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
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
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.