Sweet Taste Signalling

Sweet taste signalling is a very complex mechanism but one that has proved immensely valuable in understanding how taste stimulates the secretion of various molecules.

For many years, it was commonly believed that sugar substitutes such as artificial sweeteners and food additives had understandable functional properties. They not only enhanced flavour through the perception of sweetness but also helped reduced calorie intake.

It now seems that artificial sweeteners or noncaloric sweeteners as they are known, have significant and deeper  physiological effects. It would appear they influence glucose metabolism and modify appetite.

Initially, the perception of a sweet taste is found in the tongue of the mouth. In anatomical terms this is the oropharynx. The noncaloric sweetener molecule binds to a sweet taste receptor found on taste buds in the mouth (oral cavity) (Montmayeur & Matsunami, 2002; Zhao et al., 2003).

In Zhao’s paper of 2003, both sweet and umami taste is dependent on molecular interaction with T1R-receptors. To be more specific, these are a specific type of receptor called a G-protein coupled receptor (GPCR) which has two 7-transmembrane subunits called taste 1 receptor 3 (T1R3) and taste 1 receptor 2 (T1R2) (Chaudhari & Roper, 2010).

What then surprised receptor researchers, these sweet taste receptors started to be expressed in various extra-oral tissues including bone, brain and the small intestine (Lafitte et al., 2014).  Before that finding, functional sweetness receptors were identified in the intestinal endocrine cells of mice. These were considered to be the intestinal glucose sensor (Dyer et al., 2005).

The T1R2 and T1R3 receptors perceive noncaloric sweeteners. Binding of molecules like sucralose induces secretion of glucagon-like peptide 1 (GLP-1), GLP-2 and gastric inhibitory polypeptide (GIP). There is also enhanced expression of the active  Na⁺/glucose cotransporter (SGLT‐1) and the passive glucose transporter 2 (GLUT2) (Mace et al., 2010; Moran et al., 2014).

Glucose is absorbed in the small intestines of mammalian species using the classic pathway of active transport involving the  Na⁺/glucose cotransporter (SGLT‐1) and the apical GLUT2 pathway. 

The release of hormones also causes the up-regulation of SGLT-1 and GLUT2 genes. These hormones act as paracrine and neural signals for nearby enterocytes.

The Role Of GLP-1 

Glucagon-like peptide 1 (GLP-1) is a 30 or 31 amino-acid sized peptide which has a number of important functions in weight control. It promotes both a reduction in body weight and blood pressure, and in reducing both plasma glucose and lipids.

The impact on various organs and tissues is consistent and explains a number of changes. in the brain, release of GLP-1 increases satiety and reduces people’s appetite. In the pancreas, it stimulates insulin production and concomitant inhibition of glucagon synthesis. In the liver, fat deposition drops, it reduces inflammation and lowers insulin resistance.  For the kidneys, there is an improvement in sodium excretion and a rise in diuresis. If you consider the heart, mycocardial contractility increases whilst glucose uptake drops and in arteries, endothelial function improves, whilst arteries become more flexible and inflammation drops.

References

Chaudhari, N., & Roper, S. D. (2010). The cell biology of taste. Journal of Cell Biology, 191(2), 429 (Article).

Dyer, J., Salmon, K. S., Zibrik, L., & Shirazi‐Beechey, S. P. (2005). Expression of sweet taste receptors of the T1R family in the intestinal tract and enteroendocrine cells. Biochemical Society Transactions, 33, pp. 302–305 (Article).

Laffitte, A., Neiers, F., & Briand, L. (2014). Functional roles of the sweet taste receptor in oral and extraoral tissues. Current Opinion in Clinical Nutrition & Metabolic Care, 17(4), pp. 379–385. (Article)

Mace, O. J., Affleck, J., Patel, N., & Kellett, G. L. (2010). Sweet taste receptors in rat small intestine stimulate glucose absorption through apical GLUT2. Journal of Physiology, 582(1), pp. 379–392.

Montmayeur, J. P., & Matsunami, H. (2002). Receptors for bitter and sweet taste. Current Opinion in Neurobiology, 12(4), pp. 366–371 (Article)

Moran, A. W., Al‐Rammahi, M., Zhang, C., Bravo, D., Calsamiglia, S., & Shirazi‐Beechey, S. P. (2014). Sweet taste receptor expression in ruminant intestine and its activation by artificial sweeteners to regulate glucose absorption. Journal of Dairy Science, 97(8), pp. 4955–4972. (Article)

Zhao, G. Q., Zhang, Y., Hoon, M. A., Chandrashekar, J., Erlenbach, I., Ryba, N. J., & Zuker, C. S. (2003). The receptors for mammalian sweet and umami taste. Cell, 115(3), pp. 255–266 (Article)