Over the past several decades, the incidence of atopic diseases, such as atopic dermatitis, food allergies, and asthma has increased dramatically. Among children up to 4 years of age, the incidence of asthma has increased 160% and the incidence of atopic dermatitis has increased twofold to threefold. The incidence of peanut allergy has also doubled in the past decade. Thus, atopic diseases increasingly are a problem for clinicians who provide health care to children. Is there a way to prevent allergies developing in our high-risk population? To help address this question, several possible potential risk factors and protective measures have been studied.
One area of interest has focused on environmental exposure at a young age. Children who grow up in a farming environment show lower levels of atopic sensitization, hay fever, and asthma than children of the same age not living in such an environment. A number of investigations have provided good evidence that this is due to an early-life contact with cowsheds, farm animals, and/or consumption of products like raw milk. A recent study from The Journal of Allergy and Clinical Immunology demonstrated that exposure to two particular microbes, Acinetobacter lwoffi and Lactococcus lactic that are found commonly in cowsheds, helped to promote an immune deviation from TH2 to TH1 stimulation. A shift from TH2 toward TH1 reactions in response to a specific allergen has been shown to prevent the development of allergies. This data is in support of the” hygiene hypothesis”. The “hygiene hypothesis” was first proposed by Strachan in 1989 suggesting that infections and unhygienic contact with older siblings or through other exposures may confer protection from the development of allergic illnesses. Since 1989, new angles and aspects of the hygiene hypothesis have been proposed. These can be deduced down to three distinct claims on the proper nature of the hygiene hypothesis:
Progress in epidemiological studies over the last decades has clearly shown that the hygiene hypothesis is not one single straightforward idea, but rather a complex interaction of many factors. These include different phenotypes, timing of the exposure, various environmental exposures, and a subject’s genetic susceptibility to react to these exposures.
To date, no specific recommendations can be made on exposure to indoor environmental allergens, such as house dust mite and pets, due to insufficient evidence.
In an attempt to provide exposure to “protective bacteria,” the use of probiotics has also been addressed in atopic patients. Probiotics are defined as live microorganisms that, when administered in adequate amounts, confer a health benefit on the host, including the gastrointestinal tract. While this beneficial effect was originally thought to stem from improvements in the intestinal microbial balance, there is now substantial evidence that probiotics can also provide benefits by modulating immune functions. To date, the results of five randomized placebocontrolled clinical trials of probiotics in the primary prevention of allergic disease have been published. All of these trials enrolled infants at high risk of developing allergy. In all studies except one, the mothers received the supplement for the last weeks of pregnancy. After that, the study design becomes more variable, with the supplement being given to the lactating mother for varying periods, or to the infant directly until 6 months postnatally. Only one study supplemented until 12 months postnatally. Assessment of allergic manifestations occurred at various time points during the first and/or second year of life. In 2001, Kalliomäki et al. reported that Lactobacillus rhamnosus GG (LGG) supplementation of pregnant women and later their infants resulted in a 50% reduction in the risk of eczema by the age of 2 years. This protective effect was maintained at the ages of 4 and 7 years. However, others were unable to confirm a preventative effect of LGG supplementation, and the results of primary prevention trials using other probiotics are variable and not as impressive as those obtained by Kalliomäki et al. In the trials that examined the risk of other manifestations of allergic disease, probiotic supplementation did not show significant effects on allergic rhinitis, food hypersensitivity, or asthma. In three of the trials, the rates of sensitization also did not differ significantly between the probiotic and the placebo group. Supplementation with L. reuteri ATCC 55730 lowered the incidence of skin prick reactivity in infants with mothers with allergies. In contrast, ingestion of L. acidophilus LAVRI for the first 6 months of life raised the rate of sensitization to a variety of food and inhalant allergens tested. In addition, this and the recent LGG trial showed a significantly greater frequency of wheezing or wheezing bronchitis in the probiotic compared with the placebo groups. Of note, Kalliomäki et al. reported that the risk of both allergic rhinitis and asthma showed a tendency to be increased in the probiotic (LGG) compared with the placebo group at the 7-year follow-up. This makes it advisable to exert great caution in the use of probiotics in the prevention of allergic disease.
It has also been proposed that early childhood and maternal diet may be important in the development of both childhood and adult diseases. After an extensive review of the literature in 2008, The American Academy Of Pediatrics brought to light the following recommendations and findings (taken from Greer et al. Effects of Early Nutritional Interventions on the Development of Atopic Disease in Infants and Children: The Role of Maternal Dietary Restriction, Breastfeeding, Timing of Introduction of Complementary Foods, and Hydrolyzed Formulas. Pediatrics 2008):
These recommendations are the strongest we have to date on the possible prevention of atopic diseases. A significant amount of research is still needed in this area to clarify issues such as indoor and outdoor environmental exposure at a young age.