Many patients are unsure about the difference between food allergy and food intolerance, and may use the two terms interchangeably. In both cases the food we eat makes us unwell and triggers a number of symptoms. However the underlying causes are different even if the symptoms may look similar at first sight.
In a true allergy, the body defence mechanism reacts against a protein (allergen) that it believes can harm us by producing antibodies (IgE). The reaction is usually quick and violent. A few minutes after ingesting the offending food, the patient experiences difficulty in breathing, a swelling in the mouth, a skin rash, stomach ache, vomiting, diarrhoea, dizziness or sometimes loss of consciousness.; usually a combination of these symptoms. In extreme cases the reaction, known as anaphylactic shock, can be fatal; as in the case of severe peanut allergy. People who are susceptible to anaphylaxis must carry medication with them at all time. Usually an auto-injector containing Adrenaline (also referred to as Epinephrine) called EpiPen. It is used on the outer thigh for emergency treatment of a life-threatening allergic reaction. It is important to note that EpiPen is not a medication to treat allergic symptoms; but is prescribed by a GP only to people with a history of anaphylaxis risk.
In food intolerance, the immune system does not produce antibodies. The reaction is a result of a hypersensitivity to an ingredient contained in the food (e.g. gluten). In contrast to some food allergies, food intolerance is not life-threatening but it can severely affect your health and your quality of life. While allergic people have only to deal with a limited number of allergens (one or two usually); food intolerance can consist of a wide range of offending foods. The onset of symptoms is usually slower and may be delayed for many hours after eating. So it is not easy to pinpoint the exact allergen, only with time and experience. Unlike in food allergy, the symptoms last for several hours and sometimes days. The allergic patient is unable to tolerate any amount of the offending food, however small. Whereas with food intolerance, the patient will not react to a tiny amount. Only by eating a certain amount (depending on the individual level of tolerance), or repeatedly, will the symptoms be triggered.
Personally I suffer from both food allergy and food intolerance. I am allergic to eggs, dairy products and some fish (notably shellfish) which I have to avoid at all costs. Not only do I have to exclude these foods from my diet, but any food containing any ingredients related to them (e.g. whey, casein, albumen, etc.). For instance, eating a piece of biscuit containing one of these ingredients will trigger straight away an allergic reaction as described above. Whereas tasting a little bit of bread or any product containing gluten will not do any harm. However, if I do it repeatedly, or eat a good amount I will react slowly but surely with a headache that can last several days. I also react the same way to many other foods like sugar, yeast, pulses, rice with a multitude of symptoms like eczema flare up, stomach ache, bloating, gas, diarrhoea or constipation, etc.
Thus food intolerance is much more difficult to diagnose and treat than allergy. Allergy is caused by an immune system reaction to a protein (allergen); while food intolerance can have many causes, such as enzyme deficiency, sensitivity to natural chemicals in food, toxic substances, food additives, etc. By keeping a diary of foods eaten and symptoms for several weeks, it will be possible to start an exclusion diet. Under the supervision of a dietician, potential culprit foods can be eliminated from the diet and then reintroduced one at a time. Bear in mind that this is not a walk in the park. Just like a weight loss diet, an exclusion diet requires planning, a lot of commitment and perseverance but the outcome can be life changing. As for so called food intolerance tests, they are unreliable and not scientifically proven.