If you grew up in the ’60s or ’70s, you may never have heard the term “food allergy.” Your classmates and playmates probably were able to eat whatever they wanted free from the fear of a reaction. Perhaps your entire school had only one or two kids with food allergies if your school even had a kid with food allergies.

Over the last 20 years, however, the prevalence of food allergy appears to have risen sharply. Researchers can’t pin an absolute number on the increase, because no reliable comparative data over the 20-year period is available — studies being performed today follow different procedures from those performed 20 years ago, so you can’t really compare the numbers.

Allergists do, however, have plenty of evidence that points to a dramatic increase in food allergy:

• Evidence gathered over the last 10 years using the same methods show that the prevalence of peanut allergy has doubled in the last 5–10 years.

• Anecdotal and clinical evidence shows a significant increase in food allergy. Pediatricians tell me that they see far more food allergy than ever before. School nurses report that while a decade ago they had one or two children in the school with epinephrine prescriptions, they now have 20 or 30.

Some of this could be due to what we refer to as a detection bias; that is, increasing awareness about a problem leads to its being diagnosed more efficiently. However, most experts believe that the increase is real and not simply the result of increased awareness.

• Reliable asthma studies show at least a 100 percent increase in the prevalence of asthma (an allergy-related disease) over the last 30 years. The rise in asthma appears to have preceded the rise in food allergy (which is a source of confusion) although experts believe that similar mechanisms likely underlie the dramatic increases in all allergic diseases.

The jump in all allergic diseases across the board suggests that a collection of contributing factors not present 20 or 30 years ago is responsible. Researchers have presented many theories to explain the increase, but the only clear conclusion is that no one, single cause can explain it.

Hygiene Hypothesis

When you’re growing up, adults, whom you assume know what’s good for you tell you to practice proper hygiene. By the time you’re six years old, the phrases “Wash your hands,” “Brush your teeth,” “Don’t let the dog lick your face,” and “Stay out of the mud,” play like Zen mantras in your head.

Perhaps our obsession with cleanliness is at the root of the increase in allergies. This theory, referred to as the hygiene hypothesis, is the most popular explanation for the rise in allergies. Simply stated, the hygiene hypothesis proposes that the less the immune system is exposed to germs and bacterial by-products the more energy it has to unleash on allergens.

If this sounds kooky to you, examine the evidence before dismissing this theory:

• Allergies are much more common in developed countries, and the prevalence in allergies rises pretty much in direct proportion to the rise in development.

• Allergies are less common in children who grow up on farms, who attend daycare in early life, and who have multiple older siblings.

• Allergies may be less common in families that have pets, perhaps because pets increase exposure to bacteria and bacterial by-products.

Before you move out to the country, surround your family with livestock, and start rolling around in the pig pen, realize that the hygiene hypothesis has some holes in it, suggesting that other factors may play a role. The inner-city environment, for example, defies the hygiene hypothesis.

Children growing up in the inner-city should reap all the benefits of poor hygiene in warding off the onset of allergies, but inner-city kids have some of the highest rates of asthma and allergy anywhere. This suggests that other factors, such as air pollution or environmental allergen exposure, may trump the hygiene hypothesis.

Other Possible Suspects

When you consider all the new developments in foods, medications, and other substances introduced into our lives over the past 30 years, you can easily become suspicious of numerous items that may have influenced the rise in food allergies. In the following list, I name several of the top suspects and provide my professional opinion on just how likely they contribute to the upward trend:

• Vitamins: One study suggests that vitamin supplements may promote allergy. Another study suggests that vitamin deficiency, especially vitamin D, may promote allergy. Conclusion? We just don’t know yet.

• Antacids: Some evidence shows that antacids or other medicines designed to suppress stomach acids may promote allergy. This makes sense, because these products may reduce the breakdown of allergenic proteins in the stomach. The jury’s still out on this one.

• Immunizations or antibiotics: No evidence at this time provides reliable evidence that immunizations or antibiotics promote allergy development.

• Tobacco smoke: Of all the studies ever done, the only factor that consistently has been associated with increases in allergy is exposure to tobacco smoke. The first studies in this regard looked at childhood asthma, and nobody was surprised to learn that exposure to tobacco smoke may lead to an increased susceptibility to asthma.

This was presumed to be due to tobacco smoke’s irritating effects on the airways. Research has now shown that exposure to tobacco smoke actually stimulates the immune system to promote the development of allergy. Other air pollutants may also play a role, although most studies have shown that exposure to air pollution has relatively little effect on the development of allergy.

Families frequently ask my colleagues and me to estimate the risk that their next child will develop food allergy. These are typically families who already have at least one child with food allergy, so the odds may already be tipped against them. Although we’re reasonably good at predicting the chance of developing some form of allergic disease, our track record for specifically predicting food allergy is less impressive.

In the following sections I present data that can assist you in guesstimating the odds that your next child will develop some sort of allergic disease — based on family history and gender. I then investigate the role that age plays in the onset of an allergy-related condition. You can use the following guidelines and the graph shown in Figure below to formulate your own prediction:

• If both you and your mate have an allergic disease, your children have a 40–60 percent chance of developing some form of allergic disease, not necessarily food allergy.

• If only you or your mate has an allergic disease, the likelihood drops to 30–40 percent.

• I would estimate that in the most allergic families that already have one or two children with a food allergy, the odds are less than 20–25 percent that the next child will also have food allergy. The next child is almost guaranteed to have some form of allergic disease but not necessarily food allergy.

• All allergies, including food allergies are about twice as common in boys as girls. This evens out later in childhood, and then, by adulthood, asthma and allergies are more common in women.

In a family with several members who have food allergy, the likelihood of the next child being allergic is highly probable, but whether the allergy takes the form of food allergy is impossible to predict.

Age Factor

The age factor carries a lot of weight in determining the onset of allergies. The vast majority of allergies develop early in life. Food allergy hits its peak prevalence at one year of age, and at least 80 percent of childhood food allergies are likely to develop by the age of one. Why are young children so much more susceptible to developing a food allergy?

Because the intestinal tract of the infant provides a less complete barrier against allergenic proteins, so as proteins escape the intestinal tract, they become high-profile targets for the body’s immune system. This barrier slowly closes by about six months of age.

The only problem with this theory is that allergies very commonly worsen over the first two to three years of life, even if a child is destined to outgrow them later. A one-year-old who’s allergic to milk or egg, for example, is likely to develop a more severe allergy by the time they are two to three, even if they eventually outgrow the allergy.

A few allergies defy this rule and tend to crop up more frequently in later years.

• Shellfish and fish: Allergies to shellfish and sometimes fish most often develop later in life.

• Fruits and vegetables: Allergies to fruits and vegetables may arise later in life as a secondary allergy related to a pollen allergy that you acquired earlier in your life. Allergies to fruits and vegetables often don’t get into full swing until your pollen allergies have become sufficiently strong to spill over and cause a food allergy. Reactions to fruits and vegetables typically result in oral allergy syndrome covered later.