Food intolerance is a functional or structural intolerance to food, which is non-immunologic and non-toxic in origin, but is chemically based instead. A third type of food intolerance involves the effects produced by pharmacologically active amines and other compounds present in foods.
Thus, food intolerance describes a range of food-related symptoms that are not immunologically-mediated. These account for the majority of food reactions.
Functional food intolerance is not associated with structural abnormalities of the digestive tract, but may be due to an isolated enzyme deficiency. A prime example of functional food intolerance is lactose intolerance.
Carbohydrate intolerances involve the inability to digest and absorb sorbitol, fructose or other simple sugars. Lactase deficiency is the best known and is the cause of intolerance to milk and other dairy products, which contain lactose, also known as milk sugar.
Other examples include the isolated or combined disaccharide malabsorption syndrome, which prevents the absorption of sucrose, lactose and other disaccharides, either one in particular or all together. In addition to this, there is isolated maltase, galactase or saccharase deficiency.
When these sugars are not absorbed properly, they reach the large intestine in increased amounts, resulting in excessive fermentation and bacterial overgrowth. The production of osmotic sugars and their fermentation products in larger quantities causes symptoms, such as meteorism (i.e. the accumulation of excess gas in the gastrointestinal tract), flatulence and diarrhea, as described below.
Diamine oxidase deficiency may cause histamine intolerance.
Other than enzyme deficiencies, transporter molecule defects may be present. These include defects in GLUT 5, which transports fructose, or GLUT 2 for glucose, galactose or fructose. The inability to transport these carbohydrates into the intestinal cells results in their metabolism in the large intestine where they are transported intact in large quantities. Bacterial fermentation of these sugars produces short-chain fatty acids, methane, hydrogen and carbon dioxide. These gases and decomposition products lead to abdominal pain, diarrhea, flatulence and bloating.
Patients who are at risk for bacterial overgrowth include those who have been surgically treated by bowel resection, or are on chronic treatment with proton pump inhibitors, such as ranitidine or omeprazole. This leads to symptoms of food intolerance following ingestion of a number of foods, due to the release of excessive bacterial decomposition products in the gut.
Many foods contain compounds with intrinsic biologic activity, such as:
Other foods release histamine, such as citrus fruits or nuts. Black tea and some colorants actually inhibit diamine oxidase. In addition, factors that increase histamine release may precipitate intolerance, such as sports activity, some infections or emotional upsets.
Some substances evoke individual intolerances, such as:
Some intolerance-like symptoms are due to:
Structural intolerance is due to abnormal structure of the digestive tract; for example, this occurs with intestinal diverticula. Here food is trapped within the outpouchings with resulting bacterial overgrowth. The production of abnormal fermentation products results in secondary gastrointestinal symptoms, such as bloating and diarrhea.
Examples of structural abnormalities of one or more parts of the digestive tract include: