Preliminary data from a large, new US study that is currently under review suggests that nearly 52 per cent of American adults with a reported food allergy developed one or more food allergies after age 18.
An estimated 5 per cent of adults in the United States have a food allergy, compared with about 8 per cent of children. And while some children outgrow allergies — usually those to milk, eggs and wheat — many retain their allergies through adulthood.
Dr Ruchi Gupta, a food allergy researcher at the Ann and Robert H Lurie Children’s Hospital of Chicago and Northwestern University Feinberg School of Medicine, who led the national study, noted that at allergy meetings around the world, “you’d hear more and more about adult-onset food allergy. But this was all anecdotal. That’s the reason we did the study, to get the numbers behind how frequently.”
Last year, Gupta and colleagues from Northwestern and the NORC Survey Research Lab at the University of Chicago surveyed 40,447 adults across the United States, recruited from a nationally representative sample. They found that shellfish was the most common food allergy among adults, affecting 3.9 per cent of the population, followed by peanut allergies, at 2.4 per cent, and tree nut allergies, at 1.9 per cent.
Peanut allergies typically develop during childhood, and children less commonly outgrow them than they do other food allergies. Peanut allergy appears to be equally prevalent among American adults and children.
It has been well established that kids develop allergies to the “top eight” foods: Milk, egg, wheat, soy, peanut, tree nuts, fish and shellfish. Most of the reactions in adults occur to the top eight foods as well, according to Dr Sharon Chinthrajah, an assistant professor at the Stanford University School of Medicine and medical director of the Sean N Parker Center for Allergy and Asthma Research, which was involved in shaping the survey. That said, shellfish allergy is more common among adults than among children, as it tends to appear later in life.
Several patterns have been observed that are unique to adults who develop new food allergies. One is called oral allergy syndrome, which occurs in a small percentage of adults who have seasonal allergies. It involves your body getting tricked, said Chinthrajah. She explained that some adults might have allergies to tree pollen, for example, and some of the tree proteins are similar to those in fruits and vegetables. “So when your body eats the raw form of those foods, it thinks you’re eating tree pollen,” she added. Birch tree pollen, for instance, bears similarities to proteins found in fruits like peaches, apples and cherries. The main symptom is typically an itchy mouth or throat. Interestingly, if the fruit is first processed or cooked in any way, it denatures the protein and does not produce the same reaction.
Many adults who develop a new food allergy wonder what caused it — the “turn-on switch” as Gupta calls it. Anecdotal reports suggest that pregnancy, for example, can trigger new allergies, leading some to hypothesise that a hormonal connection may be at play. Other patients report that they noticed a new allergy after getting a viral infection. Still, it is not yet clear what causes a new reaction to a food after someone has eaten it for decades without incident.
Importantly, an allergic reaction is not the same as a food intolerance. An allergic reaction is characterised by marked symptoms, such as itching, hives, swelling, trouble breathing or vomiting, within two hours of consuming the food in question. Symptoms that appear the next day may be characteristic of a food intolerance, which Chinthrajah said researchers do not yet understand as well as they understand food allergies.
More severe allergic reactions may require epinephrine or a visit to the emergency department. But just because you have a reaction once doesn’t mean you must completely remove a food from your diet, said Gupta. Instead, if you have concerns, seek an allergist to get tested.