Oral Signals of Short and Long Chain Fatty Acids: Parallel Taste Pathways to Identify Microbes and Triglycerides

Abstract

Both short chain fatty acids (SCFAs) and long chain fatty acids (LCFAs) rely on free fatty acid receptors to signal their presence to the body, but their individual detection and putative reward systems are different. These separate, yet parallel, taste signaling pathways allow us to distinguish microbe-produced from triglyceride-based fatty acids. Free SCFAs indicate that the food has been fermented and may still contain living, probiotic microbes that can colonize the gut. Free LCFAs indicate the presence of calorie-rich triglycerides in foods. By contrast, LCFAs stimulate endocannabinoids, which reinforce overconsumption of triglycerides. Here we examine the separate oral detection and putative reward systems for both LCFA and SCFAs, and introduce a novel dietary LC:SC ratio as a guideline to improve metabolism and health.

Graphical Abstract

Introduction

Throughout the evolution of modern humans, ingestion of fatty and fermented foods as high energy and probiotic food sources has been beneficial. We propose here that there are two separate fat detection pathways in taste tissue: one for long chain fatty acids (LCFAs) and one for short chain fatty acids (SCFAs) that serve different nutritional needs. Interestingly, different taste receptors have been characterized that can identify both long and short chain fatty acids in the mouth [1]. Why do we need these receptors? LCFAs: The desire for and enjoyment of fat-rich foods has benefitted our species by providing necessary energy as well as lipid components that are crucial for cellular membranes, immune function, cell growth and repair, and brain expansion [2, 3]. Per gram, fats contain the most calories compared to other macronutrients and are an efficient way to ingest and store energy. Yet, there are few free LCFAs in food, as they are packaged as triglycerides (TGs). Recently, oral lipase was discovered to play a role in freeing fatty acids from TGs [4], thus providing ligands to stimulate LCFA taste receptors. The free LCFAs signal to the body that a high calorie meal is present, thus imparting an evolutionary advantage. The endocannabinoid system (ECS) is stimulated by the consumption of LCFAs, which activates reward circuits in the brain, suggesting a benefit of ingesting fatty foods [5]. SCFAs: Humans have also benefitted from synergistic relationships with gut-colonizing microbes. In the colon, microbes ferment fiber and other components of foods to provide humans with utilizable SCFAs. In foods, SCFAs are primarily derived from fermentation. Hence, SCFA receptors may be acting as indicators of fermentation, which conveys an evolutionary advantage of food safety and a possible source of “probiotic” microbes that can colonize the gut and promote health. Free fatty acid receptors (FFARs) and related signaling proteins (described in Table 1) provide a means of oral detection of fatty acids. The receptors and downstream effectors also act in the lower gastrointestinal system to activate satiety and reward. These detection and reward systems serve to increase intake of triglyceride-rich and fermented foods, each of which can have a selective evolutionary advantage for survival.

Table 1: Putative fatty acid receptors involved in taste signal transduction and regulation of energy metabolism.

This table represents a portion of the known free fatty acid receptors (FFARs), other G-protein coupled receptors (GPCRs), and related receptors and membrane signaling proteins involved in fatty acid signal transduction. We also indicate the receptive fields of these receptors and the functional consequences of their activations. This table is limited to the regulation of taste and energy homeostasis.

Primary Fatty Acid Type	Receptor/Protein	Primary Agonist	Expression	Functional Consequences
SCFA	FFAR2/GPR43	SCFAs C2=C3>C4>other SCFAs (human) C2>C3>C4 (mouse)	Taste bud cells Enteroendocrine cells Intestinal epithelial cells Pancreatic Beta-cells White adipocytes	Transduces taste signal? [6] Promotes PYY, GLP-1 secretion? [7] Regulates cytokine production [1] Regulates insulin secretion [8] Suppresses lipolysis, promotes lipid oxidation [9]
	FFAR3/GPR41	SCFAs C3=C4=C5>C2>C1	Taste bud cells (mouse) Peripheral nerves Enteroendocrine cells Pancreatic Beta-cells White adipocytes	Transduces taste signal? [10] Stimulates vagal afferents? [11] [12] Promotes PYY, GLP-1 secretion? [1] Suppresses GIP [1] Suppresses insulin secretion [1] Promotes leptin production [13]
	GPR109A	Niacin, C4	Colonic epithelial cells Adipose tissue	Suppresses colonic inflammation and carcinogenesis [1] Regulates lipid metabolism by regulating adipose macrophages [1]
	Olfr78 (mouse)/OR51E2 (human)	SCFAs C3>C2	Olfactory sensory neurons Enteroendocrine cells	Transduces olfactory signals [1] Regulates gut hormone secretion? [14]
	Olfr558 (mouse)/OR51E1 (human)	SCFAs C4>isovaleric>C5>C3[15]	Olfactory sensory neurons Enteroendocrine cells	Transduces olfactory signals [16] Promotes GLP-1, PYY secretion [17]
LCFAs	FFAR1/GPR40	LCFAs	Taste bud cells (mouse) Enteroendocrine cells Pancreatic β cells	Mediates taste sensitivity for fatty acids [1, 10] Promotes CCK, GIP, GLP-1 secretion [1] Enhances insulin secretion [1]
	FFAR4/GPR120	LCFAs	Taste bud cells Hypothalamus Enteroendocrine cells White adipocytes	Promotes GLP-1 release, Regulates taste sensitivity [18] Mediates energy homeostasis [19] Promotes CCK, GIP and GLP-1 secretion [20] Promotes adipocyte differentiation [1]
	CD36	LCFAs	Taste bud cells Enteroendocrine cells	Transduces taste signal [18] Regulates food intake through fat-induced satiety [18]
	DRK Channel	LCFAs (PUFAs)	Taste cells buds	Enhances LCFA taste signaling [18]
	CB1R	Endocannabinoids (eCBs)	Taste bud cells Enteroendocrine cells White adipocytes	Regulates taste sensitivity [21] Stimulates ghrelin release Inhibits CCK release [22] Promotes lipogenesis and adipogenesis [23]

Long Chain Fatty Acids (LCFAs)

The detection of LCFAs in the oral cavity likely evolved for the necessary intake of energy and essential fatty acids which are crucial for growth and development [2]. Although LCFAs, specifically omega 6 and saturated FAs, are found in natural, unprocessed foods, they are present in overabundance in processed foods and snacks (doughnuts, ice cream, etc.), adding to the growing global obesity phenomenon [24]. In particular, the involvement of the endocannabinoid system (ECS), an inherently orexigenic system that drives consumption by strengthening food intake reinforcement mechanisms [25], suggests a physiological relevance in maximizing fat intake, likely required in case of a famine.

Taste Detection and Description

The oral sensation of free fatty acids (FFAs) has been described as “scratchy,” but only LCFAs elicit “fatty” [26]. Several G-Protein coupled receptors (GPCRs) and signaling proteins respond to LCFA in the mouth and may aid in transducing these “fatty” or “scratchy” sensations [18]. The taste receptors that initiate LCFA transduction are cluster-of-differentiation 36 (CD36) and free fatty acid receptor 4 (FFAR4), although other receptors and proteins are implicated in oral LCFA transduction [18, 27] (see Table 1). CD36 transduces the LCFA signal, whereas cannabinoid 1 receptor (CB1R) and FFAR4 regulate LCFA detection sensitivity by releasing glucagon-like peptide 1 (GLP-1) from taste cells, downregulating CD36 expression (Figure 1) [18] [28].

Figure 1. Summary of the parallels between Long Chain Fatty Acids (LCFA) and Short Chain Fatty Acids (SCFA) signaling versus the different functional consequences on appetite and satiety.

Key components involved in the fatty acid-induced taste signal described in the upper boxes for both LCFA and SCFA. ‘Cluster of differentiation’ (CD36) transduces the LCFA signal, whereas cannabinoid 1 receptor (CB1R) and free fatty acid receptor 4 (FFAR4) regulate LCFA detection sensitivity. The exact signaling mechanism for SCFA in taste bud cells is under investigation. The lower boxes depict the opposing effects LCFA versus SCFA have on satiety. Tasting LCFA stimulates the release of gut endocannabinoids (eCBs), which increases ghrelin secretion and decreases FFAR1-mediated cholecystokinin (CCK) release, thus overriding satiety signals and reinforcing fat intake and fat-induced reward through vagal nerve signaling. On the other hand, SCFAs act possibly via FFAR2, FFAR3 and/or olfactory receptor 51E1 (OR51E1), to increase glucagon-like peptide-1 (GLP-1) and peptide tyrosine tyrosine (PYY) release, decrease ghrelin release and increase satiety signals via the vagal nerve. Created with BioRender.com.

Single nucleotide polymorphisms (SNPs) in CD36 and FFAR4 in humans affect LCFA absolute detection thresholds and, consequently, total fat intake [29-31]. More specifically, obese patients with the common SNP rs1761667, which reduces CD36 gene expression, displayed a higher detection threshold (lower sensitivity) and greater liking for lipids than individuals with the common variant [32, 33]. This SNP is also associated with altered endocannabinoid (eCB) levels in both obese and normal weight individuals [34]. Obesity can also alter fat preference and intake by downregulating CD36 expression [35-37] and increasing eCB tone [38], e.g. salivary eCBs are higher in obese relative to non-obese individuals [39]. Perhaps the regulation of the LCFA detection components, CD36 and FFAR4, is mediated by the changes in peripheral eCB tone and CB1R signaling. This suggests that when ECS tone is increased by obesity, LCFA perception is decreased and fat liking is increased [40].

Liking and Reward

Although LCFA ingestion increases satiety signals through FFARs (see Table 1) [1], the ability of LCFAs to stimulate eCB signaling may override these satiety signals by reinforcing reward-driven intake [22, 25]. The endocannabinoid system influences a variety of factors controlling food intake and metabolism [41]. Components of the ECS are present in the hypothalamus and brainstem, areas that regulate food intake and energy expenditure, and in the nucleus accumbens, involved in the rewarding value of food. The ECS system has also been found in taste tissue where it modulates taste sensitivity [21, 27, 28]. The presence of LCFAs in the oral cavity activates a self-reinforcing feedback loop to eat more fat through the synthesis of eCBs in the gut and, subsequently, activates reward signaling and food-reinforcement mechanisms in the brain through vagal afferents and CB1R activation [42] (Figure 1). The effects of CB1 activation include: increasing olfactory and gustatory sensitivity, increasing appetitive and reward signaling in the brain, and promoting lipogenesis, among many others [40]. This supports the idea that the ECS evolved to ensure the ingestion of essential fatty acids. Since LCFAs are presented as triglycerides in foods, this system reinforces the continued intake of calorically dense meals as well.

The continuous availability of inexpensive, fatty, palatable foods, however, drives the ECS to reinforce preference for fatty foods when it is not necessary, as no impending food shortage will afflict developed countries. This reward system is beneficial evolutionarily, but is helping to drive the current obesity epidemic [40]. The continuous intake of fat results in a decrease in LCFA detection sensitivity, which can increase total fat intake in a positive feedback loop [36]. This is caused, in part, by dysregulation of CB1 receptors in the oral cavity which impairs LCFA sensitivity (see Taste Detection and Description). Once taste sensitivity has been impaired, reward signaling is favored over satiety signaling, further exacerbating the positive feedback cycle [25]. Decreased LCFA taste sensitivity results in greater total fat intake [37]. Over time, a sustained increase in fat intake could lead to sustained increases in gut production of eCBs and CB1 activation, resulting in a greater motivation to consume fatty foods as well as a propensity towards weight gain [22]. Lastly, obesity and elevated eCB tone all impact LCFA detection sensitivity [34], contributing to the positive cycle of continued high fat intake and a dysregulated appetitive circuitry.

Short Chain Fatty Acids

SCFAs are fatty acids with fewer than six carbons, such as acetic (C2), propionic (C3), butyric (C4), and valeric (C5) acids. They are generally found in foods as byproducts of microbial fermentation. Humans obtain SCFAs from fermented foods, such as: vinegar, cheese, kimchi, natto, and sourdough bread. They are also made in our gut by the fermentation of dietary fibers by microbes. Fermentation is an important process of preserving foods that has been used since ancient times. Not only does fermentation prevent food spoilage, but it also improves the bioavailability of nutrients and produces additional vitamins and antioxidants [43]. SCFAs are a principal way that microbial fermentation affects food aroma and flavor, although there are other end-products of fermentation that play a role in flavor [44], e.g. volatile compounds, free amino acids (such as glutamate), and free ribonucleosides (such as inosine- and guanosine monophosphate).

Taste Detection and Description

Recently, several GPCRs for taste and smell have been identified that are activated by SCFAs including: FFAR2 (GPR43), FFAR3 (GPR41), GPCR109a, and the olfactory receptors OR51E1 and OR51E2 (see Table 1). FFAR2 is activated more strongly by shorter-chain fatty acids (C2 = C3 > C4 > C5), whereas FFAR3 has the opposite sensitivity (C3 = C4 = C5 > C2) [1]. The transduction mechanisms of SCFA taste and preference for them are not yet well characterized. In humans, FFAR2 has been identified in fungiform papillae of taste bud cells and its expression level is associated with dietary fat intake [6]. Among short and long chain fatty acids, the SCFAs have the lowest detection threshold [45], which may explain, in part, the perceived intensity of SCFAs.

SCFAs are associated with rancid, cheesy, or pungent flavors, which are not palatable on their own. Cheese samples elevated in the acetic, propionic, butyric, and caproic acids were associated with flavors described as rancid, pungent, acrid, and "smelly feet" [46]. Despite their seemingly unpalatable flavors, fermented foods have long been enjoyed by every culture, which appears to be a paradox. Kefir, kimchi and tempeh are now sold at mainstream supermarkets, and kombucha is found at convenience stores and “on tap” at some restaurants. The most well-known SCFA, acetic acid (vinegar), is a very common food additive in dressings and condiments (e.g. ketchup).

The amounts and ratios of individual SCFAs and other microbial metabolites produced during fermentation can vary due to the species and strains of microbes present. The specific ratios of SCFAs, along with other metabolites, can thus confer a signature flavor that indicates the microbial community that produced them [47]. Preference for certain flavors in fermented foods may affect food choices, resulting in selective colonization of the gut microbiota with specific types of microbes. Thus, there may be an interaction between our SCFA chemosensory abilities and preferences and our resultant gut microbiome.

Liking and Reward

One reason people may be attracted to these pungent, fermented foods is they are metabolically satisfying. Satiety itself can be rewarding and our brains, either consciously or unconsciously, remember our contentment after such a meal [48]. Whether they are consumed as fermented foods directly or as a high fiber food to be fermented later within the gut, SCFAs increase satiety in three ways: i) direct action on the brain, ii) stimulation of gut hormones, and iii) neural activation. Acetate (the salt of acetic acid), crosses the blood brain barrier to act directly on the hypothalamus [49]. In the gut, SCFAs increase GLP-1 and peptide tyrosine-tyrosine (PYY) release by enteroendocrine cells affecting insulin secretion and satiety, possibly via activation of FFAR2, FFAR3, and/or OR51E1 [1, 7, 14, 17, 50] (Figure 1). SCFAs also have an anorexigenic or satiating effect by inhibiting ghrelin release [51] and stimulating leptin production via FFAR2 on adipocytes [13]. The third mechanism of satiety is carried via the vagus nerve [52]. Intraperitoneal injection of butyrate (the salt form of butyric acid), suppresses appetite for 1 hour in mice by activating afferent vagal neurons and the nucleus tractus solitarius (NTS) [12]. A possible mechanism is FFAR3, which has been identified on vagal afferent neurons [11]. SCFAs signaling can create an anorexigenic effect, in contrast to the orexigenic effect of LCFAs.

Both LCFAs and SCFAs can increase GLP-1 in the blood [1]. As described above, ingesting LCFAs increases “fatty” taste sensitivity by downregulating oral CD36 expression in mice, thereby increasing fat intake [18]. It is interesting to consider whether ingesting SCFAs has a parallel effect of increasing LCFA “fatty” taste sensitivity, resulting in decreased fat intake. Without the involvement of the strong reward signaling of the ECS, as seen with LCFAs, intake of SCFAs would provide satiation without promoting overconsumption. Increasing SCFAs may help overcome disruptions in GLP-1 release and promote “fatty” taste sensitivity and satiety.

Achieving an Optimal LCFA:SCFA Ratio

The current Westernized diet provides little SCFAs as it is heavily composed of ultra-processed foods (UPFs) that are low in fiber, compared to traditional diets that are high in fiber and fermented foods. UPFs, laden with rewarding LCFAs, make up nearly 60% of total energy intake in the US [53]. We propose a new ratio of LCFA:SCFA (LC:SC) be considered to improve health. For example, Mediterranean and Japanese diets are considered healthy for a variety of reasons, perhaps among them is the LC:SC ratio. Specifically, the Japanese diet is high in SCFAs from fermented foods, such as nattō, soy, rice vinegar, pickled vegetables and fruits, fish flakes (katsuobushi), and miso [54], and low in LCFAs, which are mainly from omega 3s in fish [55]. It is worth noting that Japan has the lowest obesity rate in the world, 4.3% with a BMI above 30 kg/m² [56]. We propose that SCFAs have been under-appreciated as nutrients and should be considered when making dietary guidelines. Recommendations to increase dietary SCFAs through fermented foods and fiber would decrease the LC:SC ratio and likely improve health (see Figure 2). Furthermore, consumption of live probiotic microbes in fermented foods should also be considered under dietary guidelines.

Figure 2. LC:SC Balance in Western Diet.

The overabundance of long chain fatty acids, especially omega 6 fatty acids, in the Western Diet drives endocannabinoid system activation, which reinforces fat intake, promotes inflammation and reduces energy expenditure, ultimately contributing to diet-induced obesity. Increasing short chain fatty acid intake through fermented foods and dietary fiber would promote satiety, reduce inflammation and increase energy expenditure to protect against diet-induced obesity. Created with BioRender.com.

LC:SC=long chain fatty acid:short chain fatty acid

The LC:SC ratio is important for several reasons. SCFAs have a strong potential to benefit health by modulating metabolism [57], inflammation, and immunity via activation of FFAR2 and FFAR3 [1] (see Table 1). Once absorbed, SCFAs may activate FFAR2 and FFAR3 on pancreatic β-cells and regulate insulin secretion [8]. In adipocytes, SCFAs promote browning of white adipose tissue, increase fatty acid oxidation and energy expenditure, and protect against diet-induced obesity in mice in part through FFAR2 and FFAR3 activation [58]. Overall, SCFAs act to increase energy expenditure [9]. The beneficial effects of SCFAs can even pass through generations. In pregnant mice, SCFAs provide offspring with resistance to obesity via FFAR2 and FFAR3 [59].

Conclusion

There remains much to learn about the paradoxical taste and desirability of fermented foods and high fiber foods via the oral and post-ingestive activation of FFARs by SCFAs. Understanding the unique taste profile of SCFAs can have direct effects on food engineering. Capitalizing on the enjoyable flavor components of foods with SCFAs in food production can increase their desirability and may help re-incorporate them into current Western diets. Foods with SCFAs may play a pivotal role in preventing and treating several common metabolic and inflammatory disorders. We can also capitalize on our understanding of oral LCFA-induced reward and its dependency on the endocannabinoid system to inform dietary intake and consumption patterns. On one hand, this system reinforces the necessary intake of essential FAs, but on the other, plays a role in the overconsumption of dietary fat, especially in Western societies where ultra-processed foods are a substantial portion of the diet. Coupling the overabundance of ultra-processed foods with the endocannabinoid system is a recipe for disaster in population metabolic health. This self-reinforcing feedback loop further exacerbates the LC:SC ratio. Like the ratio for omega 6:omega 3 fatty acids, and more recently, the sodium:potassium ratio, optimal LC:SC has the potential to better guide dietary intake by shifting focus to dietary balance. We propose this intake ratio be investigated globally amongst different populations and correlated with health, obesity, and metabolic disease.