The term “food intolerance” is used to denote reactions to food which do not involve a known immune mechanism.

Sulphites

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Sulphur dioxide and other sulphites are inexpensive preservatives, very effective, and widely used.  They are both antioxidants which prevent browning of fresh fruits, and preservatives with a broad spectrum antimicrobial action as used in soft-drinks, wine and maize milling.  Sulphite preservatives are listed in Table 1.

Table 1: Sulphite Preservatives

a.) Incidence

Sulphites appear to affect mainly asthmatics, and children more than adults.  Approximately 10% of American adult asthmatics are sensitive to sulphites.  Forty three percent of children with asthma attending the Allergy Clinic at Red Cross Hospital reacted to a sulphite challenge with a fall in forced expiratory volume in 1 second (FEV₁) of 10% or more and 21% with a fall in FEV₁ of more than 20%.  Significantly, many of these patients were asymptomatic.  The sulphite challenge dose was approximately equivalent to that used in 250 mls of a common sulphite containing soft-drink.  Of interest is that more girls were affected (64.3%) than boys (30.4%).  No prevalence studies have been done on normal subjects, but of all severe sulphite reactions reported to the FDA approximately 30% occurred in non-asthmatics.

b.) Sources of Sulphite Exposure

These include industrial air pollution, foodstuffs, patient medications and cleaning agents.  Sulphite sensitive asthmatics are acutely sensitive to very small amounts of inhaled sulphite.  Severe reactions have been recorded to minute amounts found in air pollution and inhaler medications.  A sulphite sensitive nurse developed acute bronchospasm walking into a room where patients were being nebulized with a sulphite preserved bronchodilator.  These patients can often tolerate a greater concentration of ingested sulphite.

Many foodstuffs may contain sulphite preservatives.  The most important are soft-drinks, dried fruits, cold meats, wine and beer.  Some restaurants may keep their salads fresh with a sulphite preservative.

Table2:
 Possible sources of sulphites in foodstuffs

c.) Symptoms and Signs

Many symptoms and signs can be seen following ingestion of sulphites.  Reactions can be as mild as a throat irritation, to life-threatening bronchospasm and/or shock.  Coughing and/or bronchospasm are the most common reactions seen.  Other reactions seen include urticaria and exacerbation of eczema.  A variability of response occurs among sensitive patients depending on their degree of sensitivity to the sulphites and whether the sulphite is in a bound or unbound form.  Time to onset of symptoms following ingestion of a sulphite preservative can vary from 5 mins to 30 mins.  Some patients improve spontaneously within 30 minutes, but many will remain unchanged for even longer.  Those who are symptomatic will require immediate treatment.

d.) Pathophysiology

The pathophysiology of sulphite sensitivity is not absolutely clear.  Sulphite-sensitive asthmatics have decreased levels of sulphite oxidase.  This is a enzyme required for metabolism of sulphur containing amino acids, and levels vary in different tissues.  This may explain why greater levels of sulphite can be ingested than inhaled as the lung has lower levels of sulphite oxidase than the liver.  Some researchers postulate that the sulphite is eructed as sulphur dioxide fumes which then stimulate afferent fibres in the respiratory tree.  A very small group of patients are sensitive to sulphite preservatives via an IgE mechanism.

e.) Diagnosis

An accurate history of a repeatable reaction to a sulphite-containing foodstuff remains the cornerstone of the diagnosis of sulphite sensitivity.  This may be complex in mild sulphite-sensitive patients as the concentration of sulphite in foodstuffs may vary over time, and as patients have different levels of sulphite-sensitivity.  Some foods bind sulphite better than others.  Other factors that may influence sulphite levels include slow dissipation of sulphur dioxide through the walls of plastic containers, and decreased levels following cooking.  No in-vitro test is available.  It is probably better to know which patients are sensitive to sulphites than to expose them to chronic subclinical bronchospasm induced by diet.  If sensitivity to sulphite preservatives remains unclear, a sulphite challenge can be performed but this should be performed only in a clinical setting where immediate resuscitative measures can be instituted.  Prudent advice would be for all asthmatics to avoid sulphites rather than perform challenge studies.

f.) Treatment

Avoidance of sulphite preservatives remains the mainstay of treatment.  Food labels should be read carefully.  Sulphite-induced bronchospasm usually responds to a 2-agonist nebulisation.  Steroids appear to have no protective effect.  When eating in restaurants, sulphite-sensitive asthmatics should carry bronchodilator inhalers with them.  Patients with exquisite sensitivity to sulphites should wear a Medic Alert bracelet or disc.

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