The digestive system performs several distinct processes in sequence, and understanding which stage a symptom relates to clarifies what is happening.
Mouth and stomach
Chewing breaks food mechanically and mixes it with saliva containing enzymes that begin starch digestion.
The stomach adds acid and protein-digesting enzymes, and churns to produce a semi-liquid mixture.
Which means the stomach does relatively little absorption, and its main functions are breakdown, sterilisation and controlled release.
Gastric emptying rate affects how quickly nutrients reach the intestine, which affects blood glucose response.
Small intestine
Where most digestion and absorption occur.
Pancreatic enzymes and bile arrive here, breaking down proteins, fats and carbohydrates.
The enormous surface area, produced by folds, villi and microvilli, is what makes absorption efficient.
Conditions damaging this surface — coeliac disease being the clearest example — impair absorption of many nutrients simultaneously.
Large intestine
Water absorption and fermentation of material the small intestine could not digest.
Which is where the microbial population is concentrated, and where fibre is fermented into compounds absorbed by the colon.
Transit time here determines stool consistency — faster transit means less water absorbed.
Common symptoms and their stages
Reflux relates to the junction between oesophagus and stomach, where a sphincter should prevent backflow.
Bloating frequently relates to fermentation producing gas, or to altered sensitivity to normal amounts.
Altered bowel habit relates to transit time in the large intestine.
Which means symptoms in different locations have different causes and different management.
Fibre
Soluble fibre forms a gel, slowing transit and affecting cholesterol and glucose absorption.
Insoluble fibre adds bulk and speeds transit.
Which means they have different effects, and the appropriate one depends on the symptom.
Most people in developed countries consume substantially less than recommended amounts.
Increasing intake gradually with adequate fluid reduces the bloating that a sudden increase causes.
Motility
Coordinated muscular contractions moving contents along.
Affected by the nervous system, which is why stress affects digestion directly and immediately.
The gut has its own extensive nervous system, capable of coordinating without brain input.
When symptoms warrant assessment
Blood in stool, unexplained weight loss, persistent change in bowel habit, difficulty swallowing, persistent vomiting, or symptoms beginning later in life.
Which are the features that guidelines identify as requiring prompt medical assessment.
Self-diagnosis and elimination diets without assessment can delay diagnosis of treatable conditions and cause nutritional problems, and coeliac testing specifically requires gluten to still be in the diet.
The microbiome
The microbial population concentrated in the large intestine, numbering in the trillions.
Which ferments material the small intestine could not digest, producing short-chain fatty acids that the colon absorbs and uses.
Composition varies enormously between healthy individuals, which means there is no established definition of a healthy microbiome to compare against.
Diet changes it within days, which is why dietary fibre intake affects it more directly than supplements do.
Irritable bowel syndrome
Common, defined by symptoms rather than by structural findings, and now generally described as a disorder of gut-brain interaction.
Which reflects evidence of altered signalling and sensitivity rather than disease of the tissue.
Psychological treatments including gut-directed hypnotherapy have reasonable evidence, which is unusual for a gastrointestinal condition.
Dietary approaches restricting specific fermentable carbohydrates have trial evidence and are intended as short-term elimination with structured reintroduction rather than permanent restriction.
Coeliac disease
An autoimmune condition triggered by gluten, distinct from intolerance.
Which requires diagnosis before removing gluten, since testing depends on gluten still being in the diet.
Self-diagnosed removal without testing makes subsequent diagnosis considerably harder.
Stress and digestion
The gut has extensive nerve supply and responds to stress signalling directly.
Which is why stress alters motility, sensitivity and symptoms immediately and unmistakably.
This is the basis for psychological treatments having effects on gastrointestinal symptoms.
Reflux
Occurs when the barrier between stomach and oesophagus permits backflow.
Which is affected by meal size, timing, position, certain foods and abdominal pressure.
Elevating the head of the bed and avoiding large late meals have practical support.
Persistent reflux warrants medical assessment, since long-standing exposure carries risk and effective treatment exists.
Constipation
Frequently related to fibre, fluid, activity and to ignoring the urge.
Which are addressable, and it is worth addressing since chronic straining has its own consequences.
Persistent change in bowel habit is one of the features warranting medical assessment rather than self-management.
Elimination diets
Removing foods to identify triggers works when structured — removal, then systematic reintroduction one at a time.
Which is different from indefinite removal, and it requires dietitian support to avoid nutritional gaps.
Long-term restriction reduces dietary diversity, which affects the microbial population unfavourably.
Testing
Commercially marketed intolerance tests measuring antibody responses to foods have no established validity and are not recommended by allergy organisations.
Which means a positive result does not indicate intolerance, and acting on it produces unnecessary restriction.
Which is why medical assessment before elimination matters.