July 2009 Issue

The Hygiene Hypothesis
By Maggie Moon, MS, RD
Today’s Dietitian
Vol. 11 No. 7 P. 12

Ever heard the expression that a little dirt never hurt anyone? Those who are familiar with the “hygiene hypothesis” just might have some data to back that up—or do they?

The hygiene theory periodically finds its way into the news under the guise of headlines such as “Modern Hygiene’s Dirty Tricks” (Science News, 1999), “Kiddie Allergies: Let Them Eat Dirt” (What Doctors Don’t Tell You, 2002), and “From Good Hygiene Comes Bad Allergies” (Seed magazine, 2006), as well as in more metered coverage of new findings. As RDs, we can be sure that if it’s newsworthy, some clients will ask about it.
Read on for a hygiene hypothesis primer and an update on how it relates to food allergies.

The Hypothesis Uncovered
In essence, the hygiene hypothesis suggests that regular exposure to allergens early in life may help bolster one’s immune system against developing allergies. On the flip side, this means that an antiseptic environment (ie, the overly clean conditions that are possible and more common in Western nations such as the United States) may be implicated in the rise of allergies. In the realm of food allergies, what this potentially means is that further research into the effects of earlier and regular exposures to known allergens such as peanuts, eggs, and milk may one day make heroes out of villains.

But we’re not there yet. It’s important to note that the research thus far examines more than just the culprits in food allergies; it has looked at triggers for asthma, autoimmunity, and allergies in general (including, but not limited to, food allergies). Research on the relationship between exposure to foods and allergies to them is still preliminary.

History Behind the Hypothesis
In October 2008, the Centers for Disease Control and Prevention released a report indicating that food allergies increased 18% over a 10-year period in children under the age of 18. The same report found that having food allergies made kids two to four times more likely to have asthma and other allergies compared with those without food allergies.1 Naturally, people may wonder why food allergies are on the rise. What’s different now that may be behind this trend?

As it happens, a marked increase in allergies is what led scientists to the hygiene theory, which has attracted interest and investigation since the 1989 publication of a study by David P. Strachan, MD, PhD, of St. George’s University of London. Strachan aimed to offer a possible explanation for the upward trend in hay fever, asthma, and childhood eczema that had been observed during the previous three decades. The study suggested that a lack of early and repeated exposure to allergens could actually increase one’s risk of developing allergies. Strachan concluded his paper by implicating “declining family size, improvements in household amenities, and higher standards of personal cleanliness.”2

The prospective study found that participants with the most older siblings, and therefore the greatest opportunities for exposure to allergens through repeated contact, suffered the least from allergies (p < 0.01). Strachan looked at data for 17,414 British children, all born within one week of each other, who were then followed for 23 years as part of the National Child Development Study. The outcomes of interest included hay fever at 11 years of age (parental report), hay fever at 23 years of age (self-report), and eczema in the first year of life (parental recall). At both 11 and 23 years of age, analysis showed that growing up in a larger family was more protective against allergies than growing up in a small family.2 The 1989 hypothesis was enough to encourage more research on the relationship between early allergen exposure and the development of allergies.

The Research
Two leading experts in food allergy studies agree that there is evidence for the hypothesis in animal studies. Andrew H. Liu, MD, an associate professor of pediatric allergy and immunology at National Jewish Health in Denver, and Scott H. Sicherer, MD, author of Understanding and Managing Your Child’s Food Allergies, note that mice have been shown to be more allergic to food if they were treated with antibiotics early on or were raised in germ-free environments.3

Results from a 2006 study published in Pediatrics support one example of the protective effects of early exposure to food allergens. Wheat allergies were four times more prevalent in infants who were exposed to cereal grains (wheat, barley, rye, oats) after 6 months compared with those who were exposed earlier than 6 months (1.8% vs. 0.41%). The study followed 1,612 children for an average of 4.7 years, with parent surveys taken at 3, 6, 9, 15, and 24 months and yearly thereafter. Children with celiac disease were excluded, and results controlled for confounding factors such as family history of allergic disorders and history of food allergy prior to 6 months.4

However, a review of epidemiologic evidence on the hygiene hypothesis concludes that the body of evidence is not yet strong enough to make recommendations for allergy prevention or treatment.5 Liu cautions that there is no definitive proof as of yet, but he is confident that the hygiene hypothesis will continue to be studied and that “many believe it is a significant contributor to the development of allergies.”

Even though many studies look at nonfood allergies, “The hypothesis applies to all allergy, which includes allergy to foods,” says Sicherer. He adds that while “not all studies show the trend, it seems most do.” In addition, “Avoiding an allergen for prolonged periods of time (years) as a form of primary prevention was never really studied and is becoming more controversial,” he says, reporting that there are lower rates of peanut allergy in Israel where children eat peanuts earlier  (though still after weaning) than they do in the United States.

Some experimental treatments provide subjects with regular, small doses of an allergenic food with the idea that it may dampen their immune system to the point that they can tolerate the offending food. Testing is conducted in controlled situations because people’s reactions can be severe. (Both Liu and Sicherer strongly caution individuals not to try this one at home.)

Of course, no one advocates forgoing hand washing or bathing. The hypothesis is not so simplistic. An editorial that appeared in The Journal of Allergy and Clinical Immunology, “Renaissance of the Hygiene Hypothesis,” provides a good overview of the complexities of the hygiene hypothesis. It discusses the hypothesis in terms of five categories (infections, microbial components, gastrointestinal colonization, soil microbiota, and forces that reduce microbial burden, such as antibiotics) and six variables that affect them (microbial components, disease phenotype, timing, dosage and coexposures, genetic influences, and routes of exposure).6

Further Study Is Needed
At this time, experts agree that the hygiene hypothesis is intriguing enough to warrant further research but that there’s more to consider when it comes to food allergies. Check out the Resources section for information on how to stay up-to-date on emerging research and where to turn for credible allergy information.

— Maggie Moon, MS, RD, is a nutrition writer and a dietitian based in New York City.

Resources
• American Academy of Allergy, Asthma & Immunology: The largest professional medical organization in the United States devoted to the allergy/immunology specialty (www.aaaai.org)

• ClinicalTrials.gov: A broad registry of federally and privately supported clinical trials in the United States and around the world

• Consortium of Food Allergy Research: A National Institutes of Health-funded group that conducts observational and clinical studies related to food allergies (www.cofargroup.org)

• The Food Allergy & Anaphylaxis Network: An advocacy organization for those affected by food allergies and anaphylaxis (www.foodallergy.org)

Key Points
• Keep an open mind, but remember that the hygiene hypothesis is still a theory. Growing evidence seems to support it but has yet to provide definitive proof.

• Stand by current best practices for now. Although researchers are studying alternative treatments, your clients and patients with food allergies should still practice avoidance and have emergency self-injectable epinephrine readily available.

• The hygiene hypothesis’ role in food allergy development is not yet well understood in humans. There have been some supporting results from animal studies.

• Andrew H. Liu, MD, encourages RDs to stay attuned to potential new therapies but stresses that they are “not ready for prime time.”

Hygiene Hypothesis Hx6
The idea that exposure to allergens may be protective against allergies dates back further than the 1980s. Andrew H. Liu, MD, cites two 19th-century scientists who saw the trend:

• In 1828, John Bostock, MD, reported on a trend for increased allergies in populations with lower exposure to allergens, stating that some allergies “only occur in the middle or upper classes of society, some indeed of high rank.” In fact, he found not “a single unequivocal case occurring among the poor.”

• In 1873, Charles Blackley, MD, noted a marked overall increase in hay fever, while those with the most exposure to pollen (“the farming class”) had the fewest cases.

References
1. Branum AM, Lukacs SL. Food allergy among U.S. children: Trends in prevalence and hospitalizations. NCHS data brief No. 10. Hyattsville, Md.: National Center for Health Statistics; 2008.

2. Strachan DP. Hay fever, hygiene, and household size. BMJ. 1989;299(6710):1259-1260.

3. Hazebrouck S, Przybylski-Nicaise L, Ah-Leung S, et al. Allergic sensitization to bovine beta-lactoglobulin: Comparison between germ-free and conventional BALB/c mice. Int Arch Allergy Immunol. 2009;148(1):65-72.

4. Poole JA, Barriga K, Leung DY, et al. Timing of initial exposure to cereal grains and the risk of wheat allergy. Pediatrics. 2006;117(6):2175-2182.

5. Liu AH, Murphy JR. Hygiene hypothesis: Fact or fiction? J Allergy Clin Immunol. 2003;111(3):471-478.

6. Liu AH, Leung DY. Renaissance of the hygiene hypothesis. J Allergy Clin Immunol. 2006;117(5):1063-1066.