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We take digestion for granted, but it’s not an easy task: we consume a range of foods, at different times, under changing circumstances. Balancing all these variables, our guts distil macronutrients (proteins, fats and carbs) and process micronutrients and phytochemicals. For this feat, we rely on our digestive juices, secreted into the hollow inner spaces of our digestive organs by specialized glands.

Digestive juices are complex and can contain digestive enzymes, acids and pH regulators, water, electrolytes, immune-active antibodies, lubricants, protective gels, and a lot more. This content is not uniform: at any given time, and in different places along the GI tract, different components are secreted in different proportions, taking into account what type of food we’ve eaten. The purpose of the digestive juices, however, remains the same throughout: smooth chemical breakdown of food to prime the body for optimal absorption and good nutrition. To this end, our physiology uses a signal network of nerves and hormones to coordinate everything from appetite, to the rate of movement and churning in the GI tract, to the secretion of digestive juices. What current research is discovering, and what herbalists have always known, is that bitter flavors have a special role to play in this signal network, triggering its activity and inspiring the production of digestive juices across the entire gut. How does this happen? Researchers focus on bitter herbs like gentian, and bitter-tasting molecules, to gain some insight. In these cases, our bodies’ bitter taste receptors seem to be the bridge between the bitter flavor of these molecules and their supportive benefits. Let’s take a closer look.

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In the mouth, bitter taste receptors (broadly known as T2R receptors) are found on cells all across the tongue 1. These cells are connected to nerve endings (from two cranial nerves, the glossopharyngeal nerve and the facial nerve) 2 that relay the bitter sensation first to the brainstem (a specific area known as the solitary tract nucleus) 3. Here, a reflex “bounce” sends the signal right back to the salivary gland, and digestive juice secretion begins 4. Salivary secretions, like all digestive juices, are rich and complex. We find amylase, an important enzyme responsible for breaking down starches into simple sugars so, later on, they are less likely to ferment and cause gas (try chewing bread or grains for 30 seconds or so and notice the sweet taste as it develops). But the salivary glands also produce bicarbonate, to keep the mouth’s pH alkaline, and protective proteins that strengthen tooth enamel. Finally, saliva is loaded with immunoglobulin-A, an antibody that provides a first line of defense 5. Try some bitter-tasting plants like gentian or wormwood, and notice how you start to salivate 6. By working with the body, the perception of the bitter flavor helps deliver a whole lot more than just the enzyme amylase.

The activity of bitter taste receptors on the tongue doesn’t stop there, though. Remember that signal going to the brainstem? Yes, the reflex “bounce” triggers salivation right away. But the nerve signal continues past the brainstem to the structures of the limbic system, including the thalamus and hypothalamus 7. These important brain centers coordinate the function of our internal organs, and digestive organs are no exception. When the bitter taste sensation arrives there, another signal travels out of the hypothalamus on the vagus nerve and touches off the production of digestive juices in the stomach, pancreas, and liver. Within a few seconds of tasting an herb like gentian, this signal from the vagus nerve starts activating the production of pepsin (a proteolytic enzyme in the stomach), pancreatic digestive enzymes (including amylases, proteases, and lipases), and bile secreted from the liver and stored in the gall bladder 8. The whole gut is coming online. Bring on the food!

Those cells on the tongue where we find bitter taste receptors have another trick up their sleeve, one which becomes more and more of a feature as we move along the rest of the GI tract. In response to bitter-tasting molecules (researchers often use molecules like quinine or denatonium), these cells don’t just send out a nerve signal. They also produce trace quantities of important hormones, which go right into the bloodstream and are distributed widely 9. These include cholecystokinin (CCK), responsible for inducing gallbladder contraction, bile production, and pancreatic digestive enzyme production. They also include glucagon-like-peptide-1 (GLP-1), which seems to be an important regulator of appetite. These endocrine signals synergize with the nerve signals, and also begin the process of regulating consumption and movement of food.

As we move into the stomach, we also find a lot of cells with bitter taste receptors (mostly on cells known as G cells) 10. Here, the activity of these cells shifts almost completely to the production of hormones (and away from sending nerve signals), and specifically, the hormone gastrin. Gastrin activates other cells in the stomach, including parietal cells (which produce HCl, or hydrochloric acid, and intrinsic factor, needed for B12 absorption) and chief cells (which produce the precursor to pepsin, an important protease). But gastrin also sends feed-forward information to the liver and pancreas, reinforcing the neuronal signals that have been coming through the vagus nerve 11. This may be part of the mechanism that the bitter-tasting herb gentian uses to increase the production of gastric juices. Yes, we really are eating! Keep the digestive juices flowing!

The focus in the stomach is protein. Proteases (the enzymes responsible for protein breakdown) aren’t enough: we need adequate hydrochloric acid, too. This acid has the added benefit of sterilizing our food and reducing microbial load in the upper digestive tract 12. Most of the large, convoluted proteins in our food are broken into shorter, more manageable amino acid chains in the stomach. Many of the starches have already started breaking down thanks to salivary amylase. Now, all that’s left is a little attention paid to lipids (fats), and more enzyme secretion to finish breaking proteins and starches into their constituent amino acids and sugars.

 
You will recall that the vagus nerve and the hormones gastrin and CCK have been stimulating the liver, gallbladder, and pancreas to add their secretions to the pool of digestive juices. These neuroendocrine signals are triggered strongly by bitter-tasting molecules 13. So now, as food leaves the stomach and enters the duodenum for the final act of digestion, it is bathed in bile (from the liver and gallbladder) and a balanced set of digestive enzymes from the pancreas. The pancreas fine-tunes the production of these enzymes based on the makeup of the meal 14: more proteases for high-protein, more lipases for high-fat. Bile emulsifies the fats in our food, creating very small bubble-like micelles where those lipase enzymes can go to work. Put it all together, and by the time you are ready to absorb some of the nutrients from the food you just ate, they have all been effectively broken into their smallest building blocks, blocks we can now use for our own energetic and structural needs. Again we see how important it is to activate the neuro-endocrine cascades that regulate and fine-tune this whole process before we take our first bite. And again we see that bitter-tasting herbs–through the bridge of the bitter taste receptors, vagus nerve, and CCK–do just that, increasing the flow of bile from the gallbladder 15.

But in the small intestine, bitter taste receptors take it one final step further. If bitter molecules like amarogentin, quinine, or cynarin (among others) reach these intestinal receptors, they trigger the release of even more CCK and GLP-1, hormones which regulate appetite and reduce feelings of hunger. Through these hormones, the GI tract is able to ensure that we eat a healthy, normal amount of food – again, by engaging the body’s own neuro-endocrine feedback pathways16.

On any given day, we secrete around a gallon of digestive juices (most of the water is reabsorbed)17. This capacity persists with remarkable strength even as we age: pancreatic lipase secretion may drop off a little, and by and large we continue to make more than enough digestive juices throughout our whole lifetime18. In certain cases (cystic fibrosis, for example), the pancreas may be physically incapable of secreting enzymes. But aside from these cases, our mouth, stomach, liver and pancreas work in smooth, coordinated unison to produce just the right amount of digestive juices with just the right “recipe” for the task.

As an herbalist, I am drawn towards engaging and activating the whole suite of digestive juices across the entire gut by using a bitter-tasting herb like gentian. The bonus: I don’t have to guess which ingredient to use when: I can rely on the body’s wisdom and simply provide the gentle cues that support great function. Those cues are bitter plants, and like so many strategies we learn from herbal medicine, they balance digestive juice secretion by focusing on support instead of control. 

We can draw a broader lesson from observing how bitter herbs engage digestion’s neuro-endocrine regulatory system: herbalists see digestive enzymes, bile, HCl and even hormones like CCK less as “stuff” that has physiologic effects, and more as players in a process where the active, engaged coordination of all functions is the key to health. So we strive to awaken these regulatory pathways rather than using isolated “stuff”, because in a living system, attention paid to processes of coordination consistently leads to less friction well-being. And this isn’t some kind of magical coincidence: we were born this way, evolved from an environment rich in bitter cues that have always inspired great digestive juices – and so much more.

Chandrashekar, Jayaram, et al. “T2Rs function as bitter taste receptors.” Cell 100.6 (2000): 703-711.
Lewis, Dean, and Walter E. Dandy. “The course of the nerve fibers transmitting sensation of taste.” Archives of Surgery 21.2 (1930): 249-288.
Benarroch, Eduardo E. Basic neurosciences with clinical applications. Elsevier Health Sciences, 2006. Page 406
Proctor, G. B., & Carpenter, G. H. (2014). Salivary Secretion: Mechanism and Neural Regulation. Saliva: Secretion and Functions, 14–29.
Humphrey SP, Williamson RT (2001). A review of saliva: normal composition, flow and function. J Prosthet Dent 85:162-169.
In general: Spielman, A. I. “Interaction of saliva and taste.” Journal of dental research 69.3 (1990): 838-843. But also note that specific bitter botanicals have received attention for their ability to stimulate salivation (and other digestive juices, too – see (8), below)
Mennella, Julie A., et al. “The bad taste of medicines: overview of basic research on bitter taste.” Clinical therapeutics 35.8 (2013): 1225-1246.
M. Blumenthal, The Complete German E Monographs, American Botanical Council, Austin, Tex, USA, 1998.; W. Knoss and F. Stolte, Assessment Report on Gentiana lutea L., Radix, European Medicines Agency, London, UK, 2009.; J. Koch, Assessment Report on Artemisia absinthium L., Herba, European Medicines Agency, London, UK, 2009. Additionally, for a review of the pharmacology and clinical pharmacology of gentian’s effect on digestive secretions in the stomach and small intestine, see World Health Organization. WHO monographs on selected medicinal plants. Vol. 2. World Health Organization, 2007.
Herness, Scott, and Fang-li Zhao. “The neuropeptides CCK and NPY and the changing view of cell-to-cell communication in the taste bud.” Physiology & behavior 97.5 (2009): 581-591.
Wu SV, Rozengurt N, Yang M, et al. Expression of bitter taste receptors of the T2R family in the gastrointestinal tract and enteroendocrine STC-1 cells. Proc Natl Acad SciUSA 2002;99:2392–2397.
Zaterka, Schlioma, and Morton I. Grossman. “The effect of gastrin and histamine on secretion of bile.” Gastroenterology 50.4 (1966): 500-505. This effect is due, in part, to the structural similarity of gastrin and CCK: Jones, R. Scott, and William C. Meyers. “Regulation of hepatic biliary secretion.” Annual review of physiology 41.1 (1979): 67-82.
Beasley, DeAnna E., et al. “The evolution of stomach acidity and its relevance to the human microbiome.” PloS one 10.7 (2015): e0134116.
Through multiple mechanisms, starting in the mouth and stomach but continuing in the small intestine: Chen, Monica C., et al. “Bitter stimuli induce Ca2+ signaling and CCK release in enteroendocrine STC-1 cells: role of L-type voltage-sensitive Ca2+ channels.” American Journal of Physiology-Cell Physiology 291.4 (2006): C726-C739.; Jeon, Tae-Il, Young-Kyo Seo, and Timothy F. Osborne. “Gut bitter taste receptor signalling induces ABCB1 through a mechanism involving CCK.” Biochemical Journal 438.1 (2011): 33-37.
a fact first noted by Pavlov in 1879, and more recently reviewed in Corring, Tristan, Catherine Juste, and Evelyne F. Lhoste. “Nutritional regulation of pancreatic and biliary secretions.” Nutrition research reviews 2.1 (1989): 161-180.
Sternini C. 2007. Taste receptors in the gastrointestinal tract. IV.Functional implications of bitter taste receptors in gastrointestinal chemosensing. Am J Physiol Gastrointest Liver Physiol 292:G457–G461. And specifically for gentian’s use in humans, see the radiological investigation from 1967: Glatzel, H., and K. Hackenberg. “RÖNTGENOLOGISCHE UNTERSUCHUNGEN DER WIRKUNGEN VON BITTERMITTELN AUF DIE VERDAUUNGSORGANE1.” Planta medica 15.03 (1967): 223-232.
Sternini, Catia, Laura Anselmi, and Enrique Rozengurt. “Enteroendocrine cells: a site of ‘taste’in gastrointestinal chemosensing.” Current opinion in endocrinology, diabetes, and obesity 15.1 (2008): 73.
Dawes, C. (1972). “Circadian rhythms in human salivary flow rate and composition”. Journal of Physiology. 220 (3): 529–545
This has been the focus of attention for quite some time, see for instance Meyer J, Necheles H: The clinical significance of salivary, gastric, and pancreatic secretion in the aged. JAMA 115:2050-2055, 1940; but more recently Gullo L, Priori P, Daniele C, Ventrucci M, Gasbarrini G, Labo G: Exocrine pancreatic function in the elderly. Gerontology 29:407-411, 1983.

We know there are many herbs that can affect mood. Some, like coffee, are strong energizers. Others, like kava, are powerful relaxers. But there is a whole class of medicinal plants–the aromatic nervines–that act as tonics to the nervous system and spirit, helping to find the place of balance between enlivening inspiration and gentle relaxation. These nervines include lemon balm, linden, lavender, rose, skullcap and more, and the way they work in the body is fascinating: at the tissue level, they both support healthy tone and soothe excess tension. And more generally, their smell and their effects in the body synergize to deliver reliable benefits: they uplift the spirit and support a mellow mood. By and large they don’t contain alkaloids–the molecules we usually think of as psychoactive drugs. What are these sweet-scented herbs up to? 

Why do aromatic plants make these volatile molecules to begin with? One of our first clues comes by observing how they’re released: it is after damage to the trichomes that the scent appears in the air. Researchers like Richard Karban at UC Davis have observed how neighboring plants (Artemisia tridentata, specifically) are attuned to these scents, and increase the production of defensive compounds when they detect them nearby. What is even more fascinating is that this response is strongest if the scent–with its precise cocktail of volatile molecules–comes from a genetically-related plant (not necessarily the closest)3. And, as Ted Farmer and his team at the University of Lausanne in Switzerland discovered, these cocktails work by connecting with receptors on the surface of leaves. This triggers a wave (technically a depolarization wave in neuron-like structures) that travels deep into the plant to activate those defenses 4. Herbs use aromatic molecules to communicate. They recognize kin. They warn each other, and spring into action.

It is remarkable to note that, inside plant leaves, this warning is carried on an electrical wave. The structures and mechanisms involved are very similar to those in our own neurons, so you might be tempted to think that we, too, can use these aromatics to send signals across our nervous systems. But while some of our neurotransmitters share structural features with phenolic aromatics from plants, there is a crucial difference: plant volatiles lack the atom nitrogen as part of their chemical structure. So it would seem that, in most cases, these plant signals don’t work in quite the same way in human beings–they aren’t direct triggers for our nerve signal pathways.

But there is one place where we do find a direct connection: our sense of smell. In the mucous membrane of the nose are thousands of receptor sites, each fine-tuned to a different type of volatile molecule. When triggered, these receptors activate neurons that send signals to the olfactory cortex of our brain: we perceive a smell, unique and based on the cocktail of aromatics in the air. Almost immediately our limbic system, that central site of connection between sensation, organ function, and memory, layers meaning into the experience by linking it with past associations and building in emotional content. Think of key smells from your childhood, and how, when  you encounter them, your past experience comes flooding back. This is the power of the direct link between aromatics and our nervous system.

Engaging our brains through smell seems to set the stage for gentle relaxation and focused attention. This can be very useful! We can access this through a range of techniques, starting with incense, potpourri, and mulling spices (and their more modern cousin, the aromatherapy diffuser). Many strong-smelling plants are traditionally used this way. The volatile molecules in pine (a mixture of terpenes, rich in pinenes) contribute to its uplifting aroma 5. Lavender volatiles (terpenes again, with the majority being linalool) seem to be responsible for the plant’s calming scent 6. Lemon balm’s volatile chemistry (featuring the terpene citronellal, and linalool again) is similar 7. Even bergamot oil (from Citrus bergamia, source of the characteristic scent of Earl Gray tea) is sought after for these aromatic qualities 8. There are many other examples, but the general principle seems to be the same: terpenes and simple phenolics from plants speak to our spirits, even when all we do is smell them.

Another great way to deliver some of these volatile molecules to our noses is by steeping aromatic herbs in hot water: a simple cup of tea. You can detect them after a few seconds of steeping: the heat of the water liberates them from their trichomes and they rush upward on the hot steam (one reason you want to put a lid on that steeping cup). But drinking the tea takes the power of aromatic plants a step further, allowing their chemistry to come into contact with the mucous membranes and muscles of the GI tract, and later, other muscles in the body too. This is where things get really interesting, and we see that, even without a direct connection to our nerves, volatile molecules may have an effect on our mood and perception of stress.

When volatiles come into contact with muscle tissue, you get two effects. At first there is a mild pungency that “wakes up” the tissue. This effect varies from plant to plant, and results in a small, temporary increase in contractility. The muscles tighten slightly. This is followed quickly by a more pronounced, and prolonged, antispasmodic action that brings the muscle to a normal, relaxed state of tension and tone (resulting in the more familiar effect of aromatic plants). Maria Lis-Balchin, affiliated with London’s South Bank University, has investigated this and characterizes the overall effect as a “reset” of muscle tone that helps our smooth muscle to find a healthy balance point of tension 9. Others have gone into more detail, describing the mild calcium-channel blockade in muscle cells that comes from contact with the botanical terpenes of peppermint oil in gut tissue 10 or terpene-rich motherwort extracts in cardiovascular tissue 11. This may be part of our aromatic herbs’ mechanism of action, but in the end, what matters is that a warm cup of aromatic tea first engages your senses, and then supports healthy tone of the smooth muscle in our GI tract and other muscles in our body.

To maximize these benefits of aromatic plants, it helps to pay attention to two details: first, recently-dried herbal material often packs more of a punch when brewed than does fresh material. Try crushing and steeping some fresh lemon balm leaves and compare the tea to one made from dry leaves: a given weight of dry herbs has a lot more terpenes than a similar weight of fresh herbs. The tea tastes stronger and more aromatic. But second, the type of liquid the herbs steep in also determines how many volatiles are extracted: essential oils are not very water-soluble, and steeping the herbs in strong alcohol can also extract more aromatic chemistry. This is partly why herbalists will take these strong-smelling, nervine herbs and make tinctures: alcohol is a more versatile solvent, and keeps those terpenes and phenolics in solution better than water alone. With a tincture, you can add small amounts–1/4 to 1 full teaspoon–to seltzer or warm water to liberate the volatiles and enjoy the benefits.

It might seem obvious that, by setting the stage for a relaxed gut and cardiovascular system, we will help support a normal mood and reduce the effects of stress. But the reason why is not immediately obvious. To understand, we have to remember that the limbic system is constantly receiving information from all our organs, including the heart and gut. This information includes the degree of tension and tone: during stress, when circulation to the hands and feet decreases and the gut clamps down, the limbic system detects this high degree of internal tension. Conversely, when we’re relaxed, a whole different set of signals is sent up to the brain. The degree of internal tension and the information from our senses come together with our memories to help us decide: are we stressed, or not?

This blending of internal state and external input has been termed the allostatic load by Bruce McEwen 12, who has conducted pioneering research in the psychology and biology of stress since the late 1960s at Rockefeller University. When a combination of internal and external warning signals (the allostatic load) reaches a critical point, we flip more easily into fight-or-flight mode, and perceive this as “stress”. Jan Moynihan, at the University of Rochester, takes this one step further to note that when we routinely operate under high allostatic load, we see disruptions in mood and spirit 13. It’s important to note that it is the sum of internal and external input that serves as the basis for calculating allostatic load: and while we may not always be able to control our external environments, we can support healthy levels of tension internally using aromatic plants. Our aromatic nervine herbs leave us feeling more calm, warm, present, open and inspired – and this may reduce the internal component of our allostatic load. Events that once were stressful–email, traffic, the disagreeables of life–may be less likely to be perceived that way because the internal landscape is relaxed.

Roses, linden flowers, lemon balm and the citrusy lemongrass, lavender, even mint and the more camphorous mugworts and sages: all these aromatic herbs feature in the traditional herbal record as mood-supportive, stress-relieving allies. It is no wonder they are included in incense and aromatherapy blends. But these herbs can also be taken internally. Neither stimulants nor sedatives, the aromatics bring scent to our inner garden, set the stage for healthy, relaxed tone, and give us another example of the concept of “tonic herb”: just as bitters seem to be a premier digestive tonic, supporting good function by engaging our physiologic processes, so too aromatics avoid pushing our moods in one direction or the other, but rather promote good mood and low stress by engaging the limbic system and supporting normal muscle tone in our internal organs. Reach for them first, bring the tonic into your life every day, and navigate stressful times with joy.

Rohloff, Jens, et al. “Effect of harvest time and drying method on biomass production, essential oil yield, and quality of peppermint (Mentha× piperita L.).” Journal of agricultural and food chemistry 53.10 (2005): 4143-4148.
Meshkatalsadat, Mohammad Hadi, et al. “Identification of essential oil constituents of caraway (Carum carvi) using ultrasonic assist with headspace solid phase microextraction (UA-HS-SPME).” Digest Journal of Nanomaterials and Biostructures 7.2 (2012): 637-640.
Karban, Richard, et al. “Kin recognition affects plant communication and defence.” Proc. R. Soc. B. Vol. 280. No. 1756. The Royal Society, 2013
Mousavi, Seyed AR, et al. “GLUTAMATE RECEPTOR-LIKE genes mediate leaf-to-leaf wound signalling.” Nature 500.7463 (2013): 422-426. Mousavi, Seyed AR, Adeline Chauvin, François Pascaud, Stephan Kellenberger, and Edward E. Farmer. “GLUTAMATE RECEPTOR-LIKE genes mediate leaf-to-leaf wound signalling.” Nature 500, no. 7463 (2013): 422.
Manley, Charles H. “Psychophysiological effect of odor.” Critical reviews in food science and nutrition 33.1 (1993): 57-62.
There are many clinical investigations of lavender oil inhalation. Some more recent studies: Ziyaeifard, Mohsen, et al. “Evaluation of Lavender Oil Inhalation Effects on Blood Pressure and Heart Rate in Patients Undergoing Coronary Angiography.” IRANIAN HEART JOURNAL 18.4 (2017): 29-33.; Allemeier, A., et al. “EFFECTS OF LAVENDER OIL INHALATION ON HEART RATE, SPEED, AND ACCURACY DURING TIMED COGNITIVE TESTING.” International Journal of Exercise Science: Conference Proceedings. Vol. 8. No. 5. 2017.
Abuhamdah, Sawsan, and Paul L. Chazot. “Lemon Balm and Lavender herbal essential oils: Old and new ways to treat emotional disorders?.” Current Anaesthesia & Critical Care19.4 (2008): 221-226.
Watanabe, Eri, et al. “Effects of bergamot (Citrus bergamia (Risso) Wright & Arn.) essential oil aromatherapy on mood states, parasympathetic nervous system activity, and salivary cortisol levels in 41 healthy females.” Forschende Komplementärmedizin/Research in Complementary Medicine 22.1 (2015): 43-49.
Lis-Balchin, Maria, and Stephen Hart. “A preliminary study of the effect of essential oils on skeletal and smooth muscle in vitro.” Journal of ethnopharmacology 58.3 (1997): 183-187. Also, see Lis-Balchin, Maria. Aromatherapy science: a guide for healthcare professionals. Pharmaceutical press, 2006.
Hills, Judith M., and Philip I. Aaronson. “The mechanism of action of peppermint oil on gastrointestinal smooth muscle: an analysis using patch clamp electrophysiology and isolated tissue pharmacology in rabbit and guinea pig.”  Gastroenterology 101.1 (1991): 55-65.
Ritter, Malte, et al. “Cardiac and electrophysiological effects of primary and refined extracts from Leonurus cardiaca L.(Ph. Eur.).” Planta medica 76.06: 572-582.
McEwen, Bruce S. “Allostasis and allostatic load: implications for neuropsychopharmacology.”  -Neuropsychopharmacology  22.2 (2000): 108-124.
Across all ages: see O’Connor, Thomas G., Jan A. Moynihan, and Mary T. Caserta. “Annual research review: the neuroinflammation hypothesis for stress and psychopathology in children–developmental psychoneuroimmunology.” Journal of Child Psychology and Psychiatry 55.6 (2014): 615-631. and also Kobrosly, Roni W., Edwin van Wijngaarden, Christopher L. Seplaki, Deborah A. Cory-Slechta, and Jan Moynihan. “Depressive symptoms are associated with allostatic load among community-dwelling older adults.” Physiology& behavior123 (2014): 223-230.