It is conceivable that enhancing both the rate of fat digestion, and initial gastric emptying, increases the delivery of FAs to the small intestine, and as a result, potentiates the effects of fat on GI function and energy intake. One way of enhancing fat digestion is to reduce the size of fat droplets in an emulsion. This has a profound effect on lipolysis since it increases the surface area available for the binding of lipases. For example, duodenal lipolysis of a lipid emulsion containing fine droplets (0.7 μm in diameter) is ~30% greater than that of a lipid emulsion containing coarse droplets (10 μm in diameter) (Armand et al., 1999). In a recent study in our laboratory, we were able to demonstrate that the smaller the droplet size of a lipid emulsion (0.26 μm vs. 30 μm vs. 170 μm, infused isocalorically directly into the duodenum), the greater the effect on the stimulation of pyloric pressures, CCK and PYY secretion, and the suppression of hunger (Seimon et al., 2009). Similarly, enhancing the acid stability of a fat emulsion results in an initially rapid phase of gastric emptying that increases the exposure of the small intestine to fat and its digestion products (Meyer et al., 1996, 1999). Acid-stable emulsions have been shown to increase plasma CCK secretion and fullness, and suppress hunger, more potently than acid-unstable emulsions, in which the oil rapidly layers above the aqueous components and, as a result, empties into the small intestine much more slowly (Little et al., 2007; Marciani et al., 2007, 2009). Taken together, these examples illustrate strategies that could be employed to structure fat-containing foods to enhance the liberation of FAs in the intestine, thereby resulting in greater modulation of both GI function and satiety.

Once FAs have been released in the process of digestion, their effects are critically dependent upon their acyl chain length, such that only FAs with a chain length of ≥12 carbon atoms notably slow gastric emptying, stimulate GI hormone release and suppress energy intake (Hunt and Knox, 1968; Meyer et al., 1998b; McLaughlin et al., 1999; Feltrin et al., 2004). For example, in humans, intraduodenal infusion of lauric acid (C12) resulted in a much more marked increase in plasma CCK concentrations than decanoic acid (C10) (McLaughlin et al., 1999; Feltrin et al., 2004). Similarly, intraduodenal infusion of sodium oleate (C18:1), but not sodium caprylate (C8), stimulated CCK secretion (Matzinger et al., 2000). A similar pattern of effect applies to other gut hormones, e.g., C12 caused a significant increase in plasma GLP-1 and PYY concentrations, and suppressed ghrelin, whilst C10 did not (Feltrin et al., 2004, 2006). Once fatty chain length exceeds 12 carbon atoms, FAs appear to exert comparable effects on GI function. For example, the effects of isocaloric intraduodenal loads of C12 and C18:1 on the stimulation of pyloric pressures and CCK secretion are comparable (Feltrin et al., 2008).

Given the important role of fatty acid chain length for the effects of FAs on GI motility and gut hormone secretion, it is not surprising that the effects of FAs on appetite and energy intake are also dependent upon an acyl chain length ≥12 (Matzinger et al., 2000; Feltrin et al., 2004). For example, Matzinger and colleagues reported that C18:1 (infused at 8 mmol/h, corresponding to 0.76 kcal/min), but not C8, suppressed energy intake (Matzinger et al., 2000). Furthermore, we found that infusion of C12 (at a load of 0.4 kcal/min) potently reduced hunger and desire to eat, and suppressed energy intake at a buffet meal by ∼680 kcal, when compared with control (Feltrin et al., 2004), confirming that the “cut-off” between C12 and C10, described for effects on GI motility and hormone release, also applies to the effects on energy intake. Within the group of FAs ≥C12, we evaluated the comparative effects of C12 and C18:1, and found that C12, but not C18:1, suppressed energy intake (Feltrin et al., 2008), with the lack of effect of C18:1 apparently in contrast to the findings of Matzinger et al. (2000). However, we infused at a much lower load of 0.4 kcal/min, indicating that: (1) the effects of C18:1 on energy intake are load-dependent, and the threshold load required for the suppression of energy intake by C18:1 is between 0.4 and 0.76 kcal/min, and (2) that, calorie for calorie, C12 is more potent. The mechanism(s) underlying this specific effect of C12 on energy intake is(are) unclear, however, medium chain fatty acids are more extensively oxidized in the liver to anorexigenic ketone bodies (Bach et al., 1996) and undergo conversion to malonyl CoA, which is an important signal of energy stores in the hypothalamus and has been implicated in energy intake regulation (Bouchard, 2000; Lane et al., 2005). Furthermore, since C12 is not highly prevalent in the western diet, the GI tract of populations that are not regularly exposed to this FA could display “hypersensitivity” to its effects.

Intensive research has led to the identification of a number of GPCRs in the small intestine, dramatically advancing our understanding of the molecular mechanisms underlying the sensing of dietary fat. These GPCRs, including GPR120 (Hirasawa et al., 2005; Tanaka et al., 2008), GPR40 (Edfalk et al., 2008) and GPR119 (Lan et al., 2009; Lauffer et al., 2009), appear to be critical for the effects of fat on GI peptide secretion, although much of the evidence comes from cell line studies. The strongest evidence for a role in FA sensing exists for GPR120, which is activated by both saturated and unsaturated long-chain FAs (Miyauchi et al., 2010). GPR120 is expressed in the intestine of mouse and human, and in enteroendocrine cell lines (Hirasawa et al., 2005; Miyauchi et al., 2010), and in mice, is co-localized with endocrine L-cells in the large intestine (Hirasawa et al., 2005). In enteroendocrine cell lines, GPR120 is required for the fat-induced secretion of CCK (Tanaka et al., 2008) and GLP-1 (Hirasawa et al., 2005). Furthermore, GPR120 knock-out mice have impaired GLP-1 secretion in response to FA ingestion (Hirasawa et al., 2005). Thus, GPR120 appears important in the sensing of FAs by enteroendocrine cells. GPR40, which is activated by medium and long-chain saturated and unsaturated FAs (Briscoe et al., 2003), has also been reported to be expressed in enteroendocrine L cells, and GPR40^−/− mice have an impaired GLP-1 and GIP response to oral fat ingestion (Edfalk et al., 2008). However, the role of GPR40 in the secretion of other gut peptides is unclear, since knockdown of GPR40 has no effect on CCK release from STC-1 cells (Tanaka et al., 2008). GPR119 is also expressed in L-cell models, including GLUTag cells, human NCI-H716 and primary rat intestinal cell lines (Lauffer et al., 2009). An endogenous ligand for GPR119 is OEA (Overton et al., 2006). GLP-1 is secreted in response to ileal perfusion of OEA in the rat, and knock-out of GPR119 abolishes the GLP-1 response to OEA in mGLUTag cells (Lauffer et al., 2009). These studies provide strong evidence for FA sensing mechanisms in the small intestinal lumen that mediate the effects of fat on GI peptide secretion. Much further work is needed to clarify the role of these receptors, and the precise underlying mechanisms, in fat sensing, how this relates to the regulation of energy intake in humans, and whether changes in the expression and/or sensitivity of these receptors underlie the development of obesity and/or the changes in GI function and energy intake observed following over-exposure to dietary fat.

Ingestion of fat triggers the production of OEA in the small intestine (Schwartz et al., 2008). In mice, peripheral administration of OEA has been reported to prolong the time taken to initiate the next meal and decrease meal frequency resulting in an overall reduction of food intake (Fu et al., 2008; Schwartz et al., 2008) and body weight (Fu et al., 2003, 2005). The production of OEA is dependent on the FA transporter, CD36, which mediates the uptake of FAs into the enterocytes. Mutant mice lacking CD36 have impaired production of OEA and fail to reduce their food intake in response to duodenal lipid (Schwartz et al., 2008). As discussed above, OEA has been shown to activate GPR119 (Lauffer et al., 2009), however, OEA is still able to reduce food intake in GPR119^−/− mice, suggesting that this receptor does not mediate the anorexigenic effects of OEA. Instead, the effect of OEA to reduce food intake and body weight is mediated through the activation of the nuclear receptor, peroxisome-proliferator-activated receptor-α (PPAR-α) (Fu et al., 2003; Schwartz et al., 2008). The downstream target of PPAR-α is currently unclear, however, it may act by inducing apo A-IV release (Schwartz et al., 2008), a PPAR-α regulated protein that activates vagal afferents (Glatzle et al., 2004).

The uptake of FAs into the enterocyte is associated with the formation of chylomicrons and the stimulation of apo A-IV, processes that have both been demonstrated to be required for the signaling of information to the brain, mainly through the vagus nerve. For example, in rats, inhibiting chylomicron formation, using the “detergent” Pluronic L-81, attenuates the effects of lipid on the release of CCK (Raybould et al., 1998), activation of vagal afferents (Randich et al., 2000), and induction of c-fos like immunoreactivity in the NTS (Lo et al., 2007), and consequently blocks the gastric emptying (Raybould et al., 1998) and food intake (Meyer et al., 1998a) responses to fat ingestion. The induction of c-fos in the NTS by chylomicron formation and/or signaling is dependent on the CCK₁ receptor (Lo et al., 2007). Thus, the formation of chylomicrons and the stimulation of apo A-IV appear to be critical for CCK release following ingestion of long-chain FAs. Activation of CCK₁ receptors on vagal afferents, and locally on GI smooth muscle, then induces feedback effects on gastric emptying and food intake (Lo et al., 2007). In humans, the effects of FAs also appear dependent on the release of CCK. For example, the inhibitory effects of C12 on gastric emptying and the perception of intragastric volume (maximum volume tolerated) (Lal et al., 2004), and the inhibitory effects of C18:1 on energy intake (Matzinger et al., 2000), were attenuated by the CCK₁ receptor antagonist, dexloxiglumide. A recent study in humans, using functional magnetic resonance imaging, has demonstrated that C12 increases activity in the brainstem and hypothalamus, and that this effect is dependent on the CCK₁ receptor (Lassman et al., 2010), thus, in man, the sensing of lipid by the small intestine appears to also be signaled to the brainstem and hypothalamus via a CCK₁ receptor mediated pathway. As the response was rapid (occurring within 2–4 min following C12 infusion), it is consistent with a vagally mediated response.

Taken together, it appears that small intestinal FAs are detected by GPCRs located on enteroendocrine cells to stimulate GI peptide release, and also by the mobilization of OEA, and chylomicron components, in particular apo A-IV, which in turn induce gut peptide secretion. These gut peptides interact with receptors on vagal afferent nerve terminals within the intestinal wall, conveying information about the small intestinal environment to the brainstem and higher brain centers, resulting in changes in GI function and behavioral responses, including the suppression of food intake.

Oral detection of nutrients occurs as a result of the interaction of nutrients with receptors on the apical surface of taste receptor cells (TRCs). Animal studies have revealed the expression of a number of receptors, including CD36, delayed rectifying potassium channels, and a series of G-protein coupled receptors, including GPR40, GPR 41, GPR43, and GPR120 on TRCs (Mattes, 2009b). The detection of FAs by these mechanisms induces a signaling cascade activating gustatory nerves that transmit sensory information to the NTS in the brainstem (Gaillard et al., 2008), and from there to higher brain centers, such as the lateral hypothalamus and the nucleus accumbens, which play an important role in the regulation of food intake and in reward. CD36 appears to play a key role in oral fat detection; oral exposure to long-chain FAs increases [Ca²⁺] in TRCs, and c-fos expression in the NTS, an effect that is abolished in CD36 knockout animals (Gaillard et al., 2008). The role of GPCRs in fat taste signal transduction is less clear. However, GPR120 has been reported to be co-localized with phospholipase Cβ2 and α-gustducin in TRCs (Matsumura et al., 2009), both of which are involved in the transduction of other tastes, such as sweetness and bitterness.

Oral sensing of FAs appears to have two major roles: (i) mediation of cephalic phase responses and (ii) contribution to the regulation of fat and energy intake. Oral stimulation with fat, using modified sham-feeding techniques, has been reported in humans to stimulate gastric lipase secretion (Wojdemann et al., 1997), elevate serum triglycerides (Mattes, 2009a), suppress ghrelin (Heath et al., 2004), and to reduce appetite (Heath et al., 2004; Smeets and Westerterp-Plantenga, 2006; Smeets et al., 2009) and energy intake (Crystal and Teff, 2006).

Heightened preference for, and increased consumption of, fatty foods amongst the obese suggests that fat intake is poorly regulated in this group (Mela and Sacchetti, 1991), raising the question of relationships between oral fat sensitivity, fat intake and obesity. In animals, FA sensitivity appears to vary across strains; e.g., DIO-P (Osborne–Mendel) rats have decreased oral FA acid sensitivity when compared with DIO-R (S5B/PL) rats (Gilbertson et al., 2005). The greater oral sensitivity in the DIO-R rats is associated with reduced fat intake, reduced preference for fat, and reduced predisposition for obesity when compared with DIO-P rats (Gilbertson et al., 1998). These observations provide compelling evidence that oral FA sensitivity contributes to dietary fat preference, energy intake and, consequently, body weight. Similarly, human studies have identified a large diversity in oral sensitivity to FAs, with detection thresholds ranging from 0.02–12 mM, although all subjects appeared able to detect FAs across this range (Stewart et al., 2010a,b). Those individuals who were defined as “hypersensitive” to C18:1 (those able to detect C18:1 at a concentration of 1.4 mM) had significantly lower habitual dietary energy and fat intakes and body weights than those defined as hyposensitive (those unable to detect the presence of 1.4 mM oleic acid) (Stewart et al., 2010a). In a further study (Stewart et al., 2010b), obese subjects, as a group, had significantly higher detection thresholds for C18:1, and intraduodenal infusion of C18:1 was associated with reduced stimulation of pyloric motility, when compared with lean individuals, indicating that the obese are less able to sense, and induce appropriate signaling responses to, both oral and intestinal fat (Figure 2).

Given the homology between the receptors involved in oral and intestinal fat sensing, this observation raises the possibility that receptor expression/sensitivity may be altered in obesity in both the oral cavity and the GI tract. Whether those individuals prone to obesity have altered expression of fat sensing receptors (i.e., decreased expression of receptors such as CD36 and GPR120 in the oral cavity and small intestine), or whether receptor expression/sensitivity is altered by factors that contribute to the development of obesity (i.e., induced by changes in environmental factors such as prolonged exposure to a high-fat diet), is currently unknown. However, given evidence of impaired small intestinal fat sensing following high-fat diet exposure, it is conceivable that dietary over-exposure to fat also modulates oral fat sensing. In order to understand the mechanisms underlying differences in oral fat detection, research is warranted to determine whether receptor expression/sensitivity can be modulated by energy/fat restriction to restore oral and intestinal signaling responses to fat.

Food is ingested not only to replenish energy stores, but also because it is enjoyable and rewarding, and this may particularly relate to fat-containing foods, due to their high palatability. Thus, dietary fat does not only influence homeostatic pathways, as discussed above, but there is increasing evidence that it can powerfully modify food intake by activating hedonic pathways, increasing the desire to consume foods that are highly palatable (Lutter and Nestler, 2009). For example, studies using fMRI have demonstrated in humans that oral exposure to dietary fat activates reward areas, such as the lateral hypothalamus, amygdala, insula, and anterior cingulate cortex (De Araujo and Rolls, 2004), and this may be enhanced in obesity (Rolls, 2010). It is currently not known how differences in the ability to detect fat in the oral cavity contribute to differences in food preference and reward-related eating and, thus, overall energy intake. Studies are warranted to evaluate whether these hedonic effects of fat are related to oral fat taste sensitivity, i.e., whether individuals with reduced sensitivity to oral fat need to consume more fat/energy to receive the same reward/satisfaction. Furthermore, since an individual's perception of reward is also likely to be related to psychological factors, such as if they have experienced aversive responses to particular foods, or if certain foods were provided as a reward for good behavior when they were children, it will also be important to determine whether such psychological factors have the ability to influence oral sensitivity to FAs (Rolls, 2010).