Soy is a legume (like peanut and lentils) however it is also an allergen; one of the eight foods that trigger 90% of all food allergy reactions. Soy protein isolate* can be found in protein bars, meal replacement shakes, bottled fruit drinks, soups, sauces, baked goods, breakfast cereals and some dietary supplements.
*Soy protein isolate is a highly refined or purified form of soy protein with a minimum protein content of 90% on a moisture-free basis. It is made from defatted soy flour which has had most of the non-protein components, fats and carbohydrates removed. Because of this, it has a neutral flavor and will cause less flatulence due to bacterial fermentation. (Wikipedia)
When suffering from any food allergies, reading food labels becomes a necessary habit; fine-tuning this skill will allow you to spot the allergens lurking within packaged foods and prevent unwanted allergic reactions.
Knowing what to look for on labels is how you can find soy traces in:
Over 60% of foods available at grocery stores contain soy and soy byproducts as a substitute for proteins mostly used for vegan diets. Interestingly, soy beans are usually not the prime culprit of allergic reactions. It is the genetically modified (GM) ones containing anti-nutrients, which affect digestion and absorption of essential vitamins.