What is a food intolerance?
A food intolerance refers to a sensitivity to a food, most commonly a difficulty in digestion. It is important to distinguish this from food allergy, where a person’s immune system reacts to a particular food.
What are the symptoms of a food intolerance?
Most food intolerance involves the digestive system and thus cause symptoms involving the gut: bloating, gas, abdominal pain, nausea, vomiting or diarrhoea. A food intolerance can cause headaches and mimic migraines. Sometimes the person just feels miserable and unwell – very non-specific symptoms.
What are the most common causes of food intolerance?
The main causes of food intolerance are discussed below.
Deficiency or absence of an enzyme
Enzymes are the substances that digest food in the digestive tract. If an enzyme is absent or insufficient, the food cannot be digested properly, as occurs in lactose intolerance. Some people are born without the enzyme lactase, which is required to digest lactose (milk sugar). The majority of cases of lactase deficiency, however, are acquired as a result of diarrhoea or inflammatory bowel disease, and are temporary. If the lactose cannot be broken down into smaller components it remains in the digestive tract and causes upsets such as diarrhoea, bloating, gas and abdominal pain.
Sensitivity to food additives
Food additives are often blamed by people as being responsible for many symptoms, including digestive symptoms. Studies, however, suggest that this is an uncommon cause of reactions to food. An exception is sulphite sensitivity, which may affect asthmatics and cause their chests to become tight. Sulphites are often used as preservatives in food (e.g. dried fruit and processed meats) and drinks (e.g. cordials used as mixers). Sulphites may be also be used as preservatives in medicines.
Reactions to chemicals that are naturally present in foods.
Vasoactive (biogenic) amines are substances that are formed when bacteria act on food. One of these amines is histamine which may mimic allergic reactions to food. A well-known example of a histamine reaction occurs in scromboid poisoning, in which eating fish that is slightly off causes flushing, headache, skin rashes, abdominal cramps, diarrhoea and palpitations. Fish typically associated with this are yellowtail and tuna. The bacteria convert the naturally high levels of a chemical substance, histidine, in these fish species, into histamine.
Foods that are naturally high in vasoactive amines are cured meats, processed and canned fish, cheese (especially blue cheese and Parmesan), wine, beer, coffee, tomatoes, and fruits such as strawberries, pineapples, oranges and grapes.
Caffeine, especially if consumed in large quantities, may cause headache, runny nose, digestive disturbances, anxiety, palpitations and increased blood pressure, and sleep disturbances. It is present in tea, coffee, chocolate and many energy drinks. Lesser known sources of caffeine include vitamins, tonics and combination painkillers.
Salicylates occur naturally in plant foods and are commonly found in higher quantities in spices. Naturally occurring salicylates do not usually cause symptoms, but people who have proven intolerance to aspirin should avoid foods containing high levels of natural salicylates.
Gluten intolerance / sensitivity
The recently described condition of non-coeliac gluten sensitivity (NCGS) consists of symptoms involving the digestive tract as well as non-specific symptoms such as headaches, joint pain, feeling generally unwell, tiredness and depression. The symptoms occur hours to days after eating gluten, and improve with gluten avoidance. It is thought to be more complicated than just ingestion of gluten proteins, and is probably not an intolerance. In fact, there is still uncertainty as to whether the condition exists, but the reported incidence is increasing.
Distinguishing between food intolerance and food allergy
It can be difficult to distinguish between the two conditions, as symptoms may overlap. Food intolerant people may be able to tolerate small amounts of the offending food without developing any symptoms, whereas even a tiny amount of a food causing an allergic reaction may trigger a life-threatening event.
The main clue to distinguishing intolerance from allergy lies in the duration of onset of the symptoms. Generally speaking, symptoms usually only start a few hours or even days after eating the offending food or food substance in food intolerance and can last for many hours or even days. The most common form of food allergy (immediate type food allergy) results in rapid onset of symptoms.
Diagnosis of food intolerance
The history and the pattern of the symptoms may help the doctor to decide whether a person has a food intolerance or food allergy, but sometimes this is not clear. There are no definitive tests to identify food intolerance, except for lactose intolerance, and the best tool is an exclusion or elimination diet. If a food is suspected of causing a reaction, it can be removed from the diet for 2-4 weeks. If the symptoms improve or resolve, it suggests that this is the offending food. If the food is reintroduced into the diet and the symptoms recur, it confirms the diagnosis.
The CAST test is a blood test that can aid in diagnosing sulphite and salicylate sensitivity.
Immediate type food allergy is diagnosed by an allergen skin prick test or a blood test which measures the specific immunoglobulin E (IgE) antibody levels to a food, followed by a food challenge test under specialist supervision in a clinical setting if indicated.
Beware of these tests!
The IgG or IgG4 test runs a panel of about 100 foods and is advertised to the public as being diagnostic for delayed food allergy or food intolerance. There is no scientific evidence to support these tests and people should avoid them.
How is food intolerance treated?
The treatment for food intolerance is to avoid the culprit food or to eat it in smaller amounts or less frequently. This should be done under the supervision of a dietician as highly restrictive diets can lead to malnutrition, especially in children. Lactose intolerance in babies is treated by switching to a non-lactose containing milk formula, usually only for 1-2 weeks.
- Gray C, Steinman H. Food intolerances. In: Manjra A, Levin M, Gray C (Eds). ALLSA Handbook of Practical Allergy. 4th ed 2018. Allergy Society of South Africa.
- Ruemmele FM. Non-celiac gluten sensitivity: A challenging diagnosis in children with abdominal pain. Ann Nutr Metab 2018;73(suppl 4):39-46