The Physiological Path by which Bitter Herbs Affect Digestion
The process begins with transduction: the epithelial cells of our taste buds translate chemical stimuli into action potentials received by the brain. Each of our approximately 10,000 taste buds, most of which are on our tongue - but a few located in the cheeks, soft palate, pharynx and epiglotis - contains 50-100 epithelial cells that register and respond to the chemical molecules in our food (Crash Course A&P 16, n.d.)
In the taste buds, type 2 taste receptors join with G protein gustducin thereby detecting bitterness on the tonuge (Maehashi et al, 2008). In turn, phosphodiesterase is released activating secondary messengers of diglycerides (DAG) and IP3. IP3 is a molecule that transmits signals. Due to these secondary signals, potassium ion channels are closed, and a build up of potassium ions in the cell leads to the release of neurotransmitters to the brain (Maehashi et al, 2008 & Jacob, n.d.).
The neurotransmitters carry signals which are in turn received by the 7th, 9th, and 10th cranial nerves: the facial, glossopharyngeal, and vagus nerves (Crash Course A&P 16, n.d.). The gustatory area of the cerebral cortex makes sense of the bitter taste and begins to release digestive enzymes in the saliva and gastric juices in the stomach (Crash Course A&P 16, n.d.).
From an evolutionary standpoint, bitter foods were often poisonous. Even some of the bitters we eat have small amounts of phytochemicals that could be poisonous in larger amounts. For example, spinach, wood sorrel, chocolate - and in general many nightshade and cruciferous plants - all contain minute amounts of oxalic acid which at much larger amounts of 10-30 grams can cause kidney failure (CDC, 2014). Perhaps because of this potential for poison, bitters stimulate more production of digestive acids, saliva, bile, and digestive hormones and enzymes than other flavors (Mase, 2013). Additionally, bitters can lower blood sugar and signal the body that it is full. (Weil, 2014).
Specifically bitters stimulate the liver to produce bile. The production of bile not only allows the liver to function more thoroughly in cleansing the body but also helps to break down fats and make fat soluble nutrients more bioavailable (Weil, 2014).
From sensors on the tastebuds to secondary messenger molecules that stimulate neurotransmitters; from processing by the gustatory cortex to nervous system messages sent back to the digestive track: bitter flavors work along a complex pathway to stimulate digestion.
CDC (2014). Oxalic acid: Immediately dangerous to life or health concentrations (IDLH). Retrieved from: http://www.cdc.gov/niosh/idlh/144627.html
Crash Course A&P #16. Taste & smell. Retrieved from: https://curiosity.com/paths/taste-smell-crash-course-a-p-16-crash-course/#taste-smell-crash-course-a-p-16-crash-course
Jacob, Tim (n.d.). Taste (gustation). Retrieved from: http://www.cf.ac.uk/biosi/staffinfo/jacob/teaching/sensory/taste.html
Maehashi, K., M. Matano, H. Wang, L. A. Vo, Y. Yamamoto, and L. Huang (2008). "Bitter peptides activate hTAS2Rs, the human bitter receptors". Biochem Biophys Res Commun 365 (4): 851–855. doi:10.1016/j.bbrc.2007.11.070. PMC 2692459. PMID 18037373
Weil, Andrew M.D. (2014). Why bitter is better. Huffington Post. Retrieved from: http://www.huffingtonpost.com/andrew-weil-md/bitter-foods_b_5206909.html