What prompted your research into food allergies in young inner-city children?
The general interest is in trying to better define the true prevalence of food allergy; not just how common it is, but whether it's becoming more common over time.
This specific study was prompted by an opportunity that arose out of research we've been conducting on asthma in young, inner-city children.
Over the course of that study, we've been collecting information about food allergies so this is using information from a study that wasn't specifically designed to look at food allergy to address this question in a group of kids that have been very well-characterized since before they were born.
What percentage of children in the cities you studied were found to have a food allergy? How does this compare to children who don't live in cities?
At least 9.9% of children, so almost 10%, had a very convincing diagnosis of food allergy.
We say at least because we actually only studied the prevalence of three common food allergies: milk, egg and peanut. Those are the allergies we had complete data sets on.
There were certainly some children in the cohort who were allergic to other foods and, if we had full data on all foods for all children, the true prevalence would have been above 10%— probably closer to 15%.
In other studies, prevalence rates as high as 8% have been reported but, in most developed countries, they are generally 5-6% in the under-five age group.
Were you surprised by these findings?
We were and we were not. We went into it without any real expectations.
We were not surprised because this was, what we refer to as, a higher-risk birth cohort. All of these children did have a family history of allergy or asthma.
We were not surprised because other allergies and asthma have been defined as more common in children growing up in these urban areas, in cities in the United States at least.
On the other hand, in our clinical population and in some other studies, food allergy has been more associated with higher economic status and less so with children growing up in disadvantaged economic situations.
We went into it not really knowing what we would find and I wouldn't have been surprised if it was lower and I'm not surprised it was higher; we went into it with a very open mind.
What do you think are the reasons behind the high prevalence of food allergies in this group?
We would suppose that they're probably similar to the reasons why there are markedly higher rates of asthma and other allergies in children growing up in these environments. The exact reasons for those are not clear yet and are why this cohort was funded a number of years ago by the NIH here in the United States.
We had another paper, which came out two or three months ago, that looked at the same population of patients with wheezing and probable asthma as the outcome. In that paper, we found something that was completely opposite of what we expected: children who had been exposed to higher levels of certain allergens, especially allergens that are prevalent in the inner city such as mouse and cockroach, were more protected against having wheezing.
We found that exposure to high levels of certain bacteria was also protective. In this study, we didn't have the same opportunity to look at the specific bacteria but we did look at a substance called endotoxin, which is a by-product of bacteria.
We looked at this in house dust from the homes of these children and found similar results: high levels of endotoxin in the home seemed to protect against having a food allergy.
This brings us to the hygiene hypothesis, which says that, if we live in too clean of an environment, it may make us more prone to developing an allergy.
We looked at a lot of other factors we suspected could be of importance. In some other studies, we found that high levels of stress were associated with high levels of asthma and allergy. We thought that children in the inner city were possibly being fed differently; in our clinical population, we see them being introduced to a wider variety of foods early in life so we looked at that.
We looked at things like tobacco smoke and pollution exposure and none of those actually panned out to give us any sign of a relationship with food allergy.
What further research is needed to understand the causes of food allergies in young inner-city children?
This is a very important question, not just for inner city children but children in general. The ideal would be that we can come away from this study, and other studies like it, with actual strategies that we could use to try to prevent the development of food allergy.
Our dream is that, at some point, we will have identifiable factors that we can intervene upon to find ways of preventing the development of food allergy. This really requires further research into the causes and exploration of whether those causes can be reversed by treatment.
Is it true that food allergies in all children have been rising over the last couple of decades?
We believe that they have. Probably not in all children around the world, but children in developed and industrialized countries.
The studies are all consistent in their findings that food allergy has at least doubled, possibly tripled, over the last 20 years—possibly in as little as the last 15 years. We do believe that's true.
The studies aren't perfect but they have given the same general sense that there has been a significant rise in a short period of time.
What do you think are the reasons for this increase?
It goes back to the reasons why children get food allergy and, again, we don't really know. There are a dozen or so theories that are being investigated and a couple of them seem to have some evidence to support them: the hygiene hypothesis is one that has largely been supported by the research.
We think there are likely some nutritional factors that have changed over the last 20 years, and there are two that are of particular interest.
Vitamin D deficiency seems to be associated with an increased risk of allergy and we do know that vitamin D deficiency is more common now than it was 20 years ago, since we get most of our vitamin D from sunlight. As people stay indoors more, and wear sunscreen when they're outdoors, we think that could be an issue.
The other is that getting too much folic acid in your diet might actually increase your chances of developing a food allergy. As far as the relationship over the last 20 years, it was approximately 20 years ago that we began supplementing pregnant women with high levels of folic acid in prenatal vitamins. Those are some ideas.
There are a lot of other concerns: the way foods are being processed or manufactured, for example. Some don't have data to support them, like those relating to immunizations and antibiotics. There is a lot to be learned and it's most likely a number of factors—certainly not a single factor—that's led to this rise.
Your research found that breastfed children appeared to have a higher risk for developing food allergies. Were you surprised by this finding?
No. It's something that has been found in both directions in previous studies, although there are more studies that suggest breast feeding is a risk factor for food allergy.
The notion that's still widely held, that breast feeding helps prevent food allergy, is really not the case. This was not a surprising finding.
Do we understand the reasons behind this association?
No, we don't. There have been theories about how you're exposed to food proteins early in life and how that may influence the development of food allergy.
Some people have hypothesized that it's actually exposure to certain foods through breast milk that may stimulate some babies to develop food allergy, but the true reasons are unclear.
What are your thoughts on the hygiene hypothesis and did your research alter your views in any way?
My thoughts have been that the hygiene hypothesis does have some truth to it and that it is a very good explanation for why asthma and allergies may be far less common in underdeveloped countries.
In the study I mentioned earlier, the one that came out two months ago, we've done a lot of research on this inner-city environment and one of our feelings was that the inner city actually contradicted the hygiene hypothesis, in that this is an environment that is not very clean combined with the fact that there are very high rates of asthma and allergy in the United States.
For example, the rate of asthma is twice as high, three times as high in some studies, in inner-city children as it is in suburban children, so we argued that the hygiene hypothesis couldn't be true because the inner city has such high rates of asthma and allergy.
Most of these studies show that the same kind of factors appear to be present in the inner city: some homes are rich in bacteria and seem to protect against allergy, while some homes aren't necessarily clean but don't have the right bacterial exposure, and the children who live in those are more prone to developing asthma and allergy. This is, to some degree, a game changer to say that even the inner city supports the hygiene hypothesis.
Where can readers find more information?
There are some very good patient organizations in the United States. There's one called FARE: Food Allergy Research & Education.
There are similar organizations in England and other countries, so I think that's a really good starting point for patients.
About Dr. Robert Wood
Robert A. Wood, MD is Professor of Pediatrics and Chief of Pediatric Allergy and Immunology at the Johns Hopkins University School of Medicine, and Professor of International Health at the Johns Hopkins Bloomberg School of Public Health.
After receiving his medical degree from the University of Rochester School of Medicine, he completed his residency in pediatrics at the Johns Hopkins University, where he also served as chief resident in pediatrics and completed his allergy and immunology fellowship.
Dr. Wood is recognized internationally as an expert in both food allergy and childhood asthma and is a principal investigator in both the Consortium of Food Allergy Research and the Inner City Asthma consortium.
His current research focuses on novel treatment approaches for both food allergy and asthma, for which he is currently principal investigator for over 20 ongoing studies.
He has published over 200 manuscripts and book chapters as well as three books. He has served on numerous editorial boards as well as the American Board of Allergy and Immunology and the American Academy of Allergy Asthma and Immunology, and is currently on the Board of Directors of the American Board of Pediatrics.