A food allergy is a condition where exposure to certain proteins in food induces an abnormal and harmful immune reaction.
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The immune system mistakes the proteins, referred to as allergens, for harmful invaders and launches an attack that can result in mild symptoms such as hives on the skin, to severe symptoms such as difficulty breathing and loss of consciousness.
The most severe form of allergic reaction is anaphylaxis - a potentially life-threatening reaction characterized by feeling faint, difficulty breathing, racing heart rate, confusion, loss of consciousness, and collapsing.
More than 170 foods have been identified as allergenic, but the eight major allergens that are responsible for around 90% of severe reactions are milk, egg, peanut, tree nut, wheat, soy, fish, and shellfish.
Food allergy represents a major public health concern, particularly in the developed world. According to the Centers for Disease Control and Prevention (CDC), its prevalence increased by about 50% between 1997 and 2011, and now it affects around one in 13 children.
The prevalence of peanut or tree nut allergy also increased three-fold between 1997 and 2008, and between the late 1990s and mid-2000s, childhood hospitalization as a result of allergy also increased three-fold.
The prevalence of allergy in the UK and many other developed countries is also increasing. According to estimates from the Food Standards Agency (FDA), about 5 to 8% of children and 1 to 2% of adults now have a food allergy in the UK.
CDC estimates also suggest that more than 40% of US children with a food allergy have experienced a severe form of reaction, including anaphylaxis, and the number of medical procedures required as a result of anaphylaxis increased by almost five times between 2007 and 2016. Food allergy is most prevalent in Australia, with one study finding that 9% of one-year-olds had an egg allergy, and 3% had a peanut allergy.
The reasons underlying the increased prevalence of food allergy are not fully understood, but researchers around the world have proposed various theories and are working hard to combat the problem.
Delayed introduction of food allergens
One theory is that parents are avoiding introducing their children to potentially allergenic foods meaning that children react abnormally to the allergens once they do encounter them. This was the basis of a study led by Gideon Lack from King's College London called the Learning Early About Peanut Allergy (LEAP) study.
The study showed that among children who avoided eating peanuts, 17% developed a peanut allergy by the time they were age 5, whereas only 3% who had peanut introduced into their diet from the year they were born developed the allergy by age five. The child participants already had an egg allergy and/or eczema, both of which are strong predictors for nut allergy.
The theory is that eating allergenic foods during the weaning period effectively "trains" the gut's immune system to tolerate bacteria and foreign substances such as new foods.
The LEAP study findings resulted in changes to US guidelines about peanut consumption during infancy, which experts had previously spent years saying should be avoided.
The hygiene hypothesis
Another theory, referred to as the "hygiene hypothesis," posits that a lack of exposure to microorganisms during early childhood affects the gut microbiome (the total population of microbes living within and on the human body) and causes the immune system to identify food proteins as infectious agents mistakenly. Parasitic infections, in particular, are usually combated by the same immune mechanisms that are used to tackle allergy.
The theory proposes that following birth, the body comes into contact with a wide array of bacteria in the air, on the ground, and in the diet, which populate the gut to form the gut microbiota. Microbiota are so important that they outnumber the 27 to 37 trillion cells in our body by around three times, with each of us carrying approximately 3 trillion bacteria within us.
The hygiene hypothesis postulates that excess hygiene or antibiotic use practiced as part of modern life has limited exposure to a wide range of bacteria that have evolved alongside humans. This exposure helps the body develop a well-regulated immune system that does not react abnormally to harmless allergens and treat them as a threat.
Vitamin D hypothesis
A third theory, which is also linked to modern, urban living, is related to the fact that there is a geographical element to the increased prevalence of food allergy. Researchers have started to notice that food allergy prevalence seems to coincide with the availability of sunlight.
Australian researchers Katie Allen and Carlos Camargo conducted studies showing that a lack of exposure to sunlight, which can lead to vitamin D deficiency, can increase a child's risk of developing egg allergy 3-fold and the risk of developing peanut allergy 11-fold.
The idea is that vitamin D can help regulate the immune system so that the body is less susceptible to allergy. Urban living and time spent indoors reduce exposure to natural environments and sunlight, and when children are exposed to sunlight, parents often follow recommendations to cover their child's exposed skin with sunscreen.
As the population increases and becomes denser, our exposure to green spaces and natural environments reduces, meaning the immune system misses numerous opportunities to encounter microbes and to access to sunlight.
Although researchers generally think the increased prevalence of food allergy is driven by multiple factors, studies suggest that increased urbanization seems to be a strong predictor in a given population. In the United States, the prevalence of vitamin D deficiency is thought to be almost twice what it was just over a decade ago.
Allergy. British Society for Immunology. Available: www.immunology.org/.../allergy
Facts and Statistics. FARE: Food Allergy Research & Education. Available at: www.foodallergy.org/.../facts-and-statistics
Overview Anaphylaxis. NHS 2016. Available at: https://www.nhs.uk/conditions/Anaphylaxis/
Allergies: the scourge of modern life? The Guardian 2018. Available at: www.theguardian.com/.../allergies-the-scourge-of-modern-living-hay-fever-ashtma-eczema-food-peanuts-dairy-eggs-penicillin
Why the world is becoming more allergic to food. BBC News 2019. Available at: https://www.bbc.co.uk/news/health-46302780