

Fat can also affect the texture of cooked or processed food. Frying in fat brings the food to temperatures above the boiling point of water and contributes to crispiness or crunchiness in cakes, pastries, and cookies. Fat contributes to freshness and moisture by binding water molecules (Cooper, 1987). Appropriate terms for describing those texture attributes were also a part of the GF Texture Profile.

The reliance on texture cues means that an illusion of fat content can be created by making the stimulus viscous, either by gelling or by the addition of hydrocolloid thickeners. This becomes far more complex when the stimulus is solid as opposed to liquid. Drewnowski and Schwartz (1990) asked 50 young women to rate the sweetness and fat content of 15 stimuli resembling cake frostings. The samples were composed of sucrose (20%–77%, wt/wt), polydextrose (a bland, partly metabolizable starch), unsalted butter (15%–35%, wt/wt), and water.

Figure 11.2 (left panel) shows that the perception of sweetness was largely unaffected by fat content. Sweetness ratings accurately tracked increasing sweetness concentration. By contrast, the perception of fat content plunged as the sugar content increased (right panel). Perceived fatness was a combined function of fat and sugar contents that was heavily dependent on product texture. The addition of water caused a sharp decrease in fatness ratings, with the sweetest stimuli, being rated as lowest in fat content. The presence of sugar clearly distorted the ability to perceive the fat content of solid foods (reviewed in Drewnowski, 1991a).

FIGURE 11.2

Ratings of perceived sweetness intensity (left panel) and fat content (right panel) as a function of stimulus sucrose and fat levels. (From Drewnowski, A. and Schwartz, M., Appetite 14, 203, 1990. With permission.)

However, preferences for fat may be independent of the conscious ability to detect or assess the fat content of solid foods. Another study compared the ability of 25 young men and women to rate the sweetness, creaminess, and fat content of solid and liquid foods of differing sugar and fat content (Drewnowski et al., 1989). Liquid dairy products included sweetened skim milk (<0.5% fat), whole milk (around 3.5% milk fat), half-and-half (12.5% milk fat), and heavy cream (36% milk fat). Solid foods consisted of cottage cheese, cream cheese, or the two blended together to produce mixtures of comparable fat content (0.1%–52%). The sweetened blends were spread over white bread and cut into small pieces to produce the solid food samples (Drewnowski et al., 1989). Subjects reliably estimated sweetness intensity for both solid and liquid samples, but they were unable to track the increasing fat content of the solid foods. Preferences were estimated using the 9-point hedonic preference scale (Peryam and Pilgim, 1957). Hedonic preferences for high-fat samples remained high despite difficulties in estimating stimulus fat content.

Despite impaired ability to track the fat content of solid foods, as opposed to liquids, subjects generally preferred stimuli with the higher fat content (Drewnowski, 1989a). Sensory assessment of fat content in foods can be a challenge, since it relies so much on the integration of multiple interrelated sensorial signals. Further, such assessment is system-specific, and may be affected by other product attributes, such as sugar content. Finally, given that the perception of food texture is driven by the interaction between the food and the consumer, it is not surprising that humans sometimes lack an accurate, conscious judgment of how much fat is contained in the food (Blundell and McDiarmid, 1997; Schiffman et al., 1998).

11.3. INTERACTIONS BETWEEN FAT, SUGAR, AND SALT

Fat content contributes to the food’s acceptability, palatability, and enjoyment. The creation of fat replacement products with a desirable and well-rounded sensory profile is a particular challenge for food chemists (Drewnowski, 1990b). Depending on the product, desired sensations may range from creaminess and smoothness (in dairy products), to tenderness and moisture (in cakes), to crispiness and crunchiness (in baked or fried foods). A particular hedonic synergy is obtained by pairing sugar and fat (Drewnowski and Greenwood, 1983; Drewnowski, 1991a, 1995; Schiffman et al., 1998).

When it comes to sweet liquids or solids, the hedonic response typically follows an inverted U-curve as preferences increase with added sweetness and then decline when the product is perceived as too sweet. The sensory optimum, usually reached at 8%–10% of sugar in aqueous solutions, is generally referred to as the “hedonic breakpoint.” Some individual variation is observed as hedonic responses to sugar may rise and then decline (type 1 response) or rise and plateau (type 2 response). In general, children prefer much sweeter solutions than do adults and show little evidence of a hedonic breakpoint. By contrast, hedonic response curves for older adults may actually decrease as a function of sugar concentration (Drewnowski, 1991a). The hedonic breakpoint for solids is found at a much higher sugar concentration than for liquids.

Sensory evaluations of mixtures of sweetened dairy products of variable sugar and fat concentration showed no evidence for a hedonic breakpoint for fat. In these studies, normal weight men and women were asked to taste and rate 20 mixtures of milk cream and sugar. Sugar content varied from 0% to 20%, whereas fat content varied from 0.1% to 36% (Drewnowski and Greenwood, 1983). Subjects were asked to rate sweetness, creaminess, perceived fat content, and preference (hedonic response) on a 9-point category scale. Later studies, based on identical design were conducted with massively obese patients, patients who had recently lost weight and normal weight controls (Drewnowski et al., 1985).

The sensory system was highly interactive when it came to hedonic preferences. Whereas hedonic ratings for sweetness peaked at 8%–10% sugar, depending on fat concentration, no sensory breakpoint for dairy fat was observed. There was also a synergy between sugar and fat: highest preference ratings were obtained for light cream with sugar (Drewnowski and Greenwood, 1983). Hedonic preference ratings for obese patients, as compared to normal weight controls, are shown in Figure 11.3. The graphs illustrate that obese patients disliked high concentrations of sucrose in skim milk, but showed increased acceptance when the sucrose was presented in heavy cream. For both normal weight and for obese subjects, hedonic preferences for taste stimuli were a joint function of sugar and fat content.

FIGURE 11.3

Hedonic preference ratings for dairy products sweetened with different amounts of sucrose. Ratings are for obese patients and normal weight controls. (Data from Drewnowski, A. et al., Am. J. Clin. Nutr. 61, 1206, 1995.)

Modeling hedonic responses to a two-dimensional sensory system required the use of a mathematical modeling technique, the response surface method (RSM) (Drewnowski and Greenwood, 1983). This analytical procedure can be used to predict and plot the response to multiple stimuli for any combination of ingredient levels. Used in food optimization studies, RSM can determine the combination of ingredients that yield the most preferred product and can graph hedonic optima in three dimensions (Drewnowski and Greenwood, 1983; Drewnowski et al., 1985, 1987b). A contour map, sometimes called “isobologram,” joins points representing the same level of response to variable stimulus levels of sugar and fat (Gessner, 1995). Drewnowski and Greenwood (1983) showed that the peak hedonic response was obtained at high, but not highest, fat and sugar content (8% sugar and 20% fat). There was a strong interaction between the sweet and the fat component. Whereas sweetened skim milk and unsweetened heavy cream were not highly preferred, highest hedonic ratings were obtained for sweetened light cream (reviewed in Drewnowski, 1995).

Figure 11.4 (top panels) shows the three-dimensional representation of the hedonic response surface for obese patients, reduced obese patients, and normal weight controls. Isopreference contours are shown directly below (bottom panels). The data show that reduced obese patients had the highest hedonic ratings, selecting high energy density stimuli that were highest in sugar and fat content (Drewnowski et al., 1985).

FIGURE 11.4

Hedonic preference ratings for sweetened dairy products of differing sugar and fat content. The data are for normal weight subjects, obese patients, and reduced obese patients. Data are presented as three-dimensional hedonic response surfaces (top panels) (more...)

Sensory responses to sugar/fat mixtures were further explored as a function of extremely low body weight (Drewnowski et al. 1987a,b). Those clinical studies investigated female patients with anorexia nervosa and bulimia nervosa, as compared to normal weight age-matched female controls. A negative relation (r = −0.43) between sugar/fat ratios and degree of overweight confirmed clinical reports of differential response to dietary fat by anorectic and obese women. Whereas anorectic patients tended to reject fat-rich foods, obese patients, on the contrary showed enhanced fat preferences. An association between high preference for fats and overweight had also been shown in other studies (Mela and Sacchetti, 1991; Drewnowski and Holden-Wiltse, 1992). Formerly obese patients maintaining weight loss showed enhanced sensory preferences for both sugar and fat (reviewed in Drewnowski, 1991a).

A majority of laboratory studies have supported the link between obesity and liking for higher fat foods (Drewnowski and Greenwood, 1983; Drewnowski et al., 1985; Mela and Rogers, 1998; Rissanen et al., 2002; De Graaf, 2005). However, the findings were not universal (Salbe et al., 2004). Obese persons tend to consume more energy-dense diets and formerly obese subjects show enhanced responsiveness to fat (Drewnowski et al., 1985; Cox et al., 1999; Rissanen et al., 2002). However, some cross-sectional and longitudinal studies failed to show a convincing link between BMI values and self-reported liking for high-fat foods.

Sensory studies on fat and salt mixtures are extremely limited. Warwick and Schiffman (1990) studied sensory responses for fat/sugar and fat/salt mixtures in young and elderly subjects of different body weights. The stimuli contained 0.5%–36% fat by weight, and 0%–20% sucrose, or 0%–0.6% NaCl. Younger subjects preferred sweeter stimuli, consistent with previous studies (Drewnowski, 1991a; Cox et al., 1999). Hedonic ratings of elderly subjects were not influenced by fat content. Younger subjects showed a link between preferences for fat/salt mixtures but the elderly did not.

11.4. MECHANISMS UNDERLYING PREFERENCES FOR FAT: PALATABILITY OR SATIETY?

There is no question that dietary sugars and fats are associated with overconsumption of energy. Whether this is due to excess palatability or deficient satiety is an unresolved issue. On one hand, fat and sugar mixtures are among the most preferred of foods (Drewnowski and Greenwood, 1983; Drewnowski, 1991a). The foods most preferred by young children are energy-dense, familiar, and sweet. Typically, such foods include chocolate, cookies, jellies, and candy (Drewnowski, 1989a). High-fat foods were not only the most preferred, but induced the least neophobia (Rankin and Mattes, 1996). Children quickly learned to select flavors associated with high-energy content, that is, foods rich in fat (Drewnowski, 1989a; Johnson et al., 1991; Birch, 1992).

Highly preferred foods can stimulate consumption independent of energy deficit or perceived hunger (Sørensen et al., 2003). Palatable foods can stimulate appetite and eating rate, leading to excessive food consumption (Yeomans et al., 1997, 2001, 2004). This overriding of hedonic responses over metabolic responses has been termed passive overconsumption (Blundell and Mcdiarmid, 1997), or more recently, nonhomeostatic eating (Mela, 2006).

Research has begun to integrate the homeostatic (energy) and the hedonic (pleasure) components of appetite control, assigning a role to weight status. Mela (2006) has noted that obesity seems to be linked to an increased “wanting” of highly palatable foods (motivation for eating) that is not necessarily linked to higher “liking” of those foods. Studies by Blundell et al. support the notion that wanting (how hard you are prepared to work for a food) and liking (the actual pleasure effects of eating the food) are two independent components of appetite, which can be measured separately (Blundell and Finlayson, 2004; Finlayson et al., 2007).

Studies on endogenous opioid peptides seem to support the hedonic or “liking” viewpoint (Reid, 1985; Cooper et al., 1988; Drewnowski et al., 1992a, 1995; Will et al., 2004; Ward and Dykstra, 2005). Endogenous opioid peptides are pleasure-enhancing molecules produced in the human brain. Early studies in rodents identified the link between endogenous opioid peptides and sugar and fat intake (Blass, 1987). Later clinical studies (Drewnowski et al., 1992a, 1995) examined sensory preferences for fat and sugar following administration of opioid peptide agonists (butorphanol) and antagonists (naloxone).

Those studies were conducted in normal weight and obese bulimic patients and nonbinging controls. Bulimic women consumed more sweet, high-fat foods under laboratory conditions than did nonbinging controls. Following naloxone administration, taste preferences for sugar/fat mixtures were suppressed by the opiate blockade. Food consumption of sweet high-fat foods (mostly chocolate) was also selectively suppressed, but only in bulimic females. The conclusion was that the opoid antagonist naloxone selectively affected the hedonic component of overeating: Stronger suppression was obtained for sweet- and fat-containing foods than for bland breadsticks and popcorn. Since sensory preferences were affected as well, the conclusion was that naloxone exerted its effects by reducing the hedonic appeal of palatable foods. By contrast, no differences in sensory or consumption responses were obtained between obese women and women of normal body weight. In retrospect, body weight was unrelated to the wanting of palatable foods, at least, under those particular laboratory conditions.

Butorphanol, an opiate agonist, administered in very small doses, had no impact on taste preferences or food selection. These clinical studies showed that sensory preferences for sweet and high-fat foods did involve opioid peptides that are implicated in food reward. Naloxone had less effect on foods that were high in either sugar or fat than on foods that contained both. The sweet high-fat foods, mostly chocolate, were the most preferred, most palatable combinations. In other studies also, naltrexone, another opioid antagonist, only had an effect on the consumption of most preferred foods (Yeomans and Gray, 1996).

Considering the growing body of research linking food reward with the endogenous opioid system in humans (Reid, 1985; Drewnowski et al., 1985, 1992a; De Araujo and Rolls, 2004; Davies et al., 2007) and animals (Will et al., 2004; Mizushige et al., 2006, 2007, Sclafani, 2007), it seems safe to conclude that both oral (including hedonics) and postingestive (including neuropeptides) stimuli are likely to play an interactive role and that hedonic factors are likely to override homeostatic controls on food intake at least under particular psychological conditions. In particular, dopamine, a neurotransmitter involved in the reward response, has been implicated in fat overconsumption and positive energy balance in humans (Davis et al., 2007). Low levels of brain dopamine have been shown to predict overeating and obesity in women (Goldfield et al., 2007). Dopamine has also been implicated in the concept of food addictions, generally involving sweet and high-fat foods (Drewnowski and Bellisle, 2007).

Additional information has come from recent neuroimaging studies. The anorectic peptide YY (PYY), associated with satiety, may downregulate preferences for high-fat foods in laboratory animals (Chandarama and Batterham, 2008). Functional brain imaging studies indicate that an exogenous analog to PYY is able to activate the orbitofrontal cortex; reduce motivation for seeking high-fat foods, and induce weight loss. These findings highlight the tightly interconnected roles for the hedonic and homeostatic pathways. In fact, recent studies in animals have shown that neurons in the homeostatic (“metabolic”) and reward (“cognitive”) brain areas are in close anatomic contact (Zheng et al., 2007).

Satiety is another mechanism advanced to explain human overconsumption of fats. Fats are also said to be less satiating than either carbohydrate or protein (Blundell et al., 1993; Rolls, 1995). The last point is something of a puzzle, since fats contain more energy per gram than does protein; slow down gastric emptying, and very effectively delay the onset of the next meal. Furthermore, palatability and satiety are generally measured in terms of food consumption. Higher intakes mean that a food is palatable; lower intakes mean that it is satiating. By definition then, palatability is the inverse of satiety, such that having a satiating yet palatable food is a contradiction in terms.

Distinguishing between satiety and satiation resolved the paradox. Whereas satiation refers to internal sensations responsible for terminating the current meal, satiety refers to processes that influence the time delay until the following meal and subsequent meal size. Fats were decreed to have an impact on satiety but not on satiation, neatly sidestepping the paradox. Of course, this makes it impossible to distinguish between high palatability and low satiation, especially since they are both measured in terms of food consumption and meal size.

The concept of sensory-specific satiety (SSS), first described by Rolls et al. (1981), integrates current pleasure and past experience. The current response may be influenced by postingestive consequences of recently consumed foods. SSS is defined as a decrease in pleasantness for foods just eaten, without a decrease in pleasantness for new foods (Rolls et al., 1981). The salient sensory mechanism may be taste, but flavor/aroma, texture, and even color may contribute as well (Johnson and Vickers, 1992; Guinard and Brun, 1998). Sensory satiety can be prolonged, lasting from up to 24 h (single exposure) to 2 weeks for repeated exposures (Kamphuis et al., 2001; Weenen et al., 2005). It is not clear if ingestion of calories is even necessary to induce sensory satiety. Drewnowski et al. (1982) had called a similar phenomenon flavor fatigue, as it applied to reduced preferences following repeated tasting without ingestion of a variety of soft drinks.

The evolutionary significance of sensory satiety may lie in variety seeking and the drive to secure a diet containing all essential nutrients (Rolls, 2007). Exposure to a wide variety of palatable foods, representing an array of tastes, flavors, and textures, has led to higher food intakes in rats (Rolls et al., 1983) and may reduce SSS in humans (Romer et al., 2006). However, studies have found that taste can have a stronger effect on SSS than fat content (Snoek et al., 2004) and texture (Guinard and Brun, 1998). A study of pleasantness ratings comparing ham sandwiches made with soft versus crusty bread and apples versus apple sauce showed that subjects could experience texture fatigue, independent of the taste fatigue. Another study of potato chips (Maier et al., 2007) showed that flavor was a strong component of SSS in salted high-fat foods; however different textures were not compared.

The distinction between palatability and satiety makes the argument that fats are highly palatable but have little impact on satiation (as distinct from satiety). However, in purely practical terms, palatability and satiation are inextricably linked since both are measured in terms of meal size and food consumption. Studies have failed to show differences in SSS profiles between obese subjects and lean controls (Snoek et al., 2004; Brondel et al., 2007).

The newer distinction between wanting and liking attempts to account for the fact that the obese are equally sensitive to taste sensations and like high-fat foods not more that lean controls, based on hedonic ratings and self-reports. Yet the obese appear more susceptible (or less resistant) to the drive to eat, that is, to the wanting component of appetite for energy-dense high-fat foods. Judging by published reports, this wanting seems to be unconscious and distinct from cognitive, conscious wanting, and may be related to the postingestive effects of eating pleasurable food itself, rather than oronsensory stimulation (Berridge, 2004; Mela, 2006). In addition to liking and wanting, it is evident that learned behaviors (Herman et al., 2005; Brunstrom, 2007), as well as external associations and cues (cultural background, time, and social environment for example) also modulate eating behavior.

11.5. FAT PREFERENCES AND AVERSIONS IN EATING DISORDERS, STRESS, AND OVERWEIGHT

11.6. CONCLUDING REMARKS

The aroma, texture, and mouthfeel of foods are often influenced by their fat content. Human preferences for fat in foods shape dietary choices and may affect energy balance and the long-term regulation of body weight. Understanding the mechanisms by which we detect, perceive, and enjoy fat-rich foods may help us predict food choices and their multiple influences on diets and health.

This chapter explored the perceptions and preferences for dietary fats, using findings from genetic, physiological, and behavioral research on food preference. Traditionally, studies on the sensory perception of fat have focused on diverse aspects of food texture; however, the response to fats also depends on taste and olfaction and on synergistic interactions between fats and other food components. Given that fat in foods can give rise to a variety of textures and flavors, it can be difficult to decide which particular percept is most closely associated with fat content. Fat perception is also influenced by physical form (liquid or solid) and by the presence of other gustatory cues, such as sweetness.

Genetic, developmental, attitudinal, and physiological factors also influence fat preferences. Fat preferences are shaped by postchildhood experiences, remain stable in adult life and do not decline with age. Hedonic preferences for fat-containing foods among adults are influenced by multiple sensory stimuli. In particular, studies have identified a hedonic synergy in sugar/fat mixtures. Mixtures of cream and sugar were universally better liked than sweetened skim milk or unsweetened heavy cream (Drewnowski and Greenwood, 1983). Although studies have generally failed to link sugar consumption to overweight, several cross-sectional studies have pointed to an association between overweight and excess consumption of dietary fats.

Human preferences for energy-dense sweet and high-fat foods may have evolved for reasons of survival. The relative contributions of hedonic (preference or “liking”) versus homeostatic (physiological or “wanting”) processes in determining preference and choice for high-fat foods have not been defined (Mela, 2006). The hedonic response to fat seems to be strongly linked to the endogenous opioid reward system. This reward system may be independent from motivation to eat at least in the obese population, contributing to increased energy intakes.

Our understanding of fat chemosensation has helped to model the development of a preference for dietary fats in humans. Physiological and gender variations in fat consumption need to be examined further in the context of food culture. The amount and type of fat consumed is strongly influenced by geography, culture, and cost (Perrin et al., 2002; Serra-Majem et al., 2007). Fat consumption is a classic example of how human innate preferences have been aligned with current trends in the food supply and the economics of food choice.

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