Some people have an adverse reaction on exposure to certain foods that can make them unwell. This can be a recurring reaction, happening each time that person ingests the food. Symptoms depend on the mechanism of reaction; however, they range from vomiting and diarrhoea and skin reactions such as eczema and urticaria to dramatic angio-oedema, severe respiratory distress and anaphylaxis.
One way to classify adverse reactions to foods is as either:
- Immunological reactions - both IgE (acute, often rapid, onset) and non-IgE-mediated (delayed and non-acute reactions).
- Non-immunological reactions.
The vocabulary one uses is often confusing. Food allergy should be reserved for immunologically mediated reaction to food allergens. Food intolerance is a rather vague term and needs a fuller explanation specifying whether immunologically mediated or not.
- Food allergy is thought to affect 5 in 100 young children and 3-4 in 100 adults in westernised countries.
- Levels of food allergy seem to be rising.
- Reasons for this are unclear but it may be connected with the processed nature of the western diet and its effects on the gut microbiome.
- In the UK about 2 in 100 infants develop cow's milk protein allergy.
- It is difficult to find accurate numbers for prevalence of food intolerance or allergy. Some studies have shown that self-reporting of food-related symptoms may not be confirmed in food challenge studies.
When presented with a case of possible food allergy or intolerance, a careful symptom history is essential.This should seek to identify possible allergens and whether the reaction is likely to be IgE-mediated and thus the patient may be at risk of anaphylaxis.
- Why is a food allergy suspected?
- What foods do they feel are implicated? A complete list is needed, including how the food has been prepared, to try to identify the likely ingredient.
- What are the symptoms that occur after eating the food?
- At what age did the symptoms start?
- How much food is needed to cause symptoms?
- Do symptoms occur every time the food is eaten?
- How long does it take for symptoms to occur?
- How long do the symptoms last?
- Do the symptoms follow a particular pattern or sequence?
- Frequency of occurence - does it happen every time after exposure?
- Setting of reaction (eg, home, school, anywhere)?
- What is the worst reaction that the person has had?
- Is there a personal or family history of allergy?
- Feeding history, age of weaning, formula or breast-fed (in which case, consider the mother's diet)?
- Previous treatments. Have any exclusion diets been tried?
- Is their diet nutritionally adequate?
|Symptoms Suggestive of IgE and Non-IgE-mediated Food Allergy|
|Pruritis, erythema, diarrhoea and abdominal pain are common to both types.||Pruritus, erythema, diarrhoea and abdominal pain are common to both types.|
Food diary - this simple activity may be helpful in some cases. It involves noting what foods are eaten and any reaction to them. It is cheap but often not diagnostic.
Physician-supervised oral food challenges are a key investigation, alongside skin-prick testing and food-specific serum IgE testing.Concordance between the results of skin prick testing and serum IgE levels is not always good and sometimes both tests need to be carried out.
- If an IgE-mediated food allergy is suspected:
- Arrange a skin prick test and/or blood tests for specific IgE antibodies to the suspected foods and likely co-allergens, depending on which is more acceptable to the patient or on which is available for a given food.
- Both skin prick tests and IgE testing measure sensitisation to an allergen, rather than clinical food allergy.
- Skin prick tests must be done where there are facilities to deal with a possible anaphylactic reaction and the expertise to interpret the results:
- A drop of liquid or solid food is placed on the skin of the forearm.
- A lancet or needle is used to prick the skin through the allergen solution.
- A saline-based control solution is also used at a separate site on the forearm.
- The reaction is usually 'read' after 15 to 20 minutes.
- The result should be interpreted with referral to the history.
- Note that food extracts are not available and skin prick testing has not been validated, for all foods.
- There have been some cases of systemic reactions and anaphylaxis in food allergen testing.
- Measurement of serum allergen-specific IgE:
- These are enzyme-linked immunosorbent assay (ELISA) and fluorescent enzyme immunoassay (FEIA) tests. They are only available for a small number of foods, are more expensive than skin tests and specificity and sensitivity both vary according to test manufacturer and allergen.
- There may be clinically insignificant cross reactions with other allergens, so results must be interpreted with caution and always in conjunction with the clinical history. However, this test is safe and can be used in people who have widespread skin disease or a history of severe anaphylactic reaction.
- Foods commonly involved:
- Fish and seafood
- Tree nuts
- Soy beans
- Kiwi fruit
- Atopy patch testing or oral food challenges should not be used to diagnose IgE-mediated food allergy in primary care or community settings.
- If a non-IgE-mediated food allergy is suspected:
- When the allergen is clear, arrange a trial elimination diet (normally for between 2-6 weeks) to see if symptoms improve, and then reintroduce after the trial to see if symptoms return. Seek advice from a dietician to ensure adequate nutrition during the trial and to provide dietary follow-up. Examples are:
- Food protein-induced enterocolitis - presents with projectile vomiting, diarrhoea and failure to thrive in the first few months of life. Cow's milk and soy protein formulas are usually responsible, although it can also be triggered by solid foods.
- Eosinophilic oesophagitis and gastroenteritis - there can be nausea, abdominal pain, reflux, and failure to thrive. There is no response to antacids. Eosinophilia may be found on FBC in some or at gastrointestinal biopsy.
- Coeliac disease - this is not strictly an allergy but occurs because of an immune response induced by exposure to gluten in genetically predisposed individuals.
Clinical Editor's comments (October 2017)
Dr Hayley Willacy draws your attention to the recently released guideline from the British Society for Allergy and Clinical Immunology dealing with peanut and tree nut allergy. The guideline underlines that current clinical experience suggests peanut allergy in teenagers and adults rarely resolves. The quality of life of the affected patients and their families is decreased because of the need for constant vigilance over food choices and the perceived likelihood of anaphylaxis, alongside the dietary and social restrictions that accompany food allergy. Current best management is directed at educating patients, families and caregivers on food allergen avoidance and how to treat food allergy emergencies.
The management of an anaphylactic reaction is discussed in the separate article Anaphylaxis and its Treatment.
Referral to secondary or specialist careReferral should be made if:
- The child has faltering growth with one or more of the gastrointestinal symptoms above.
- They have not responded to a single-allergen elimination diet.
- They had one or more acute systemic reactions or severe delayed reactions.
- They have IgE-mediated food allergy and concurrent asthma (assumes IgE tests are available in primary care).
- There is significant atopic eczema where multiple (or cross-reactive) allergies are suspected.
- There is clinical suspicion of multiple food allergies.
- There is ongoing diagnostic uncertainty - eg, persisting parental suspicion of food allergy, difficult or perplexing symptoms (despite a lack of supporting history) or a strong clinical suspicion of IgE-mediated food allergy but allergy test results are negative.
Management of food allergy and intolerance may include one or more of the following:
- Food avoidance: the only real treatment for food allergy and intolerance is food avoidance. This is particularly crucial in the case of a previous anaphylactic reaction to a food substance and can prove very difficult when eating out and when buying food, as cross-contamination can occur. Food labelling is also an issue and laws on this are being tightened.
- Dietician referral: this should be considered. Patients should be taught to read food labels carefully. Detailed written advice on avoidance strategies can be helpful. Dietary deficiencies can be anticipated and prevented. The possibility of allergen cross-reactivity should also be discussed; for example, almost all of those allergic to cow's milk are also allergic to goat's milk.Advice about possible sources of contamination should be given.
- Antihistamines: if symptoms are less severe (for example, just pruritus or urticaria), antihistamines can be helpful. However, caution should be used as they may mask more serious reactions and the patient's degree of sensitivity can increase over time.
- Adrenaline (epinephrine): if respiratory symptoms or severe anaphylactic reactions have occurred, or the allergy is to foods such as peanuts that commonly cause severe reactions, or there is a history of asthma, the need for adrenaline (epinephrine) - eg, EpiPen® - should be considered. Advise that this is to be carried at all times and give full instructions for use.
- Medical emergency identification bracelet or similar: should be worn by people at risk of anaphylaxis.
- Patient and parent, relatives, schools and carer education: anyone closely involved with the person who has the food allergy, particularly if there is risk of an anaphylactic reaction, should be informed and educated about what to do if a reaction occurs. A written emergency plan is helpful.
- Support groups: there are many groups available for patients and their families.
Injection immunotherapy (desensitisation) has been used with some success to treat pollen and insect venom allergies but, at present, is not widely used to treat food allergy because of the risk of anaphylaxis. However, studies looking at oral desensitisation are underway.
- Most children 'grow out' of their allergy to eggs, milk, wheat and soya. This may be due to maturation of the gut or maturation of immune responses that are responsible for allergy.[14, 15]
- Adults with food allergy can develop tolerance after appropriate food elimination diets have been implemented. One third of all adults and children lose their clinical reactivity to food allergens after 1-2 years of food elimination diets.
- Sensitivity to peanuts, seafood, fish and tree nuts is rarely lost.
There are a number of unanswered questions about how to prevent allergies developing. The guidance has changed several times over recent years, causing some confusion, and new research often reaches surprising conclusions.
Breast-feeding for 6-12 months is promoted as a way to prevent food allergy and atopy but there has been some controversy surrounding this. Debate exists as to whether breast-feeding is protective against the development of allergies, or is, in fact, sensitising. However, in general, studies show that infants who are fed cow's milk or soy protein formula have a higher incidence of atopic dermatitis and wheezing illness in early childhood. This supports the fact that breast-feeding should be encouraged.The Department of Health (DH) in the UK does encourage breast-feeding for the first six months of life.
It used to be advised that pregnant and breast-feeding mothers should avoid allergenic foods such as peanuts and shellfish. In 2009, the DH changed its advice. It now states that pregnant or breast-feeding mothers may eat peanuts or foods containing peanuts irrespective of whether they have a family history of allergies.
Delayed introduction of solid foods until the child is 6 months old, as an allergy prevention measure, is also controversial.
- A systematic review published in 2006 showed that early solid feeding before 4 months may increase the risk of eczema but there was no evidence to show an association between early solid introduction and the development of food allergy. The review pointed out that the methods in most of the studies reviewed were flawed and that additional controlled trials are needed.
- However, the DH recommends that solid food should not be introduced before a baby is 6 months of age.
- It also recommends that, after six months, you should continue breast-feeding and/or giving your baby breast milk substitute alongside solid food for up to 2 years of age or beyond.
- They do say that, if solid foods are to be introduced before 6 months (after talking to a health visitor or GP), peanuts or other allergens such as nuts, seeds, milk, eggs, wheat, fish or shellfish should not be introduced.
- They also advise that parents or carers talk to their GP before introducing peanuts into their child's diet if their child has already been diagnosed with an allergy or there is a history of allergy in the child's family.
- Sensible advice to give to parents is also that, when introducing solid foods that are known to be potential allergens, such as nuts or eggs, introduce them one at a time so that any reaction can be noticed.
- One study suggests that early introduction of peanuts into the diet of children at risk of allergy may actually be protective.
There is also some controversy about at what age a child with a family history of atopy should be introduced to certain foods that are linked to food allergy, such as milk, peanut, eggs, nuts and fish. The Isle of Wight prevention study looked at the prevention of allergic disease during childhood by allergen avoidance and concluded that allergic diseases can be reduced, for at least the first eight years of life, by combined food and house dust mite allergen avoidance in infancy.
Some Cochrane reviews on allergy prevention have concluded that:
- Evidence is lacking to support feeding with a hydrolysed formula for the prevention of allergy compared with exclusive breast-feeding to prevent allergy. If a high-risk infant cannot be breast-fed completely, there is limited evidence that feeding with a hydrolysed formula compared with a cow's milk formula reduces infant and childhood allergy and infant cow's milk allergy.
- Likewise, soy formula cannot at present be recommended for prevention of allergy or food intolerance in infants at high risk of allergy or food intolerance. However, further research is needed to look at the role of soy formulas for the prevention of allergy or food intolerance in infants unable to be breast-fed who have a strong family history of allergy or cow's milk protein intolerance.
- There is insufficient evidence to determine the role of prebiotic or probiotic supplementation of infant formula for prevention of allergic disease and food hypersensitivity. Studies have shown some possible positive outcomes regarding reduction in eczema but more research is needed.[25, 26]
Non-immunologically mediated food intolerance
This can be due to one of the following reactions:
Lactase deficiency, leading to lactose intolerance, is an example of a metabolic food intolerance. It causes diarrhoea and abdominal symptoms after milk is ingested. See separate article Lactose Intolerance for further details.
Food additives and chemicals in foods can cause pharmacological food intolerance reactions. For example, consumption of monosodium glutamate in foods can cause flushing, headache and abdominal symptoms in some people. Reactions to a number of different chemicals can occur. There may also be a family history of the problem. Substances commonly involved include:
- Artificial food colours.
- Glutamates including monosodium glutamate.
- Vasoactive amines.
- Flavour enhancers.
- Artificial sweeteners.
Histamine intolerance is a reaction to histamine which is present in many foods including alcoholic drinks, pickled and cured foods, mushrooms and Quorn®. Histamine-induced symptoms include rashes, headaches and abdominal symptoms. Some foods, including a range of fruits and vegetables, can stimulate the release of histamine from mast cells in susceptible individuals.
Toxic reactions can occur when foods are contaminated by toxins, or by viruses, bacteria or parasites. For example, lectins found in undercooked beans can cause gastrointestinal symptoms; aflatoxins found in some mouldy foods can cause liver disease.
Psychological reactions to foods are also known as 'food aversion'. This refers to the dislike of, or an emotional response to, a particular food. The symptoms may be nonspecific and do not occur when the person is faced with a blinded food challenge.
Almost any system in the body can be affected. Symptoms can depend on the level of exposure to the problem food (how much is ingested) and usually take hours, or sometimes days, to become apparent. They commonly include:
- Gastrointestinal disturbance
- Behavioural problems
- Triggering of asthma
This will also involve avoidance of dietary triggers. Referral to a dietician may be appropriate.
Further reading and references
; Food Standards Agency
; Managing cows' milk allergy in children. BMJ. 2013 Sep 16347:f5424. doi: 10.1136/bmj.f5424.
; British Society for Allergy and Clinical Immunology (2014)
; NICE Clinical Guideline (February 2011)
; Food allergy. J Allergy Clin Immunol. 2010 Feb125(2 Suppl 2):S116-25. Epub 2009 Dec 29.
; Food intolerance and allergy: increased incidence or contemporary inadequate diets? Curr Opin Clin Nutr Metab Care. 2014 Sep17(5):442-7. doi: 10.1097/MCO.0000000000000086.
; New guidelines for managing cow's milk allergy in infants. J Fam Health Care. 200818(1):27-30.
; Food allergy. BMJ. 2002 Dec 7325(7376):1337.
; Diagnosis and management of food allergies: new and emerging options: a systematic review. J Asthma Allergy. 2014 Oct 247:141-64. doi: 10.2147/JAA.S49277. eCollection 2014.
; The diagnosis of food allergy. Am J Rhinol Allergy. 2010 Nov-Dec24(6):439-43. doi: 10.2500/ajra.2010.24.3515.
; Skin prick test and specific serum IgE in the diagnostic evaluation of suspected cow's milk and hen's egg allergy in children: does one replace the other? Clin Exp Allergy. 2012 Aug42(8):1266-72. doi: 10.1111/j.1365-2222.2012.04046.x.
; Adverse reactions to skin prick testing in children - prevalence and possible risk factors. Pediatr Allergy Immunol. 2009 May20(3):273-8. Epub 2009 Feb 10.
; BSACI guideline for the diagnosis and management of peanut and tree nut allergy. Clin Exp Allergy. 2017 Jun47(6):719-739. doi: 10.1111/cea.12957.
; [Use of goat's milk in patients with cow's milk allergy]. An Pediatr (Barc). 2003 Aug59(2):138-42.
; Oral immunotherapy for milk allergy. Cochrane Database Syst Rev. 2012 Nov 1411:CD009542. doi: 10.1002/14651858.CD009542.pub2.
; The natural history of egg allergy. J Allergy Clin Immunol. 2007 Dec120(6):1413-7.
; The natural history of IgE-mediated cow's milk allergy. J Allergy Clin Immunol. 2007 Nov120(5):1172-7. Epub 2007 Nov 1.
; The role of breast-feeding in the development of allergies and asthma. J Allergy Clin Immunol. 2005 Jun115(6):1238-48.
; Dept of Health, May 2003 (archived)
; Dept of Health, August 2009 (archived)
; Timing of solid food introduction in relation to atopic dermatitis and atopic sensitization: results from a prospective birth cohort study. Pediatrics. 2006 Feb117(2):401-11.
; Systematic review of the relationship between early introduction of solid foods to infants and the development of allergic disease. Arch Pediatr Adolesc Med. 2006 May160(5):502-7.
; Randomized trial of peanut consumption in infants at risk for peanut allergy. N Engl J Med. 2015 Feb 26372(9):803-13. doi: 10.1056/NEJMoa1414850. Epub 2015 Feb 23.
; Prevention of allergic disease during childhood by allergen avoidance: the Isle of Wight prevention study. J Allergy Clin Immunol. 2007 Feb119(2):307-13.
; Formulas containing hydrolysed protein for prevention of allergy and food intolerance in infants. Cochrane Database Syst Rev. 2006 Oct 18(4):CD003664.
; Soy formula for prevention of allergy and food intolerance in infants. Cochrane Database Syst Rev. 2006 Oct 18(4):CD003741.
; Prebiotics in infants for prevention of allergy. Cochrane Database Syst Rev. 2013 Mar 283:CD006474. doi: 10.1002/14651858.CD006474.pub3.
; Probiotics in infants for prevention of allergic disease and food Cochrane Database Syst Rev. 2007 Oct 17(4):CD006475.
; Adverse reactions to food: allergies and intolerances. Dig Dis. 200826(2):96-103. doi: 10.1159/000116766. Epub 2008 Apr 21.
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