Can Probiotics Prevent Allergies?
A Review of Studies on Atopic Disease Risk
CONTEXT
In 1989, David Strachan, PhD, introduced the “hygiene hypothesis.” Dr. Strachan was a British professor of epidemiology, and his hypothesis was based on his findings that children living in large households with older siblings had fewer instances of hay fever and eczema than children without siblings. Dr. Strachan proposed that the reason might be higher exposure to germs in the larger families.
The hygiene hypothesis states that early exposure to microbes is an essential step in training the immune system. Without this exposure, the immune system never learns to respond appropriately. In essence, cleanliness leads to an increased risk of the immune disruption we see in allergies and other atopic diseases.
Since Dr. Strachan published his research, additional studies have confirmed the hygiene hypothesis. Children in the developing world experience fewer allergies than children in developed countries. And children growing up on farms and who are exposed to animals have fewer allergies than those growing up isolated from animals.
In the wake of the hygiene hypothesis comes the rapidly evolving science of the human microbiome. Like the germs that children are exposed to from family members or pets, the trillions of bacteria and fungi that inhabit the gut and other surfaces of the human body are instrumental in training the immune system.
Studies have found that the human microbiome interacts with the immune system in complex ways. Some studies suggest that beneficial bacteria help to shift the balance of Th1 to Th2 cells, increase interleukin-1, or support the production of T-regulatory cells.
The potential for beneficial microbes to help prime or balance the immune response has led researchers to consider probiotics as a strategy to decrease the risk of allergies and atopic diseases.
OBJECTIVE
This article examined the role of probiotics in reducing the risk of eczema, asthma, allergic rhinitis, and food allergies in children.
STUDY DESIGN
Narrative review
KEY FINDINGS
Eczema. The article summarized three systematic reviews of probiotics for the prevention of eczema.
The first review included 17 randomized, controlled trials and 4,755 children who were at high risk for allergic diseases. Mothers took probiotics during pregnancy or beginning in the first week of infancy, and infants also received probiotics. Compared with the children in a control group, the children in the probiotics group developed less eczema (28 percent versus 36 percent). The analysis showed that 13 children needed to be given probiotics for one to benefit.
The second review found that babies had a 28 percent reduced risk of eczema if their mothers took probiotics during pregnancy. Probiotics taken during breastfeeding were even more beneficial, reducing the risk by 39 percent. Giving probiotics directly to the babies offered some benefit, with a 19 percent reduced risk of eczema compared with placebo.
The third review included 27 randomized, controlled trials and one controlled cohort study. The conclusion was that prenatal and postnatal probiotics both helped reduce eczema in infants and children by about 33 percent.
Asthma and allergic rhinitis. Only one meta-analysis of probiotics for the prevention of asthma was cited. It concluded that probiotics in children had no effect on the risk of asthma.
There was also minimal information on probiotics for allergic rhinitis. One study found that children who had been given prenatal and infant diet supplementation with probiotics experienced the same lifetime prevalence of allergic rhinitis as those who hadn’t received probiotics. Surprisingly, between the ages of 5 and 10, the children who had been in the probiotics group as infants experienced more allergies than those in the placebo group (37 percent versus 29 percent).
Food allergies. A 2016 meta-analysis of 17 different trials reported that a combined approach of prenatal probiotics, and then probiotics after the baby was born, showed some benefit for reducing the risk of food allergies. However, there was no reduction in risk when probiotics were given either prenatally or postnatally alone.
In children with an allergy to cow’s milk, one study compared the use of extensively hydrolyzed casein formula (EHCF) alone or combined with the probiotic L. rhamnosus GG (LGG). The researchers found that the addition of the probiotic to the EHCF decreased the incidence of allergic conditions (including asthma, eczema, and allergic rhino-conjunctivitis) over three years. The number of children needed to be treated to see a beneficial effect was four.
CLINICAL RELEVANCE
This is a recent and comprehensive review of probiotics as a strategy to reduce the risk of atopic diseases. There are more data to support a role for probiotics in the prevention of eczema than any of the other allergic diseases.
There is little to no evidence that probiotics are useful for preventing asthma or allergic rhinitis. However, when given to children with a milk allergy, probiotics were found to be quite effective at reducing the risk of other allergic manifestations.
Current guidelines from most medical societies, including the American Academy of Pediatrics and the National Institute of Allergy and Infectious Disease, do not recommend the use of probiotics for the primary prevention of allergic disease. The World Allergy Organization (WAO) agrees, except for high-risk infants. In this specific population, WAO favors probiotics to reduce the risk of eczema.