Bringing a food back

01Watching for late changes

Some changes come hours or days after eating. A guideline on milk allergy in young children puts fast symptoms within minutes to an hour or two, and slower ones anywhere from a couple of hours to a few days (Meyer et al., 2023). In a study of young children who had milk brought back, most of the symptoms that returned came late, over the following weeks (Lins et al., 2010).

Skin can change late too. Two European allergy groups advise checking it 24 hours after a food challenge, and later (Werfel et al., 2007). In one study, late flares commonly came about 24 hours after eating, sometimes with no earlier sign (Breuer et al., 2004).

These timings come from studies of young children under medical care, not from tests people run on their own. There, research keeps watching for at least a day after a food comes back, and for days or weeks where an effect may build slowly (Werfel et al., 2007; Meyer et al., 2023; Lins et al., 2010). A late change is harder to read, though, because more else has happened in between.

02One food at a time, with a gap before the next

Research lets one test finish before the next begins, and starts each from a settled level. In one study, milk came back only after at least 15 days without it and without symptoms (Lins et al., 2010). In a blinded trial, each test week was followed by a break of at least seven days, until any symptoms from it had gone (Skodje et al., 2018). A blinded test set out by an expert panel leaves a full week without the test food between its two test weeks (Catassi et al., 2015).

Each test then starts from the same settled level, so one doesn’t spill into the next. Bringing back one food at a time also keeps a change from being mixed up with another new food, and keeping the rest of what is eaten the same makes one day easier to compare with the next.

03When a food belongs with a doctor

A food brought back at home is an open test: the person knows when it is back, and knowing can change what they report (de Graaf et al., 2024). Research regards a double-blind test, where neither the person eating nor the people watching know which portion holds the food, as the gold standard (Meyer et al., 2023). So a home test can’t show that a food is safe for you, or stand in for a test in a clinic.

Allergy clinics run a different test, under medical supervision (Sampson et al., 2024). In challenges for fast-onset allergy, doses rise in steps, typically 20 to 30 minutes apart, with set rules to go on, stop or keep watching (Sampson et al., 2024). Australia’s allergy specialists list a history of allergic reactions to a food as a reason to test it under medical supervision, in a clinic with immediate access to emergency care (ASCIA, 2023). Some people who have avoided a food for a long period react to it when they eat it again (ASCIA, 2023).

Ever had a fast reaction after a food, like hives, swelling, trouble breathing or feeling faint? A mild one counts too. Or has a doctor told you to avoid a food?

Then don’t run a trial on that food. Ask your doctor about that food.

There is more in Safety, and when to involve a clinician.

04What the studies found

• A 2023 World Allergy Organization guideline on cow’s milk allergy in infants and young children puts fast (IgE-type) symptoms within minutes to 1–2 hours, and slower ones from a couple of hours to a few days (Meyer et al., 2023). It keeps challenges for the fast type under medical supervision (Meyer et al., 2023). For some slower types it lets milk come back at home under professional guidance (Meyer et al., 2023). After a few weeks without milk, that means a full daily amount for a week; after months without it, a slow step-up from small amounts, to see whether milk is tolerated again (Meyer et al., 2023). Several authors declared ties to companies, including infant-formula makers.
• A 2007 position paper from two European allergy groups, on reactions to food in atopic eczema, advised scoring the skin 24 hours after a food challenge and later, because otherwise a worsening would be missed (Werfel et al., 2007).
• A 2004 study looked back at 106 blinded food challenges in 64 children with atopic dermatitis, median age 2 (Breuer et al., 2004). Of these, 49 (46%) brought a reaction. A late skin flare commonly came 24 hours after eating, and in 12% of reactions it was the only sign.
• In a 2010 study, 65 young children whose symptoms had been put down to cow’s milk had milk brought back openly, at least 15 days after it was stopped and symptoms had gone (Lins et al., 2010). Symptoms came back in 35 within 4 weeks, 27 of them late: 18 in the first week, 3 in the second and 6 in the third.
• In 2015, experts meeting in Salerno, Italy, set out a blinded test for suspected non-coeliac gluten sensitivity (Catassi et al., 2015). It runs a week of gluten or a look-alike placebo, a week back on a strict gluten-free diet, then a week of the other. A difference of at least 30% in the main symptoms counts as positive, a threshold the experts called somewhat arbitrary. The Dr. Schär Institute funded the meeting.
• In a 2024 trial at three European universities, 83 adults with self-reported non-coeliac gluten sensitivity were told they would or would not eat gluten, and separately did or did not (de Graaf et al., 2024). Rated hourly for 8 hours, gut symptoms were highest in those who expected gluten and got it, which the authors linked to expectation, without ruling out an effect of gluten. Bakery and wheat-industry bodies were among the funders.
• In a 2018 double-blind trial, 59 people with self-reported non-coeliac gluten sensitivity ate muesli bars hiding gluten, fructans or neither for a week each (Skodje et al., 2018). Between weeks came a break of at least seven days, until any symptoms had gone.
• In 2024, the US and European allergy academies updated their joint guidance on food challenges (Sampson et al., 2024). It puts safety first, raises doses in steps typically 20 to 30 minutes apart for fast-onset (IgE-type) allergy, and uses go-on, stop and observation rules to cut false alarms and severe reactions (Sampson et al., 2024).
• The professional body for allergy specialists in Australia and New Zealand says food challenges give rising amounts of a food over two to three hours, under medical supervision, with emergency care at hand (ASCIA, 2023). It lists a history of allergic reactions to the food as one reason for a supervised challenge, and notes that avoiding a food for a long time can lead to reactions when it is eaten again (ASCIA, 2023).

05Sources on this page

Each source is listed once, with links to its record. The reference list says what was read for each, and when it was checked.

1. ASCIA, 2023Australasian Society of Clinical Immunology and Allergy. 2023. Position Paper - Oral Food Allergen Challenges. Australasian Society of Clinical Immunology and Allergy (ASCIA). Content updated April 2023; formatting and links updated July 2024.allergy.org.au
2. Breuer et al., 2004Breuer K, Heratizadeh A, Wulf A, Baumann U, Constien A, Tetau D, et al. 2004. Late eczematous reactions to food in children with atopic dermatitis. Clinical & Experimental Allergy 34(5):817–824.DOI 10.1111/j.1365-2222.2004.1953.x · PubMed 15144477
3. Catassi et al., 2015Catassi C, Elli L, Bonaz B, Bouma G, Carroccio A, Castillejo G, et al. 2015. Diagnosis of Non-Celiac Gluten Sensitivity (NCGS): The Salerno Experts' Criteria. Nutrients 7(6):4966–4977.DOI 10.3390/nu7064966 · PubMed 26096570 · PMC4488826
4. de Graaf et al., 2024de Graaf MCG, Lawton CL, Croden F, Smolinska A, Winkens B, Hesselink MAM, et al. 2024. The effect of expectancy versus actual gluten intake on gastrointestinal and extra-intestinal symptoms in non-coeliac gluten sensitivity: a randomised, double-blind, placebo-controlled, international, multicentre study. The Lancet Gastroenterology & Hepatology 9(2):110–123.DOI 10.1016/S2468-1253(23)00317-5 · PubMed 38040019
5. Lins et al., 2010Lins Md, Horowitz MR, Silva GA, Motta ME. 2010. Oral food challenge test to confirm the diagnosis of cow's milk allergy. Jornal de Pediatria 86(4):285–289.DOI 10.2223/JPED.2011 · PubMed 20508909
6. Meyer et al., 2023Meyer R, Venter C, Bognanni A, Szajewska H, Shamir R, Nowak-Wegrzyn A, et al.; WAO DRACMA Guideline Group. 2023. World Allergy Organization (WAO) Diagnosis and Rationale for Action against Cow's Milk Allergy (DRACMA) Guideline update - VII - Milk elimination and reintroduction in the diagnostic process of cow's milk allergy. World Allergy Organization Journal 16(7):100785.DOI 10.1016/j.waojou.2023.100785 · PubMed 37546235 · PMC10401347
7. Sampson et al., 2024Sampson HA, Arasi S, Bahnson HT, Ballmer-Weber B, Beyer K, Bindslev-Jensen C, et al. 2024. AAAAI–EAACI PRACTALL: Standardizing oral food challenges—2024 Update. Pediatric Allergy and Immunology 35(11):e14276.DOI 10.1111/pai.14276 · PubMed 39560049
8. Skodje et al., 2018Skodje GI, Sarna VK, Minelle IH, Rolfsen KL, Muir JG, Gibson PR, et al. 2018. Fructan, Rather Than Gluten, Induces Symptoms in Patients With Self-Reported Non-Celiac Gluten Sensitivity. Gastroenterology 154(3):529–539.e2.DOI 10.1053/j.gastro.2017.10.040 · PubMed 29102613
9. Werfel et al., 2007Werfel T, Ballmer-Weber B, Eigenmann PA, Niggemann B, Rancé F, Turjanmaa K, et al. 2007. Eczematous reactions to food in atopic eczema: position paper of the EAACI and GA2LEN. Allergy 62(7):723–728.DOI 10.1111/j.1398-9995.2007.01429.x · PubMed 17573718