Heritable Variation in Fat Preference

For humans, eating is often a group rather than a solitary activity, and it is inevitable when eating with others that individual differences in food preferences become obvious. These food preferences form early in life (Mennella et al., 2001) and persist into adulthood (Nicklaus et al., 2004). People like to eat familiar foods that are safe and avoid foods associated, even indirectly, with illness. However, pleasant experiences and time also help to form food preferences. For instance, the ability to tolerate and even like bitterness increases as children grow to adulthood, and the liking for sweet and sour decreases (Desor and Beauchamp, 1987; Liem and Mennella, 2003). Over a lifetime, new foods are tried, rejected, or incorporated into the diet. Against this backdrop of development and environment, there are inborn differences in food likes and dislikes which may be due to genetic constitution. There is a genetic basis to bitter detection in humans (Bufe et al., 2005) and given that fat intake is moderately to highly heritable, it is likely that genotype contributes to food selection and, by extension, to fat preference. The focus here is on how individual differences in fat preference are formed and, in particular, the evidence that the liking for fat is influenced by genotype. The interest in dietary fat arises because its intake is tied to metabolic syndrome, a constellation of disorders that feature obesity, diabetes, and hypertension. An adage is that everything that tastes good is bad for you, and the liking for fat fits well into this viewpoint: fat is desirable and when people are given the opportunity to do so, many will adopt a high-fat diet. Two aspects of fat make it attractive, its sensory qualities (Reed et al., 1992b) and postingestive consequences (i.e., feelings of satiety). Fat is sensed in the mouth and although the texture is a key feature of its sensory properties, fat itself may be a legitimate taste stimulus. The evidence for this assertion is recent and reviewed below, but it is useful to know that fat has been considered a taste by some through the ages. For instance, Fernel wrote “There are nine classes of flavors, and the sense of taste recognizes no others: acrid, tart, fatty; salty, sour, and sweet; bitter, pungent, and insipid” (Fernelius, 1581). While controversies arise when applying the term “taste” to fat, and the issue is dealt with elsewhere in this book, “umami” as a taste was equally controversial but it was readily adopted as a fifth basic taste once the receptor(s) was identified (Chaudhari et al., 2000; Nelson et al., 2002). Likewise, when the oral receptors for fat are unequivocally identified, its place as a basic taste will probably become equally well accepted. What is known about fat as a taste is outlined below. Taste is both the gatekeeper and advance messenger of ingestion, keeping out bad food and warning the gastrointestinal system about the impending rush of nutrients. One of the effects of fat stimuli in the mouth is to prepare the body for calories, setting off a cephalic phase response. This cascade of events may be a general response to incoming dietary fat in mammals because it is found in rats (Ramirez, 1985) as well as people (Mattes, 2001; Crystal and Teff, 2006). Under normal circumstances, once fat is ingested, it is briefly held in the stomach and then absorbed in the intestines. From here it is either oxidized for energy or stored, primarily in adipose tissue. In some abnormal states, such as untreated diabetes, fat is more easily oxidized than carbohydrate and is thus preferred, at least in experiments using animal models (Tordoff et al., 1987). In addition to the other benefits of fat, it contains pharmacologically active substances, for instance, olive oil has an anti-inflammatory agent (Beauchamp et al., 2005). These compounds may also contribute to the human liking for fat.