Hormones, the cycle and the gut

Healthy state

The gut has receptors for sex hormones, in the muscle layer, on nerve cells and on immune cells of the gut wall. Oestrogen and progesterone therefore influence how fast it works, how sensitively it reports pain and how the immune system reacts. The cycle has four stages: the period, the first half up to ovulation with rising oestrogen, ovulation itself and the second half, in which progesterone dominates. Progesterone relaxes smooth muscle, including that of the gut wall. In the second half of the cycle the gut therefore runs a little more sluggishly in many people, with more gas and firmer stool. That is why digestion changes across the cycle and across life.

At the start of the period both hormones fall, and the womb makes prostaglandins, messengers that make its muscle contract. According to the usual explanation they also act on the gut and contribute to looser stool and cramps. That is one reason why diarrhoea and period pain often appear together.

Conversely, gut bacteria influence hormones. With an enzyme, beta-glucuronidase, they make oestrogen active again that the liver had actually released into the gut for excretion. The sum of bacterial genes that handle oestrogen is called the estrobolome. How much this matters in humans is still open. It comes mainly from laboratory and animal data, and it would be premature to draw conclusions about your hormone balance from a stool test.

The thyroid also sets the pace. Its hormones determine how fast smooth muscle works: an underactive thyroid slows the gut and often causes constipation, an overactive one speeds it up and often causes diarrhoea. In underactive thyroid a higher risk of small intestinal bacterial overgrowth Small intestinal bacterial overgrowth (SIBO) Too many bacteria in the small intestine, where normally few live. Bloating and diarrhoea are typical; it is usually diagnosed with a breath test. More in the chapter “When the small intestine is too densely colonised” has also been described, probably because the cleaning wave between meals slows down too.

Hormones and stress are closely interlinked: the stress axis and the sex hormones influence each other, which is why lack of sleep and constant stress can disturb the cycle and digestion at the same time. In men the link between sex hormones and digestion is much less studied; IBS Irritable bowel syndrome Recurring abdominal pain linked to bowel movements, without tests finding another cause. Today it is understood as a disorder of gut–brain interaction. More in the chapter “The gut-brain axis” is more common in women. How hormonal contraception affects the gut is also hardly researched. Some people report changes when starting or switching; there are no solid studies on this here.

Disrupted state: causes and effects

Before and during the period the hormone levels fall. About a third of otherwise symptom-free women notice digestive symptoms then, and almost half of women with IBS report more symptoms around the period. Typical are looser stool, more bloating and abdominal pain. The gut is more sensitive to pain at this time. On the Bristol stool 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. More in the chapter “Constipation: more than too little fibre”, types 3 and 4 are considered ideal; around the period, stool in many people slides towards types 5 to 6.

Endometriosis, tissue similar to the womb lining outside the womb, can affect the gut and mimic IBS symptoms. Women with endometriosis have about three times as often a diagnosis of IBS, almost one in four. This often delays the correct diagnosis. A review also describes a shared nerve pathway through which the two conditions might reinforce each other. Clues are symptoms that clearly come and go with the period, pain on passing stool during the period, very severe period pain and pain during sex. Less often, bowel involvement also causes blood in the stool with the period.

In pregnancy constipation is common: 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. it affected about one in three pregnant women, most often in the second trimester. High progesterone slows the gut, later the womb also presses, and iron supplements often make it worse. Added to this are nausea in the first trimester and heartburn, because progesterone also relaxes the sphincter at the entrance to the stomach.

In the menopause oestrogen falls permanently. In studies the gut flora Microbiome, gut microbiota The community of all microbes in the gut, mainly bacteria, plus viruses, fungi and archaea. Most live in the colon. More in the chapter “Building microbial diversity” after the menopause is less diverse and more similar to that of men. Whether and how this explains symptoms is little studied, but many women report more bloating and constipation during this time, also because sleep, stress and pelvic floor tissue change. The evidence here is thin, and the symptoms still deserve attention.

The thyroid deserves a thought when symptoms are new: an underactive thyroid causes constipation, tiredness and feeling cold, an overactive one diarrhoea, weight loss and a racing heart. Both are well treatable, and digestion often normalises once the hormones are adjusted.

Two misunderstandings: “Period symptoms are just normal” is true only up to a point. Common does not mean normal, and severe pain should be checked. And “hormones are to blame” falls short: hormones amplify or trigger symptoms but do not replace the search for other causes such as IBS, intolerances or inflammation.