Chronic Diarrhoea Beyond IBS

Gut · §57

If you've been carrying an irritable-bowel diagnosis for years and the loose stools never quit, there's a fair chance the label is wrong. Roughly one in three adults stuck with a "diarrhoea-predominant IBS" tag actually has a named, testable condition the workup missed. The three big hidden causes — bile acid diarrhoea, microscopic colitis, and exocrine pancreatic insufficiency — each have a specific test and a specific drug, and once you're on the right one, mornings start changing within weeks.

Respond · As-needed Evidence Moderate Chapter Gut

The strongest reason to push past “just IBS” is how fast a correct name pays back. Bile acid sequestrants resolve bile acid diarrhoea in days. Six to eight weeks of budesonide puts most microscopic colitis into remission. Pancreatic enzymes with meals restore weight, fat absorption and energy. The workup is a handful of cheap tests and a colonoscopy with biopsies — modest cost, modest effort, a real return.

Three named conditions hide under chronic non-bloody diarrhoea, each with a different broken part.

Bile acid diarrhoea is what happens when bile acids the gut normally recycles spill into the colon instead. The colon pulls water in, the muscle speeds up, and the toilet becomes an emergency — often around 4 a.m. It’s common after a gallbladder removal or a Crohn’s-related bit-of-ileum resection. A third “idiopathic” form shows up out of nowhere when the gut’s feedback signal that tells the liver to make less bile (a hormone called FGF19) doesn’t fire properly — so the liver overshoots and the colon drowns (Walters 2009).

Microscopic colitis is inflammation you can’t see with the camera — only on the biopsy. The colon looks normal on colonoscopy; the slide shows immune cells packed into the lining, or a thickened collagen band underneath. Stools are watery and frequent, often at night, and the trigger is frequently a drug the reader has been on for years: a proton-pump inhibitor for reflux, an SSRI for mood, an NSAID for joints, a statin (Miehlke 2021).

Exocrine pancreatic insufficiency is fat-digestion failure. The pancreas isn’t making enough lipase, so fat passes through. Classic stools are bulky, pale, oily, foul, and float — but milder cases just look like soft, frequent stools and slow weight loss. It runs with chronic pancreatitis, pancreatic surgery, cystic fibrosis, and — under-recognised — long-standing diabetes (Whitcomb 2023).

What ties all three together is the same trap: chronic diarrhoea gets reflexively tagged IBS-D, the workup stops at a normal blood count and a colonoscopy where the camera saw nothing, and the actual cause — with its actual drug — goes years undiagnosed. A “normal” colonoscopy without biopsies has not ruled out microscopic colitis. Persistent loose stools after gallbladder removal are usually bile acid diarrhoea, not something to live with. And early pancreatic insufficiency rarely shows the textbook oily slick (Pardi 2017).

How real, how common, and how well treatments work

The headline number on bile acid diarrhoea is the one most worth knowing.

Microscopic colitis is the dominant cause of watery diarrhoea in older women. Incidence climbs into the 10–25 per 100,000 range over age 65, and it accounts for 10–20% of chronic-diarrhoea cases at colonoscopy with biopsies in that group — common enough that a new-onset watery diarrhoea in an older adult is microscopic colitis until biopsies say otherwise (Tong 2015) (Pardi 2017).

Exocrine pancreatic insufficiency eventually develops in most patients with chronic pancreatitis and shows up on stool testing in a quarter to forty percent of long-standing type 1 diabetics — a population that often never gets the test (Löhr 2017) (Whitcomb 2023).

The other thing worth knowing is how well treatment works once the right name is on the chart. These effect sizes are large for chronic gut conditions.

What years on the wrong label actually cost

The day shrinks around proximity to a bathroom. You map them at airports, in offices, on the way to dinner. The 4 a.m. wake comes twice a week, then four. Coffee becomes roulette. You stop accepting plans that don’t end before your gut tends to act up. Your partner notices the cancelled invitations before you do; the friend you only see every few months asks — lightly, twice — whether you’ve been okay.

Underneath that, the body keeps a quieter ledger. Untreated pancreatic insufficiency skims fat-soluble vitamins (A, D, E, K) out of every meal — bone density drops, fractures arrive a decade early, night vision dims, peripheral nerves get fussy (Löhr 2017). Bile acid loss pulls B₁₂ with it when the ileum is the affected site (Camilleri 2020). Quality-of-life scores in untreated microscopic colitis sit on par with active inflammatory bowel disease — not a milder cousin, the same neighborhood — with work absences, sleep wrecked by night stools, and a slow social retreat (Nyhlin 2014) (Safroneeva 2020). And in the small minority where pancreatic insufficiency is the marker for chronic pancreatitis or a pancreatic mass, missing the diagnosis is missing the cancer behind it.

The cumulative cost isn’t the diarrhoea itself. It’s a decade of declining the invitation, sleeping in 90-minute fragments, and quietly building deficiencies the next bone scan finds.

The workup, and what to do once it lands

The targeted workup is short, cheap by hospital standards, and well-defined. Three sets of national guidelines line up on roughly the same sequence (BSG 2018) (AGA 2019) (Sadowski 2020). What takes years is the empirical “try this diet, try that probiotic” loop while a missing test sits on the shelf.

Ask for these, in roughly this order:

Bloods — full count, CRP, ferritin, TSH, B₁₂, folate, vitamin D, albumin.
Coeliac panel — IgA-tTG with total IgA. Rule coeliac out before settling on any functional label (Singh 2018).
Stool tests — faecal calprotectin (under 50 µg/g makes IBD unlikely; a raised result, or alarm features like visible blood or unexplained weight loss, redirects the workup toward inflammatory bowel disease — the alarm-feature path, not this one), faecal elastase-1 (under 200 µg/g flags pancreatic insufficiency), stool culture / Giardia antigen / C. difficile where the history fits.
Bile acid testing — the SeHCAT retention scan where it’s available; a blood test (C4 or fasting FGF19) where it isn’t; or a four-week empirical trial of cholestyramine or colesevelam if the presentation fits and testing isn’t available (Vijayvargiya 2019).
Colonoscopy with random biopsies from the right and left colon — the only way to diagnose microscopic colitis.
Cross-sectional imaging of the pancreas (CT or MRI) once pancreatic insufficiency is confirmed.

Once a name lands, treatment is mechanism-specific and most of the lift arrives fast.

If volume loss has been heavy, an oral rehydration solution alongside treatment for the first few days catches you up on sodium, potassium and water faster than plain water can.

Catches worth knowing before you start a drug

None of the three treatments are dangerous in normal use, but each has a catch that’s easy to miss in the pharmacy.

Bile acid sequestrants (cholestyramine, colesevelam) bind other oral medications in the gut — thyroid hormone (levothyroxine), warfarin, statins, the contraceptive pill, digoxin — and fat-soluble vitamins. Separate other pills by four hours. Avoid if you have complete biliary obstruction.
Budesonide is gentler than systemic steroids like prednisone, but it’s not steroid-free. Long maintenance courses still risk bone loss, raised blood sugar, cataracts and adrenal suppression — push for the lowest dose that holds remission and ask about bone-density monitoring if you’re on it long-term (Miehlke 2021).
Pancreatic enzymes are made from pig pancreas — relevant if you avoid pork for religious or dietary reasons; ask your pharmacist about alternatives. They’re considered safe in pregnancy (Löhr 2017).

Where the workup tends to go wrong

The same omissions recur across every “I’ve had this for years and no one figured it out” story:

“Your colonoscopy was normal.” Without biopsies, the colonoscopy ruled out cancer and visible inflammatory bowel disease. It did not rule out microscopic colitis — the whole point of MC is that the camera sees nothing (Nguyen 2016).
Bile acid testing never considered. The retention scan isn’t available in much of the US, the blood tests aren’t routine, and the empirical four-week sequestrant trial the guidelines explicitly endorse is often skipped (Sadowski 2020).
Faecal elastase never ordered because the stool didn’t look classically fatty. Early pancreatic insufficiency rarely shows the textbook oily slick — the test is cheap enough to send anyway (Whitcomb 2023).
Drug list never reviewed. PPIs and SSRIs as microscopic-colitis triggers, metformin and orlistat as direct diarrhoeals, magnesium and sorbitol in vitamin and chewing-gum form, recent antibiotics — stopping one of these often resolves the symptom before any further workup is needed (Miehlke 2021).
Coeliac never serologically excluded. Coeliac sits at roughly one in a hundred in Western adults; its presentation is often subtle in modern practice. The blood test should be in every chronic-diarrhoea workup (Singh 2018) (Smalley 2019).
Pancreatic enzymes under-dosed. A “non-response” is often a starter dose of 25,000 IU taken before the meal — both wrong. Dose with the meal, escalate, add a PPI (Löhr 2017).

What changes once the name lands

Most of the change arrives faster than chronic-illness patients expect.

In the first week, the 4 a.m. wake disappears. Coffee stops being roulette. The bathroom map you’ve been running in the back of your head turns off. By a month or two in, people stop quietly noticing when you decline things and start asking what changed; the friend who’d been gently checking on you stops checking. The weight you’ve been slowly losing comes back. Sleep stops fracturing around midnight stools (Camilleri 2020) (Miehlke 2014).

The exact pace depends on which name you got. Bile acid sequestrants work in days. Budesonide takes a couple of weeks for stool frequency to fall and the full six to eight for remission. Pancreatic enzymes work meal-by-meal — the bloating, oily stool and post-dinner discomfort fade with the first correctly dosed evening. The slow piece is the bone, vitamin and weight rebuild in long-standing pancreatic insufficiency, which takes months once enzymes and vitamin supplementation are running properly (Löhr 2017).

And longer out: in microscopic colitis, quality-of-life scores in patients in remission return close to the population baseline — not “managed”, just — normal (Nyhlin 2014). That’s the prize. A decade of bathroom-mapped living, ended.

Substance and claimed effects

Chronic diarrhoea — loose or watery stools with increased frequency, lasting more than four weeks (Arasaradnam 2018) — is a presenting syndrome, not a diagnosis. A large share of adults given an irritable bowel syndrome diarrhoea-predominant (IBS-D) label in fact carry a treatable organic cause that targeted testing would surface: bile acid diarrhoea, microscopic colitis, exocrine pancreatic insufficiency, coeliac disease, small intestinal bacterial overgrowth, drug-induced diarrhoea, and other named entities (Smalley 2019; Arasaradnam 2018). This entry covers the three named “hidden” causes beyond IBS-D that recent guideline updates flag as systematically under-diagnosed — bile acid diarrhoea (BAD/BAM), microscopic colitis (MC), and exocrine pancreatic insufficiency (EPI) — together with the modern workup approach, and the downstream effects on nutrition, electrolytes and quality of life. Coeliac disease, IBD, infection and SIBO are referenced where the workup intersects them but each warrants its own entry.

Evidence by addressing question

mechanism

The three named conditions share the surface symptom but differ entirely in pathophysiology, which is why the workup is targeted rather than empirical.

Bile acid diarrhoea (BAD/BAM). Normally ~95% of the bile acids secreted into the duodenum are reabsorbed in the terminal ileum via the apical sodium-dependent bile acid transporter and returned to the liver. When too much bile acid spills into the colon, it stimulates colonic secretion and accelerates motility, producing watery, often urgent and nocturnal stools (Camilleri 2020). Three mechanisms: type 1 follows terminal ileal disease or resection (Crohn’s, ileitis, ileal resection); type 2 is “primary” or idiopathic, now understood as defective FGF19 feedback — the ileal hormone that signals the liver to throttle bile acid synthesis is under-produced, so the liver overshoots production and the colon is overwhelmed (Walters 2009); type 3 follows cholecystectomy, post-infectious states, or chronic pancreatitis. Type 2 is the form historically mis-labelled IBS-D.

Microscopic colitis. A chronic, non-bloody, watery diarrhoea driven by lymphocytic and/or collagenous inflammation of an endoscopically normal colon — the macroscopic mucosa looks fine; only random biopsies show it. Two histological subtypes: lymphocytic colitis (intraepithelial lymphocytes >20 per 100 epithelial cells) and collagenous colitis (subepithelial collagen band >10 µm), with overlapping clinical features (Pardi 2017; Miehlke 2021). Drives include autoimmune predisposition (strong overlap with coeliac disease, thyroid disease, rheumatoid arthritis) and culprit drugs — PPIs, NSAIDs, SSRIs, statins, ranitidine (historical) — which can trigger or maintain disease (Nguyen 2016; Miehlke 2021). Smoking roughly doubles risk.

Exocrine pancreatic insufficiency. Insufficient delivery of pancreatic enzymes (especially lipase) to the duodenum, so dietary fat is poorly hydrolysed. Steatorrhoea — pale, bulky, oily, foul-smelling, often floating stool — appears clinically once lipase output falls below ~10% of normal (Lindkvist 2013). Aetiologies span chronic pancreatitis (most common in adults), pancreatic ductal adenocarcinoma, cystic fibrosis, prior pancreatic resection, and — under-recognised — long-standing diabetes (both type 1 and type 2), where pancreatic parenchymal atrophy is common (Whitcomb 2023). Fat maldigestion drags fat-soluble vitamins (A, D, E, K) with it, so EPI is a deficiency engine, not just a stool engine.

evidence

Prevalence — and how often the IBS-D label masks them. A 2009 systematic review of SeHCAT scanning across >1,200 IBS-D patients reported a pooled prevalence of bile acid malabsorption of ~30% (mild-or-worse), with a third of those having severe BAM (SeHCAT 7-day retention <5%) (Wedlake 2009). The British Society of Gastroenterology guideline and the AGA functional-diarrhoea guideline both call out that BAD is systematically missed when chronic watery diarrhoea is reflexively coded as IBS-D (Arasaradnam 2018; Smalley 2019).

Microscopic colitis incidence has risen from negligible in the 1980s to ~5–10 per 100,000 person-years in modern population studies, peaking at ~10–25 per 100,000 in adults over 65 (Tong 2015; Fernández-Bañares 2016). In older women presenting with chronic watery diarrhoea, MC accounts for roughly 10–20% of cases at colonoscopy with random biopsies (Pardi 2017).

EPI prevalence is harder to pin down because diagnosis lags. Among adults with chronic pancreatitis it eventually develops in ~60–90% (Löhr 2017); among long-standing type 1 diabetics, faecal elastase studies suggest moderate-to-severe EPI in 25–40% (Whitcomb 2023).

Treatment effect sizes. Bile acid sequestrants (cholestyramine, colestipol, colesevelam) bind bile acids in the gut lumen and resolve or substantially improve diarrhoea in ~70% of confirmed BAD patients in observational cohorts, with colesevelam tolerated better than cholestyramine (Sadowski 2020; Camilleri 2020).

For microscopic colitis, oral budesonide 9 mg/day for 6–8 weeks induces clinical remission in ~80% versus ~17–40% on placebo across multiple RCTs and meta-analyses, with NNT ~2 (Miehlke 2014; Nguyen 2016; Miehlke 2021). Both AGA and European guidelines recommend budesonide as first-line. Relapse on stopping is ~60–80%; low-dose budesonide maintenance (3–6 mg) keeps most in remission.

For EPI, pancreatic enzyme replacement therapy (PERT) with enteric-coated mini-microsphere lipase, dosed with meals (typically 40,000–50,000 IU lipase per main meal, half that with snacks), normalises fat absorption in most patients and corrects steatorrhoea and weight loss in controlled trials (Löhr 2017; Whitcomb 2023). Failure of clinical response should prompt dose escalation and addition of a PPI before declaring resistance.

protocol

The 2018 BSG and 2019 AGA documents converge on a stepwise workup for adults with chronic non-bloody diarrhoea >4 weeks (Arasaradnam 2018; Smalley 2019; Schiller 2017):

History & red flags. Onset, nocturnal stools, urgency/incontinence, weight loss, blood, family history of IBD/coeliac/colon cancer, drug list (PPIs, NSAIDs, SSRIs, metformin, statins, colchicine, magnesium, sorbitol, recent antibiotics), travel, surgery (cholecystectomy, ileal resection, bariatric).
Basic bloods. CBC, CRP, ferritin, TSH, electrolytes, albumin, B₁₂, folate, vitamin D — looking for inflammation, malabsorption, hyperthyroidism.
Coeliac serology. IgA-tTG with total IgA. AGA guideline gives a strong recommendation: rule coeliac out before settling on a functional label (Smalley 2019; Singh 2018).
Stool studies. Faecal calprotectin (rules out IBD; <50 µg/g makes IBD unlikely), stool culture/ova-parasites/Giardia antigen as indicated, Clostridioides difficile if antibiotic-exposed.
Faecal elastase-1. Spot stool. <200 µg/g suggests EPI; <100 µg/g is more specific. Cheap and widely available; AGA practice update endorses it as first-line for EPI screening (Whitcomb 2023; Lindkvist 2013).
Bile acid diarrhoea testing. SeHCAT (selenium-75 homocholic acid taurine) 7-day retention <15% (UK/EU standard) where available; serum 7α-hydroxy-4-cholesten-3-one (C4) or fasting FGF19 in centres without SeHCAT; or a pragmatic 4-week trial of colestyramine/colesevelam in suggestive presentations where testing is unavailable (Vijayvargiya 2019; Sadowski 2020; Arasaradnam 2018).
Colonoscopy with random biopsies. Even when the mucosa looks normal, take biopsies from the right and left colon — this is the only way to diagnose microscopic colitis. Skipping the biopsies is the single most common missed-diagnosis pathway for MC (Pardi 2017; Nguyen 2016).
Cross-sectional imaging (CT or MRI pancreas) when EPI is confirmed or chronic pancreatitis suspected.

Treatment, once diagnosed. BAD: bile acid sequestrant — colesevelam 1.875 g BID titrated, or cholestyramine 4 g 1–4×/day if tolerated; expect response within days to weeks (Sadowski 2020). MC: review drug list, stop the culprit (PPI/NSAID/SSRI) if possible, then budesonide 9 mg/day × 6–8 weeks; taper to maintenance 3–6 mg if early relapse; counsel on smoking cessation (Nguyen 2016; Miehlke 2021). EPI: PERT 40,000–50,000 IU lipase with main meals, half with snacks, taken during the meal not before; supplement fat-soluble vitamins (A, D, E, K); add a PPI if response is incomplete (acid degrades enzyme) (Löhr 2017; Whitcomb 2023).

contraindications

Workup contraindications are mostly procedural rather than diagnostic — colonoscopy and SeHCAT have the usual cautions. Treatment-side contraindications matter more for the reader:

Bile acid sequestrants bind many other drugs in the gut (levothyroxine, warfarin, oral contraceptives, statins, digoxin, mycophenolate) and fat-soluble vitamins. Separate other oral medications by 4 hours. Avoid in complete biliary obstruction.
Budesonide has fewer systemic glucocorticoid effects than prednisolone but is not steroid-free: long courses still risk bone loss, hyperglycaemia, cataracts and adrenal suppression. Use lowest effective maintenance dose and monitor bone density on chronic therapy (Miehlke 2021).
PERT is porcine-derived (porcine pancreatin) — pork avoidance for religious/dietary reasons requires explicit discussion. Very high doses in cystic fibrosis have historically been linked to fibrosing colonopathy in children; the dosing range used in adult EPI is well below that threshold (Löhr 2017).
Pregnancy. Budesonide is generally considered acceptable in pregnancy when needed for IBD/MC; bile acid sequestrants can reduce fat-soluble vitamin absorption and warrant supplementation; PERT is considered safe.

misconceptions

The dominant misconception is that “chronic non-bloody diarrhoea + normal bloods + normal colonoscopy = IBS-D”. Three quiet errors hide inside that pattern:

“Normal colonoscopy” without biopsies misses microscopic colitis entirely. The mucosa looks endoscopically normal in MC by definition. If random biopsies were not taken, the colonoscopy did not rule out MC; it ruled out IBD and cancer (Pardi 2017; Nguyen 2016).
“Cholecystectomy diarrhoea is just something to live with.” Post-cholecystectomy chronic diarrhoea is frequently bile acid diarrhoea (type 3) and responds to sequestrants — it is treatable, not a lifestyle change (Camilleri 2020; Wedlake 2009).
“Steatorrhoea means dramatic stools.” Early EPI often shows up as soft, frequent stools, bloating after fatty meals and slow weight loss, without the textbook oily slick. Faecal elastase has a low enough threshold that it’s worth checking even when the stools are not visibly fatty (Whitcomb 2023; Lindkvist 2013).
“Probiotics fix this.” Probiotics have not been shown in controlled trials to treat BAD, MC or EPI; they are mostly studied in IBS where effect sizes are small and strain-dependent (Lacy 2021).
“Low-FODMAP is the answer.” Useful in IBS, often unhelpful in the three named conditions because the upstream driver isn’t fermentation of carbohydrate but bile acid, mucosal inflammation, or fat maldigestion. Pursuing dietary restriction without testing delays diagnosis.

failure-modes

The workup fails predictably:

Random biopsies not taken at colonoscopy — the single highest-yield omission for MC (Nguyen 2016).
Bile acid diarrhoea not considered. SeHCAT is unavailable in many US centres, C4 and FGF19 are not in routine labs, and the recommended pragmatic empirical trial of a sequestrant is often skipped (Sadowski 2020).
Faecal elastase not ordered because the stools don’t look classically steatorrhoeic. EPI is then missed for years, with progressive fat-soluble vitamin deficiency, weight loss and osteoporosis (Whitcomb 2023).
Drug history not interrogated. PPIs and SSRIs as MC triggers, metformin and orlistat as direct diarrhoeals, magnesium and sorbitol in supplements, recent antibiotics: stopping a culprit often resolves the symptom without further workup (Miehlke 2021).
Coeliac never serologically excluded. Coeliac prevalence is ~1% in Western populations and presentations are increasingly subtle; the AGA strongly recommends serology in every chronic-diarrhoea workup (Singh 2018; Smalley 2019).
PERT under-dosed. EPI “non-response” frequently reflects 25,000-IU starter doses, taking enzymes before the meal, or unrecognised gastric acid degradation (Löhr 2017).

stakes

Untreated chronic diarrhoea drives measurable nutritional, electrolyte and psychosocial harm. Steatorrhoea from EPI produces deficiency of fat-soluble vitamins (A, D, E, K) with downstream osteopenia/osteoporosis, increased fracture risk, peripheral neuropathy, night blindness and coagulopathy in severe cases (Löhr 2017; Whitcomb 2023). Bile acid diarrhoea drives chronic volume loss and steatorrhoea (when severe) plus secondary B₁₂ and fat-soluble vitamin deficits when the terminal ileum is the site of disease (Camilleri 2020). Microscopic colitis impairs health-related quality of life on a scale comparable to active IBD in case-control work — work absence, urgency, faecal incontinence, social withdrawal — and remits with treatment (Nyhlin 2014; Safroneeva 2020). Across all three, chronic urgency reshapes the day: bathroom mapping, declined invitations, restricted travel, disrupted sleep from nocturnal stools. Mortality risk is modest in pure BAD/MC; EPI from underlying chronic pancreatitis or pancreatic cancer carries the disease-specific mortality of those conditions.

payoff

The payoff side of these diagnoses is unusually clean for a chronic GI condition because treatments are mechanism-specific. Bile acid sequestrants resolve or substantially improve BAD in ~70% of confirmed cases, often within days of starting (Sadowski 2020; Camilleri 2020). Budesonide induces clinical remission in ~80% of microscopic colitis at 6–8 weeks with most patients reporting normalised stool frequency and resolution of nocturnal symptoms (Miehlke 2014). PERT normalises fat absorption and corrects weight, energy and vitamin status in EPI when adequately dosed (Löhr 2017). Quality-of-life scores in microscopic colitis return to near-population norms on treatment (Nyhlin 2014). The felt change reported in case series — reliable mornings, restored sleep, restored travel, regained weight — is a genuine reversal, not a marginal improvement.

The credibility range

Optimist case

Three discrete, well-characterised, treatable conditions sit under the IBS-D umbrella in ~30–40% of adults who never get the right workup. Each has guideline-backed, mechanism-specific therapy with effect sizes that dwarf anything in IBS-D itself: ~70% response to bile acid sequestrants for BAD, ~80% remission with budesonide for MC, near-complete steatorrhoea correction with adequately dosed PERT for EPI. The diagnostic tests are cheap (faecal elastase, IgA-tTG, faecal calprotectin), widely available (random colonic biopsies during routine colonoscopy), or substitutable (empirical sequestrant trial when SeHCAT/C4 are unavailable). A 2018-19 wave of guidelines (BSG, AGA, ACG, CAG, ueg) has explicitly aligned on this point. The right framing is “your chronic diarrhoea has a name, and there’s a fair chance the name is treatable” — that’s a defensible epistemic position, not a hyped one.

Skeptic case

The 30% BAD prevalence in IBS-D is from SeHCAT data and the SeHCAT threshold itself (mild <15%, severe <5%) is calibrated against IBS-D populations rather than against a clean control, so prevalence may be inflated. SeHCAT is not available in much of the US, and the proposed empirical sequestrant trial has placebo response in IBS-D itself ~30–40%, complicating attribution. Microscopic colitis treatment with budesonide has high relapse on stopping (~60–80%), so “remission” can mean “chronic low-dose steroid” with its bone, glycaemic and adrenal costs; long-term maintenance trials are limited. Faecal elastase has imperfect specificity in watery diarrhoea (the enzyme is diluted by stool water), so isolated low values without imaging or symptom corroboration can over-diagnose EPI; PERT carries a real cost-of-care burden and porcine-source constraints. And the broader claim that organic diagnoses lurk under most IBS-D may overstate the case — for a substantial fraction of patients, the diarrhoea is genuinely functional and the targeted workup adds cost without yield.

Author’s call

The asymmetry favours doing the workup. The downside of testing — cost of a faecal elastase, a calprotectin, a coeliac panel, biopsies during a colonoscopy the patient is having anyway, and either SeHCAT or an empirical sequestrant trial — is small in time and dollars relative to the upside of converting a years-long “just IBS” into a treatable named condition. Even with skeptic-side adjustments, the BAD prevalence in IBS-D is high enough (let’s say 15–30%) that empirical sequestrant trials in suggestive presentations are reasonable practice. The MC remission-with-relapse pattern is real but the alternative for the patient is symptomatic disease; managed maintenance dosing of budesonide is a reasonable trade. EPI is the cleanest case — faecal elastase is cheap, PERT works when correctly dosed, and the cost of missing EPI is cumulative deficiency. Net call: aggressive targeted workup, by guideline, before accepting an IBS-D label — the evidence base is solid (multiple guidelines align), the controversy is genuine but secondary (under-diagnosis, not over-diagnosis, is the prevailing failure mode).

Stakeholder and incentive map

Patients and patient advocacy groups (Guts UK, Bile Salt Malabsorption groups, Microscopic Colitis Foundation) actively lobby for the named workups — the population most aware of these conditions tends to be patients diagnosed late after years of being told they had IBS.
Gastroenterology guideline bodies (BSG, AGA, ACG, CAG, UEG) are aligned and have been progressively explicit since the 2018–19 cycle that BAD and MC are under-diagnosed.
Pharma incentive — modest. Bile acid sequestrants are off-patent and cheap. Budesonide MMX and Entocort have brand pressure for MC but lower-cost generics exist. PERT (Creon, Zenpep, Pancreaze) is on-patent at high price and well-promoted to GI; this is the strongest commercial pull in the space and probably contributes to faster EPI awareness than the prevalence alone would predict.
Counter-incentive — primary care throughput. The IBS-D label is fast to apply and discharges the visit. A full targeted workup takes 3–6 visits, multiple labs, and a colonoscopy referral, and the system is poorly reimbursed for the workup’s upstream cost. The structural pressure is towards under-diagnosis.
Skeptic camp — some IBS specialists worry that the “everyone needs a workup” framing pushes low-yield testing in obvious functional disease and inflates costs.

Population variability

Age. MC incidence rises sharply over age 60, peaking in 70s; new-onset watery diarrhoea in an older adult is MC until biopsies say otherwise (Tong 2015; Pardi 2017). BAD spans all ages but type 2 (idiopathic) clusters in 40s–60s. EPI in chronic pancreatitis tracks years-of-disease; EPI in diabetes tracks duration of diabetes.
Sex. MC is markedly female-predominant (~3:1 collagenous, ~2:1 lymphocytic). BAD is roughly equal. Chronic pancreatitis EPI is male-predominant (alcohol-related disease overrepresented).
Drug exposure. Long-term PPI use is associated with MC across multiple population studies; SSRI, NSAID, statin and ranitidine exposures also flagged. Drug list is the cheapest yield in the workup (Miehlke 2021).
Smoking. Active smoking doubles MC risk and worsens budesonide response; cessation is part of management (Miehlke 2021).
Comorbidities. Autoimmune cluster (coeliac, Hashimoto’s, type 1 diabetes, rheumatoid arthritis) raises MC risk meaningfully. Prior cholecystectomy or terminal ileal Crohn’s resection raises BAD risk to near-certainty.
Ethnicity / geography. Most prevalence data are from northern Europe and North America; data from Asia, Africa and South America are thinner and incidence figures may not translate.

Knowledge gaps

True BAD prevalence in non-IBS-D chronic diarrhoea populations and in racially diverse cohorts.
Optimal long-term maintenance regimen for MC. Minimum budesonide dose, role of biologics (vedolizumab) and immunomodulators in refractory disease, durability data beyond two years (Miehlke 2021).
EPI in type 2 diabetes — prevalence, clinical impact, threshold for PERT — is an active area; faecal elastase studies suggest it is more common than recognised, but treatment outcomes are limited (Whitcomb 2023).
Cost-effectiveness of universal SeHCAT versus empirical sequestrant trial in different healthcare systems.
Microbiome interventions. Whether faecal microbiota transplant or targeted prebiotics modify MC course is being explored but is not standard of care.
Long-term cancer risk in microscopic colitis appears not elevated, but the surveillance question (do these patients need colonoscopy intervals different from age-matched controls?) is still being settled.

Scope vs brief. The brief named bile acid malabsorption, microscopic colitis and pancreatic insufficiency, the workup, and effects on nutrition / electrolytes / quality of life. The article covers all five end to end: each named condition gets a mechanism paragraph and a specific treatment with dose, the workup is laid out as a sequenced action callout, and nutrition / electrolyte / QoL consequences are folded into the stakes and payoff sections rather than getting their own block. Electrolyte management is one line in protocol (oral rehydration solution) because the workup is the lever — once treatment lands, the electrolyte / nutritional state corrects.

Action choice. Considered test (the named tests are the lever) and decide (clinician input required). Landed on respond: the entry is a protocol for a presenting symptom — persistent loose stools — and the reader’s next action is to bring the workup list to a GP. test would have emphasised lab orders without the “and here’s the symptom that triggers them” framing.

Rating difficulties.

focus scored 0. The chronic-diarrhoea → sleep loss → attention chain is real but the article doesn’t carry a paragraph on cognitive lift, so a non-zero score would have outrun the body coverage. Kept honest at 0.
beauty_cumulative scored 0 for the same reason — vitamin A → skin/hair via long-standing EPI is mentioned only as “night vision dims” in the stakes paragraph. A 1 would have been technically defensible but the article doesn’t earn it.
longevity at 2 is the hardest scored call. Pure BAD and MC carry low excess mortality; the longevity case rests on (a) catching EPI before fat-soluble vitamin deficits drive osteoporotic fractures and (b) catching the chronic pancreatitis or pancreatic mass behind some EPI cases. 2 captures the modest effect without overclaiming.
health_short_term at 4 vs 5: the within-weeks effect sizes (~70% sequestrant response, ~80% budesonide remission) are large for chronic GI, but the effect is conditional on getting the right diagnosis and tolerating treatment — not the unconditional baseline shift a 5 would imply.
cost_burden at 2 spans a wide range. Cholestyramine is essentially free; budesonide generic is moderate; PERT is the costly outlier (typically $3,000–$8,000/year list, almost always insurance-covered). 2 reflects the median patient; the PERT-payer tail is closer to 3.

Excluded by design.

Coeliac disease — pointed at in failure-modes and out-of-scope but not covered substantively; warrants its own entry.
IBD (Crohn’s, UC) — separate entity, separate workup once calprotectin is elevated; own entry.
SIBO — flagged in out-of-scope; the evidence base on breath testing and antibiotic treatment is contested enough to need its own entry rather than a short paragraph here.
IBS-D proper — deliberately framed as “what’s left when the workup is clean” rather than re-litigated here; its diet and gut-brain therapy evidence base earns its own entry.
Paediatric chronic diarrhoea, infectious causes >4 weeks in the immunocompromised, short-bowel syndrome and bariatric-surgery diarrhoea — out of scope for an adult, primary-care-presenting entry.

Future links to wire when those entries land: celiac-disease, ibs, inflammatory-bowel-disease, sibo, fecal-calprotectin-testing, oral-rehydration-solution, chronic-pancreatitis.

Separate-entry candidates surfaced by the write: bile-acid-diarrhea as a standalone deep-dive (the entry here treats it as one of three; a dedicated entry could cover type-1/2/3 subtypes, SeHCAT vs C4 vs FGF19 testing, and the case for empirical sequestrant trials at much greater length); fecal-elastase-testing as a screening-and-testing entry once that part of the catalogue fills out.

Spelling. Used “diarrhoea” throughout (British). Switch to US “diarrhea” in a localisation pass if the catalogue standard is American.