By Reed Mangels, PhD, RD

QUESTION: “Two of my family members developed serious allergic reactions to soy foods. Have you heard of a lot more of this sort of thing happening?”

D.B., via e-mail

ANSWER: In the United States, approximately 1 percent of people have a soy allergy. Soy is one of the eight most common foods that trigger allergic reactions. The others are cow's milk, eggs, tree nuts, peanuts, wheat, fish, and shellfish.

Allergies to soy most commonly occur in children; many outgrow this allergy by age 3, although it can continue into adulthood. Soy allergies can also develop in adults for unknown reasons.

As soy products are more commonly eaten, the incidence of soy allergy may increase because more people are exposed to soy. The authors of Food Allergy Survival Guide (Healthy Living Publications, 2004) suggest that our hygienic lifestyle, pollution, genetic modification, exposure to pesticides, injury or infection of the intestinal tract, medication use, and stress are factors that could possibly explain the increased incidence of allergies in general. In addition, peanuts and soybeans have similar amino acid sequences, so people who are allergic to peanut protein may also react to soybean protein. As peanut allergies become increasingly common, soy allergy rates may also rise.

A food allergy is commonly defined as an abnormal immune reaction to a food, usually to a food protein. A reaction can be triggered by even the smallest amount of that food imaginable. Symptoms can include hives; swelling of the lips, face, tongue, or throat; wheezing; difficulty breathing; dizziness; or fainting. Anaphylaxis is a very serious and potentially fatal allergic reaction that involves a sudden drop in blood pressure, loss of consciousness, and body system failure.

Food allergies are relatively uncommon. In contrast, many people report reactions such as nausea, vomiting, diarrhea, and stomach pain to specific foods. The distinction between an allergy and an intolerance is an important one because many people with a food intolerance can eat small amounts of the food without having symptoms. Anaphylaxis does not occur with food intolerance.

Food scientists are working to reduce the allergenicity of soybeans. This area of research appears promising, although so far no technique has been found that completely eliminates potential allergens in soybeans.

QUESTION: “I read an article stating that a person's age at the time of their exposure to soy could be very important with regard to their risk of developing cancer. For instance, one study has shown that eating a lot of soy in adolescence is good and may lower the risk, but when you start as an adult, it may have no impact or actually increase your risk. As a person who became a vegetarian during my early 30s, I am concerned that I am placing myself at an increased risk of developing cancer. Soy products constitute a sizable portion of my daily diet. Are you aware of any research regarding this effect?”

C.K., via e-mail

ANSWER: The study that you read about was most likely a study of breast cancer. Several studies have reported that soy intake during adolescence is more important than soy intake later in life in terms of breast cancer risk reduction.1,2 What about those like you who come to soy later in life? This has not been well studied, but one study has compared breast cancer incidence in women who ate little soy as adolescents and in adulthood with women who ate little soy as adolescents but who ate high amounts as adults.3 The women who ate generous amounts of soy as adults had a slightly lower, but statistically insignificant, risk of developing breast cancer. These results certainly do not suggest that soy products increase risk of breast cancer.

Studies of Asian women find a marked reduction in breast cancer risk among the highest consumers of soy products, although these results could, at least partially, be due to soy consumption in adolescence.4 Soy intake appears to have little effect on breast cancer risk in Western women whose intake of soy was quite low4 or in British women, some of whom were vegetarians, with higher intakes of soy isoflavones.4,5

Based on what we know now, soy products do not appear to increase the risk of breast cancer and may be slightly protective. You mentioned that soy products are a sizable part of your diet. In the interest of variety, perhaps you should consider using other foods in addition to soy, including dried beans, nuts, and wheat gluten, as protein sources.


1 Shu XO, Jin F, Dai Q, et al. 2001. Soyfood intake during adolescence and subsequent risk of breast cancer among Chinese women. Cancer Epidemiol Biomarkers Prev 10:483-88.
2 Wu AH, Wan P, Hankin J, et al. 2002. Adolescent and adult soy intake and risk of breast cancer in Asian-Americans. Carcinogenesis 23:1491-96.
3 Wu AH, Yu MC, Tseng CC, et al. 2007. Body size, hormone therapy and risk of breast cancer in Asian-American women. Int J Cancer 120:844-52.
4 Wu AH, Yu MC, Tseng CC, et al. 2008. Epidemiology of soy exposures and breast cancer risk. Br J Cancer 98:9-14.
5 Travis RC, Allen NE, Appleby PN, et al. 2008. A prospective study of vegetarianism and isoflavone intake in relation to breast cancer risk in British women. Int J Cancer 122:705-10.