When people are diagnosed with Irritable Bowel Syndrome (IBS), it often feels unsatisfying. IBS is a functional gastrointestinal disorder, meaning symptoms are real and disruptive, but routine tests may not reveal a single structural disease. This does not mean “doctors don’t know anything”—rather, it means IBS arises from multiple interacting systems, including digestion, motility, gut bacteria, and the gut–brain axis.
Understanding IBS requires zooming out. The gastrointestinal tract is a long, complex system—stretching over 20 feet—whose job is to digest food, absorb nutrients, manage microbes, and move waste efficiently. Problems can arise not only from what happens chemically during digestion, but also from how food physically moves through the system.
This article explores IBS through that dual lens: chemical processes and mechanical function.
Why Gut Problems Are So Difficult to Resolve
The digestive tract is essentially a continuous tube from mouth to anus that is constantly exposed to the outside world through food, liquids, medications, and microbes. Unlike most organs, it rarely gets to “rest.”
Digestion involves:
- Strong acids in the stomach
- Alkaline secretions from the pancreas and bile system
- Enzymes that break food into absorbable components
- Coordinated muscular contractions that move contents forward
When any part of this system is even slightly out of sync, symptoms such as bloating, pain, diarrhea, or constipation may emerge—especially in people with heightened gut sensitivity.
Chemical Digestion: When Processing Goes Off Balance
The Role of Stomach Acid
Stomach acid plays a critical role in digestion. An adequately acidic environment:
- Activates pepsin for protein digestion
- Helps release vitamin B12 from food
- Limits the survival of ingested microbes
If stomach acidity is reduced—whether due to medications, chronic stress, or other factors—digestion may be less efficient. This does not automatically cause IBS, but it may contribute to downstream symptoms in some individuals.
Bile, Pancreatic Enzymes, and the Small Intestine
As partially digested food enters the small intestine, it is mixed with:
- Bile, which helps digest fats
- Pancreatic bicarbonate, which neutralizes stomach acid
- Digestive enzymes that further break down proteins, fats, and carbohydrates
The small intestine—particularly the jejunum and ileum—is where most nutrient absorption occurs through finger-like projections called villi. When digestion or motility is impaired, nutrients may not be fully absorbed, and undigested material can pass into the large intestine.
The Large Intestine, Bacteria, and Fermentation
In the colon, trillions of bacteria ferment undigested carbohydrates and fiber. This process produces gases and short-chain fatty acids (SCFAs) such as butyrate, acetate, and propionate, which help nourish the colon lining and support gut health.
Gas production and water absorption are normal processes. However, in people with IBS, the gut may be hypersensitive, making normal amounts of gas feel painful or excessive.
Mechanical Factors: Motility and Flow Matter Too
Constipation and Diarrhea Are Not Always Opposites
Constipation and diarrhea can coexist. In some cases, loose stool passes around harder, retained stool—a phenomenon known as paradoxical diarrhea. IBS symptoms may shift over time, reflecting changes in gut motility rather than a single fixed problem.
Chronic Constipation and Pelvic Floor Dysfunction
Long-standing constipation can sometimes be associated with pelvic floor dyssynergia, also called dyssynergic defecation. In this condition, the muscles that should relax during a bowel movement instead tighten, making evacuation difficult.
A key muscle involved is the puborectalis, part of the pelvic floor. When it fails to relax appropriately, people may experience:
- Excessive straining
- Incomplete evacuation
- Rectal discomfort
- Bloating
This is a recognized medical condition and is distinct from IBS, though the two can overlap.
Diagnosis and Treatment of Pelvic Floor Dyssynergia
Diagnosis is typically made using tests such as:
- Anorectal manometry
- Balloon expulsion testing
The most evidence-supported treatment is pelvic floor biofeedback therapy, which retrains coordination between the muscles and nervous system. Pelvic floor physical therapy may also help improve awareness, relaxation, and function.
Other interventions—including medications or, in select cases, botulinum toxin injections—may be considered by specialists when conservative treatments fail. These approaches must be carefully individualized and supervised.
IBS as a Whole-System Condition
IBS does not arise from a single malfunction. Instead, it reflects a dynamic interaction between:
- Digestive chemistry
- Gut motility
- Microbial activity
- Nervous system signaling
- Muscular coordination
For some individuals, chemical digestion plays a dominant role. For others, mechanical issues such as constipation or pelvic floor dysfunction are more significant. Many experience a combination of both.
Understanding IBS through this broader systems-based framework helps explain why:
- Symptoms vary widely between individuals
- No single treatment works for everyone
- Improvement often requires addressing multiple contributing factors
IBS is real, complex, and highly individualized. Progress often comes not from searching for one hidden cause, but from identifying which parts of the digestive system are most out of balance in a given person—and addressing them thoughtfully.