It is unsurprising that food frequently is thought to be the culprit behind an eczema flare, especially in infants. Indeed, it often is said that infants do only 3 things: eat, sleep, and poop.1 For those unfortunate enough to develop the signs and symptoms of atopic dermatitis (AD), food quickly emerges as a potential culprit from the tiny pool of suspects, which is against a cultural backdrop of unprecedented focus on foods and food reactions.2 The prevalence of food allergies in children, though admittedly fraught with methodological difficulties, is estimated to have more than doubled from 3.4% in 1999 to 7.6% in 2018.3 As expected, prevalence rates were higher among children with other atopic comorbidities including AD, with up to 50% of children with AD demonstrating convincing food allergy.4 It is easy to imagine a patient conflating these 2 entities and mistaking their correlation for causation. Thus, it follows that more than 90% of parents/guardians have reported that their children have had food-induced AD, and understandably—at least according to one study—75% of parents/guardians were found to have manipulated the diet in an attempt to manage the disease.5,6
Patients and parents/guardians are not the only ones who have suspected food as a driving force in AD. An article in the British Medical Journal from the 1800s beautifully encapsulated the depth and duration of this quandary: “There is probably no subject in which more deeply rooted convictions have been held, not only in the profession but by the laity, than the connection between diet and disease, both as regards the causation and treatment of the latter.”7 Herein, a wide range of food reactions is examined to highlight evidence for the role of diet in AD, which may contradict what patients—and even some clinicians—believe.
No Easy Answers
A definitive statement that food allergy is not the root cause of AD would put this issue to rest, but such simplicity does not reflect the complex reality. First, we must agree on definitions for certain terms. What do we mean by food allergy? A broader category—adverse food reactions—covers a wide range of entities, some immune mediated and some not, including lactose intolerance, irritant contact dermatitis around the mouth, and even dermatitis herpetiformis (the cutaneous manifestation of celiac disease).8 Although the term food allergy often is used synonymously with adverse food reactions, the exact definition of a food allergy is specific: “adverse immune responses to food proteins that result in typical clinical symptoms.”8 The fact that many patients and even health care practitioners seem to frequently misapply this term makes it even more confusing.
The current focus is on foods that could trigger a flare of AD, which clearly is a broader question than food allergy sensu stricto. It seems self-evident, for example, that if an infant with AD were to (messily) eat an acidic food such as an orange, a flare-up of AD around the mouth and on the cheeks and hands would be a forgone conclusion. Similar nonimmunologic scenarios unambiguously can occur with many foods, including citrus; corn; radish; mustard; garlic; onion; pineapple; and many spices, food additives, and preservatives.9 Clearly there are some scenarios whereby food could trigger an AD flare, and yet this more limited vignette generally is not what patients are referring to when suggesting that food is the root cause of their AD.
The Labyrinth of Testing for Food Allergies
Although there is no reliable method for testing for irritant dermatitis, understanding the other types of tests may help guide our thinking. Testing for IgE-mediated food allergies generally is done via an immunoenzymatic serum assay that can document sensitization to a food protein; however, this testing by itself is not sufficient to diagnose a clinical food allergy.10 Similarly, skin prick testing allows for intradermal administration of a food extract to evaluate for an urticarial reaction within 10 to 15 minutes. Although the sensitivity and specificity vary by age, population, and the specific allergen being tested, these are limited to immediate-type reactions and do not reflect the potential to drive an eczematous flare.
The gold standard, if there is one, is likely the double-blind, placebo-controlled food challenge (DBPCFC), ideally with a long enough observation period to capture later-occurring reactions such as an AD flare. However, given the nature of the test—having patients eat the foods of concern and then carefully following them for reactions—it remains time consuming, expensive, and labor intensive.11
To further complicate matters, several unvalidated tests exist such as IgG testing, atopy patch testing, kinesiology, and hair and gastric juice analysis, which remain investigational but continue to be used and may further confuse patients and clinicians.12