Sweet sensation, the detection of sugars, is one of the five basic tastes. Sweet tastes are appetitive, in that they are generally regarded as pleasant. The ability to detect sugars has likely been advantageous through human evolution by facilitating the identification of substances with high caloric value. The standard substance that is used to measure sweetness is sucrose (table sugar), which is designated as having a sweetness index of 1. Common sugars range in sweetness from 0.15 (lactose) to 1.7 (fructose), and common artificial sugars range from 180 (aspartame; brand name Equal®) up to 8,000 (neotame; brand name NutraSweet®).
The relative sweetness of a molecule is determined by its affinity for the receptor for sweet sensation, the heterodimer of the Taste Receptor 1 Member 2 (T1R2) and Taste Receptor 1 Member 3 (T1R3) G-protein coupled receptors. The T1R2+R3 receptor is expressed on the surface of taste receptor cells (TRCs). TRCs that express the T1R2+R3 receptor sense sweetness, and TRCs that detect sweetness do not detect other tastes.
Different TRCs are responsible for all five of the primary tastes. TRCs are organized into taste buds, which contain 50—150 TRCs each. Taste buds throughout the tongue contain TRCs for all five primary tastes; therefore, there is no topographic taste map. The specificity of the TRC to one taste alone is essential for the way that the different taste sensations are encoded.
Depolarization of the sweet TRCs following a signaling cascade triggered by the taste receptor transmits a signal to afferent neurons. The signal is relayed by fibers of the facial nerve (cranial nerve VII) in the anterior two-thirds of the tongue and the glossopharyngeal nerve (cranial nerve IX) in the posterior third of the tongue. Sensory afferents synapse in the rostral portion of the nucleus of the solitary tract in the brainstem and are relayed to the thalamus with projections to the primary gustatory cortex.
The T1R2+R3 receptor broadly detects different sugars and artificial sweeteners. In general the sensation of one sugar is indistinguishable from another, though artificial sweeteners may produce a slightly different taste than natural sugars. It is possible that the homodimer of T1R3 can detect natural sugars but not artificial sweeteners, or bitter receptors may be cross-activated by artificial sweeteners at high concentrations.
Foods and beverages provide calories and nutrients that are essential for humans to survive. Sweet foods generally are more calorie-rich than others and are considered appetitive or pleasant in taste. This sidebar focuses on the perception of sweet foods and how this perception changes when a natural antagonist—gymnemic acid—blocks sweet taste receptors.
Gymnemic acid is a glycoside that can be isolated from the leaves of the Gymnema sylvestre plant, which grows in central and southern India. Gymnemic acid reduces the taste of sugar when placed in the mouth by blocking sweet receptors found in taste buds. Gymnema sylvestre leaves are often chewed in India to fight sweet cravings.
1 Gymnema sylvestre capsule (it should contain at least 450 milligrams of Gymnema sylvestre leaf extract, 25% gymnemic acid in each capsule. This can be purchased from an organic food store and found in the vitamin aisle.)
2 small paper cups
1 to 2 tablespoons of water
2 to 4 tablespoons of soda (such as a cola type)
2 pieces of chocolate (dark chocolate will have a more pronounced effect than milk chocolate)
Pour the water into one of the paper cups. Open the Gymnema sylvestre capsule and dump the powder into the water. Stir until mostly dissolved and set aside. This will look like an herbal tea mix and may have a bitter taste. Pour the soda into the other paper cup and set aside. As a control for this experiment, eat one of the pieces of chocolate. This should taste sweet. Next, drink some of the soda. Again, this should taste sweet. Next, for about 1 minute, swish and coat your mouth with the Gymnema sylvestre liquid. You want to ensure that the gymnemic acid binds to your sweet receptors, which are located on the tongue, cheeks, and the roof of the mouth. Gargling will also work. If the taste of the liquid is too pungent, plug your nose to reduce the bitterness. For best results, ensure you take your time with this step. Swallow the Gymnema sylvestre liquid. You may take a small drink of water (about 1 to 2 tablespoons) to wash down the remaining Gymnema sylvestre liquid in your mouth.
Next, eat the last piece of chocolate. This should now taste bitter and not sweet! It is because the gymnemic acid has blocked the sweet taste receptors, and the sugar in the chocolate can no longer bind. Next, drink some of the soda. Again, this should taste bitter or very acidic. If the experiment was done right, sweet perception will be blocked for about an hour.
Jennifer L. Hellier
The T1R2+R3 receptor detects sugars with relatively low affinity when compared to the affinities of the bitter receptors and bitter tastants. This means that substances need relatively high concentrations of sugar to be identified as sweet, consistent with the need to identify foods that are sufficiently nutritive.
The ability to detect sweet, along with the other tastes, declines with age. This may result in the ingestion of more sweet foods, which could exacerbate obesity and diabetes in older individuals. More recently, it has been identified that sugars can be detected by taste receptors in the gut, but not consciously sensed. Gut sensing of sugar may have important implications for regulation of appetite and metabolism.
Michael S. Harper
See also: Facial Nerve; Glossopharyngeal Nerve; Supertaster; Taste Aversion; Taste Bud; Taste System; Type II Taste Cells
Joesten, Melvin D., John L. Hogg, & Mary E. Castellion. (2007). Sweetness relative to sucrose (table). The World of Chemistry: Essentials (4th ed., p. 359). Belmont, CA: Thomson Brooks/Cole.
Mayo Clinic. (2015). Artificial sweeteners and other sugar substitutes. Retrieved from http://www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/in-depth/artificial-sweeteners/art-20046936
McLaughlin, Susan K., & Robert F. Margolskee. (1994). The sense of taste. American Scientist, 82(6), 538—545.