Peanuts and soy are both common allergens and both legumes, and one frequent pattern among allergens is that foods that are closely related sometimes have higher-than-average risks of causing allergens in the same person. (Examples of this phenomenon include high risks of multiple tree nut allergies in a person who's diagnosed with allergies to any one particular tree nut allergy and a similar issue with shellfish allergies.) With peanuts and soy being close biological relatives, can soy cause reactions in people with peanut allergies? Should kids diagnosed with peanut allergies pre-emptively avoid soy out of an abundance of caution?

The good word, according to the Food Allergy & Anaphylaxis Network: not unless your allergist has specifically recommended you do so. The majority of people with peanut allergies tolerate soy well, and vice versa. Peer-reviewed studies have not demonstrated that soy can trigger a reaction in people with peanut allergies.

While a 2003 study raised questions about whether early exposure to soy (as in soy formula) might be a risk factor for peanut allergies, the authors of that study noted that "there is a low prevalence of clinical reactivity to soy in infants with peanut allergy." And a larger 2005 study (mentioned by FAAN) showed no correlation between early introduction to soy and peanut allergies.

What's the downside of restricting soy from the diet if you've got a peanut allergy in the family? Well, consider one common nutritional argument made by folks who disagree with peanut-restriction policies in schools: the argument that nuts are a potent source of inexpensive, high-quality protein. If you restrict soy unnecessarily, you're cutting out another nutritious protein source, as well as a ubiquitous ingredient in processed foods and many national cuisines.

_Sources:

Klemola, Timo, et al. "Feeding a soy formula to children with cow's milk allergy: The development of immunoglobulin E-mediated allergy to soy and peanuts." Pediatric Allergy & Immunology. Dec. 2005. 16(8): 641-46. 30 Nov. 2007.

Lack, Gideon, et al. "Factors Associated with the Development of Peanut Allergy in Childhood." New England Journal of Medicine. Mar. 13, 2003. 348(11): 977-85. 30 Nov. 2007.

More on Peanut Allergies and Soy Allergies:

• Food Allergy & Anaphylaxis Network: Soy and Peanut Allergy
• Peanut Allergies
• Soy Allergies