Interesting story:

Earlier this year, the Centers for Disease Control and Prevention (CDC) published a major study on children and allergies. The results were astounding. Among other things, the government survey suggested: 1 in 20 children have food allergies (up 50 percent from the 1990s). 1 in 8 have skin allergies or eczema (that number is up 69 percent). There has been no increase in hay fever or other respiratory allergies In some ways, these numbers are no surprise.

Most of us with young children know families with food or skin allergies. To protect those children, it is now common practice in many childcare and school settings to ban or severely restrict potential allergens like tree nuts from all children's lunches.

But we have to be cautious. It's hard to tell how much allergies have actually increased, and how much the reporting of allergies has increased due to greater awareness and/or a better understanding of the science.

In fact, the CDC study itself was based on a survey of parents who self-reported allergies in their children, and the study did not ask if those allergies were diagnosed by a medical professional. It seems highly likely that some cases will be a misdiagnosis by hypersensitive parents.

Experts do tend to agree, however, that instances of both food allergies and hereditary atopic disorders such as eczema and asthma have seen a dramatic increase in recent decades, at least in so called "developed" economies. And herein may lie a big clue: Westerners may be experiencing more allergies because our culture has become too effective at controlling or eliminating exposure to microbes.

Microbes, you see, are central to training our immune system to deal with allergens.

Nevertheless, researchers like SF Bloomfield of the London School of Hygiene and Tropical Medicine in London warn against too simplistic a reading of this so called "hygiene hypothesis." If your child develops allergies, it's not necessarily because you cleaned your house too well. It's more likely the result of broader, societal changes.

In a research paper entitled Too clean, or not too clean, Bloomfield et al suggest the hypothesis would actually be better renamed:

The increase in allergic disorders does not correlate with the decrease in infection with pathogenic organisms, nor can it be explained by changes in domestic hygiene. A consensus is beginning to develop round the view that more fundamental changes in lifestyle have led to decreased exposure to certain microbial or other species, such as helminths, that are important for the development of immunoregulatory mechanisms. Although this review concludes that the relationship of the hypothesis to hygiene practice is not proven, it lends strong support to initiatives seeking to improve hygiene practice. It would however be helpful if the hypothesis were renamed, e.g. as the 'microbial exposure' hypothesis, or 'microbial deprivation' hypothesis, as proposed for instance by [Bengt] Bjorksten [professor emeritus at the Karolinska Institute in Sweden]. Avoiding the term 'hygiene' would help focus attention on determining the true impact of microbes on atopic diseases, while minimizing risks of discouraging good hygiene practice.

Rural kis have less allergies. Studies would seem to back up this broader, societal reading of the "hygiene hypothesis."

Kids are less likely to develop asthma if they grow up on farms, for example. And children in cities have more food allergies than those in the country. (Exposure to air pollution in cities may also be a factor in this latter case.) There have even been studies suggesting that babies born in hospitals, and particularly those born by cesarean section, may have a different balance of gut bacteria making them more prone to allergies.

Finally, it's worth noting recent major changes to recommendations as to how and when to introduce potential food allergens to a child's diet.

As my wife Jenni, a registered dietitian, wrote over at Parentables a while back, it used to be recommended that parents avoid giving children cow's milk until the age of 1, and tree nuts before the age of 2. In 2008, the American Academy of Pediatrics revised its guidelines, stating that unless there was a strong family history of allergies, potential allergens could be introduced as and when developmentally appropriate. (Parents should still avoid whole nuts and thick layers of peanut butter with young children as they are a potential choking hazard.)

Mostly, it appears that these guidelines were revised because there was simply no correlation between the onset of allergies and the withholding of certain allergens. Yet some researchers like Morton Galina, a pediatric allergist at Atlanta's Emory School of Medicine, have even suggested that there may be more going on - and that the previous practice of not giving children allergens may in fact have increased, not decreased, their risk factor. Here's Galina discussing the recent CDC study to USA Today:

Emory's Galina said the new CDC statistics may reflect a recent "sea change" in the recommendations for when young children should first eat certain foods. In families with a history of eczema or food allergies, parents were advised to wait for years before introducing their young children to foods tied to severe allergies, like peanuts, milks and eggs. But professional associations changed that advice a few years ago after research suggested that allergies were more likely in those kids when the foods were delayed. The old advice "was exactly the wrong thing to do," and could have contributed to some of the increased cases, Galina said.
Exactly what is going on with allergies and our children will remain a topic of much study and fierce debate. We certainly shouldn't take concern over the "hygiene hypothesis" as a reason to throw the baby out with the (dirty) bath water. It's still a good idea to wash your hands after going to the bathroom. You still want to watch what your child puts in his mouth. And good food hygiene can be crucial in protecting your child's health.

As with most things in life, it seems the best advice is to practice moderation.

Let your children play in the dirt, and then call them inside to wash their hands. Go ahead and feed them a PB&J sandwich; just make sure they are old enough to chew it properly.