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. An accurate history of a repeatable reaction to a sulphite-containing foodstuff remains the cornerstone of the diagnosis 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.