Poor Food Absorption Explained: Causes, Symptoms, Tests, and Fixes

You can eat a decent diet and still be undernourished if your gut isn’t absorbing what you eat. That’s the nasty trick behind poor food absorption: normal meals, abnormal payoff. This guide breaks down the science in clear English, shows you the usual culprits, the tests that actually help, and the fixes you can start today. Expect practical steps, not miracle claims. If you’re dealing with weight changes, low energy, bloating, or stubborn deficiencies, this will help you figure out what matters and what’s noise.

• Poor absorption stems from problems with digestion (enzymes, bile, acid), the gut lining (inflammation, coeliac), bugs (SIBO), or speed (too fast).
• Classic sign cluster: chronic bloating, loose or fatty stools that float/shine, unexplained anaemia, easy bruising, brittle hair/nails, and weight loss.
• First-line tests (via GP): bloods (FBC, ferritin, B12, folate, vitamin D, albumin), coeliac antibodies, stool elastase, calprotectin; consider breath tests.
• Fixes often work fast: targeted enzymes, MCT oil, treat bile-acid diarrhoea or SIBO, correct deficiencies, and personalise carbs (lactose/FODMAP).
• Red flags need urgent review: blood in stool, fever, night sweats, unintentional weight loss, persistent vomiting, severe pain, or signs of dehydration.

What poor absorption actually is (and how it shows up)

Food has to be broken down, dissolved, transported, and then absorbed through your small intestine. When any step stumbles, you get malabsorption. Think of four checkpoints:

• Breakdown: stomach acid, bile, and pancreatic enzymes turn food into small bits.
• Mixing: small intestine churns and mixes fats with bile, carbs with enzymes, proteins with proteases.
• Entry: the intestinal lining-with its villi and enzymes-lets nutrients in.
• Transport: the blood and lymph carry absorbed nutrients away.

If bile is low, fats don’t emulsify. If pancreatic enzymes are low, fats, carbs, and proteins aren’t fully digested. If the villi are flattened (as in coeliac disease), surface area shrinks. If bacteria take over the small bowel (SIBO), they steal nutrients and gas out the place. If transit is too fast (post-infection, thyroid issues, stress), there’s not enough time to absorb.

So what does it look like in real life?

• Fat malabsorption: pale, bulky, hard-to-flush, oily or floating stools; weight loss; greasy toilet water; easy bruising (vitamin K), bone aches (vitamin D/calcium), night blindness (vitamin A).
• Iron, B12, or folate issues: fatigue, brain fog, shortness of breath on stairs, paler skin, numbness/tingling (B12), sore tongue, brittle nails.
• Protein loss: ankle swelling, muscle loss, hair thinning, slow wound healing.
• Carb problems: bloating within 30-120 minutes of eating, cramping, diarrhoea, lots of gas (often lactose or FODMAP sensitivity, or SIBO).

Quick self-check (not a diagnosis):

• The “toilet test”: do stools often float, shine, or leave an oily ring? That points to fat malabsorption.
• The “energy gap”: are you eating normally but losing weight or staying tired? Consider iron/B12/folate or thyroid issues.
• The “timing clue”: bloating that hits within two hours of meals hints at carbohydrate digestion or SIBO.

How common is this? In the UK, coeliac disease affects about 1 in 100 people (NICE, 2023), and many don’t know they have it. Bile acid diarrhoea is underdiagnosed and may account for roughly a third of chronic diarrhoea labelled as IBS-D (BSG guidance, 2024). Exocrine pancreatic insufficiency is common in chronic pancreatitis and after pancreatic or gastric surgery. SIBO prevalence varies by condition-higher in those with IBS, diabetes with slow gut, or after bowel surgery (BSG, 2024).

Root causes: how to spot the likely culprit

Think in buckets. When you know the bucket, you can pick the right test and treatment faster.

• Digestion problems (before absorption): low stomach acid (older age, PPIs), low bile (gallbladder issues), low pancreatic enzymes (pancreatitis, diabetes, cystic fibrosis, post-surgery).
• Gut lining problems (the wall): coeliac disease, Crohn’s affecting small bowel, infections (e.g., Giardia), radiation injury, short bowel after surgery.
• Microbiome problems: SIBO over-ferments carbs, steals B12, and produces gas; post-infectious IBS can speed things up.
• Transport and speed: hyperthyroidism, stress, medications (metformin, orlistat), rapid transit after surgery.
• Specific enzyme deficits: lactase deficiency (lactose intolerance), sucrase-isomaltase deficiency (less common in adults but real).

Clue-based shortcuts you can use:

• Fatty, floating stools + weight loss + bloating → suspect pancreatic enzyme shortage, bile acid issues, or coeliac.
• Chronic watery diarrhoea soon after meals with urgency, often without severe pain → think bile acid diarrhoea.
• Bloating that arrives fast after dairy → try lactose reduction for two weeks; consider a hydrogen breath test later.
• Bloating plus mixed bowel habits (constipation and diarrhoea), lots of gas, worse on onions/beans/apples → consider FODMAP sensitivity or SIBO.
• Long-standing anaemia that doesn’t shift with iron → check coeliac antibodies; also consider ongoing blood loss.
• Joint pains, mouth ulcers, perianal issues, or fever → consider inflammatory bowel disease; don’t self-manage-see your GP.

Medication wildcards to check: metformin (loose stools), orlistat (fat in stool by design), PPIs (can shift microbiome), NSAIDs (gut injury), alcohol (pancreas and gut lining). If symptoms began after a new med, tell your GP.

How doctors test for poor absorption (and what results mean)

Here’s a simple, UK-friendly pathway you can discuss with your GP. It’s evidence-backed (NICE 2023-2025; BSG 2024) and keeps costs sensible.

1. History and red flags
   • Red flags: blood in stool, black tarry stools, fever, persistent vomiting, unintentional weight loss, severe pain, night sweats, dehydration. These need urgent assessment.
   • Clues: onset after infection or antibiotics; surgery history; gallbladder removal; pancreatitis; family history of coeliac, IBD, or autoimmune thyroid.
2. Baseline bloods
   • FBC and ferritin (iron), B12, folate.
   • Vitamin D (25-OH), calcium, magnesium, phosphate, albumin (protein status).
   • CRP/ESR (inflammation), thyroid (TSH), coeliac screen (tTG-IgA plus total IgA).
   • Glucose/HbA1c (diabetes can affect gut motility and pancreas).
3. Stool and breath tests
   • Faecal elastase for pancreatic enzyme output (low suggests exocrine pancreatic insufficiency).
   • Faecal calprotectin to rule in/out inflammation suggestive of IBD.
   • Faecal fat (72-hour) is rarely done now; clinical signs often guide.
   • Hydrogen/methane breath tests for lactose malabsorption or SIBO when history fits.
4. Imaging and scopes (as needed)
   • Upper endoscopy with duodenal biopsies if coeliac serology positive (or strong suspicion despite negative bloods).
   • Colonoscopy if red flags or calprotectin raised.
   • Ultrasound, MRCP, or CT if pancreatitis or biliary issues suspected.
5. Specific bile acid tests (UK)
   • SeHCAT retention scan (7-day): low retention supports bile acid diarrhoea.
   • Serum C4 and FGF19 are alternatives in some centres.

How to read a few common results in plain language:

• Low faecal elastase: your pancreas isn’t making enough enzymes. Enzyme replacement helps most people.
• Normal calprotectin but ongoing symptoms: IBD less likely; focus on function (IBS), SIBO, bile acids, or coeliac if not already checked.
• Positive coeliac antibodies: don’t go gluten-free until after biopsies, or you may get a false negative biopsy.
• Breath test positive for lactose: try a lactose-reduced diet; many can still handle hard cheeses/yoghurt.
• SeHCAT/C4 consistent with bile acid diarrhoea: bile acid binders (by prescription) usually work well.

On evidence and credibility: the approach above aligns with NICE coeliac guidelines (updated 2023), BSG guidance on chronic diarrhoea and bile acid diarrhoea (2024), and UK gastroenterology practice. Breath testing standards continue to evolve (2024-2025), so local lab protocols vary.

Fixes that work: diet tweaks, supplements, and when to seek help

No single diet cures every case. The trick is to match the fix to the fault. Here’s a practical menu of options you can tailor with your GP or dietitian.

Simple nutrition strategies that help most people:

• Smaller, more frequent meals: aim for 4-6 mini-meals. Easier on enzymes, bile, and motility.
• Add MCT oil (start 1 tsp with meals, build to 1-2 tbsp/day if tolerated): MCTs absorb without bile or pancreatic enzymes. Useful in fat malabsorption.
• Protein steady across the day: about a palm-sized portion each meal. Helps preserve muscle when absorption is patchy.
• Low-lactose trial for two weeks if dairy triggers bloating: choose lactose-free milk, hard cheeses, live yoghurt. Reintroduce to your comfort level.
• Cooked, peeled, and blended veg if fibre is irritating; gradually reintroduce variety as symptoms settle.
• Hydration with electrolytes if diarrhoea: add a pinch of salt and a squeeze of citrus to water, or use an oral rehydration solution.

Targeted interventions by likely cause:

• Exocrine pancreatic insufficiency: pancreatic enzyme replacement therapy (PERT). A common starting point is 40,000-50,000 units of lipase with main meals and 25,000 units with snacks-adjust to stool consistency and meal fat. Take capsules with the first bites and consider splitting the dose during longer meals. Your GP or gastro team prescribes and adjusts.
• Bile acid diarrhoea: bile acid binders (e.g., colesevelam/colestyramine) via prescription. Take away from other meds and vitamins (2-4 hours) so they’re not bound up. Many see improvement within days.
• SIBO: treatment is medical (antibiotics guided by your clinician) plus diet as needed. Don’t cycle antibiotics without supervision; relapses are common if underlying issues (slow motility, adhesions) aren’t addressed.
• Coeliac disease: strict gluten-free diet for life after biopsy confirmation. Work with a dietitian to avoid hidden gluten and to meet fibre and micronutrient needs.
• Lactose intolerance: most tolerate up to 12-18 g lactose (about 250 ml milk) if spread out. Lactase tablets can help when eating out.

Supplement smart, not random:

• Vitamin D3: many adults need 1,000-2,000 IU (25-50 mcg) daily in winter; dosed higher short-term if levels are low, per GP advice.
• Iron: ferrous bisglycinate or ferric maltol tend to be gentler. Take with vitamin C, away from tea/coffee/calcium. If you can’t tolerate or don’t respond, ask about IV iron.
• Vitamin B12: if low due to malabsorption, injections may be needed; high-dose oral (1,000 mcg/day) works for some, but check with your GP.
• Calcium and vitamin K if on bile acid binders or with fat malabsorption-only after checking bloods and diet.
• Electrolytes: consider oral rehydration after bouts of diarrhoea to stop the energy crash.

Short, safe experiments (2-4 weeks) to clarify triggers:

• Lactose-reduced trial if dairy bloats you.
• Low-FODMAP phase 1 (with a dietitian if possible): 2-4 weeks max, then structured reintroduction to identify which FODMAP groups are your issue. Don’t stay restrictive long-term.
• Fat-titration with enzyme support and MCT: if stools normalise as you match enzymes to meal fat, you’re on the right track.

Decision hints (simple and practical):

• If your main issue is greasy stools + weight loss → ask for faecal elastase and fat-soluble vitamins; trial PERT under supervision.
• If diarrhoea arrives fast after meals (especially fatty ones) → ask about SeHCAT/C4 or an empirical bile acid binder trial.
• If dairy triggers gassy bloating → run a lactose-reduced trial; consider a breath test later.
• If bloat dominates and calprotectin is normal → discuss SIBO breath testing or a cautious empirical course, depending on your GP’s view.
• If iron deficiency won’t budge → re-check coeliac serology and look for blood loss.

Side effects and pitfalls to avoid:

• Don’t go gluten-free before coeliac testing-you may mask the diagnosis.
• Don’t stay on a strict low-FODMAP diet long-term; it can starve helpful gut bacteria.
• Enzymes work only if taken with food; taken after a meal, they underperform.
• Bile acid binders can block vitamins and meds-separate dosing and monitor vitamins A, D, E, K.
• Random mega-dosing of vitamins can backfire; check levels first when possible.

Quick FAQ

• Can stress alone cause poor absorption? It won’t flatten villi, but it can speed transit and increase gut sensitivity, which reduces contact time and increases symptoms.
• Are probiotics a fix? They can help some IBS symptoms, but evidence in SIBO and frank malabsorption is mixed. If you try one, pick a single strain and test it for 3-4 weeks.
• Is weight gain impossible with malabsorption? Not at all. Once the cause is treated and you match enzymes/diet to your needs, weight and energy usually improve.
• Does fibre help or hurt? Both. Soluble fibre (oats, psyllium) often helps diarrhoea; very high insoluble fibre can worsen bloating in sensitive guts. Adjust to comfort.
• Is this forever? Many causes are treatable: coeliac improves on a gluten-free diet, bile acid diarrhoea often responds to binders, and enzyme therapy is very effective for pancreatic issues.

Next steps and troubleshooting for common scenarios:

• Busy parent with new bloating, no red flags: ask GP for FBC, ferritin, B12/folate, vitamin D, thyroid, coeliac screen; try a two-week lactose-reduced trial; keep a simple symptom-food diary.
• Post-gallbladder surgery with urgent diarrhoea: request SeHCAT/C4 via GP; ask about bile acid binder trial; reduce very high-fat meals short term.
• History of pancreatitis, greasy stools: request faecal elastase; discuss PERT dosing; add MCT; monitor fat-soluble vitamins.
• Long-standing IBS label, poor response to generic advice: check calprotectin (if not done), coeliac serology, breath test for SIBO/lactose as indicated; do a short, structured low-FODMAP trial with reintroduction.
• Vegetarian with iron/B12 issues: consider iron forms that absorb better and B12 supplements or injections; pair iron with vitamin C; avoid tea/coffee with iron-rich meals.

When to seek urgent care: signs of dehydration, black or bloody stools, severe abdominal pain, persistent fever, or rapid weight loss. For everyone else, starting with GP-led tests and the targeted steps above is usually enough to find the cause and get you feeling human again.

Sources behind the advice include NICE guidance on coeliac disease (2023), British Society of Gastroenterology guidance on chronic diarrhoea and bile acid diarrhoea (2024), and UK gastroenterology clinic pathways as of 2025. Use this as a plan to discuss with your GP or dietitian, not a replacement for care.