One of the most exciting potential applications of probiotics is in the area of allergy. The need is great for safe interventions targeting allergy prevention that lasts or treatment of symptoms. But studies showing that probiotics can improve allergy symptoms or reduce incidence of childhood allergies beyond infant eczema is lacking.
The strongest evidence for probiotics is primary prevention of eczema in infants given probiotics several months after birth and whose mothers were exposed to probiotics 2-4 weeks prior to birth. A 2015 meta-analysis concluded that “Probiotics used by pregnant women or breast-feeding mothers and/or given to infants reduced the risk of eczema in infants; however, the certainty in the evidence is low. No effect was observed for the prevention of other allergic conditions.” When study participants were followed for up to 10 years of age, researchers found that protection did not extend to other measured expressions of allergy, such as asthma or food allergies.
A 2016 review by Zhang et al. took a different look at these studies. They looked at probiotic intervention trials that examined food hypersensitivity and concluded they were underpowered with regard to this endpoint. So they pooled results of multiple studies to provide insight into the relationship of early probiotic exposure and food hypersensitivity. Their systematic review found 17 randomized, double-blinded, placebo-controlled studies that assessed the impact of probiotics on prevention of atopic sensitization, as defined by positive skin prick test and/or elevated immunoglobulin E levels to tested allergens. Nine of these also reported on food hypersensitivity.
Atopic sensitization is a less interesting outcome as it is a biomarker of allergy, but not an actual measure of allergy. In these studies, atopic sensitization was measured by the skin prick test. But to be clear, it is not the same as allergy. In fact, in only about 40-50% of the time, does a positive skin prick test associate with a food allergy.
The analysis of all studies pooled together found that probiotics did not have an impact on atopic sensitization or food hypersensitivity. But in both cases, when studies where probiotics were administered both prenatally and postnatally were separated out from studies where probiotics were administered only postnatally to the infant or only prenatally to the pregnant mother, an impact of probiotics was found on both food hypersensitivity and atopic sensitization.
The authors conclude, “Probiotics administered prenatally and postnatally appears to be a feasible way to prevent atopy and food hypersensitivity in young children.”
As with the 2015 meta-analysis mentioned above, Zhang et al. concluded that due to study quality issues, additional, well-controlled trials are needed to increase confidence in these conclusions. It is unfortunate that many of the quality issues uncovered were due to ‘unclear risk of bias’ in different categories. This may be due to improper conduct of the study. Or it may be due to failure to make quality issues clear in the published paper, which is a regrettable and ultimately wasteful occurrence.
In short, we need more research. But the Zhang study suggests that early exposure of the pregnant mother and the newborn infant to probiotics might be useful for allergy prevention later in life.