Food allergies in adults can manifest in different ways. An adult-onset food allergy describes the development of a new allergy to a food that had been eaten for years without a reaction or, more commonly, when ingesting a new food as an adult causes a reaction, most likely due to a previous environmental or cross-reactive exposure. Adults can also experience oral allergy syndrome or food intolerance, which can mimic some of the symptoms of food allergies, so an accurate diagnosis, including a history and testing, is key to appropriate management.

Food allergies in adults can be separated into adult food allergies and adult-onset food allergies. An adult food allergy describes a persistent condition such as a milk, peanut, or cashew allergy that an adult has had since childhood. Adult food allergies are certainly real and can be serious. Young adults tend to have more severe allergic reactions than children. This may be because proposed risk factors for more severe reactions, such as alcohol use, exercise, lack of sleep, and not carrying an epinephrine device, are more common in this age group.

Adult-onset food allergies are somewhat different. An adult-onset food allergy describes a situation in which a patient who has eaten a certain food for many years develops a new-onset allergy to that food as an adult. This is uncommon but well documented. Shellfish, particularly shrimp, is the classic example, although adult-onset food allergies can occur with other types of seafood.

A much more common scenario describing an adult-onset food allergy is an exposure to a food as an adult that the patient has never eaten before that causes a reaction. So, it is really the first exposure to eating the food, but the patient may have been sensitized before through environmental exposure. It should also be noted that many adult-onset food intolerances such as lactose intolerance are not true food allergies.

Then there is oral allergy syndrome, which is more commonly known as pollen food allergy syndrome (PFAS). Patients with PFAS are classically allergic to birch pollen, although ragweed, grass, and other types of pollen can also be involved. Some foods, including fresh apples, contain proteins that are similar to those found in birch pollen. When these foods are eaten, the body thinks that it is consuming birch pollen, causing symptoms such as an itchy mouth. Some patients with severe birch pollen allergies can experience abdominal pain, and those with extremely severe birch pollen allergies can experience true anaphylaxis. However, the vast majority of people with PFAS only experience an itchy mouth when eating a fresh food such as an apple but not when eating prepared foods such as apple sauce or a baked apple.

The diagnosis of adult-onset food allergies is more or less the same as the diagnosis of food allergies in children and is based mainly on a careful history followed by IgE testing. Allergy component testing often can differentiate true food allergies from PFAS. This should be followed by oral food challenges, although most facilities will not perform these in adults. In pediatrics, we look for objective symptoms to avoid the need for blinded food challenges, where the patient and/or provider do not know what is being tested. For adult patients, we often end up needing to perform blinded food challenges because symptoms such as abdominal pain are very subjective.

Regarding the management of food allergies, there are often different strategies used in children and adults. The data for the use of immunotherapy in pediatric patients are excellent. The data are not as good in adult patients. Some adults can develop tolerance through the use of immunotherapy, although there are often more side effects, and it is a very time-consuming procedure requiring many visits. Therefore, in adults, food avoidance is relatively more common. The effectiveness of the anti-IgE monoclonal antibody omalizumab has been well demonstrated in children, but the data in adults are currently limited.