Mixed IBS: Understanding the Diarrhoea-Constipation Cycle
Diarrhoea one day, constipation the next? Mixed IBS affects 20–25% of people with irritable bowel syndrome. Here's what you need to know.
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When Your Gut Changes Direction Without Warning
One morning you're rushing to the loo. A few days later, you're waiting in vain. This uncomfortable back-and-forth between diarrhoea and constipation has a name: mixed irritable bowel syndrome, also known as IBS-M or alternating IBS. Far from being rare, it accounts for 20 to 25% of all IBS cases — a form that is often considered harder to live with than the "pure" subtypes.
Understanding why your gut alternates in this way is the first step towards managing your daily life more effectively.
What the Science Says: The Rome IV Criteria
IBS-M is not diagnosed lightly. Clinicians rely on the Rome IV criteria, the international reference standard for functional bowel disorders. A diagnosis requires:
- Recurrent abdominal pain, present on at least 1 day per week over the previous 3 months
- Associated with at least 2 of the following: a relationship with defaecation, a change in stool frequency, or a change in stool consistency or appearance
In the case of IBS-M specifically, stools cycle between loose or liquid consistency (diarrhoea) and hard, lumpy stools (constipation) according to the Bristol Stool Scale — often within the same month, or even the same week.
Warning signs never to ignore: blood in the stools, unexplained weight loss, persistent fever, or being woken at night by pain. These symptoms require urgent medical attention to rule out other conditions.
Three Mechanisms That Explain the Alternation
1. An Erratic Gut Motility
The gut does not contract in a uniform way. In IBS-M, it oscillates between hyperactive phases — which speed up transit and trigger diarrhoea, often in the morning or after meals — and hypoactive phases, where contractions are too weak to move stools along, leading to constipation with discomfort and a sense of blockage.
2. Heightened Visceral Hypersensitivity
People with IBS-M frequently experience visceral hyperalgesia: their intestines react in an exaggerated way to stimuli that others would not even notice. A simple movement of gas, a slight distension, and the pain is perceived as intense. This phenomenon explains the cramping, the feeling of a band across the abdomen, and the recurring bloating that accompany alternating episodes.
3. A Disrupted Gut-Brain Axis
The third — and by no means least important — factor is the connection between the brain and the gut. The gut microbiome, neurotransmitters, and the enteric nervous system are in constant communication with the brain. In IBS-M, these exchanges are disrupted. Stress, anxiety, and depression — which are frequently associated with IBS — are not merely consequences: they actively worsen symptoms by altering gut motility and sensitivity.
Day-to-Day Symptoms: Far More Than Irregular Stools
IBS-M is not limited to bowel habits alone. Nearly 90% of those affected report associated symptoms:
- Persistent bloating and flatulence
- Abdominal cramping before or after bowel movements
- Chronic fatigue, which is often underestimated
- Frequent headaches
- Anxiety or heightened sensitivity to stress
The sensation of incomplete evacuation — the feeling that you haven't quite finished, even after going to the loo — is also highly characteristic and particularly uncomfortable to live with day to day.
Managing IBS-M: A Multi-Pronged Approach
There is no cure for IBS-M. It is a chronic functional condition, with periods of flare-up and periods of relative calm. The goal is to reduce the frequency and intensity of episodes.
Validated approaches include:
- Antispasmodics to ease pain and cramping
- Osmotic laxatives (gentle) during constipation-dominant phases
- Antidiarrhoeals during periods of accelerated transit
- Visceral pain modulators to address hyperalgesia
- Stress management: cognitive behavioural therapy, heart rate coherence techniques, and mindfulness — all of which have clinically documented benefits
- Dietary monitoring: identifying trigger meals, particularly for postprandial diarrhoea
The low-FODMAP diet, whilst not specifically detailed in official guidelines for IBS-M, is frequently recommended in clinical practice to reduce intestinal fermentation and associated bloating.
Key Takeaways
Mixed IBS is a complex, often debilitating condition that deserves to be taken seriously — by healthcare professionals and loved ones alike. Understanding its mechanisms (variable motility, visceral hypersensitivity, the gut-brain axis) helps explain why symptoms fluctuate and how to act on several fronts simultaneously.
If this clinical picture sounds familiar, speak to a gastroenterologist. A multidisciplinary approach — combining medical, nutritional, and psychological support — remains the most effective strategy for improving quality of life.