It's Important to Know the Difference

Harvard Health Letters

It's no fun to live in fear of food. If you can't tolerate certain foods, you probably dread the gastrointestinal distress they can cause. If you have a food allergy, the stakes are higher: a meal could end in a trip to the emergency room. Or, like many people, you could be uncertain whether your symptoms are due to an allergy (which requires eliminating all traces of the food from your diet) or an intolerance (which can be managed with less drastic measures).

An analysis revealed that while 13 percent of adults described themselves as allergic to peanuts, milk, eggs, fish, or shellfish, only 3 percent truly were. Other studies have shown that undetected food allergies may play a role in several medical conditions.

The prevalence of food allergies -- or at least diagnosed food allergies -- has increased steadily since the early 1990s; an estimated 5 percent of children and 4 percent of teens and adults are now affected. Food intolerance is harder to track, but estimates of its prevalence range from 2 percent to 20 percent.

In December 2010, the National Institute of Allergy and Infectious Diseases (NIAID) published guidelines for the diagnosis and management of these conditions. The guidelines explain how to distinguish a food allergy, which can be fatal, from a food intolerance, which can cause a great deal of discomfort but rarely has serious health consequences. The difference: Food allergies are orchestrated by the body's immune system; food intolerance results from an inability of the gut to digest food normally.

HOW FOOD ALLERGY WORKS

An allergy is the immune system's overreaction to a normally harmless substance. In people predisposed to food allergy, the immune system creates an antibody called immunoglobulin E (IgE) when it is first exposed to the offending food, or allergen. IgE antibodies stick to the surface of mast cells -- immune cells that surround nerves and blood vessels in the skin, airways, and gut. When the allergen reappears, IgE signals the mast cells to release a barrage of chemicals, including histamines (which stimulate nerves and dilate blood vessels) and leukotrienes (which contribute to inflammation).

The resulting symptoms may include lightheadedness; itching, hives or rash; swelling of the lips, tongue, and throat; and nausea, vomiting, and intestinal upset, including cramping and diarrhea. Rarely, a food allergy will trigger a life-threatening, whole-body reaction known as anaphylaxis, in which airways constrict and blood pressure drops precipitously, causing shock, loss of consciousness, and sometimes death.

People with food allergies must avoid the dietary culprit, but in case of accidental exposure, a self-injecting device (EpiPen, Twinject) can supply enough epinephrine to dampen the reaction until medical help arrives.

MYTHS ABOUT FOOD ALLERGY

1. A tiny taste won't hurt. Some people react to a single bite, or even an inhaled food particle.

2. Problem foods are less likely to cause allergic reactions if they're cooked. Food proteins are the allergens, and most proteins aren't altered by cooking.

3. Adults don't develop allergies. Although allergies usually begin in infancy, adults can develop them, too -- especially fish and shellfish allergies, which are more common in women.

4. Lactose intolerance is the same as milk allergy. Milk allergy is an immune reaction to the protein in milk. Lactose intolerance is caused by a deficiency of the enzyme that breaks down milk sugar in the intestines.

5. Allergic reactions become increasingly severe after each exposure to the problem food. The severity and symptoms are unpredictable. A food may produce a mild rash one time and full-blown anaphylaxis at another time (before or after).

WHO GETS FOOD ALLERGIES?

More than 170 foods have been associated with allergic reactions, but 90 percent of all cases involve milk, eggs, peanuts, tree nuts, fish, shellfish, wheat, or soy. Food allergies usually begin in the cradle, and most children outgrow them by adolescence. But the risk continues throughout life for some people, especially those allergic to peanuts and tree nuts.

Others develop food allergies in adulthood, usually to fish or shellfish. (These allergies are more common in women.) The later in life an allergy begins, the more likely it is to persist. Food allergies are more common in people who have other allergies, eczema, hay fever, or asthma. They can also trigger anaphylaxis induced by exercise, which is twice as common in women as in men.

DIAGNOSING FOOD ALLERGIES

When a food allergy is suspected, the NIAID guidelines recommend two tests to narrow the field of potential culprits: a skin-prick test and a blood test. In the skin-prick test, a small amount of the suspect food is scratched into the skin to see whether it causes a rash or hives. The blood test looks for IgEs -- that is, antibodies to the food. Positive results are verified by what's called an oral food challenge.

In this procedure, you're asked to eat increasing doses of the suspected allergen while medical professionals look for signs of an allergic reaction and, when necessary, provide epinephrine and other medical attention to prevent anaphylaxis. If you can eat a normal portion of the food without symptoms, you're not (or no longer) allergic to it, and the allergy is not a serious problem.

Other food/immune system-related conditions

There are two other disorders in which long-lasting health consequences can result from interactions between food and the immune system:

1. Celiac disease

This is an autoimmune disorder in which gluten, a protein found in many grains, including wheat, barley, and rye, triggers an attack by the immune system on the lining of the small intestine. The symptoms include abdominal pain, bloating, diarrhea, and fatigue.

Because the injured intestine can't adequately absorb vital nutrients (such as iron, calcium, and vitamin D), untreated celiac disease can lead to weight loss, iron deficiency anemia, osteoporosis, lactose intolerance, and other chronic health problems. Celiac disease is diagnosed in two stages. First comes a blood test for tTG-IgA, an antibody for anti-tissue transglutaminase, the enzyme that attacks the intestinal lining. If that test is positive, a small bowel biopsy -- a relatively simple procedure that requires only local anesthesia -- is performed to confirm damage to the intestinal lining. The damage can be arrested, though usually not fully reversed, by eliminating gluten from the diet.

2. Eosinophilic gastrointestinal disorders (EGIDs)

These are rare conditions in which eosinophils -- immune cells not usually found in the digestive system -- get into the tissues of the esophagus, stomach, or intestine. The most common forms of EGID, eosinophilic esophagitis and eosinophilic gastroenteritis, are named for the digestive organs in which they occur. Patients with eosinophilic esophagitis may have difficulty swallowing or experience food impaction (food lodged in the esophagus). Patients with eosinophilic gastroenteritis have abdominal pain, bloating, gas, and diarrhea.

EGIDs are known as mixed-immune disorders because people who have them often have IgE antibodies to certain foods as well. Eosinophils attracted by the chemicals released from mast cells in turn release their enzymes, perpetuating the inflammation and causing damage to surrounding tissues. Eliminating a food trigger identified by a blood test for IgE antibodies can end the inflammation and allow the tissues to heal.

TREATING FOOD INTOLERANCE

Three types of therapy have shown varying degrees of success in reducing gas, bloating, and other symptoms:

1. Enzyme supplementation.

Using lactase-enhanced supplements or dairy products can help you digest dairy foods. Taking alpha-galactosidase (Beano) tablets before meals can help you to metabolize fermentable carbohydrates in bran, fruits, beans, and other vegetables.

2. Probiotics.

Adding lactobacillus acidophilus and other "friendly" bacteria can help improve microbial balance in the gut. There's some evidence that this may help relieve lactose intolerance. However, there's no standard formulation for probiotics, and finding one that's right for you can be a hit-or-miss affair.

3. Rifaximin.

This is the only antibiotic that isn't absorbed into the bloodstream; it remains in the digestive system, where it can destroy many gas-producing bacteria.

WHAT CAUSES FOOD INTOLERANCE?

Food intolerance usually results from the inability to digest or metabolize a food completely. The symptoms -- gas, bloating, nausea, and diarrhea -- overlap those of irritable bowel syndrome (IBS), and food intolerance can trigger episodes of IBS. Its causes include the following:

-- Lactase deficiency. Some people don't produce enough lactase, the enzyme that breaks down lactose, or milk sugar, into smaller molecules that are more readily absorbed from the intestines. Lactase production may be diminished by intestinal infections or conditions like celiac disease; it can also decline with age. In people with lactase deficiency, lactose sits in the intestines longer and is broken down mainly by intestinal bacteria. The bacteria give off hydrogen gas, producing bloating, flatulence, and diarrhea. Lactase deficiency can be diagnosed by measuring the amount of hydrogen concentration in the breath. The condition is more common in blacks, Asians, and Native Americans than in whites.

-- Impaired complex carbohydrate digestion. People who eat large quantities of fermentable carbohydrates, such as beans, bran, fruit, or sugars and sugar alcohols (fructose, sorbitol, mannitol, and xylitol), may develop symptoms similar to those of lactose intolerance. In this case, the body's enzymes simply can't handle the volume of carbohydrates in the digestive system, and the intestinal bacteria pick up the slack.

-- Histamines in foods. Some foods, including alcoholic beverages, aged cheeses, cured meats, yeast products, spinach, and tomatoes, contain chemicals that are broken down into histamines. Normally, the histamines are deactivated in the intestines by the enzyme diamine oxidase (DAO). But in people who have low DAO levels because of illness or who take drugs that block DAO, histamine-rich foods can trigger flushing, headaches, diarrhea, and other symptoms that resemble allergic reactions. (DAO-blocking drugs include the antibiotics isoniazid and clavulanic acid; the antinausea drug metoclopramide; and verapamil, an antihypertensive drug.) Antihistamines may alleviate these symptoms, and a histamine-free diet can prevent them altogether.

-- Undefined food intolerance. Researchers have documented reactions to various food additives, including sulphites, nitrites, nitrates, monosodium glutamate, and certain food colorings. The symptoms include runny noses, sneezing, and migraines. The mechanism is unknown.

-- Gluten sensitivity. Some people can't tolerate gluten but repeatedly test negative for celiac disease. Eliminating gluten from the diet significantly reduces gastrointestinal symptoms and fatigue in these patients. According to a study, gluten sensitivity and celiac disease are distinct disorders caused by different intestinal responses to gluten.

THE IMPORTANCE OF TESTING

If you have troublesome reactions to certain foods, it's important to determine whether the problem is an allergy, celiac disease, or an EGID. If you're diagnosed with any of these conditions, you must avoid the food trigger or risk anaphylaxis (due to food allergy) or gastrointestinal damage (due to celiac disease or EGIDs). Otherwise, you are free to experiment with dietary changes and other remedies for symptom relief.

 

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