Classification of Food Allergies
It is useful to first separate out the classic IgE-mediated food allergy reactions that are common. In these immediate-type reactions, a person sensitized to a food protein will develop characteristic cutaneous and/or extracutaneous reactions such as urticaria, angioedema, and even anaphylaxis, usually within minutes of exposure. Although it is possible that an IgE-mediated reaction could trigger an AD flare—perhaps simply by causing pruritus, which could initiate the itch-scratch cycle—because of the near simultaneity with ingestion of the offending food and the often dramatic clinical presentations, such foods clearly do not represent “hidden” triggers for AD flares.3 The concept of food-triggered AD (FTAD) is crucial for thinking about foods that could result in true eczematous flares, which historically have been classified as early-type (<2 hours after food challenge) and late-type (≥2 hours after food challenge) reactions.13,14
A study of more than 1000 DBPCFCs performed in patients with AD was illustrative.15 Immediate reactions other than AD were fairly common and were observed in 40% of the food challenges compared to only 9% in the placebo group. These reactions included urticaria, angioedema, and gastrointestinal and respiratory tract symptoms. Immediate reactions of AD alone were exceedingly rare at only 0.7% and not significantly elevated compared to placebo. Just over 4% experienced both an immediate AD exacerbation along with other non-AD findings, which was significantly greater than placebo (P<.01). Although intermediate and late reactions manifesting as AD exacerbations did occur after food ingestion, they were rare (2.2% or less) and not significantly different from placebo. The authors concluded that an exacerbation of AD in the absence of other allergic symptoms in children was unlikely to be due to food,15 which is an important finding.
A recent retrospective review of 372 children with AD reported similar results.4 The authors defined FTAD in a different way; instead of showing a flare after a DBPCFC, they looked for “physician-noted sustained improvement in AD upon removal of a food (typically after 2–6-wk follow-up), to which the child was sensitized without any other changes in skin care.” Despite this fundamentally different approach, they similarly concluded that while food allergies were common, FTAD was relatively uncommon—found in 2% of those with mild AD, 6% of those with moderate AD, and 4% of those with severe AD.4
There are other ways that foods could contribute to disease flares, however, and one of the most compelling is that there may be broader concepts at play; perhaps some diets are not specifically driving the AD but rather are affecting inflammation in the body at large. Although somewhat speculative, there is evidence that some foods may simply be proinflammatory, working to exacerbate the disease outside of a specific mechanism, which has been seen in a variety of other conditions such as acne or rheumatoid arthritis.16,17 To speculate further, it is possible that there may be a threshold effect such that when the AD is poorly controlled, certain factors such as inflammatory foods could lead to a flare, while when under better control, these same factors may not cause an effect.
Finally, it is important to also consider the emotional and/or psychological aspects related to food and diet. The power of the placebo in dietary change has been documented in several diseases, though this certainly is not to be dismissive of the patient’s symptoms; it seems reasonable that the very act of changing such a fundamental aspect of daily life could result in a placebo effect.18,19 In the context of relapsing and remitting conditions such as AD, this effect may be magnified. A landmark study by Thompson and Hanifin20 illustrates this possibility. The authors found that in 80% of cases in which patients were convinced that food was a major contributing factor to their AD, such concerns diminished markedly once better control of the eczema was achieved.20