There's no one reason for the increase, says Stephan Meller, an allergist and senior doctor at Dusseldorf University Clinic.

Contributors include the environment, food and habits that might mean a person typically avoids something.

Since 2008, the German Allergy and Asthma Association (DAAB) has been marking the illness - which affects hundreds of thousands of people and for which there is no cure - with a food allergy awareness day on June 21.

"The only thing that works is for the patient to know what he or she is allergic to and to avoid that foodstuff," says DAAB's Sonja Laemmel.

That means the ingredient that causes the reaction must be clearly marked on packaging or on labels for loose products in restaurants and bakeries, which is not always the case.

In Germany, stricter rules have been in force since December: ingredients that could potentially cause a reaction have to be written in bold or underlined on packaging labels.

That includes gluten, milk, eggs, fish, nuts, soya, celery and sulfite.

There's still a problem with products that are sold loose however: sausages from the butcher and rolls from the baker should also be labelled according to the December regulation, but this hasn't necessarily been happening. Customers have to double-check.

"It's a catastrophe for people who are allergic," says Andrea Wallrafen, the director of DAAB.

Allergic reactions to food can be confusingly diverse. They range from itching, redness and welts to sneezing fits and runny noses, coughing and breathlessness as well as diarrhoea, vomiting and nausea.

The worst form of allergic reaction is anaphylactic shock, which can be fatal.

People's allergies vary according to age: for babies, cow's milk and chicken's eggs are the most common culprit.

Nuts, fish and wheat are often problems for children.

Teenagers and adults react more often to raw fruit and vegetables, spices and nuts. In 60 per cent of cases, people are also allergic to pollen.

There is a particularly mysterious form of wheat allergy that tends to appear only in combination with exercise.

It emerged around three years ago and since then has become more common, says Laemmel.

Exercise, alcohol and certain medicines can act as triggers and young people are worst affected, Laemmel said.

Meller sees such cases around once a month. "Somebody eats a bread roll and then does some sport. It can cause an extreme reaction," he says.

The phenomenon has been dubbed Wheat Dependent Exercise Induced Anaphylaxis, or WDEIA for short.

In these kinds of cases the doctor quite often has to convince the patient that he or she is allergic to wheat. "No, I was fine with it yesterday," Meller often hears.

"It's a puzzle," he says.

And an increasingly common one.

AAP