Do you have a food allergy? If you do, you're not alone. The prevalence of food allergies seems to be growing. But so is the number of people who believe they have one but who really don't.

A recent survey of Americans has found that more than 10 per cent of people there have a true food allergy. On top of that, though, close to 20 per cent believe they do. Spot that gap?

Genuine food allergy rates are climbing worldwide for reasons that are not well understood. Experts suggest links with genetic factors - children of parents with allergies are at higher risk – but, according to Allergy New Zealand, it's also generally acknowledged that environmental factors "associated with a westernised lifestyle" are driving this epidemic.

Factors include "the hygiene hypothesis" (the idea that we're too clean for our own good); lifestyle changes leading to lack of Vitamin D; pollutants; and the effect of stress on the immune system.

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Whatever the causes, it's clear two things are happening: more people than in the past are suffering, and there's a lot of people who believe they're allergic to a food, who probably are not.

In New Zealand we know that up to 10 per cent of children have a food allergy, and between 2 and 4 per cent of adults. That difference reflects the fact that children often outgrow food allergies as they grow up.

But adults can also develop them. According to the new research, certain allergies such as shellfish may be more likely to develop during adulthood. Researchers found nearly half of food-allergic adults were allergic to more than one food, and the same number had at least one adult-onset food allergy. We don't know if this is true for New Zealand, but it's not an unreasonable assumption to make.

So what of all those people who wrongly believe they are sufferers?

It's possible they may be suffering from intolerance or some other reaction to foods. But an allergy is a severe immune reaction, and tends to produce severe (and sometimes life-threatening) symptoms.

In the US survey, a true allergy was recorded if the person had one or more of these symptoms: hives, lip or tongue swelling, difficulty swallowing, throat tightening, chest tightening, trouble breathing, wheezing, vomiting, chest pain, rapid heartbeat, fainting or low blood pressure.

Feeling nauseous or having diarrhoea is not in the same category.

The survey didn't answer why so many people wrongly believe they're allergic, but we can make some guesses. Self-diagnosis – maybe based on Doctor Google – is one. So is shonky diagnosis, maybe by sending off a hair sample or visiting a naturopath. These are not scientifically accurate methods of diagnosis, and will probably only end up costing money and stress.

If you think you or your child may be allergic to foods, the best and only place to start is at the doctor.

*Niki Bezzant is editor-at-large for Healthy Food Guide www.healthyfood.co.nz.