How digestion works
Your stomach mixes food with acid and enzymes, the small intestine absorbs nutrients with help from pancreatic enzymes and bile, and the colon reabsorbs water and hosts most of your gut microbes. Your liver processes what is absorbed; fatty liver disease is covered in Metabolic health, where the optimal plan is a FIB-4 liver score every year from 35.
About 68% of adults worldwide cannot fully digest lactose, the sugar in milk. It causes gas and bloating, not damage, and lactose-free dairy, yogurt, and hard cheeses are usually well tolerated.
The microbiome
Your colon holds roughly 38 trillion bacteria, about as many as your own cells. They ferment the fiber you cannot digest into short-chain fatty acids such as butyrate, which fuel the cells lining the colon and influence immunity and metabolism.
Variety may matter as much as quantity. In the American Gut Project, a citizen-science study of more than 10,000 people, those who ate more than 30 types of plants a week had different gut microbes and fewer antibiotic resistance genes than those who ate 10 or fewer. It was observational, so treat 30 plants as a useful target, not a proven threshold. Weak
Restoring a damaged microbiome can be real medicine: in 2023 the FDA approved Vowst, an oral product made from donor stool, to prevent repeat C. difficile infection.
Microbiome tests
Direct-to-consumer stool tests promise a personalized diet. A 2025 consensus of 69 experts found insufficient evidence to recommend routine microbiome testing, discouraged ordering a test without a clinical reason, and discouraged the popular Firmicutes-to-Bacteroidetes ratio. Weak
Fermented foods
Fermented foods such as yogurt, kefir, kimchi, sauerkraut, and kombucha contain live microbes. In a 2021 Stanford trial, 36 healthy adults were randomized to a high-fiber or high-fermented-food diet for 17 weeks. The fermented-food group, who worked up to about 6 servings a day, saw microbiome diversity rise steadily and 19 of 93 inflammatory proteins fall, including IL-6. The high-fiber group, who roughly doubled fiber to 45 g a day, did not gain diversity during the trial. Moderate
The trial was small and measured markers, not disease, but fermented foods are cheap and safe. A serving or two a day of unsweetened yogurt, kefir, or fermented vegetables is a reasonable habit.
Probiotics
Probiotic supplements are a different matter. The American Gastroenterological Association (AGA) recommends them for IBS, Crohn's disease, ulcerative colitis, and C. difficile treatment only within clinical trials, with a weak suggestion of specific strains to prevent C. difficile during some antibiotic courses. For healthy people, the evidence is weak. Weak
They may even slow recovery: after antibiotics, people given a multistrain probiotic had delayed and incomplete recovery of their own gut microbes. Product quality varies; see Supplements.
Constipation
About 1 in 10 adults meets formal criteria for chronic constipation, women about twice as often as men. It means hard or lumpy stools, straining, a sense of incomplete emptying, or fewer than three bowel movements a week for at least three months.
What helps
The 2025 British Dietetic Association guideline, the first to grade diet treatments for constipation, recommends psyllium, kiwifruit, rye bread, and mineral water high in magnesium, among others. Fiber supplements worked at more than 10 g a day for at least four weeks, but not at lower doses.
| Option | What to do | Evidence |
|---|---|---|
| Food fiber | At least 30 g a day from beans, vegetables, fruit, whole grains, nuts, and seeds (Fiber) | Moderate |
| Psyllium | Over 10 g a day with a full glass of water, for at least 4 weeks (Psyllium) | Moderate |
| Kiwifruit | 2 to 3 a day for at least 4 weeks | Moderate |
| Prunes | Worked about as well as psyllium in a 79-patient trial, with kiwifruit best tolerated | Moderate |
| Footstool | Raised knees reduced straining and time on the toilet in a 52-person study | Weak |
| Polyethylene glycol (Miralax) | Strongly recommended by the AGA and ACG | Strong |
| Bisacodyl or sodium picosulfate | Strongly recommended for short-term or rescue use | Moderate |
Add fiber gradually and drink enough fluid as you do. Senna, magnesium oxide, and lactulose get conditional recommendations, and prescription drugs exist when these fail.
Lifelong sluggishness is rarely dangerous. A new and lasting change in your bowel habit is different, especially with blood in the stool, weight loss, or iron deficiency; see Warning signs.
Reflux and GERD
Gastroesophageal reflux disease (GERD) means stomach contents rise into the esophagus often enough to cause symptoms or damage. It affects 18–28% of adults in North America.
Lifestyle measures
The American College of Gastroenterology (ACG) 2022 GERD guideline recommends:
- Lose weight if you have overweight or obesity (strong recommendation, moderate evidence). Moderate
- Stop eating 2 to 3 hours before bed. Weak
- Avoid tobacco and any foods that reliably trigger your symptoms. Weak
- Raise the head of your bed with blocks or a wedge for nighttime symptoms; lying on your left side also reduces reflux compared with your right. Weak
For classic heartburn without warning signs, the ACG recommends an 8-week trial of a proton pump inhibitor (PPI) taken once daily before a meal, and then trying to stop it if symptoms resolve. If symptoms return or never respond, testing off the drug, with endoscopy or pH monitoring, confirms whether you actually have GERD before you commit to long-term treatment.
Screening for Barrett's esophagus
Barrett's esophagus is a change in the esophageal lining caused by chronic reflux, and it is the precursor of esophageal adenocarcinoma, a deadly cancer. The ACG's 2022 guideline suggests a single screening test for people with chronic reflux symptoms plus three or more risk factors: male sex, age over 50, White race, smoking now or in the past, obesity, or a parent, sibling, or child with Barrett's esophagus or esophageal adenocarcinoma.
The test can be an upper endoscopy or, as an accepted alternative, a swallowed capsule sponge that collects cells for a biomarker test. If it is negative, the ACG suggests not repeating it. The evidence is very low quality, but the test is done once and the cancer is lethal, so this is the optimal plan for people who qualify. Weak
Long-term acid suppression
Proton pump inhibitors such as omeprazole and pantoprazole are taken long term by millions of people. Observational studies have linked long-term use to fractures, kidney disease, dementia, pneumonia, and infections, but they cannot fully separate the drug's effects from the health problems of the people who take it.
A randomized trial is the better test. COMPASS assigned 17,598 people with cardiovascular disease to pantoprazole 40 mg or placebo for a median of three years and tracked pneumonia, fractures, kidney disease, dementia, diabetes, cancer, and death. The only difference was a small rise in intestinal infections (1.4% versus 1.0%). Strong
| Situation | What the AGA advises |
|---|---|
| Uncomplicated reflux that responded to a short course | Try to stop or reduce the dose (AGA 2017) |
| Severe erosive esophagitis, stricture, or Barrett's esophagus | Generally stay on a PPI (AGA 2022) |
| High risk of ulcer bleeding, such as NSAIDs or blood thinners | Stay on a PPI |
| No clear reason to be on one | Try stopping, by tapering or all at once |
| Worried about side effects but have a real indication | Keep taking it; the AGA says to stop only for lack of an indication, not out of fear |
Expect rebound when you stop. In healthy volunteers given a PPI for eight weeks, 44% developed acid symptoms in the two to four weeks after stopping, versus 15% on placebo, so ride out that period before deciding you still need the drug. An H2 blocker such as famotidine can bridge the gap; ranitidine (Zantac) was withdrawn in 2020 because of a contaminant.
Two or more years of acid suppression was linked to 65% higher odds of vitamin B12 deficiency, so long-term users should check B12 yearly; that test is in Blood count, iron, thyroid, and vitamins.
Irritable bowel syndrome
Irritable bowel syndrome (IBS) is recurrent belly pain, on average at least one day a week for three months, linked to bowel movements or to changes in stool frequency or form. Under these Rome IV criteria it affected 4.1% of adults in a large international survey. It is a disorder of gut-brain communication, not imagination, and it does not damage the bowel.
Diagnosis
The ACG's 2021 guideline favors a positive diagnosis from symptoms over endless tests. With diarrhea, it suggests a celiac blood test and a fecal calprotectin, a stool marker of inflammation that helps rule out inflammatory bowel disease. Warning signs such as bleeding, weight loss, or iron deficiency call for more testing.
Low-FODMAP diet and other treatments
FODMAPs are fermentable carbohydrates found in foods such as wheat, onions, garlic, beans, some fruits, and dairy. The ACG recommends a limited trial of a low-FODMAP diet, and the AGA calls it the most evidence-based diet for IBS. It has three phases:
- Restriction for no more than 4 to 6 weeks.
- Reintroduction, testing one FODMAP group at a time.
- Personalization, eating as broadly as your tolerance allows.
The AGA advises working with a dietitian and warns that restrictive diets are a poor choice for people at risk of malnutrition or with an eating disorder.
Other options with evidence include soluble fiber such as psyllium, gut-directed psychotherapy such as hypnotherapy, rifaximin (an antibiotic) for IBS with diarrhea, and prescription drugs for IBS with constipation. Gluten-free diets give mixed results in trials.
Diverticular disease
Diverticula are small pouches in the colon wall. More than 30% of US adults aged 50 to 59 and more than 70% over 80 have them, and fewer than 5% develop diverticulitis, inflammation of a pouch, which puts about 200,000 Americans in the hospital each year.
Diet and habits do matter. In the EPIC-Oxford cohort of 47,033 adults, people in the highest fifth of fiber intake (about 26 g a day or more) had a 41% lower risk of hospitalization or death from diverticular disease than those under 14 g. In US men, the highest red meat intake was linked to a 58% higher risk of diverticulitis. Moderate
After diverticulitis, the AGA advises a high-quality diet, a normal weight, activity, no smoking, and no NSAIDs other than aspirin prescribed for the heart. Mild attacks may not need antibiotics, and surgery should not be triggered by a set number of episodes.
Celiac disease
Celiac disease is an immune reaction to gluten, the protein in wheat, barley, and rye, that damages the small intestine. About 1.4% of people worldwide test positive on blood tests, and 0.7% have biopsy-confirmed disease. Symptoms range from diarrhea and bloating to fatigue, anemia, or nothing at all, which is why the NICE list of reasons to test is so broad.
Who should get a celiac test
Guideline minimum: the USPSTF found the evidence insufficient to judge screening people without symptoms, and the ACG recommends case finding but not mass screening.
The optimal plan is to test once if anything on the UK NICE guideline's broad list applies.
| Group | Recommendation |
|---|---|
| Parent, sibling, or child with celiac disease | Test; 7.5% of first-degree relatives have it, including about 1 in 7 sisters |
| Unexplained iron, B12, or folate deficiency | Offer testing |
| Type 1 diabetes or autoimmune thyroid disease | Offer testing at diagnosis |
| IBS symptoms, persistent unexplained gut symptoms, fatigue, or weight loss | Offer testing |
| Low bone density or osteomalacia | Consider testing |
| Unexplained raised liver enzymes | Test (ACG 2013) |
| Unexplained nerve symptoms, subfertility, or recurrent miscarriage | Consider testing |
The AGA also suggests celiac testing in adults with iron deficiency anemia; who should get a ferritin test is in Blood count, iron, thyroid, and vitamins. Relatives who test negative should be retested if symptoms appear. Moderate
How to test
- Eat gluten first. NICE advises eating gluten in more than one meal a day for at least six weeks before testing.
- Blood test: tissue transglutaminase IgA (tTG-IgA) is the preferred single test, with total IgA measured when suspicion is high, because some people make no IgA and get a false negative.
- Confirmation: a positive blood test is confirmed with an upper endoscopy and several small-bowel biopsies.
- Already gluten-free? A genetic test for HLA-DQ2 and DQ8 can rule celiac disease out before you consider a gluten challenge. A negative result ends the question; a positive one means you need the challenge and testing to know.
Treatment is a strict lifelong gluten-free diet, and the ACG also recommends the pneumococcal vaccine.
H. pylori and stomach cancer
Helicobacter pylori is a stomach bacterium that infected an estimated 4.4 billion people in 2015, more than half the world. It causes indigestion, stomach ulcers, and stomach cancer, which will strike about 31,510 Americans and kill 10,740 in 2026.
Treating it prevents cancer. A Cochrane review of seven trials, six in Asia, found that eradication cut stomach cancer by 46% and stomach cancer deaths by 39% in healthy infected people. In a Korean trial of 1,676 infected people whose parent or sibling had stomach cancer, treatment lowered stomach cancer from 2.7% to 1.2% over 9.2 years. Strong
Who should get an H. pylori test
The US has no population screening. The optimal plan is a one-time breath or stool test if any of these apply:
- Medical reasons in ACG guidance: a history of ulcers, ongoing indigestion, unexplained iron deficiency, long-term low-dose aspirin or starting regular NSAIDs, or a low platelet disorder called ITP. Indigestion under 60 is managed by testing for H. pylori and treating it if found, per the ACG and Canadian guideline.
- Higher stomach cancer risk in the AGA's 2025 guidance: a parent, sibling, or child with stomach cancer, being a first-generation immigrant from a region with high stomach cancer rates, such as parts of East Asia, possibly other non-White racial and ethnic groups, and certain hereditary cancer syndromes.
- A household member who has tested positive; the AGA suggests considering family testing.
The ACG's 2024 guideline prefers 14-day bismuth quadruple therapy when antibiotic resistance is unknown, and recommends a test afterward for everyone treated to confirm the infection is gone.
Endoscopic screening for high-risk people
For the same high-risk groups, the AGA advises that upper endoscopy be considered to screen for stomach cancer and its precursors, such as gastric atrophy and intestinal metaplasia, with surveillance about every three years if severe changes are found. It does not set a starting age, so discuss timing with a gastroenterologist; pairing it with a screening colonoscopy saves a second sedation. Weak
Inflammatory bowel disease
Inflammatory bowel disease (IBD) includes Crohn's disease, which can affect any part of the gut, and ulcerative colitis, which affects the colon. About 2.39 million Americans, or 0.72%, have a diagnosis, and new cases peak in the 20s.
Unlike IBS, IBD causes visible inflammation and can damage the bowel. Clues include diarrhea lasting weeks, blood or mucus in the stool, waking at night to pass stool, weight loss, and pain or abscesses around the anus. A fecal calprotectin stool test and blood work are the usual first steps, followed by colonoscopy.
IBD raises the risk of colorectal cancer, so people with colitis need colonoscopies earlier and more often than average; see Colorectal cancer. Many IBD treatments suppress the immune system, which makes keeping up with vaccines more important.
Warning signs and specialists
Hemorrhoids or something worse
Hemorrhoids are swollen veins around the anus that can bleed bright red blood, itch, or prolapse. About half of adults over 50 have them, and straining, constipation, and low-fiber diets make them worse. In seven trials, fiber cut the risk of persistent symptoms by 47% and bleeding by 50%. Moderate
The risk is mistaking cancer for hemorrhoids. Colorectal cancer is now the leading cause of cancer death in Americans under 50, and in a study of early-onset cases, abdominal pain, rectal bleeding, diarrhea, and iron deficiency anemia were the red flags; having three or more raised the risk 6.5-fold. Rectal bleeding at any age deserves an examination.
The optimal colorectal screening plan is a first colonoscopy at 40, repeated every 5 years if normal, with an endoscopist whose adenoma detection rate is at least 35%; stool tests are the fallback for people who will not have a colonoscopy. Guideline minimum: any approved test from 45, or colonoscopy every 10 years. Details are in Colorectal cancer, and other warning symptoms in Symptoms you should never ignore.
When to see a gastroenterologist
| Symptom or finding | Why it matters | Usual next step |
|---|---|---|
| Food sticking, or trouble or pain swallowing | Stricture, eosinophilic esophagitis, or cancer | Upper endoscopy soon |
| Black stools or vomiting blood | Bleeding ulcer or other upper gut bleeding | Same day, emergency care if heavy |
| Rectal bleeding or a lasting change in bowel habit | Cancer, polyps, or IBD | Colonoscopy |
| Iron deficiency anemia in a man or a woman after menopause | Hidden gut bleeding or celiac disease | Colonoscopy and upper endoscopy (AGA 2020) |
| New indigestion at 60 or older | Ulcer or cancer | Upper endoscopy (ACG and CAG) |
| Reflux that doesn't respond to 8 weeks of a PPI | Not GERD, or more severe disease | Endoscopy or pH testing off the drug |
| Chronic reflux plus 3 or more risk factors | Barrett's esophagus | One screening test |
| Unexplained weight loss, or a parent or sibling with a gut cancer | Earlier or extra screening may be needed | Specialist review |
| FIB-4 of 1.3 or more | Liver scarring | Liver stiffness scan (Fatty liver) |
GLP-1 drugs and your gut
GLP-1 drugs such as semaglutide and tirzepatide slow stomach emptying, so nausea, vomiting, diarrhea, and constipation are common while the dose is rising; see Side effects. In a claims study, users had higher rates of pancreatitis, bowel obstruction, and gastroparesis (stalled stomach emptying) than people on another weight-loss drug, though these events were rare and the estimates imprecise. Severe belly pain or persistent vomiting needs prompt evaluation.
Because food can linger in the stomach during sedation, tell your endoscopist you take one. A 2024 multisociety guidance says most people can keep taking their GLP-1 drug before procedures, but those at highest risk of stomach problems should follow a liquid diet for 24 hours beforehand.