Examining the 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.

Sudden Rise in Food Allergies Phenomenon

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.

Triggering allergies through exposure

A genetic predisposition to a food allergy is like a genetic predisposition to bruising. Even if you bruise easily, you won’t get a bruise unless you get whacked with something. With food allergy, you’re highly unlikely to develop an allergy unless you’re predisposed to react to a particular allergen and then exposed to that allergen. This sensitizes your immune system to the allergen, making you susceptible to future reactions.
The formula that causes the onset of a food allergy is well known:
Genetic Predisposition + Exposure = Sensitization
After your immune system is sensitized to a particular allergen, exposure to that allergen potentially leads to symptoms. The exposure piece of the equation gets pretty complicated. For those who are predisposed to developing a food allergy, the type of exposure may influence the likelihood that the exposure triggers onset, as the following general tendencies reveal:

• Repeated low-dose exposure to an allergen early in life is most likely to sensitize you to a specific allergen. In other words, you’re more likely to develop a food allergy to an allergen that repeatedly enters your system in small amounts, such as in breast feeding, trace amounts in foods, or even incidental contact.
• Large-dose exposures early in life may make you less likely to develop a food allergy. Odd, but true — increased exposure to an allergen may actually make you less sensitive to that food. Hold on. Don’t start feeing your baby peanut-based formula or advising nursing mothers to gobble up more peanuts. At this point, doctors have no reliable way to implement this observation in a preventive treatment plan. Based on the best information currently available, the recommendation is still to avoid peanut and other common allergens early in life.
• Avoidance diets may or may not help ward off the development of a food allergy. Although my colleagues and I recommend that parents limit exposure to common food allergens early in their children’s lives, research results waffle on the conclusion. While some studies show that avoidance diets early in life ward off the onset of food allergies, others have failed to uphold these results. We commonly see children who are born into allergic families where exposure has been virtually or completely eliminated develop the allergy. The mother may never have eaten peanut during pregnancy or breast feeding, all peanut has been banned from the premises, and incidental contact is highly unlikely, but the child still develops a peanut allergy. My belief is that these children are so genetically prone to developing the allergy that even inhaling a few errant molecules of peanut protein in the grocery store or shopping mall may be enough to trigger the sensitization process.

Discovering the genetic connection through peanut studies on twins

Studies of peanut allergy in twins have provided the strongest proof that genes play a role in developing food allergy but are not exclusively responsible. An important study shows that an identical twin has a 64 percent chance of sharing a peanut allergy with the twin sibling who is allergic to peanut. With nonidentical twins the risk drops to a mere 7 percent.
The study clearly demonstrates a strong genetic link to peanut allergy but also proves that food allergy is not purely a genetic disease. What leads one of the identical twins to develop the allergy and spares the second is unknown. Allergy specialists believe that exposure plays a role in developing a food allergy, but determining exactly what each twin is exposed to in the uncontrolled testing environment of daily living is nearly impossible.

What causes you to develop an allergy in the first place?

Is it your genetic makeup? Something in our food supply? Something you ate too much of as a kid? A great deal of research has been devoted to answering these questions, and the best answer that researchers have developed so far is that food allergies are caused by a combination of nature (your genes) and nurture (what you eat). The best evidence available for a genetic connection comes from studies of peanut allergy in twins. The medical community knows that allergies run in families, so the genetic link is well established, but we also know that children in the same families —even twins — do not always share the same allergies. This has led researchers to suspect that genes account only for a susceptibility to food allergies. If parents have hay fever or asthma, for example, their children are more prone to developing an allergic disease, including hay fever, asthma, eczema, or food allergy. One child may develop food allergy, another may develop hay fever, and a third may be allergy-free. Why this happens is still largely a mystery.