A cross-talk between fat and bitter taste modalities

Highlights

• Fat and bitter taste modalities are altered in obesity.

• The two taste modalities might interact with each other to give rise “bitter fat taste”.

• The interaction between bitter and fat taste might involve/share signaling mechanisms, downstream to receptor activation.

Abstract

The choice of food is governed largely by the sense of taste. To date, five basic taste modalities have been described; however, there is an increasing agreement on the existence of a 6^th fat taste. The taste modalities might interact with each other and also with other senses. The advancements in cellular and molecular biology have helped the characterization of taste signaling mechanisms, down to the receptor level and beyond. CD36 and GPR120 have been shown to be involved in the detection of fat taste while bitter taste is perceived by a number of receptors that belong to a family of taste-type 2 receptors (T2R or TAS2R). Hence, the most common role is played by TAS2R16 and TAS2R38 in bitter taste perception in humans. Increasing evidences from behavioural studies suggest that fat and bitter taste modalities might interact with each other, and this interaction might be critical in obesity. In the current review, we will discuss the evidence from genetic and behavioural studies and propose the molecular mechanism of a cross-talk between fat and bitter tastes.

Introduction

Humans are omnivorous food-seeking species that have developed through evolution a sense of taste to survive in a complex environment. Taste is a sensory modality, involving the oral perception of food-derived chemicals that stimulate receptor cells within taste buds [1]. Taste not only helps decide whether a food is noxious or palatable but also prepares the gastrointestinal tract for post-ingestive metabolic events. The 5 basic taste modalities have been demonstrated. Simple carbohydrates are perceived as sweet, sodium salts and salts of few cations are perceived as salty, acids are perceived as sour while amino acids like glutamate and aspartate elicit a savoury taste [2]. Moreover, during the recent couple of years, there has been an increasing agreement that fatty acids might trigger a lipid taste [3]. Recent evidence indicates that CD36 and GPR120 might play a non-overlapping role in the detection of a fat taste [4].

The taste receptor (TR) cells are regrouped in taste buds which are the units of the papillae, distributed on the tongue. There are three kinds of papillae: fungiforme (localized in the frontal part of the tongue), foliate (localized in the lateral region of the tongue) and circumvallate (present on the anterior part of the tongue). There are three types of taste cells wherein type I cells are considered as supporting cells, type II cells respond to bitter, sweet and umami, and type III cells are involved in making synaptic junctions with afferent nerve fibres. In this review, we will not go into the details of perception of different taste modalities, as several recent review articles can be consulted on this subject [2,[5], [6], [7]]. Briefly, the heterodimeric T1R2/T1R3 detects the sweet taste whereas, the T1R1/T1R3 heterodimer is sensitive to umami taste [2]. Salt activates sodium channels, while the acidic compounds induce depolarization by blocking potassium channels [7]. TAS2R receptors have been shown to be involved in the perception of bitter taste. Bitter molecules belong to the largest family of tastants and humans possess more than 25 functional bitter taste receptor genes to interact with such large group of taste substances [1]. Therefore, most of the noxious and poisonous substances are experienced as bitter.

The majority of laboratory studies on taste perception are based on the use of candidate molecules. However, in the real world, food, most often, comprises of a mixture of agents that can trigger different taste sensations at the same time. The sense of taste can interact with other senses, and different taste modalities may interact with each other [8,9]. For instance, salt taste has been shown to inhibit bitterness in food and enhances flavour [10,11]. Similarly, salty and sour tastes have been shown to interact each other with the enhancement of their effect at low concentrations, while causing an inhibition or no effect at high concentrations [12]. It is interesting to mention that some amino acids like l-arginie or l-lysine may modulate the oro-sensory perception of 5 taste modalities. The exogenous l-Arg was found to increase the perception of sucrose, umami, sodium chloride, salt and bitterness of caffeine only in tasters, and to decrease citric acid sourness [13]. Indeed, l-arginine has been considered as “carrier” as its addition to bitter molecules increases the bitterness as this agent makes hydrogen-bonded adducts that increase the solubility of PROP and its interaction with the receptor [14]. A detailed understanding of various taste interactions is out of the scope of the current appraisal and can be found elsewhere [12]. We will focus on the cross-talk between bitter and fat taste for the remaining part of this review.