Nonimmunological mechanisms

Enzymatic mechanisms

Enzymatic food intolerance is due to an enzyme defect which could result from an inborn error of metabolism or could be secondary to a number of disorders. The most common food intolerances in this category are disaccharide deficiencies, galactosemia and phenylketonuria. Amongst disaccharide deficiencies, lactose intolerance is the most common. Lactose deficiency can be congenital, persisting in the neonatal period, or can be acquired where it presents later.19 These deficiencies are genetically based and not due to an environmental factor. Secondary lactase deficiency often occurs following an episode of gastroenteritis.

Galactosemia is also a form of carbohydrate deficiency.20 Here the genetic imbalance is expressed as a deficiency of either galactokinase, galactose 1-phosphate uridyl-transferase or uridine diphosphate galactose 4-epimerase. These enzymes are responsible for converting galactose to glucose. So in individuals with this deficiency this process fails. The clinical manifestations are toxicity syndromes when exposed to galactose. These include failure to thrive, vomiting and liver disease.

Phenylketonuria is caused by a gene mutation which suppresses the activity of phenylalanine hydroxylase enzyme.21 Approximately 4-5% of amino acids in all food protein are phenylalanine. Restriction of phenylalanine intake to 0.4 mmol/l throughout life ensures almost normal physical and mental development.

Pharmacological mechanisms

Many foods contain pharmacologically active components. A pharmacological food intolerance is usually evident soon after eating the food responsible. The amount of food ingested to elicit a reaction varies from person to person and may even vary in the same individual over time. The pharmacological components can either initiate a reaction directly themselves or activate the host's mediator system indirectly and hence induce a reaction.

The largest class of substances that are found in many foods responsible for inducing pharmacological food intolerance are vasoactive amines. Other substances involved are methylxanthines, capsaicin and ethanol. Vasoactive amines include histamine, tyramine, tryptamine and serotonin. Foods such as tuna, cheese (in particular Parmesan and Roquefort), yeast extracts such as Marmite, and red wine such as Burgundy and Chianti are rich in these amines. Ingestion of large amounts of these foods can be followed by toxic symptoms.

The best example is scombroid poisoning, due to an excessive amount of histamine in some species of fish such as tuna and mackerel.22 Because histamine is a mediator released from mast cells in food allergy, sometimes pharmacological food intolerance due to histamine is confused with allergic-type reactions. In scombroid poisoning, when fish is inadequately refrigerated, marine bacteria convert the amino acid histidine to histamine. This will generate a histamine concentration greater than the body's normal capacity to metabolise and hence the individual will suffer from the full spectrum of histamine effects, including flushes, vomiting and diarrhoea.

Some foods that do not have a high histamine content themselves contain compounds that can indirectly induce degranulation of mast cells and histamine release. These foods include chocolate, ethanol, tomatoes and crustaceans.

Vasoactive amines such as tyramine and serotonin are found mainly in fermented food.

Food Allergies

Food Allergies

Peanuts can leave you breathless. Cat dander can lead to itchy eyes, a stuffy nose, coughing and sneezing. And most of us have suffered through those seasonal allergies with horrible pollen counts. Learn more...

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