The easiest way to figure out whether your food is making you sick is to try an elimination diet, which simply means avoiding all of the foods that are known to cause bad reactions in other people. A registered dietitian can help you figure out which foods to avoid during the elimination diet. The elimination diet must be followed for at least a week, to allow the offending allergens to be cleared from the body.
One elimination diet protocol is Dr. John McDougall’s Diet for the Desperate. It eliminates all of the usual suspects. It excludes all of the foods that come from animals, including dairy products. It also excludes some commonly troublesome plant-based foods, such as wheat, rye, and barley, which are the grains that cause problems in people with celiac disease. However, the diet does include plenty of other starchy staple foods, such as brown rice and sweet potatoes. It includes most vegetables and most fruits, except for citrus fruits. Dr. McDougall says that all of the vegetables and fruits that are eaten during the elimination diet phase should be fully cooked. Cooking alters the proteins in foods and can thus make them less likely to provoke an allergic reaction. During the elimination diet phase, people should avoid spices and other condiments, such as mustard and vinegar. Water is the only beverage permitted.
If a problem is due to a food allergy, the individual may start to feel better after staying on the elimination diet for at least a week. At that point, you may wish to start reintroducing some of the foods that you have been avoiding. For testing purposes, it is best to reintroduce each food one at a time. Dr. McDougall recommends eating a large serving of the test food three times a day for two days. If the symptoms return, that food should be avoided in the future. The person should go back to eating only the safe foods for at least a week, to allow the body to recover, before reintroducing another food.
A registered dietitian can give you detailed advice on how to follow an elimination diet, and how to reintroduce suspicious foods. Also, if you have any health problems or are taking any prescription medicines, you should talk to the healthcare professional who prescribed those medications before making any major change in diet.