Some reactions to peanuts can lead to anaphylaxis, where breathing becomes difficult.
Now, a UK team that carried out a study in mice, has found roasted peanuts are more likely to trigger an allergy to peanuts than raw peanuts and suggests dry-roasting produces chemicals that sensitize the immune system to both dry roasted and raw peanuts.
The study is published in the journal Allergy and Clinical Immunology.
According to Allergy UK, allergy to peanut and tree nuts is the most common food allergy, and allergy to peanuts affects around 1 in 50 young children.
Most allergic reactions to peanuts and tree nuts are mild, but some can be severe and can lead to anaphylaxis, where breathing becomes difficult due to asthma-like symptoms or throat swelling, and blood pressure can also drop.
High-temperature roasting of peanuts changes their proteins
Children are more likely to develop allergy to peanuts if they already have a known allergy such as eczema or an intolerance to another food, or if their immediate family has a history of allergies such as asthma, eczema or hay fever. However, allergy to peanuts can also involve a trigger of some sort, as first author Dr. Amin Moghaddam, a senior postdoctoral research scientist at Oxford University says:
"Allergies in people are driven by multiple factors including family genetic background and exposure to environmental triggers. In the case of peanut allergy, we think we may have discovered an environmental trigger in the way that peanuts are processed by high-temperature roasting."
There is already evidence that roasting peanuts changes their proteins, in turn changing the way the immune system recognizes them. But until this study, we did not know that those changes can trigger an allergic response.
For their study, Dr. Moghaddam and colleagues exposed groups of mice to purified proteins from raw peanuts or dry-roasted peanuts - for instance by applying to broken skin or via skin injection, or directly into the stomach.
They then measured the immune responses in the mice to further peanut extracts given later.
The mice previously exposed to dry-roasted peanut proteins had a much stronger immune reaction to peanut extracts than mice that had only previously been exposed to raw peanut proteins.
First direct demonstration of a potential trigger for peanut allergy
Senior author Quentin Sattentau, a professor of immunology at Oxford, says:
"This is the first time, to our knowledge, that a potential trigger for peanut allergy has been directly shown."
But he adds the research is still at an early stage, and it would be premature to urge people to avoid roasted peanuts until the findings have been more robustly confirmed.
He says he and his team have identified the chemical reactions that occur in dry roasting proteins that trigger allergic reactions, and they are now looking for ways the food industry can eliminate the proteins.
Dry roasting involves heating the peanuts to temperatures of 160 to 170°C and higher. Above 130°C, peanuts undergo a reaction that changes the chemical composition of specific groups of proteins. The team suggests it is these products that trigger the strong immune reaction.
Funds for the study came from the National Institute for Health Research (NIHR) Oxford Biomedical Research Centre, the US National Institutes of Health and the Swiss National Science Foundation.
Meanwhile, in January 2014, a trial conducted by another UK team showed exposure to peanuts builds immunity in allergic children if they consume increasingly larger amounts of peanut protein on a regular basis.
The researchers in that study strongly urged people not to try it at home and reminded parents and child carers that currently, the only way for children allergic to peanuts to avoid severe reactions is for them to completely avoid foods that contain them.