The gut is a long passage designed to break down and process food, absorb nutrients and reject waste. Like an assembly line in reverse, it will only work efficiently if the delivery of material to the next stage is coordinated closely with the optimum rate of process at that stage. This subtle coordination is achieved by a robust and remarkably reliable network of control systems, orchestrated by a range of neurochemical and endocrine responses reacting to the material in the gut and managed by the complex circuitry and programmes of the enteric nervous system. Smooth muscle cells and the recently reviewed nerve-like interstitial cells of Cajal¹ spontaneously generate electrophysiological slow waves that are then modulated by a network of nerves, ganglia and neurohormones within the abdomen and the intestinal wall that is complex enough to merit the description ‘intelligent’. Yet this is a decentralised system, not controlled entirely by the autonomic or higher nervous systems (the gut has approximately 10⁸ nerve cells but only thousands of nerve fibres connecting to the central nervous system, CNS), and most ‘decisions’ are made at a very local level rather than relying on central controls.²

Thus when functions are disturbed, treatments at a local level may have significant impact. Plant constituents have a unique range of topical effects on the gut. The case will be made that, because of the fundamental linkages between gut and other body functions, these effects can account for not only a valuable contribution to the therapeutics of the digestive system, but also for a very wide range of systemic activity as well (see also later).

In phytotherapy there is a traditional emphasis on normalising the functions of the digestive system. This accords well with its dynamics: like other complex dynamic systems in nature, the digestive system is essentially self-correcting. A gentle trigger stimulation of an appropriate reflex response or the temporary dampening of an inappropriate response may be all that is required to prompt or allow the digestive system to revert to normal patterns of behaviour. Plant constituents seem well suited to such tasks. The following text will review some of the areas of the gut where they may work.

Normal peristalsis is a simple example of the self-correcting and automatous nature of the digestive system. Gut activity is coordinated by a vast complex of nerve fibres, intrinsic fibres within the digestive tract linking to networks of extrinsic fibres, these in turn linked to ganglia within the abdominal cavity. Ascending intrinsic pathways are excitatory on the gut musculature and descending pathways inhibitory, but both are activated by distension. Peristalsis follows the activation by a bolus of food of ascending excitatory pathways proximal to each point along the intestine and the simultaneous inactivation of the descending inhibitory pathways distal to the contraction. The propagation of the circular muscle contraction stops when there is no longer a sufficient distension stimulus ahead.³ The result of this arrangement is known as the ‘law of the gut’. Any bolus of food material in the gut, simply by being there, is normally propelled in one direction, towards the lower bowel;⁴ the absence of such a bolus leads to quiescence.

As well as provoking muscular activity, or motility, the presence of food in the gut also stimulates secretomotor reflexes via submucous neurons and causes a proportion of water and electrolytes that are absorbed with nutrients such as glucose to be returned to the lumen or instead for more to be lost in diarrhoea.⁵ Observations in healthy volunteers point to innate rhythms of absorption, secretion and motility in the small intestine and biliary tract during fasting, these marked by variations in plasma levels of gastrointestinal hormones. Current evidence indicates that this periodicity is generated within the intrinsic innervation,⁶ that is probably by local environmental factors, and that disturbances in these rhythms are potent factors in the aetiology of gastrointestinal and hepatic disease.⁷ There is a significant correlation between motility and secretory modes, so that they are often coupled as the ‘secretomotor’ mode. As elaborated below, cholinergic neurons seem to mediate the shift in their direction from the absorptive mode.⁸

Carminative herbal remedies, such as the warming spices, reduce motility⁹ (and thus, possibly, secretory activity) and, at least in the case of ginger, increase absorptive activity (see the ginger monograph). Increases in secretory activity and motility may be seen after the prescription of bitters, cholagogues and stimulating laxatives. However, these responses are not always predictable (in the case of bitters and cholagogues they may even be the opposite reaction in some cases) and it is also observed that reactive bowel looseness may follow a much wider variety of treatments. It appears that increases in gut activity are programmed as a response to any potentially perturbing influence, presumably as a simple defence mechanism.

If there is gut infection or food intolerance, luminal antigens or bacterial products may be detected by the immune system. This may trigger a cascade of events associated with the release of inflammatory mediators. These mediators lead to increased motility and secretory patterns that are characterised by strong muscular contractions, copious secretion and diarrhoea.^10,11 The close connection between the gut-associated immune responses and the enteric nervous system is confirmed as important in the symptomatology of several functional disorders.^12,13 The enteric nervous system is also increasingly recognised as a regulator of epithelial barrier integrity, especially when the latter is exposed to inflammatory challenge; chronic inflammation has further disabling effects on enteric neuronal function.¹⁴ All this provides further support for an integrated functional model in managing gut and bowel disturbances.

Diarrhoea is an extreme pattern of fluid and electrolyte loss which, as a consequence of enteric infections and malnutrition, is still the most common immediate cause of death around the world. In developed societies it is rarely as dangerous as it is in impoverished regions, where rehydration with fluids and electrolytes is a critical life-saving measure (as seen in the sugar and salt solution widely used by aid agencies around the world for the purpose). Nevertheless, diarrhoea is relatively common, reflecting a wide variety of events in the gut, from food poisoning and enteric infection through hepatobiliary activity (bile being a prominent potential irritant of the gut wall), food intolerance, to irritable bowel behaviour.¹⁵ Often diarrhoea is of short duration and is not explained. It is even seen as a benign transient cleansing event in some healing strategies (‘better out than in’).

Any looseness of the bowel involves the loss of considerable amounts of fluid and electrolytes which, with the certainty of the excretion of a wide range of other materials, means that the gut can dwarf the function of the kidneys. Even modest fluctuation in bowel consistency and frequency can have significant impact on fluid balance in the body.

Some herbal traditions use remedies that loosen or bulk the bowel contents to reduce fluid accumulation and retention and even in some circumstances reduce weight. Cathartic remedies were included among those with drastic diuretic effects in the Chinese tradition, and stimulating anthraquinone laxatives clearly lead to fluid and electrolyte loss. In the French tradition, even bulk laxatives and fibre are used to reduce weight associated with fluid retention.

By contrast, there are also a range of approaches to reducing bowel looseness if this is seen as harmful. These include the temporary use of astringent tannins that reduce reflex irritation of the bowel from the higher reaches of the gut in the case of gastroenteritis, the aromatic digestives and volatile antispasmodics, and the use of demulcent and topically anti-inflammatory remedies such as Althaea (marshmallow), Filipendula (meadowsweet), Glycyrrhiza (licorice), Calendula (marigold) and, for short-term use, Symphytum (comfrey) (see Chapter 2).

The stomach’s functions are closely coordinated with the rest of the digestive tract. It stores masticated food as a hopper, delivering its acidic contents into the alkalinised duodenum only at a rate that can be alkalinised: too fast and it damages the duodenal wall, too slow and it leads to discomfort and inhibits digestion of the following meal.

The stomach is also the first significant point of contact with most herbal remedies. Many have immediate effects on its function, increasing secretions of acid, pepsin and mucus (bitters and pungent constituents), calming activity (mucilaginous, astringent and some aromatic and antispasmodic remedies) or stimulating motility (saponins and emetic alkaloids). It is often highly sensitive to even minor doses of herbal remedies.

Healthy upper digestive function is important for maintaining health and preventing disease. Low acidity can lead to poor nutrient absorption and abnormal bowel flora. Patients with reduced gastric secretion are more susceptible to bacterial and parasitic enteric infections.¹⁶ The contribution of poor upper digestive function to the chronicity of intestinal dysbiosis is often overlooked by therapists: the herbal practitioner has bitters and pungent principles to employ.

The coordination of digestive processes is mediated in the first instance by batteries of neurohumoral agents, chemicals that range from familiar endocrine hormones, standard neurotransmitters and chemical mediators, found also in the CNS and other body tissues, to gut-specific chemical agents. Each may be elicited by several receptor types in the gut wall and thus may be sensitive to different secreted, metabolised, dietary or pharmacological agents. These interactions, although highly complex and multilayered, are all integrated into a whole control mechanism for digestive activity.¹⁷ Many constituents of herbal remedies are likely to interact with chemical mediators and this may provide some mechanisms for their effect on gut function. When these interactions are combined with similar examples in the nervous, endocrine, immunological and reproductive systems it appears that herbal medicine might genuinely provide a unique ‘psychoneuroimmunological’ strategy for the widest body disharmonies, one moreover that is centred on the gut.¹⁸

One common phytotherapeutic constituent has already established a number of contrasting effects on several neurohumoral mechanisms. Capsaicin, from Capsicum spp., at quite low concentrations blocks the release of calcitonin gene-related peptide, a peptide found in enteric ganglia and known to be elicited directly by local mucosal irritation leading to increased peristaltic and secretomotor activity. Capsaicin thus results in a concentration-dependent decrease in peristaltic activity following mucosal stimulation.¹⁹ Another recent discovery is the reflex release by capsaicin-stimulated nerve fibres of somatostatin, a systemic anti-inflammatory and analgesic with modulating effects on various digestive and pancreatic secretions.²⁰ Capsaicin also blocks an alternative stimulant to intestinal contraction, vasoactive intestinal peptide (VIP): VIP mediates the effects of noradrenaline and local stressors (such as acute hypoxia in vitro) which both depress cholinergic transmission and stimulate non-cholinergic contractions.^21,22 On the other hand, capsaicin is also known to induce intestinal contractions as a stimulant of sensory substance P-containing neurons,²³ which are also activated by various luminal stimuli such as the presence of food metabolites and simple physical distension to stimulate intestinal contractions. The consumption of hot spices has long been a feature of traditional herbal therapeutics (see also the discussion of ‘acupharmacology’ below) and the effects of Capsicum on the wider physiology have been confirmed in studies of cutaneous blood flow after ingestion of chillies; variable increases in blood flow have been observed, most consistently in the abdominal area.²⁴

One of the most important hormonal regulators of the digestive process is cholecystokinin. This hormone is concentrated in the wall of the upper small intestine and is secreted into the blood on the ingestion of proteins and fats and has also been implicated in the action of bitters. The physiological actions of cholecystokinin include stimulation of pancreatic and gastric acid secretion and gallbladder contraction and regulation of gastric emptying, gastrointestinal motility and satiety, as well as augmenting symptoms of fear and anxiety.²⁵ (Plants with anxiolytic reputations have been shown to bind to cholecystokinin receptors and one, gotu kola, has been shown to attenuate startle response in human subjects.²⁶) It is produced particularly by carbohydrate foods and appears to promote a feeling of fullness.²⁷ In humans it has been found that foods that lead to high levels of both cholecystokinin and satiety (such as bran) are associated with lower postprandial blood sugar and insulin levels, compared with refined carbohydrate.²⁸ This may be due to the physical characteristics of the carbohydrate but may also suggest that cholecystokinin helps to suppress hyperinsulinaemia.²⁹ Cholecystokinin has also been observed to suppress taste when feeding strongly tasting foods to animals.³⁰ There are suggestions that an increase in cholecystokinin production with age may contribute to the relative anorexia in the elderly.³¹

Gastrin is a polypeptide hormone which is secreted following vagal nerve activity and by the bulk presence of food in the stomach. Some food extractives, including partially digested proteins, alcohol and caffeine have an additional effect and some of the more stimulating plant constituents such as resins, spice ingredients and some saponins are likely to compound this activity.³² Gastrin stimulates gastric acid and pepsin secretion. It extends and indeed multiplies the short-term effect of vagal stimulation on gastric secretions.

When gastric hydrochloric acid spills over into the intestine it lowers intestinal pH and stimulates the secretion of secretin. This acts to stimulate Brunner’s glands and bicarbonate-rich bile and pancreatic secretions so as to alkalinise the intestinal contents. Secretin is also a potential mediator of the antiulcer actions of mucosal protective agents; it appears that secretin inhibits gastric acid secretion via endogenous prostaglandins.³³ Glycyrrhiza (licorice) stimulates the release of secretin in humans.³⁴

Serotonin or 5-HT receptors have been shown to mediate the emetic reflex³⁵ and antagonists of the main receptor concerned, type 5-HT₃, have been developed to reduce emesis of cancer chemotherapy. (These are also found to have anxiolytic effects, confirming the overlap in enteric and higher nervous mediators.³⁶) There is evidence, referred to in its monograph later in this book, that ginger also acts on this receptor. 5-HT is also a likely mediator in the lower bowel of both sennoside activity and the diarrhoeal response.³⁷ It is possibly implicated in the action of the emetic plant remedies.

There are a number of herbal constituents shown to have effects on neurosynaptic mediators. These effects are also likely to impinge on gut function, as many mediators have been found common to both. Gamma-aminobutyric acid (GABA), probably affected by a number of plant constituents, is produced in the myenteric plexus in the gut wall and acts via GABA receptors to both stimulate cholinergic and relax VIP motor neurons, contributing to both components of the peristaltic reflex.^38,39 Among many other neuroactive substances and receptor sites now identified as important modulators of gut function, the endocannabinoids⁴⁰ and fasting-associated orexins⁴¹ are promising further templates for phytochemical activities. Certain Echinacea alkylamides are cannabinoid 2 receptor agonists (see Echinacea monograph).

The role of the gut wall is to allow for selective absorption of nutrients while providing vital protection against intrusion into the body tissues of harmful substances from the lumen. Non-steroidal anti-inflammatory drug (NSAID) treatment has adverse effects on enterocyte mitochondria, which may predispose the mucosa to absorption of bacterial and other large molecules that provoke a local inflammatory response. A similar mechanism may operate in patients with untreated Crohn’s disease, who show abnormally high permeability. Remission of Crohn’s induced by treatment with elemental diets coincides with a reduction in permeability. Significant correlations have been seen between permeability and plasma IgA concentrates in kidney disease and between permeability and the passage of neutrophil chemotactic agents.⁴²

It is likely that some plant constituents could reduce excessive intestinal permeability. Tannins are likely to have a limited short-term effect at least in the upper reaches of the tract and healing plants such as Matricaria recutita (chamomile), Filipendula (meadowsweet), Ulmus spp. (slippery elm),⁴³ Glycyrrhiza (licorice), Calendula and Symphytum (comfrey) have long been applied with this effect in mind. In theory, local anti-inflammatory activity might effectively reduce some types of increased permeability. However, the most promising effect on intestinal permeability is likely to lie in changing biliary constituents, using hepatics and choleretics (see later in this chapter).

The balance of flora populations in the gut is highly complex but, in steady-state health and dietary conditions, probably reasonably stable. In humans, for example, there seems to be a moderately predictable sequence of colonisation after birth and through to adulthood, with fluctuations in the relative numbers of aerobic and anaerobic bacteria in newborns up to a total of 10¹⁰/g wet weight, reaching in adulthood between 10⁵ and 10⁷/g wet weight of aerobic bacteria and between 10¹⁰ and 10¹¹/g wet weight of anaerobic bacteria.⁴⁴

The benefits of a healthy bacterial population in the gut are clear. Anaerobic bacteria in particular are shown to be responsible for considerable secondary digestion and to decrease intestinal transit time.⁴⁵ Normal bacteria like Escherichia coli, Enterococcus faecalis and Bacteroides distasonis have been shown to help protect the gut from pathogenic infiltration and there are a number of other non-specific defences known.⁴⁶ Mechanisms are likely to include modification of bile acids, stimulation of peristalsis, induction of immunological responses, competition for substrates and possible elaboration of various bacteriostatic substances.⁴⁷ The intestinal flora also contributes to non-specific defences against immunological challenge from dietary antigens by helping to reduce their uptake across the mucosal barrier.⁴⁸

The populations of bacteria and other organisms are, however, obviously dependent on their food supply, the dietary material and its metabolites, reaching the lower intestine. There is a potential for rapid responses to diet: a general reduction in bacteroidetes and increases in firmicutes, clostridia, bacilli and erysipelotrichi are apparent within 1 day of moving the diet from plant based to ‘Westernised’.⁴⁹ Bowel flora in African populations with a plant-based diet are also found to be much richer in bacteroidetes than Western groups.⁵⁰ Probably the most widespread impact on bacterial populations in the gut in modern times, however, is the use of antibiotics. It has long been established that this has an adverse effect on normal gut flora.⁵¹ Antibiotic use can lead to secondary hypersensitivities in the body, presumably through their effects on intestinal flora.⁵² For example, they can lead to increased activity of Candida, which has been shown to be a factor in driving allergic responses.⁵³

The relationship of bile products with intestinal flora is complex and works in two directions (see also p. 209). Bile salt metabolites variably stimulate growth in bacterial populations,⁵⁴ while anaerobic bacteria act on bile products to produce volatile fatty acids that control other pathogenic bacteria.⁵⁵ A particularly revealing insight into the relationship is seen in the case of bowel cancer. There are three known endogenous components that affect development of colorectal cancer – colonic bacteria, the mucus layer and bile acids. The major effects of the bacteria are deconjugation and reduction of bile acids, activation of mutagen precursors, fermentation and production of volatile fatty acids, formation of endogenous mutagens and physical adsorption of hydrophobic chemicals. The mucus layer covering the surface acts as a barrier and its composition changes in premalignant and malignant colon tissue. The secretion of protective mucus is elevated by plant cell wall components in the diet. Mucus has some hydrophobic properties and its presence may alter the effect of bile components and bacterial metabolites on the gut wall.

Bowel bacteria have been linked to another cancer. In looking for reasons to explain the epidemiological link between high-fibre diets and lower risks of breast cancer, it was found that both raising fibre content in the diet and suppressing microflora with antibiotics led to reduced intestinal reabsorption of oestrogens and lower levels circulating in the blood. It was concluded that intestinal microflora raise oestrogen levels by deconjugating bound oestrogens that appear in the bile, thereby permitting the free hormones to be reabsorbed.⁵⁶ The beneficial levels of a high-fibre diet are likely to be the dominant factor in women susceptible to breast cancer, especially as there is evidence that bacterial flora actually enhance some of its wider benefits.⁵⁷

There are several ways popularly promoted to correct disturbed or damaging bowel flora. Current evidence indicates that varying probiotic strains mediate their effects by a variety of different effects that are dependent on the dosage employed as well as the route and frequency of delivery. Some probiotics act in the lumen of the gut by elaborating antibacterial molecules such as bacteriocins; others enhance the mucosal barrier by increasing the production of innate immune molecules; and other probiotics may mediate their beneficial effects by promoting adaptive immune responses (for example, secretory immunoglobulin A, regulatory T cells, interleukin-10). Some probiotics have the capacity to activate receptors in the enteric nervous system, which could be used to promote pain relief in the setting of visceral hyperalgesia.⁵⁸ The value of a high bulk diet with reduced simple sugars intake is, however, more accepted. The phytotherapist might combine the benefits of such dietary moves with attendance to hepatic and biliary function and with bitter or aromatic digestive insurance that food matter is well rendered in the upper digestive tract. The value of direct agents such as Artemisia absinthium (wormwood), Marsdenia (condurango) and Allium sativum (garlic) on disruptive bowel flora is likely to be upheld (see also later under Intestinal dysbiosis).

Plant fructo-oligosaccharides are associated with ‘prebiotic’ properties; that is, they selectively stimulate growth and/or activities of health-giving microbial species in the gut.⁵⁹ They are a mixture of oligosaccharides typically consisting of glucose linked to fructose units. They are widely distributed in plants such as onions, asparagus and particularly cereals,⁶⁰ and in herbal remedies such as Cichorium (chicory) and Taraxacum (dandelion root).⁶¹ They are not hydrolysed by human digestive enzymes but are utilised by intestinal bacteria such as Bifidobacteria, Bacteroides fragilis group, Peptostreptococci and Klebsiellae. In clinical studies, improvement of faecal microflora was observed on oral administration of fructo-oligosaccharides at 8 g⁶² and 12.5 g⁶³ per day; the population of bifidobacteria in faeces increased substantially compared with before the administration. Mucilages can also be used to ‘feed’ healthy bacteria.

Modern insights into the fate of much plant material in the digestive tract support a view that a herbal remedy mostly affects the gut and its immediate surroundings. Adding what is known of the interrelationships between digestive activity and the wider body’s physiology allows the modern phytotherapist to develop a rationale for the effect of herbal medicines on the body that is both unique to these remedies and provides potentially very powerful therapeutic strategies.

The body presents two distinct surfaces to the outside world, each of which has its own triggers to initiate reflex responses. The skin is a sensitive template from which it is possible to trigger a wide range of reflex responses elsewhere in the body. When one touches a very hot object, for example, the response is both complex and predictable. More positively the benefits of touch, caressing and massage are increasingly well understood. There are a number of established mechanisms by which cutaneous stimulation can have an impact on both specific internal functions and on general well-being: for example, via dermatomes and spinal afferents, stretch sensor stimulation and somatopsychological connections. These have been used to explain the potential benefits of hands-on therapies.

There are other less well-established mechanisms, possibly involving neurohormonal reflexes, that have been suggested as underpinning acupuncture, acupressure and shiatsu and possibly reflexology – the claim that stimulating particular points on the body surface can effect substantial benefits elsewhere. If stimulation of certain points on the body surface can lead to changes in neurohormones such as enkephalin, then the fact that the stimulant was a steel, stone or gold needle, a warming moxa or finger pressure seems less important than the point that was stimulated. It appears that such reflexes are programmed, even wired, into the system.

If the skin has the potential for mediating complex effects within the body then the second of the body’s surfaces has much greater potential. The lining of the gastrointestinal tract has a hundreds of times greater surface than the skin. It is also a dramatically more complex surface structure. Unlike the skin, which has primarily a protective function, the gut surface has a literally intimate engagement with the outer world. It provides by far the largest exposure of the body’s immune system and other defences to the outside world.

The main inputs into the decision-making processes involved in digestion are a vast array of receptors and sensory tissues along the gut wall. Each of these provides signals for some effector function elsewhere in the gut or indeed elsewhere in the body. The effects of the following archetypal plant constituents discussed in this chapter and in Chapter 2 can clearly be seen to work primarily on the digestive tract.

There are several general ways by which stimulation along the digestive tract can influence wider body functions, some of which were reviewed earlier.