A food allergy is an immediate or delayed adverse reaction to the ingestion of a specific food. Common signs and symptoms of such allergies include dark circles and puffiness under the eyes, chronic diarrhea, various inflammations, headaches (including migraine), chronic runny nose, itchy eyes, asthma, hives, poor digestion, mental and physical fatigue, inflammatory diseases, and chronic infections. Food allergy sensitivity usually forms slowly as a result of repetitive consumption of a food.
To test for a hidden food allergy, remove the item completely from your diet for two weeks, making sure to real all labels to ensure compliance. After two weeks, eat a moderate to large amount of the food in question and see if the re-introduction of the food causes a noticeable adverse reaction over the next 24-48 hours. Common reactions include diarrhea, headache, nausea, hyperactivity, hives or skin itching, fatigue, irritability, bags under the eyes and insomnia. If any of these symptoms appear, you are most likely allergic to the food and should remove it from your diet permanently. The two-week abstinence period allows the body to mount a stronger response, allowing for easier identification. If there is no response, you are not allergic to this food, and may resume regular consumption. You may want to repeat the test if you are unsure of the result.
Once you have eliminated the offending food from your diet, you may find that after 3-6 months you can eat it occasionally without experiencing an adverse reaction. Eat the food no more than once every four or five days--several days in a row will probably re-activate the allergy and cause a return of symptoms. It is also possible to react to an entire class of foods, such as all milk products, citrus fruits, nuts, or chemical preservatives. Remember to check all labels of the foods you eat to make sure none of the foods you are allergic to are hidden in the ingredients.
For a list of common food allergens, go here