Most food intolerances come down to dose, while gluten sensitivity remains a puzzle

Key takeaways

• In a Munich hospital study, 63% of adults with suspected histamine intolerance had symptoms after drinking plain peppermint tea, and doctors ruled the diagnosis out for 85%¹.
• About 68% of adults worldwide don’t fully digest lactose, a 2017 review covering 89 countries found, yet a 2010 NIH panel’s draft statement said people with lactose intolerance can generally handle about a cup of milk’s worth⁴⁵.
• In a 2007 study, healthy adults fully absorbed 15 grams of fructose, while 50 grams caused malabsorption in 60% to 80% and symptoms in about half⁶.
• In a 2018 blinded trial in Oslo, fructans, a carbohydrate in wheat, raised bloating more than gluten did, and gluten scored no worse than a placebo¹¹.
• The American Academy of Allergy, Asthma & Immunology (AAAAI) recommends against IgG food tests for diagnosing intolerance and says IgG likely just reflects eating a food¹⁵.

Between 2016 and 2021, 59 adults came to the dermatology and allergy department at the Technical University of Munich suspecting that histamine in their food was making them sick¹.

They followed a low-histamine diet for 10 to 14 days and were admitted to the hospital for testing. Then each drank peppermint tea and, later, tea with rising doses of histamine. They weren’t told which was which¹.

Nearly two-thirds had symptoms after the plain tea¹. Only 4 people, about 7%, had clear physical signs such as flushing or diarrhea after histamine alone¹. For 85% of them, the doctors ruled histamine intolerance out¹.

That doesn’t mean their symptoms were fake. It means the label didn’t fit most of them, and a wrong label can cost someone a long list of foods.

Food intolerances are real and common. They aren’t allergies, and they usually depend on how much you eat. Here are the main ones, the tests that help and the strange case of gluten sensitivity.

An intolerance isn’t an allergy, and dose is the clue

A food allergy is an immune reaction. A milk allergy, for example, is an immune system disorder that usually shows up in a baby’s first year, notes NIDDK, the federal digestive disease institute³. Lactose intolerance is a digestion problem that usually appears later³.

A suspected allergy needs an allergist, not a home experiment. Our article on medically necessary elimination diets covers allergy testing and the conditions where cutting a food is required.

Intolerances mostly work by dose. A small amount may cause nothing, while a large one brings gas, belching, bloating or diarrhea⁶. That’s good news, because you may be able to keep a food you love in smaller amounts.

One more distinction: not fully absorbing a sugar isn’t the same as being intolerant of it. Many people don’t absorb all their lactose yet feel fine, NIDDK explains³. Intolerance means you get symptoms.

Lactose: two-thirds of adults malabsorb it, far fewer need to quit dairy

In most people, the small intestine makes less lactase, the enzyme that splits milk sugar, after infancy. NIDDK calls this lactase nonpersistence, and it’s the most common cause of low lactase³.

A 2017 review in The Lancet Gastroenterology & Hepatology pooled studies of 62,910 people in 89 countries. It estimated that 68% of adults worldwide malabsorb lactose⁴. Rates were lowest, at 28%, across western, southern and northern Europe and highest, at 70%, in the Middle East⁴.

How you measure it changes the answer. Gene tests put the share at 74%, while breath tests and blood-sugar tests found 55% to 57%⁴. A gene result tells you about your enzyme, not whether a latte will bother you.

Symptoms are a separate question. In 2010, a panel convened by the National Institutes of Health found the research too thin to set a firm count of people with true intolerance⁵. Older estimates based on malabsorption had overstated how many people get symptoms from normal servings, the panel said⁵.

In its draft statement, the panel also said people with lactose intolerance can generally handle at least 12 grams of lactose, about a cup of milk, with minor symptoms or none⁵. The news release we found was distributed by MilkPEP, a group funded by milk processors, but those conclusions were the NIH panel’s⁵.

Lactose trouble can also be temporary. Injury to the small intestine from an infection, Crohn’s disease or celiac disease can lower lactase, and symptoms may ease once the cause is treated³.

Fructose: even healthy guts have a limit

U.S. fructose intake has climbed, mainly from high-fructose corn syrup, fruits and juices, and crystalline fructose, a 2005 study in the Journal of the American Dietetic Association noted⁷. Your small intestine can absorb only so much at once.

In 2007, researchers led by Satish Rao gave 20 healthy adults different doses of fructose in a double-blind study⁶. Nobody malabsorbed 15 grams. At 25 grams, 2 of 20 showed malabsorption on a breath test, and none had symptoms⁶.

At 50 grams, 60% to 80% malabsorbed it, depending on how concentrated the drink was, and about half had symptoms such as gas, bloating or diarrhea⁶.

The 2005 study, with 15 healthy adults, saw a different picture: more than half malabsorbed 25 grams⁷. Both studies were small, so the fair conclusion is that limits vary by person.

They agree on one point. Breath test results don’t line up neatly with symptoms⁶⁷. Some people “fail” the test and feel fine, so Rao’s team warned that a breath test alone may overstate fructose intolerance⁶.

So the test dose matters. Rao’s group recommended 25 grams, with breath samples every 30 minutes for about three hours⁶. A test with a huge dose may only tell you that you’re human.

FODMAPs: one family behind many “intolerances”

Lactose and fructose belong to a larger group of fermentable carbs called FODMAPs. The name stands for fermentable oligosaccharides, disaccharides, monosaccharides and polyols, according to Monash University’s gastroenterology department⁸.

High-FODMAP foods on Monash’s 2026 list include onion, garlic, wheat-based breads, apples, pears, mango, cow’s milk, most legumes, honey and high-fructose corn syrup⁸. Many of these are nourishing foods, which is why the goal is to find your limit, not to cut them all.

The Monash approach has three steps⁸. First, you swap high-FODMAP foods for lower ones for 2 to 6 weeks. Then, over 8 to 12 weeks, you test one FODMAP group at a time, eating one test food a day for 3 days⁸.

Finally, you bring back what you tolerate and limit only what you must for relief⁸. Monash recommends doing this with a trained dietitian⁸. Our upcoming article on low-FODMAP diets covers the steps and the research.

Gluten sensitivity: real symptoms, an uncertain culprit

Some people without celiac disease or a wheat allergy feel worse after eating wheat and better without it. Doctors call this non-celiac gluten sensitivity. A 2015 expert group acknowledged that no one knows how common it is⁹.

The group’s Salerno Experts’ Criteria set out how to test it. First, celiac disease and wheat allergy must be ruled out while you’re still eating gluten⁹. Then you go gluten-free for at least six weeks to see whether your main symptoms improve by at least 30%⁹.

If they do, a blinded challenge follows. You get a week of 8 grams of gluten a day, hidden in food with no FODMAPs, and a week of an identical placebo⁹. A 30% gap between the two counts as positive, though the authors called that cutoff somewhat arbitrary⁹.

Gluten often fails to stand out in blinded tests. A 2013 study of 37 people by Biesiekierski and colleagues found that once participants were on a low-FODMAP diet, gluten made no difference. Symptoms rose during every blinded diet, including the one with no gluten at all¹⁰. Our article on testing your own patterns covers that study and a 2024 nocebo trial.

Then came a 2018 trial at Oslo University Hospital in Norway¹¹. Fifty-nine adults who had put themselves on gluten-free diets ate muesli bars for a week at a time. Some bars had 5.7 grams of gluten, some had 2.1 grams of fructans, a FODMAP in wheat, and some had neither¹¹.

Fructans raised bloating more than gluten did, and overall gut symptoms too, though that gap was borderline¹¹. Gluten scored no worse than placebo. Yet fructans didn’t clearly beat placebo either, and placebo produced the worst week for 22 of the 59 people¹¹.

The trial was funded by Norwegian foundations and the Norwegian Celiac Association. One author, Peter Gibson of Monash, disclosed that his university and department earn royalties from the Monash FODMAP app¹¹.

A 2017 meta-analysis by Molina-Infante and Carroccio pooled 10 blinded gluten challenge trials with 1,312 adults¹². Of 231 people diagnosed with gluten sensitivity, only 38, or 16%, had symptoms specific to gluten¹². And 40% of those 38 reacted as much or more to placebo¹².

Still, most of those trials did find that gluten raised symptom scores more than placebo on average¹². A small group of people may truly react to gluten. For many others, wheat’s fructans or the expectation of harm explain more than gluten does.

Histamine, sulfites and caffeine: three different stories

Histamine. Low-histamine diets often cut foods such as fermented dairy, fish, tomatoes, fermented vegetables and fermented soy². The theory of histamine intolerance holds that some people can’t clear food histamine well because diamine oxidase, or DAO, an enzyme that breaks it down, runs low¹².

The idea is plausible. The tests are not.

In May 2026, experts from two Polish medical societies said blood tests for DAO have low accuracy, swing from day to day and shouldn’t be used to diagnose children². The Munich study found low DAO in people with and without the condition¹. A proposed prevalence of 1% to 3% has never been confirmed, the AAAAI notes¹.

The Polish groups found no validated test of any kind. For children, they say a low-histamine diet may be considered only as a short trial of 10 to 14 days after other causes are ruled out, followed by bringing foods back². Histamine food lists disagree, and no safe threshold has been set, so they warn against cutting foods based on histamine content alone².

DAO supplements, sold to break down food histamine in the gut, have only a few small randomized trials in adults and none in children².

Sulfites. Sulfites are preservatives found in foods such as wine, dried fruit and some potatoes cut for frying¹³. They’re a real hazard for some people with asthma. In 1986, the FDA banned sulfites on fruits and vegetables meant to be eaten raw, after asthma reactions were linked to salads¹³.

The FDA has said it knows of deaths among sulfite-sensitive people with asthma¹³. Packaged foods must declare sulfites at 10 parts per million or more, and alcohol labels must list them, too¹³. If you have asthma and react to wine or dried fruit, see an allergist rather than testing it at home.

Caffeine. The FDA says about 400 milligrams a day, roughly two to three 12-ounce cups of coffee, isn’t generally linked to harm in healthy adults¹⁴. But people vary widely in how sensitive they are and how fast they clear it¹⁴. Too much can bring an upset stomach, nausea, jitters and poor sleep¹⁴.

A 12-ounce brewed coffee can hold 113 to 247 milligrams, so “one cup” can mean very different doses¹⁴. If you’re pregnant or breastfeeding, or you take certain medicines, ask a clinician about your limit¹⁴.

Your food intolerance table

This is our own summary of the sources above. Use it to plan a fair test and a talk with your clinician.

Component	Common sources	Typical timing	How it’s confirmed	Dose tips
Lactose	Milk, yogurt, ice cream8	Within a few hours3	Breath test, or a dairy break and then reintroduction3	Most tolerate about 12 g, roughly a cup of milk5
Fructose	High-fructose corn syrup, fruit and juice, honey78	Breath hydrogen peaked about 1.5 to 2 hours after a test dose7	25 g breath test, read alongside your symptoms6	Healthy adults absorbed 15 g fully; 50 g overwhelmed most6
Fructans and other FODMAPs	Wheat, rye and barley breads, onion, garlic, legumes8	Not well defined; Monash tests each food over 3 days8	Low-FODMAP phase, then one group at a time8	Limit only as much as you need8
Gluten, without celiac disease	Wheat, rye, barley	Not well defined; blinded challenges last a week9	Rule out celiac and wheat allergy, then a blinded challenge9	Wheat’s fructans may be the real trigger11
Histamine	Foods often cut: fermented dairy, fish, tomatoes, fermented vegetables and soy2	Not well defined	No validated test; short trial diet, then reintroduction2	No proven threshold; don’t cut foods by lists alone2
Sulfites	Wine, dried fruit, some potatoes for frying13	Varies	Allergist review, especially with asthma	Labels must list 10 ppm or more13
Caffeine	Coffee, tea, energy drinks, soft drinks14	Varies by person14	Cut back and watch symptoms	About 400 mg a day is the FDA’s general adult marker14

Which tests and claims hold up

Test or claim	Rating	Why
Most people with lactose intolerance can keep some dairy	Strong evidence	NIH panel’s review of the research5
Fructose breath test at a 25 g dose	Promising but early	Useful, but small studies disagree and results don’t track symptoms well67
Short low-FODMAP phase with planned reintroduction	Promising but early	Structured method; our low-FODMAP article covers the trials8
Blinded gluten challenge after ruling out celiac disease	Promising but early	Expert criteria; few people test positive, and placebo reactions muddy results91112
Gluten causes most self-reported gluten sensitivity	Evidence says no, or unsafe	Only 16% had gluten-specific symptoms in pooled trials; fructans did more in one trial1112
Blood DAO test for histamine intolerance	Evidence says no, or unsafe	Low accuracy; results overlap with people who don’t have it12
DAO supplements	Not enough evidence	A few small adult trials; none in children2
IgG food sensitivity panels	Evidence says no, or unsafe	Not shown to diagnose intolerance; allergy societies advise against them15
Avoiding sulfites if they trigger your asthma	Strong evidence	Documented severe reactions led to a 1986 FDA ban13

IgG panels deserve one more line. An IgG antibody to a food most likely shows only that you’ve eaten it, and higher IgG4 levels may even point to tolerance, the AAAAI says¹⁵. Our upcoming article on food sensitivity tests covers them in depth.

Paying close attention to how food makes you feel is a natural-health strength. The research adds a second step: check the pattern before you give up a food for good. That protects the variety many readers prize, and it keeps yogurt, aged cheese, bread and fermented vegetables on the table when they don’t need to go.

Questions readers ask

The bottom line

Food intolerances are real, and most depend on dose. Lactose and fructose have useful but imperfect breath tests. FODMAPs are best sorted with a structured trial and reintroduction.

Histamine intolerance and gluten sensitivity are harder. They lack validated tests, and blinded studies show that expectation plays a large role. Rule out allergy and celiac disease first, test one food group at a time, and look for your own limit rather than a lifetime ban.

Related reading

• Elimination diets are real medicine for some conditions and a risk for others
• Your body makes digestive enzymes to spare, and pills help when it can’t
• The chocolate may not cause the migraine, and you can test your own patterns
• When the company reading your labs also sells the fix
• How gut motility works, and why a cup of coffee can wake your colon like a meal

Sources

1. Bent RK, Kugler C, Faihs V, Darsow U, Biedermann T, Brockow K. “Placebo-Controlled Histamine Challenge Disproves Suspicion of Histamine Intolerance.” Journal of Allergy and Clinical Immunology: In Practice, 2023. education.aaaai.org; with the AAAAI research summary “Histamine Intolerance: Fact or Fiction?” August 28, 2023. aaaai.org
2. Horvath A, Stróżyk A, Sybilski A, et al. “The Limited Clinical Relevance and Potential Risks Associated With the Diagnosis of Histamine Intolerance in Children: A Position Statement of the Food Allergy Section of the Polish Society for Pediatric Gastroenterology, Hepatology and Nutrition and the Pediatric Section of the Polish Society of Allergology.” Alergologia Polska – Polish Journal of Allergology, 2026;13(2):80-93. termedia.pl
3. National Institute of Diabetes and Digestive and Kidney Diseases. “Lactose Intolerance” (all content). Last reviewed February 2018. niddk.nih.gov
4. Storhaug CL, Fosse SK, Fadnes LT. “Country, Regional, and Global Estimates for Lactose Malabsorption in Adults: A Systematic Review and Meta-Analysis.” Lancet Gastroenterology & Hepatology, October 2017;2. RELX SDG Resource Centre record.
5. Perishable News. “Avoiding Dairy Due To Lactose Intolerance Is Unnecessary And May Risk Health” (news release on the NIH Consensus Development Conference on Lactose Intolerance and Health, distributed by the milk processors’ group MilkPEP). February 26, 2010. perishablenews.com
6. Rao SSC, Attaluri A, Anderson L, Stumbo P. “Ability of the Normal Human Small Intestine to Absorb Fructose: Evaluation by Breath Testing.” Clinical Gastroenterology and Hepatology, 2007;5(8):959-963. pmc.ncbi.nlm.nih.gov
7. Beyer PL, Caviar EM, McCallum RW. “Fructose Intake at Current Levels in the United States May Cause Gastrointestinal Distress in Normal Adults.” Journal of the American Dietetic Association, 2005;105(10):1559-1566. em-consulte.com
8. Monash University, Department of Gastroenterology. “3-Step FODMAP Diet” information sheet. 2026. monashfodmap.com
9. Catassi C, Elli L, Bonaz B, Bouma G, Carroccio A, Castillejo G, et al. “Diagnosis of Non-Celiac Gluten Sensitivity (NCGS): The Salerno Experts’ Criteria.” Nutrients, 2015;7(6):4966-4977. Harvard DASH repository. pubmed.ncbi.nlm.nih.gov
10. Brett AS. “Gluten Sensitivity Without Celiac Disease: Real or Not?” (summary of Biesiekierski JR, et al., Gastroenterology, 2013;145:320). NEJM Journal Watch, September 19, 2013. clinician.nejm.org
11. Skodje GI, Sarna VK, Minelle IH, Rolfsen KL, Muir JG, et al. “Fructan, Rather Than Gluten, Induces Symptoms in Patients With Self-Reported Non-Celiac Gluten Sensitivity.” Gastroenterology, 2018;154:529-539. wheatfoundation.org
12. Molina-Infante J, Carroccio A. “Suspected Nonceliac Gluten Sensitivity Confirmed in Few Patients After Gluten Challenge in Double-Blind, Placebo-Controlled Trials.” Clinical Gastroenterology and Hepatology, 2017;15:339-348. University of Palermo repository. iris.unipa.it
13. Food Allergy Research and Resource Program, University of Nebraska–Lincoln. “Sulfites – USA.” Accessed October 2026. farrp.unl.edu
14. U.S. Food and Drug Administration. “Spilling the Beans: How Much Caffeine Is Too Much?” Content current as of August 28, 2024. fda.gov
15. American Academy of Allergy, Asthma & Immunology. “IgG Food Test.” Conditions library, accessed October 2026. aaaai.org