What Causes Constipation

What Causes Constipation: A Comprehensive Guide to Understanding Digestive Dysfunction

Constipation affects approximately 15% of adults worldwide, making it one of the most common digestive complaints. If you’ve been struggling with infrequent bowel movements, straining, or feeling like you can’t fully empty your bowels, you’re not alone. Understanding what causes constipation is the first step toward finding effective relief.

Jump ahead to Treatment Implications to learn about specific recommendations to support constipationn based on the underlying cause.

What Is Constipation?

Constipation isn’t just about how often you go to the bathroom. According to the Rome IV criteria, chronic constipation is diagnosed when you experience two or more of the following symptoms for at least three months:

• Straining during more than 25% of bowel movements
• Lumpy or hard stools more than 25% of the time
• Sensation of incomplete evacuation
• Feeling of blockage or obstruction in the anorectal area
• Needing manual maneuvers to facilitate defecation
• Fewer than three bowel movements per week

Many people focus solely on stool frequency, but symptoms like straining, incomplete evacuation, and the need for digital assistance are equally important indicators of constipation.

Primary Causes of Constipation

1. Colonic Motor Dysfunction (Slow Transit Constipation)

One of the most significant causes of chronic constipation is impaired colonic motility—essentially, when your colon doesn’t move stool through efficiently.

Research shows that patients with slow transit constipation have reduced colonic contractions, particularly high-amplitude propagated contractions (HAPCs), which are essential for moving stool through the colon. These powerful contractions normally occur about six times per day, often after meals or upon waking, and are responsible for mass movement of colonic contents.

Studies using colonic manometry have revealed that many constipated patients have:

• Reduced propagated and non-propagated colonic activity
• Decreased contractile responses to meals
• Impaired responses to stimulants like bisacodyl

Interestingly, colonic motor dysfunction can exist even when colonic transit time appears normal, making it a complex condition to diagnose and treat. What this means is you might be having a regular bowel movement (i.e having a poo every day) but what’s coming out has been in the colon for longer than it should be, sometimes days.

2. Gut Microbiome Imbalances

Your gut microbiome plays a crucial role in maintaining regular bowel function, and disruptions to this delicate ecosystem can contribute significantly to constipation.

Research has identified several microbiome alterations in people with constipation:

Decreased beneficial bacteria: Studies consistently show reduced levels of Bifidobacterium and Lactobacillus species in constipated individuals. These bacteria produce short-chain fatty acids (SCFAs) like butyrate, which stimulate colonic motility.

Reduced butyrate-producing bacteria: The relative abundance of butyrate producers such as Faecalibacterium and Roseburia is significantly lower in patients with functional constipation. Butyrate serves as the primary fuel for colonocytes and helps regulate colonic contractions.

Increased methanogens: Patients with slow transit constipation often have elevated levels of methane-producing bacteria. Methane can slow intestinal transit by acting as a neuromuscular transmitter that impairs gastrointestinal motility.

Altered Firmicutes to Bacteroidetes ratio: The balance between these two major bacterial phyla is often disrupted in constipation, affecting various aspects of gut function.

Groundbreaking research using germ-free mice has demonstrated that transferring the fecal microbiome from constipated patients to these mice results in slow colonic transit, proving that the microbiome directly influences bowel function.

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3. Defecatory Disorders (Pelvic Floor Dysfunction)

Defecatory disorders, also called pelvic floor dysfunction or dyssynergia, affect the muscles and nerves involved in the act of defecation. These conditions are surprisingly common and often go undiagnosed.

The problem typically involves:

Paradoxical contraction: Instead of relaxing during defecation, the pelvic floor muscles and external anal sphincter contract or fail to relax adequately, creating resistance to stool evacuation.

Inadequate propulsive force: Reduced rectal propulsive forces can make it difficult to expel stool effectively.

Structural abnormalities: Issues like rectoceles (bulging of the rectal wall), excessive perineal descent, or rectal intussusception can develop, sometimes as secondary consequences of chronic straining.

Up to 50% of patients with defecatory disorders also have slow colonic transit, which can be either a cause or consequence of pelvic floor dysfunction. The retained stool from incomplete evacuation can physically obstruct passage and trigger reflexes that further slow colonic transit.

A digital rectal examination can identify pelvic floor dyssynergia with 75% sensitivity, though specialised anorectal testing provides more definitive diagnosis.

4. Enteric Nervous System Dysfunction

The enteric nervous system (ENS)—sometimes called the “second brain”—is a complex network of neurons embedded in the walls of your digestive tract. It regulates virtually all aspects of gastrointestinal function, including motility.

Research indicates that the gut microbiome profoundly influences ENS development and function. Studies in germ-free mice show weakened excitability of enteric neurons and reduced intestinal transit rates compared to mice with normal microbiomes. When these mice are colonized with beneficial bacteria, the density of the ENS network improves and gut motility increases.

Several mechanisms link the microbiome to ENS function:

Toll-like receptor (TLR) signaling: Gut bacteria influence gastrointestinal motility through TLR-mediated effects on the ENS. Beneficial bacteria and pathogenic bacteria regulate TLR expression differently in enteric cells. Toll-like receptors are receptors found on various immune cells that recognise components of pathogens such as viruses and bacteria.

Serotonin (5-HT) pathways: The microbiome helps maintain serotonin networks in the intestine. Serotonin is crucial for initiating contractions and regulating the smooth muscle of the gut.

Neurotrophic factors: Bacterial effects on ENS are mediated by glial cell-derived neurotrophic factor (GDNF), which supports neuronal development and survival.

5. Gut-Brain Axis Disruption

The bidirectional communication between your gut and brain—the gut-brain axis—plays a vital role in bowel function. Disruptions to this communication can manifest as constipation.

The gut microbiome is a key regulator of this axis, influencing:

Vagal nerve signalling: Bacterial metabolites activate receptors on vagal afferent fibers, transmitting signals between the gut and central nervous system.

Neurotransmitter production: Gut bacteria produce or influence the production of neurotransmitters like serotonin, which affects gut motility and can also influence mood and pain perception.

Stress response: The gut-brain axis is sensitive to psychological stress, which can alter gut motility patterns. Many constipated patients report that stress worsens their symptoms.

This explains why constipation often coexists with conditions like anxiety and depression, and why psychological interventions can sometimes help improve bowel function.

6. Impaired Intestinal Barrier Function and Inflammation

Emerging research suggests that intestinal barrier dysfunction and low-grade inflammation contribute to constipation.

Studies show that constipated patients have:

• Increased intestinal permeability (often called “leaky gut”)
• Elevated numbers of immune cells (CD4+, CD8+, CD25+ T cells) in the colonic mucosa
• Activation of inflammatory pathways

The gut microbiome plays a crucial role in maintaining the intestinal barrier through several mechanisms:

Tight junction regulation: Beneficial bacteria directly modulate the expression of tight junction proteins like zonula occludens-1 and claudin-3.

Mucus layer maintenance: Bacteria influence goblet cells to secrete mucin (especially MUC2), which forms the protective mucus layer over the intestinal epithelium.

SCFA production: Bacterial fermentation produces SCFAs that strengthen the epithelial barrier through mechanisms like activating AMP-activated protein kinase.

When this barrier is compromised, bacterial components can trigger inflammation that affects enteric nervous system function and alters gut motility.

7. Metabolite Deficiencies

Your gut bacteria produce numerous metabolites that directly influence colonic function. Deficiencies in key metabolites can contribute to constipation.

Short-Chain Fatty Acids (SCFAs): Acetate, propionate, and especially butyrate are critical for:

• Providing energy to colonocytes
• Stimulating colonic contractions through G-protein-coupled receptors (GPR41, GPR43)
• Regulating serotonin biosynthesis
• Enhancing the excitability of cholinergic neurons in the enteric nervous system

Studies show that constipated patients often have altered fecal SCFA levels, particularly reduced butyrate.

Tryptophan metabolites: Bacterial metabolism of tryptophan produces compounds like indole, indole-3-carbinol, and tryptamine, which:

• Activate aryl hydrocarbon receptors (AHR) in myenteric neurons, triggering colonic peristalsis
• Stimulate serotonin receptors (5-HT4R) on colonic epithelial cells, increasing fluid secretion and accelerating transit
• Support intestinal barrier integrity and immune homeostasis

Dysregulation of tryptophan metabolism has been identified as a key factor in colonic motility disorders.

Bile acids: Gut bacteria modify primary bile acids into secondary bile acids, which activate receptors like TGR5 on enteroendocrine cells and myenteric neurons, stimulating serotonin release and accelerating transit.

8. Reduced Intestinal Secretion

Normal bowel movements require adequate fluid content in the stool. Alterations in intestinal fluid and electrolyte secretion represent another pathophysiologic mechanism in constipation.

Research indicates that the gut microbiome influences colonic water content through several pathways:

Aquaporin expression: Bacteria can regulate the expression of aquaporin water channels. Certain bacterial profiles (like the Prevotella enterotype) are associated with improved fecal water content. Aquaporins are proteins that form channels in cell membranes, acting as “water gates” to allow rapid, controlled movement of water (and sometimes other small molecules like glycerol) in and out of cells.

Serotonin-mediated secretion: Bacterial metabolites regulate the expression of serotonin receptor subtypes (particularly 5-HT3R and 5-HT4R) that modulate intestinal fluid secretion.

Direct secretagogue effects: Metabolites like tryptamine activate G-protein-coupled receptors on colonic epithelial cells, elevating cyclic AMP levels and increasing fluid secretion.

Studies suggest that impaired intestinal secretion in constipation may result from bacterial metabolite deficiencies rather than inherent defects in epithelial transport.

9. Hormonal Influences

The higher prevalence of constipation in women highlights the role of sex hormones in bowel function.

Progesterone effects: Elevated progesterone levels, particularly during pregnancy or the luteal phase of the menstrual cycle, can slow intestinal transit. Research indicates that over-expression of progesterone receptors (rather than progesterone levels alone) may be responsible for impaired gut motility.

Estrogen influences: Estrogen affects colonic motor function through both genomic (nuclear receptors) and non-genomic (membrane receptors like GPER) mechanisms. Estrogen can promote nitric oxide release from myenteric nerves, which may inhibit colonic propulsion.

Microbiome-hormone interactions: Fascinatingly, the gut microbiome influences hormone metabolism through:

• Steroid enterohepatic circulation
• Bacterial enzymes like β-glucuronidase that affect estrogen reabsorption
• Direct effects on hormone receptor expression

The bidirectional relationship between gut bacteria and hormones represents an emerging area of constipation research, particularly relevant for understanding why constipation is more common during pregnancy and varies with the menstrual cycle.

Secondary Causes of Constipation

Beyond these primary mechanisms, numerous medical conditions and medications can cause or contribute to constipation:

Medical Conditions:

• Endocrine disorders: Hypothyroidism, diabetes mellitus, hypercalcemia
• Neurological conditions: Parkinson’s disease, multiple sclerosis, spinal cord injuries
• Connective tissue disorders: Scleroderma, amyloidosis
• Structural abnormalities: Colon cancer, strictures, large rectoceles

Medications:

• Opioid pain medications
• Anticholinergic drugs
• Calcium channel blockers
• Antidepressants (particularly tricyclics)
• Iron supplements
• Antacids containing aluminum or calcium

Lifestyle Factors:

• Inadequate fiber intake: Less than 25-30g daily
• Insufficient fluid consumption
• Physical inactivity
• Ignoring the urge to defecate
• Restrictive dieting or inadequate caloric intake

The Interconnected Nature of Constipation Causes

It’s important to understand that these causes don’t exist in isolation. Constipation typically results from multiple interconnected factors:

A person might have slow transit constipation due to reduced butyrate-producing bacteria, which leads to decreased SCFA production, impairing both colonic motor function and enteric nervous system excitability. This slow transit could then contribute to bacterial overgrowth and further microbiome imbalances, creating a self-perpetuating cycle.

Similarly, someone with pelvic floor dysfunction might develop slow colonic transit as a secondary consequence of incomplete evacuation, with retained stool triggering inhibitory reflexes and providing more opportunity for water reabsorption, making stools harder and more difficult to pass.

When to Seek Medical Evaluation

While occasional constipation is common and usually harmless, you should consult a healthcare provider if you experience:

• Constipation lasting more than three weeks
• Severe abdominal pain
• Blood in your stool
• Unintentional weight loss
• New onset constipation after age 50
• Constipation alternating with diarrhea
• Family history of colon cancer or inflammatory bowel disease

These warning signs may indicate more serious conditions requiring medical investigation.

Diagnostic Approaches

Understanding the specific cause of your constipation often requires specialised testing:

Anorectal manometry and balloon expulsion test can identify defecatory disorders by measuring the coordination and strength of pelvic floor muscles during evacuation.

Colonic transit studies using radiopaque markers or scintigraphy can distinguish normal transit from slow transit constipation.

Gut microbiome testing can reveal bacterial imbalances, reduced diversity, or deficiencies in beneficial species that may be contributing to your symptoms.

SIBO breath testing can identify intestinal methanogen overgrowth, which can cause constipation along with bloating and other digestive symptoms.

Treatment Implications Based on Causes

Understanding what’s causing your constipation is crucial for selecting the most effective treatment:

For microbiome-related constipation:

• Targeted probiotics (particularly Bifidobacterium and Lactobacillus strains)
• Prebiotics to feed beneficial bacteria
• Dietary modifications to support SCFA production

Recommended probiotic: UltraFlora

Recommended prebiotic: PHGG

Recommended butyrate (a SCFA): Sodium Butyrate

For slow transit constipation:

• Osmotic laxatives (polyethylene glycol)
• Stimulant laxatives (bisacodyl, senna)
• Prokinetic agents (prucalopride)
• Secretagogues (linaclotide, plecanatide, lubiprostone)

For defecatory disorders:

• Biofeedback therapy (first-line treatment)
• Pelvic floor physical therapy
• Specific exercises to improve coordination

For metabolite deficiencies:

• Increasing dietary fiber to boost SCFA production. Two good fibre supplements to boost intake are PHGG and/or psyllium husk.
• Tryptophan-rich foods or supplements. Foods include turkey, chicken, fish, eggs, and dairy (cheese, milk).
• Butyrate supplements

Browse our selection of targeted supplements designed to support gut motility and microbiome balance.

Check out our more comprehensive blog How To Get Rid Of Constipation for more recommendations.

The Role of Probiotics in Addressing Constipation Causes

Given the central role of gut microbiome imbalances in constipation, probiotics have emerged as an important therapeutic option. Research demonstrates that specific probiotic strains can:

Modulate the microbiome: Studies show that supplementation with Bifidobacterium and Lactobacillus species increases the abundance of beneficial bacteria while reducing potentially pathogenic organisms.

Increase SCFA production: Certain probiotic strains, particularly Lactobacillus plantarum and Bifidobacterium animalis, enhance levels of acetate and other SCFAs that stimulate colonic motility.

Support ENS function: Probiotics like Clostridium butyricum regulate TLR2 signaling pathways that promote intestinal motility, while Lactobacillus rhamnosus GG enhances choline acetyltransferase expression in enteric neurons.

Strengthen intestinal barrier: Probiotic bacteria augment tight junction protein expression and stimulate mucin secretion, reducing inflammation and supporting normal motility.

Modulate serotonin pathways: Some probiotic strains influence serotonin biosynthesis and receptor expression, affecting both gut motility and visceral sensation.

Clinical trials have demonstrated that specific probiotics can improve bowel movement frequency, stool consistency, and overall constipation symptoms. However, effects are strain-specific, emphasising the importance of selecting evidence-based formulations. Read our blog on the best probiotics for gut health for specific probiotic recommendations for specific symptoms.

Dietary Strategies Targeting Root Causes

Addressing constipation through diet involves more than just adding fiber:

Fermentable fiber: Foods rich in fermentable fiber (such as legumes, oats, and certain fruits) feed beneficial bacteria that produce SCFAs. Aim for 25-30g of total dietary fiber daily, with a good portion coming from fermentable sources.

Polyphenol-rich foods: Berries, green tea, and dark chocolate contain polyphenols that selectively promote the growth of beneficial bacteria like Bifidobacterium and Lactobacillus.

Resistant starch: Found in cooled potatoes, green bananas, and legumes, resistant starch reaches the colon intact where it’s fermented by bacteria to produce butyrate.

Probiotic foods: Fermented foods like yogurt, kefir, sauerkraut, and kimchi provide live beneficial bacteria that can help restore microbiome balance.

Adequate protein: Tryptophan from protein sources serves as a precursor for both serotonin production and bacterial tryptophan metabolites that influence gut motility.

Conclusion: A Personalised Approach to Constipation

Constipation is a complex condition with multiple potential causes, from colonic motor dysfunction and microbiome imbalances to pelvic floor disorders and metabolite deficiencies. Rather than viewing it as a simple mechanical problem, we now understand constipation as a disorder involving intricate interactions between gut bacteria, the enteric nervous system, the gut-brain axis, immune function, and metabolic processes.

This comprehensive understanding opens the door to more targeted, personalised treatments. By identifying the specific mechanisms contributing to your constipation—whether through microbiome testing, colonic transit studies, or anorectal function tests—you and your healthcare provider can develop an individualised treatment plan that addresses the root causes rather than just managing symptoms.

If you’ve been struggling with chronic constipation, consider taking a proactive approach to understanding what’s happening in your gut. Start with our Ultimate Gut Health Test to uncover potential microbiome imbalances, or our SIBO test if you suspect bacterial overgrowth may be contributing to your symptoms.

Remember, the journey to improved bowel health often requires patience and a multifaceted approach, but with proper understanding and targeted interventions, most people with constipation can achieve significant improvement in their symptoms and quality of life.

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