Adults can develop new food allergies, even to foods they have safely consumed for decades. This phenomenon, known as adult-onset food allergy, affects nearly half of all food-allergic adults. A food allergy is defined as an adverse health effect arising from a specific immune response that occurs reproducibly upon exposure to a given food. This process is distinct from a food intolerance, as it involves the immune system creating an antibody, typically Immunoglobulin E (IgE), to identify a particular food protein as a threat. When an individual encounters the food again, the immune system releases chemicals like histamine, triggering symptoms that can range from mild hives to life-threatening anaphylaxis.
The Reality of Adult-Onset Food Allergies
The development of a new food allergy in adulthood is not uncommon; 48% of food-allergic adults developed their condition after the age of 18. While the overall prevalence of food allergies is slightly higher in children, adult-onset cases represent a substantial health issue. Childhood allergies, such as those to milk or egg, are frequently outgrown, but adult-onset allergies tend to be life-long conditions.
Adult-onset allergies can manifest with severe reactions, with over half of food-allergic adults reporting at least one severe reaction in their lifetime. These reactions can appear suddenly and without any previous warning signs. This sudden shift requires a precise diagnosis and immediate adjustment to dietary habits and emergency preparedness.
Common Adult-Onset Food Allergens
The profile of foods triggering new allergies in adults differs from those most common in children. Shellfish is consistently the most frequent cause of adult-onset food allergy, including crustaceans like shrimp, crab, and lobster, and is a common trigger for severe reactions. Tree nuts are also highly represented in adult-onset cases, though the frequency of new peanut allergy development is lower in adults compared to children.
A particularly common form of adult-onset allergy is Oral Allergy Syndrome (OAS), also known as Pollen-Food Allergy Syndrome (PFAS). This condition typically occurs in individuals who already have a seasonal allergy, such as to birch or ragweed pollen. The immune system reacts to fresh fruits, vegetables, or nuts because their proteins closely resemble those found in the triggering pollen. Symptoms are generally localized to the mouth, lips, and throat, causing tingling or itching, and often diminish when the food is cooked.
Mechanisms Behind Developing Adult Allergies
The abrupt loss of immunological tolerance in adulthood involves a complex interplay of genetic, environmental, and biological factors. The immune system is constantly adapting, and changes can lead it to incorrectly classify a harmless food protein as an invader. Environmental exposures, including changes in the gut microbiome or interaction with new chemicals, can alter the delicate balance of the immune response.
A significant biological mechanism is the concept of cross-reactivity, where the immune system’s IgE antibodies bind to similar proteins found in different substances. For example, an existing sensitivity to birch pollen may create IgE antibodies that subsequently recognize the structurally similar proteins in fresh apples or carrots. This recognition is what triggers the allergic response in the mouth and throat typical of OAS.
The loss of immune suppression involves regulatory T-cells, which normally prevent the immune system from overreacting, becoming less effective. This failure allows the production of IgE antibodies directed against previously tolerated food proteins. The development of a compromised skin barrier, such as through eczema, can also increase the risk by allowing food allergens to enter the body through the skin, potentially leading to sensitization.
Diagnosis and Ongoing Management
The diagnostic process for a new adult-onset food allergy begins with a detailed medical history, linking specific symptoms to the ingestion of a suspected food. An allergist will then use initial testing, such as a skin prick test or a specific IgE blood test, to look for evidence of sensitization to the food protein. These tests measure the presence of allergen-specific IgE antibodies, but a positive result alone is not sufficient for a diagnosis of a true clinical allergy.
The gold standard for confirming a food allergy is the oral food challenge, a procedure where the patient consumes gradually increasing amounts of the suspected food under strict medical supervision. This confirms the diagnosis and prevents the unnecessary avoidance of foods. Once a food allergy is confirmed, the primary management strategy is the strict avoidance of the offending food.
Patients must be trained to carefully read food labels, which in many countries are required to clearly list the nine major food allergens. Emergency preparedness is necessary, and individuals with a risk of severe reaction or anaphylaxis should be prescribed and carry an emergency epinephrine auto-injector. The proper use of this life-saving medication and the development of a personalized action plan are fundamental to safely managing an adult-onset food allergy.