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A differential diagnosis means distinguishing food allergy from food intolerance or other illnesses. If a patient goes to the doctor's office and says, "I think I have a food allergy," the doctor has to consider the list of other possibilities that may lead to symptoms that could be confused with food allergy.
One possibility is the contamination of foods with microorganisms, such as bacteria, and their products, such as toxins. Contaminated meat sometimes mimics a food reaction when it is really a type of food poisoning.
There are also natural substances, such as histamine, that can occur in foods and stimulate a reaction similar to an allergic reaction. For example, histamine can reach high levels in cheese, some wines, and in certain kinds of fish, particularly tuna and mackerel. In fish, histamine is believed to stem from bacterial contamination, particularly in fish that hasn't been refrigerated properly. If someone eats one of these foods with a high level of histamine, that person may have a reaction that strongly resembles an allergic reaction to food. This reaction is called histamine toxicity.
Another cause of food intolerance that is often confused with a food allergy is lactase deficiency. This most common food intolerance affects at least one out of ten people. Lactase is an enzyme that is in the lining of the gut. This enzyme degrades lactose, which is in milk. If a person does not have enough lactase, the body cannot digest the lactose in most milk products. Instead, the lactose is used by bacteria, gas is formed, and the person experiences bloating, abdominal pain, and sometimes diarrhea. There are a couple of diagnostic tests in which the patient ingests a specific amount of lactose and then the doctor measures the body's response by analyzing a blood sample.
Another type of food intolerance is an adverse reaction to certain products that are added to food to enhance taste, provide color, or protect against the growth of microorganisms. Compounds that are most frequently tied to adverse reactions that can be confused with food allergy are yellow dye number 5, monosodium glutamate, and sulfites. Yellow dye number 5 can cause hives, although rarely. Monosodium glutamate (MSG) is a flavor enhancer, and, when consumed in large amounts, can cause flushing, sensations of warmth, headache, facial pressure, chest pain, or feelings of detachment in some people. These transient reactions occur rapidly after eating large amounts of food to which MSG has been added.
Sulfites can occur naturally in foods or are added to enhance crispness or prevent mold growth. Sulfites in high concentrations sometimes pose problems for people with severe asthma. Sulfites can give off a gas called sulfur dioxide, which the asthmatic inhales while eating the sulfited food. This irritates the lungs and can send an asthmatic into severe bronchospasm, a constriction of the lungs. Such reactions led the U.S. Food and Drug Administration (FDA) to ban sulfites as spray-on preservatives in fresh fruits and vegetables. But they are still used in some foods and are made naturally during the fermentation of wine, for example.
There are several other diseases that share symptoms with food allergies including ulcers and cancers of the gastrointestinal tract. These disorders can be associated with vomiting, diarrhea, or cramping abdominal pain exacerbated by eating.
Gluten intolerance is associated with the disease called gluten-sensitive enteropathy or celiac disease. It is caused by an abnormal immune response to gluten, which is a component of wheat and some other grains.
Some people may have a food intolerance that has a psychological trigger. In selected cases, a careful psychiatric evaluation may identify an unpleasant event in that person's life, often during childhood, tied to eating a particular food. The eating of that food years later, even as an adult, is associated with a rush of unpleasant sensations that can resemble an allergic reaction to food.
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