In fish, histamine is believed to stem from bacterial contamination, particularly in fish that hasn't been refrigerated properly. If someone eats one of these foods with a high level of histamine, that person may have a reaction that strongly resembles an allergic reaction to food. This reaction is called histamine toxicity. Another cause of food intolerance that is often confused with a food allergy is lactase deficiency. This most common food intolerance affects at least one out of ten people.
Lactase is an enzyme that is in the lining of the gut.
This enzyme degrades lactose, which is in milk. If a person does not have enough lactase, the body cannot digest the lactose in most milk products. Instead, the lactose is used by bacteria, gas is formed, and the person experiences bloating, abdominal pain, and sometimes diarrhea. Another type of food intolerance is an adverse reaction to certain products that are added to food to enhance taste, provide color, or protect against the growth of microorganisms. Compounds that are most frequently tied to adverse reactions that can be confused with food allergy are yellow dye number 5, monosodium glutamate, and sulfites.