Gut function, introduction

The gut is an adaptive interface, an important part of the digestive system which separates the outside world from our internal world: a tube with two endings. The mouth at one end and the anus at the other. The inside of the tube may, in fact, be considered external to the body, because the influence of the outside environment can be felt inside this tube.

If we look at the gut in a simplistic way, it acts as border control, only allowing “passport-holders” to enter. One individual (or nutrient) can pass this border at a time, in the same way we cross border control at an airport. This means that large groups of people (large molecules) must be broken, and this is what the upper part of the digestive tract is entrusted with: getting each individual (nutrient) ready for border control (absorption).

It has been known for millennia that “all disease begins in the gut” a common phrase attributed to Hippocrates (460–377 BC), but we are only beginning to truly understand the pivotal role of the gut in health and disease. Imagine if large groups were able to push through border control and cross the frontier, and carry their luggage with them (as observed in increased intestinal permeability, or leaky gut syndrome). Imagine individuals carrying weapons and attacking border control staff (reactive molecules like gluten and those found in dairy products, as well as toxic substances like some additives, traces of solvents manufactured from petrochemicals, and pesticides). Imagine a loud, problem-making group of individuals (sugar), disturbing the entire environment and driving chaos. Imagine this happening all day every day. Border control would become a place at war, desperately trying to hold their position while being attacked on every front, with commonly innocent individuals (lectins, phytic acid, histamine) and residents (opportunistic pathogens) throwing all they got (microbial inflammatory byproducts) at border control in order to maintain chaos, which can only get worse if the perpetrators are not removed. Once eliminated, the inflammatory situation may still remain for a while, but, eventually, the area will return to normal and order may be restored. This is not to say that future outbreaks are impossible, especially since those ‘seemingly innocents’ have experienced the full extent of their power.

Back to our physiology.
The gut is designed to process food in order to provide the energy, building blocks and cofactors essential in every biological process. It is also a two-way interface that allows for the elimination of metabolic waste and other substances. It is also a barrier against ingested pathogens and other factors. The gut, therefore, plays crucial roles in digestion and absorption, integrity and immunity, motility, and balancing the gut microbiota. Any issues arising from any one of these key roles will have an impact on the assimilation of essential nutrients and on our overall health and well-being.

The impact of our modern world on our gut environment

It is not surprising that our gut and its integrity have been put to the test since the Industrial Revolution. Since then, new processing methods and new molecules have been invented to increase standardisation and profitability by using cheap ingredients and the manufacture of convenience and ultra-processed food products. A dramatic shift happened in human history at this time. We became disconnected from our roots, from nature, with the masses moving to the cities, and from food. Eating has become associated with pleasure and is no longer seen as a necessity to fulfil our most basic physiological requirements. Meal settings and habits were also broken. We started to eat much faster, often eating on the go or at our desks, and more often, a shift also triggered by the addictive ingredients newly incorporated into our food chain.[1]

Today, less than 50% of people eat fresh foods[2] and far too few consume 5 of their 5-a-day. In fact, NHS UK displays alarming statistics: In 2018, only 28% of adults and 18% of children aged 5 to 15 were eating the recommended five portions of fruit and vegetables per day.[3]

Eating less nutritious foods, like fruits and vegetables, and eating a greater amount of ultra-processed manufactured food products known to contain a multitude of additives and addictive substances, is raising serious safety concerns.[4] In fact, many additives have been shown to impact the constitution of the gut microbiome and play a part in diseases involving microbial imbalances, inflammation and stress.[5,6]

Eating faster, mixing too many food groups in one sitting and the increased complexity of food pose a bigger challenge for our digestive system and it is likely that we have not yet adapted to those changes. This may correlate with the explosion of metabolic disorders and increased rates of obesity and diabetes.[7]

At the same time, malnutrition is back in the Western parts of the world where food abundance is highly common. Therefore, malnutrition is no longer linked to not having access to food but to particular dietary choices, including poorly nutritious diets revolving around ultra-processed manufactured food products, or other types of restrictive diets (crash dieting, calorie-controlled diet, strict diets like paleo, ketogenic or vegan diets, or supervised or unsupervised therapeutic diets, like the FODMAP diet).

While our diet is having a major impact on our health, one of the key components of health that is the subject of much dispute is fibre. Many studies have demonstrated that excess intake of fibre does not protect the body from metabolic disorders and may actually be harmful, particularly for gut health.[8] When it comes to research, the invested parties, the choice of the studied substance, and conflicts of interest must be accounted for, and this is why there are so many contradictory conclusions.[9,10] It may be anticipated that using highly reactive molecules in studies, like gluten found in wheat cereal products, may cause inflammatory responses that negate the health-supportive effect of dietary fibre.

Fibre: Insoluble or soluble?

Fibre is essential to create bulk and maintain peristalsis, the one-way movement of the digestive tract, generated by stress impacted to the cell wall by food or stools. In response to the pressure, cells of the digestive tract wall contract, pushing the matters forward, in a — domino-like effect — series of wave-like muscle contractions, until most or all nutrients from food and water have been extracted, and the stools reach the rectum, ready for elimination.

Insoluble fibre, on the other hand, does not participate in the formation of stools. This type of dietary fibre is often referred to as prebiotic fibre and, therefore, plays a pivotal role in the balance of the gut microflora. This type of fibre is essential to life. We are clearly outnumbered. We harbour a multitude of microbial communities which shadow our own cells, and so, the impact of the gut microbiome on our body, in health or disease, cannot be any more apparent. Feeding the gut microflora with the right kind of raw ingredient is, therefore, crucial to maintaining a healthy gut milieu that contributes to our health and overall well-being. It is now understood that the gut microbiome has an effect on our mood and our emotional balance, as well as our cognition. Poor cognition and neurodegeneration[11] have been linked to dysbiosis (a disturbed gut milieu), increased intestinal permeability and a sugar-rich diet,[12] among other key dietary components, like a very low (soluble) dietary fibre intake.[13,14,15]

A quick insight into the digestive system

Digestion involves the mechanical and chemical breakdown of food into smaller components that can be more easily absorbed and assimilated by the body. A host of digestive organs work in harmony (including the digestive tract, pancreas, liver, and gallbladder) to ensure that we are able to extract the nutrients from food to power the demands of the body. Once food has been efficiently digested, it needs to be absorbed into the systemic circulation.[16]

The upper part of the digestive tract, consisting of the mouth, oesophagus, stomach and the initial part of the intestines (the upper duodenum), is where most chemical digestion takes place. Digestion begins in the mouth, with the mechanical action of chewing, enzymes contained inside saliva (amylase enzymes initiate the breakdown of starches). The stomach, via the release of acid release, continues the digestive process by allowing the breakdown of proteins and fats (due to the effect of pepsin and gastric lipase respectively), and mineral dissociation (stomach acid is required to assimilate minerals). The release of the intrinsic factor by stomach cells is essential to chaperone B12 and allow for its absorption by the colon. Highly corrosive stomach acid also sterilises the milieu, the first barrier against pathogenic microbes ingested with food.

The pancreas secretes alkalising compounds to increase the pH of chyme (a mixture of partially digested food and stomach juices), as well as digestive enzymes that allow for the continuous breakdown of food. The brush border, the upper part of the intestinal tract, also secretes digestive enzymes.

At the same time, the liver produces bile, which is collected and distributed in great amounts by the gallbladder, allowing for the emulsion of fats (breakdown of fat into fatty acids) and the absorption of fat-soluble compounds (e.g., fat-soluble vitamins). Bile also displays mild laxative capabilities.

Nutrients are now ready to be absorbed. Most nutrient absorption occurs in the jejunum, the central area of the small intestine. The formation of cellular villi and microvilli increases the surface of absorption and the motility of food through the small intestinal tract.

By the time food remains enters the colon, it is ready for elimination. However, the colon is also the part of the digestive tract where useful material is reabsorbed. This includes water (the large intestine plays a crucial role in maintaining positive water balance), electrolytes and certain vitamins. While the colon doesn’t produce digestive enzymes, the digestive process is continued, being taken over by the resident microflora. Gut bacteria release short-chain fatty acids and a multitude of other health-supporting metabolites feeding on dietary fibre and other nutrients (e.g., sugar and undigested food components).

Compromised digestion

The timely movement of food and fluid through the digestive tract is essential to digestion, absorption and elimination. Disrupted peristalsis and the consequent slowing down or speeding up of this process has been associated with many conditions including gastroesophageal reflux disease (GERD), SIBO/SIFO, constipation and/or diarrhoea, and complications related to malabsorption and impaired detoxification.

These processes can be disrupted by many factors, including stress (anxiety, depression, overwhelming life demands and poor coping mechanisms), eating too fast, eating too much and mixing too many food groups in one sitting, drinking too much liquid with a meal, eating on the go or eating while working. Again, relying on poorly nutritious ultra-processed food products may irritate the lining of the digestive tract, fuel inflammatory responses and disrupt enzymatic and absorptive capabilities. A lack of dietary fibre intake will also play is part in poor motility and waste retention.

Low stomach acid can have a knock-down effect on all processes occurring at any stage of digestion. As explained earlier, stomach juices are essential to break down proteins and fats and the assimilation of minerals and vitamin B12. Low stomach acids due to suppressed digestive capabilities under the action of stress (or chronic inflammation, or certain drugs) can lead to proteins putrefying inside the digestive tract and producing gas, leading to bloating and abdominal discomfort and, in severe cases, debilitating pain. The absorption of nutrients and amino acids (the building blocks of life) can be greatly compromised as a result, even more so if sugar is present. Sugar fuels inflammatory processes and also plays a key role in dysbiosis, immune dysregulation and increased intestinal permeability.

Constipation

Constipation is one of the most common medical complaints in the western parts of the world. A significant portion of the population describes being constipated continuously or frequently.

Internationally, the incidence of constipation varies, depending largely on dietary habits. Constipation can occur at any age and is more common among individuals who resist the urge to move their bowels at their body’s signal, especially in the morning. Also, this often happens when children start school or enter daycare and feel shy about asking permission to use the bathroom or are prevented from doing so. It also can happen to adults who are in jobs where they cannot take bathroom breaks at will. Constipation is more common in women than in men and is especially likely to occur during pregnancy. Dietary changes linked to ageing may also be involved. People over age 65, particularly women, are more likely to experience constipation.[17]

Constipation is an acute or chronic condition in which bowel movements occur less often than usual or consist of hard, dry stools that are painful or difficult to pass. It is characterised by infrequent bowel movements (less than 3 times per week), stools that are difficult to pass or have a hard consistency, and a sense of incomplete evacuation, as well as abdominal bloating.[18]

When waste sits or moves very slowly throughout our digestive tract, it gets fermented by microbes and becomes a literal breeding ground for all kinds of opportunistic and often pathogenic microbial species. These species then produce all kinds of toxins that create a stress response in our body and increase whole-body inflammation levels. Therefore, constipation is a very serious condition, especially if chronic.

Clinical observations display a very concerning picture with many people only going to the toilet once a week or even less often. This problem can significantly impact the quality of life for all ages, and evidence shows that it is affecting 20% of the population.

Some people may be dealing with bouts of constipation and diarrhoea, where compacted stools may prevent colonic emptying — only watery stools can escape.

Because the colon is involved in water balance, dehydration will have a more or less severe impact on the hardness of stools, and so constipation. Again, a diet low in dietary fibre may not provide sufficient bulk for regular stool formation. Food hypersensitivities to gluten, dairy and other specific foods can increase the likelihood of chronic constipation.

Solutions

Changes in stool frequency and consistency can have an effect on your overall health, driven by, driving and perpetuating digestive disorders in a self-feeding cycle, where poor digestive capabilities increase stress and inflammation, and inflammation and stress suppress digestive processes.

If you are taking drugs that suppress stomach acid production (e.g., omeprazole and PPIs), it is very likely that your digestion is not optimal and the cause of bloating, flatulence and constipation. It may be important to address the causes of heartburn and GERD to re-establish normal stomach function and reduce the sensation of burning due to low-stomach acid (NOT excessive stomach acid production).

Increase your water intake. Again, dehydration is one of the main culprits in chronic constipation. Keeping a bottle of water by your side at all times can be a great reminder to drink more. Setting regular alarms during the day can also remind you to drink. Drink plenty of water or herbal teas in the day to stay hydrated.

Ensure you consume your five-a-day, and aim to increase to ten portions of vegetables a day and a couple of fruits (no more than three). A handful of berries or a cup of vegetables qualifies as a portion. For indication, one apple makes one of your five-a-day. Two apples still make one of your 5-a-day. So a varied, balanced diet with as many different colours of vegetables is highly recommended.

Increase dietary fibre intake. However, it is important to consider that insoluble fibre feeds for the ‘good’ and the ‘bad’ bacteria. Yet, the good kind has a greater affinity with soluble fibre. Starting low and slow is of the utmost importance to prevent symptoms from worsening. Studies have shown that increasing fibre intake to around 29 grams per day is optimal. Intake of over 38 grams doesn’t display any benefits and may lead to diarrhoea. If your diet mainly revolves around ready-made meals, takeaways and ultra-processed manufactured food products, start by incorporating fresh fruits and vegetables in your diet. Then, aim for 50% of your plate to be vegetables. Mix a variety of raw (salads, fermented foods like kimchi) and cooked (soups and broths, steamed or baked) vegetables. Antioxidant-rich they also play a key role in balancing inflammatory responses and reducing free radical damage; therefore, always aim to consume one vegetable of each colour (Orange: sweet potatoes, squash, oranges, papaya. Blue: blueberries, blackberries, blue carrots and tomatoes. Red: tomatoes, red peppers, radishes and beetroots. Green: especially vegetables from the Brassicaceae family like broccoli, Brussels sprout, cauliflower, kale and cabbages, and spring/collard greens. Yellow: yellow peppers, beans and courgettes, pineapple, mango, and pumpkin).

If you are choosing to supplement with prebiotic fibre (GOS, FOS and XOS), again start low and slow, and build up to optimum dosage. The severity of symptoms appears to be dose-dependent. However, symptoms associated with increased dietary fibre intake may dissipate within a two-week period so, daily intake is highly recommended to offer any benefit and allow for beneficial bacteria to change the gut milieu to a more acidic environment (lactobacilli and bifidobacteria produce lactic acid), which is essential to displace and keep oxygen-breathing pathogenic microbes under control.

Consider the following foods, which are high in insoluble fibre:

• Flax/linseeds, sunflower and chia seeds

• Almonds, hazelnuts and walnuts

• Oat/wheat bran and germ

• Beans, lentils and legumes

• Dry fruits (sulphite-free)

• Apple and pears (also contain sorbitol a type of sugar that many people cannot digest, leading to bloating and gut discomfort).

• Greens (spinach, lettuce, kale, mesclun, collards, arugula, watercress, etc.)

• Peas, snow peas, snap peas

• Green beans (also an excellent source of silica)

• Bell peppers

• Aubergines

• Celery

• Onions, shallots, leeks, scallions, garlic

• Brassicaceae: broccoli, Brussels sprouts, cabbage, cauliflower, kale, pak choi

If you suffer from any gut disorder, start with the following vegetables that are high in soluble fibre, but lower in insoluble fibre (these tend to be safer for those with gut issues):

• Carrots, sweet potatoes and squashes (peeled)

• Asparagus tips

• Okra

• Avocados

• Starchy tubers (yams, sweet potatoes, potatoes) • Turnips and greens

• Rutabagas (Swedish turnips)

• Parsnips

• Beets

• Plantains

Keep away from food to which you may be hypersensitive or those that are known to promote inflammation (e.g., gluten, dairy products) until you have resolved gut issues and constipation.

Lastly, any food that has an effect on liver function and the production and flow of bile is also highly recommended. Remember that bile acts as a mild laxative and is also essential for the absorption of nutrients and elimination. Choleretics (substances that increase the volume of secretion of bile from the liver) and cholagogues (stimulate gallbladder contraction to promote bile flow) should, therefore, be incorporated in the diet daily. Consider the following in cooking and in herbal preparations like teas or tinctures (always consult your health practitioner before supplementing with extracts):

• Chamomile (Chamomilla recutita)

• Elecampane (Inula helenium)

• Dandelion (Taraxacum officinalis)

• Fennel

• Artichokes, including Jerusalem artichokes

• St. John’s wort (Hypericum perforatum)

• Artemisia sp.

• Yarrow (Achillea millefolium)

• Rosemary (Rosmarinus officinalis)

• Chelidonium majus (from the poppy family)

• Any bitter foods (e.g., wild rocket leaves, watercress, Swedish bitters)

If you suffer from gut disorders, soothing the digestive tract may be fundamental to recovery and reduce constipation. A demulcent is a herb rich in mucilage and can soothe and protect irritated or inflamed internal tissue. Include those (always consult your health practitioner before supplementing with extracts):

• Marshmallow leaf or root (Althea officinalis)

• Slippery Elm (Ulmus fulvus)

• Liquorice (Glycyrrhiza glabra)

• Flax/linseeds and chia seeds also provide soothing mucilage (mucous-like substance)

Other herbs to consider include mint, ginger, turmeric and fennel seeds to further reduce inflammation, improve digestion, reduce gas and bloating

It may also be recommended to reduce caffeine and alcohol consumption to allow for the gut to heal and restore peristalsis. Avoid sugar as much as possible.

Remember to eat until satiated (just before you feel full), so that you supply enough roughage to increase bulk.

Reduce the load on the digestive tract by consuming pre-digested foods. For example, fermented foods like kimchi and sauerkraut, as well as broth, stews and slow-cooked preparations. You may also benefit from supplementing with digestive enzymes if you are under stress, anxious or taking certain drugs that can impede with digestion.

Practice mindful eating. Mark a pause before touching your food, to break your state from hyperactivity to a more relaxed state, which is essential for digestive function. Chew each mouthful and take your time to eat.

Do not mix too many food groups in one sitting and do not drink more than one glass of water with your meal.

Exercise is key for movement. A sedentary lifestyle is associated with a sluggish immune system, stagnation (retention of waste) and constipation. Aim to take part in any physical activity every day — adapt your regime to your abilities. Haven’t exercised in a while? Consider stretching and yoga to get you on the way to a more healthy lifestyle and healthier mind.

Consider gentle enemas to improve regularity.

Magnesium deficiency has also been linked to constipation and is also involved in the stress response and relaxation. Supplement with magnesium if necessary.

References

1. Hammons, AJ. Fiese, BH. (2011). Is frequency of shared family meals related to the nutritional health of children and adolescents? Pediatrics. 127(6): e1565–e1574.

2. BBC Good Food Nation survey 2016: How we eat now. Available at: https://www.bbcgoodfood.com/article/bbc-good-food-nation-survey-results

3. Source: NHS Digital. (2021). Available at: http://healthsurvey.hscic.gov.uk/data-visualisation/data-visualisation/explore-the-trends/fruit-vegetables.aspx

4. Lerner A, Matthias T. Changes in intestinal tight junction permeability associated with industrial food additives explain the rising incidence of autoimmune disease. Autoimmun Rev. 2015;14(6): 479-89.

5. Roberts, CL. et al. (2013). Hypothesis: increased consumption of emulsifiers as an explanation for the rising incidence of Crohn’s disease. J Crohns Colitis. 7(4): pp. 338-341.

6. Source: Nutrunity UK. (2021). Available at: https://www.nutrunity.com/updates/freedom-food-summit

7. Radzevičienč, L. Ostrauskas, R. (2013). Fast eating and the risk of type 2 diabetes mellitus: A case-control study. Clinical Nutrition. 32(2): pp. 232-235.

9. Alberts, DS. et al. (2000). Lack of effect of a high-fiber cereal supplement on the recurrence of colorectal adenomas. Phoenix Colon Cancer Prevention Physicians' Network. N Engl J Med. 342(16):1156–1162.

10. Jacobs, ET, et al. (2002). Intake of supplemental and total fiber and risk of colorectal adenoma recurrence in the wheat bran fiber trial. Cancer Epidemiology, Biomarkers & Prevention. 11(9): pp. 906-914. Erratum in: Cancer Epidemiology, Biomarkers & Prevention. 11(12): 1699.

11. Iadecola, C. (2015). Sugar and Alzheimer's disease: A bittersweet truth. Nature neuroscience, 18(4), pp. 477–478.

12. Source: Diabetes.co.uk. (2018). Available at: https://www.diabetes.co.uk/in-depth/link-between-sugar-and-alzheimers

13. Saji, N. et al. (2019). The relationship between the gut microbiome and mild cognitive impairment in patients without dementia: A cross-sectional study conducted in Japan. Scientific Reports. 9: 19227.

14. Tooley, KL. (2020). Effects of the human gut microbiota on cognitive performance, brain structure and function: A narrative review. Nutrients. 12(10), 3009.

15. Caracciolo, B. et al. (2014). Cognitive decline, dietary factors and gut–brain interactions. Mechanisms of Ageing and Development. Volumes 136–137, pp. 59–69.

16. Source: Biocare. (2018). Digestion in Practice.

17. Source: The Gale Encyclopedia of Medicine. (2015). 5th ed. Blackbirch Press.

18. Sharma, A. Rao, S. (2017). Constipation: Pathophysiology and current therapeutic approaches. Handbook of Experimental Pharmacology. 239: pp. 59–74.