In a bid to discover why rates of reported childhood food allergy have risen in recent decades, scientists compared children's blood samples from 20-30 years ago to some taken 10 years ago and were surprised to find no evidence that rates of sensitivity to foods have gone up.
The researchers say their findings suggest either that parents and medical professionals have got better at recognizing and diagnosing food allergies, or that the relationship between allergy markers in the blood and food allergy symptoms has changed.
The study, led by Johns Hopkins University School of Medicine, Baltimore, MD, is published in The Journal of Allergy and Clinical Immunology: In Practice.
Since the 1990s, studies have shown that the proportion of children in the United States found to have one or more food allergies has risen by at least 50 percent.
But it is not clear why allergies have increased, and many studies have tried to find out what might be causing this apparent epidemic.
Today, it is thought around 5 percent of American children are allergic to one or more foods - the most common being milk, eggs, peanuts, and shellfish.
The researchers wondered if the increase in reports of children with food allergies is matched by an increase in the proportion of children who have evidence of food allergy - in the form of antibodies called immunoglobulin E (IgEs) - in their blood.
An allergic reaction occurs when the body comes into contact with a substance - an allergen - that provokes an overreaction in the immune system. The immune system "identifies" the allergen by producing IgEs to instruct immune cells to release chemicals. There are different IgEs for different allergens.
No increase in prevalence of allergy markers for peanuts, milk, or eggs
To answer their question, the team analyzed blood samples and data from nearly 8,000 children aged 6-19 years who took part in the National Health and Nutrition Examination Survey (NHANES).
They compared the food-specific IgE content of blood samples taken in the period 1988-1994 to samples taken during 2005-2006.
The samples taken in the earlier period had been stored and were analyzed for the study. The samples taken in the later period were analyzed at the time.
To their surprise, the researchers found no increase - between the two periods - in the proportion of children sensitized to peanuts, milk, or eggs, while the proportion with antibodies to shrimp actually decreased.
If the reported prevalence of food allergies consisted just of self-reports, then there might be a case for saying something has happened to increase self-reporting - but not necessarily actual prevalence - of food allergies.
But, as senior author Corinne A. Keet, an associate professor of pediatrics from Johns Hopkins, notes:
"It hasn't only been self-reported food allergy rates that have risen but emergency room visits and hospitalizations for food allergies as well."
Could raised awareness be a reason?
One possible explanation for the findings, say the researchers, is that parents and health professionals are more aware of food allergies today than they were a few decades ago.
In the past, perhaps people just avoided food that made them unwell but did not call it an allergy, whereas today, if a child has a rash or other symptom after eating a certain food, we are quick to suspect allergy as a cause.
However, the team also mentions another possibility. Although scientists know that food-specific IgE antibodies are necessary to cause the extreme immune response seen in allergic reactions, many people with IgE antibodies to certain foods can still eat those foods without experiencing allergic reactions.
First author Emily McGowan, assistant professor of medicine at the Johns Hopkins, notes:
"These results raise the question of whether something has changed in the relationship between food-specific IgE and clinical food allergy over the past few decades."
She refers to a recent study that found introducing peanuts early to babies prevented most peanut allergies - but did not necessarily reduce the proportion that developed IgE antibodies to peanuts. Prof. McGowan concludes:
"We don't really understand all of the reasons why one person with IgE to a food will have serious reactions to the food, while another can eat it without problems. This is an area that needs more research."
Prof. Keet emphasizes the need to find out what the reason could be, because if sensitization has not changed, then looking for environmental factors that affect sensitization may not explain the rise in food allergy.