Constipation: more than too little fibre

Healthy state

A meal usually needs one to three days from mouth to exit. The contents spend most of this time in the large intestine. There, water and salts are drawn out until the liquid mush becomes formed stool. About three quarters of stool is water; of the dry matter, bacteria make up a large share, plus indigestible plant fibres and shed cells. A few times a day, strong mass movements run through the colon, especially after getting up and after breakfast (the gastrocolic reflex: stretching of the stomach during eating sets the colon in motion).

This is controlled by the nervous system of the gut wall, which handles many processes by itself, by the vagus nerve Vagus nerve The tenth cranial nerve and the most important nerve of the rest-and-digest system. It connects the brainstem and the abdominal organs; most of its fibres tell the brain what is happening in the gut. More in the chapter “The vagus nerve: the data line between belly and head” and by pelvic nerves. Pacemaker cells in the muscle set the rhythm, and serotonin from the lining triggers the waves. Microbes help too: short-chain fatty acids Short-chain fatty acids Acids such as acetate, propionate and butyrate that gut bacteria make from fibre. They nourish the gut wall and act throughout the body. More in the chapter “Butyrate and other short-chain fatty acids” from fibre digestion nourish the lining and influence motility, and bile acids Bile acids Substances made by the liver that make fat digestible. About 95 percent are reabsorbed in the last part of the small intestine; gut bacteria transform the rest. More in the chapter “Bile acids: when bile causes diarrhoea” that were not recovered in the small intestine stimulate water release and movement in the colon. The body clock makes the gut active in the morning and quiet at night.

You have a simple measuring tool yourself: the shape of your stool. The Bristol scale Bristol stool form scale A classification of stool into seven types, from hard lumps (1) to liquid (7). Types 3 and 4 are considered ideal. runs from type 1 (hard lumps) to type 7 (liquid). In a study of 66 volunteers whose transit was measured, stool form was clearly related to transit time: the harder and smaller, the longer it had been on its way. Types 3 and 4 are considered ideal.

Anything between three times a day and three times a week is considered normal, as long as you can pass stool without straining hard. The range is wide: on average women have a slower gut than men, in the second half of the cycle and in pregnancy the hormone progesterone slows the gut muscles, and digestion becomes more sluggish with age. So what matters is less the number than the change from what is normal for you.

Disrupted state: causes and effects

Constipation is not a single disease but an umbrella term for three things that often mix. In normal-transit constipation the gut runs at the usual speed, but stool feels hard or the belly bloated. In slow-transit constipation the movement of the colon itself is too weak. In a defecation disorder, stool cannot get out at the exit because the pelvic floor does not let go. Which type it is decides the treatment, and it is often recognised only after the simple measures have failed.

Common reasons are little fibre, little fluid and little exercise. Suppressing the urge (on the road, on unfamiliar toilets, under stress) also makes the reflex quieter over time. Medications slow the gut: opioids most strongly, but also iron, drugs for cramps or bladder weakness, some antidepressants, certain blood pressure drugs and aluminium- or calcium-containing antacids. Others are hormonal problems such as an underactive thyroid, diabetes, nerve diseases such as Parkinson’s or multiple sclerosis, and too little food, for example in eating disorders. With sudden constipation without an obvious reason, especially from about age 50, seeing a doctor makes sense.

An often overlooked reason is a defecation disorder: the pelvic floor does not relax when pushing, but tightens. Then fibre hardly helps, but targeted pelvic floor training does (see the chapter on the pelvic floor). More fibre helps many, but not all. With slow transit or a blocked exit, larger amounts can increase bloating and fullness without improving bowel movements.

Microbes play a part too. Some people have many methane-producing microbes (archaea, single-celled organisms that convert hydrogen into methane). In a meta-analysis Meta-analysis An analysis that combines the results of several studies. It is only as good as the studies it includes. , methane on the breath test Breath test After drinking a sugar solution, hydrogen and methane in the breath are measured several times. This can reveal sugar intolerances and signs of bacterial overgrowth. More in the chapter “Gut tests: what they show and what they do not” goes along with more constipation; whether methane itself slows the gut or is merely a companion is still open. Too few bile acids in the colon also make stool harder, and with a sluggish small intestine overgrowth and constipation can reinforce each other.

Constipation can turn into a vicious circle. Hard stool requires straining, straining promotes haemorrhoids, tears at the anus and, in the long run, pouches of the gut wall (diverticula Diverticula Small pouches of the gut lining that bulge through weak spots in the colon wall. When they become inflamed, it is called diverticulitis. More in the chapter “Diverticula, haemorrhoids and fissures: what can go wrong at the end of the gut”). Because of the pain, the urge is suppressed and the stool becomes even harder. With severe blockage, especially in older people and children, liquid stool can leak around the hard plug. It looks like diarrhoea and is yet a sign of constipation.

Have a doctor check constipation that appears new and without a reason from about age 50, blood in the stool, unintended weight loss, iron deficiency, pencil-thin stools, pain at night or bowel cancer in the family. Usually something harmless is behind it, but you should know.