What is the difference between food allergy and food intolerance?
Although the symptoms are often very similar, they differ in biochemical mechanism. Food allergy is caused by an overactive immune system, whilst certain chemicals contained in food are responsible for one’s symptoms with food intolerance.

What causes food intolerance?
These are three causes:
A) The excessive build up of certain chemicals naturally found in food. This is because of either a deficiency or abnormality of certain enzymes required for the processing of these chemicals. As it usually happens in the intestines, symptoms of heartburn, bloating, nausea and diarrhoea are common.
B)  The presence of certain chemicals naturally contained in food that one happens to be sensitive to. Examples are:

• Foods high in histamine: cheddar cheese, tuna, mackerel, spinach, eggplant and red wine.
• Foods high in serotonin: chocolate, banana, pineapple, avocado and tomato.
• Foods high in tyramine: Camembert cheese, chicken liver, pickled herring, yeast extract and Chianti wine.

C) The presence of certain chemicals not naturally found in food, but that have been added by the food processing industry.  Examples are:

• Tartrazine (used in colouring)
• Sulphur (used as a preservative in soft drinks, processed meat, vegetables and fruit)
• MSG
• Monosodium glutamate (frequently used as a flavour-enhancer, especially in dried herbs, spices and Chinese foods)
• Sodium benzoate (used as a preservative in soft drinks)
• BHA and BHT (anti-oxidants often found in margarines)

What causes food allergy?
Food allergy is caused by an overactive immune system. There are two types:
1)    Type 1 hypersensitivity.
2)    Type 3 hypersensitivity.

Type 1 hypersensitivity
This is the most severe kind of food allergy, caused by an antibody called immunoglobulin E (IgE).

What is an antibody?
Antibodies are specially designed protein molecules have the ability to recognise certain target or enemy molecules, called antigens, amongst millions of other molecules. Once an antigen has been identified, the immune system is activated the antigen is attacked and destroyed.

Antibodies are crucial to our immune systems and responsible for the life-long immunity that we develop after having recovered from past infections. Like other antibodies, IgE is manufactured by specialised cells called B cells. Their behaviour differs from that of other antibodies (IgA, IgM and IgG), in that they have the ability to attach themselves to two other types of immune cells, namely “mast cells” and “basophils”. Whilst basophils can be found floating around the body carried by the blood stream, mast cells are embedded in the solid tissues of our body. One mast cell can host as many as 100 000 IgE molecules at a time, which would have been manufactured by different B cells. Combined as a unit they have the ability to recognise many different enemy antigens.

How does it cause symptoms?
Each mast cell contains numerous pockets of powerful inflammatory chemicals, the most famous one being histamine. When two identical and neighbouring IgE molecules, sitting on the outside of a mast cell, both come into contact with the same antigen that they have been programmed to recognise, a message is sent to the mast cell, whereupon it releases the contents of these chemical pockets into the system. This triggers an allergic reaction, dependent on the site of activation. Contact with allergy-producing food in the mouth causes swelling of the lips, tongue and sometimes the throat. In the stomach and intestines it leads to diarrhoea, cramps and vomiting. Once absorbed, it reaches the airways where it causes spasm or bronchospasm of the airways. Itchy bumps (hives) and other rashes occur on the skin. In severe forms, the vascular system collapses and shock follows, called anaphylactic shock.

Some foods contain substances that can trigger mast cells directly without the aid of an IgE molecule. This is called false food allergy and the exact mechanism for this process is still not clearly understood.

2) Type 3 hypersensitivity.
With this type of reaction antibodies other than IgE are responsible for the symptoms. After a meal, undigested food molecules (also called antigens) are absorbed from the intestines and transported throughout the body by the blood stream. These new molecules invariably come into contact with the immune system, whose task it is to scrutinise all newcomers and destroy those potentially dangerous to the system. The antibody IgA is responsible for this duty. Once a foreign food molecule has been identified, IgA binds to it, and in doing so, forms a new molecule called an antibody-antigen-complex. This new molecule attracts the attention of large, specialised white blood cells which swallows or ingests the entire antibody-antigen-complex whole and destroys it, thus removing it from the system.

Another type of antibody called IgG can also bind with these food molecules, but the resulting immune reaction is not quite so gentle. Unlike IgA, IgG triggers a vicious biochemical process called the complement system, whereby various highly energetic and potentially “toxic” chemicals are activated. Their action destroys not only the food molecule, but also injures normal bodily tissues such as the lining of capillaries, resulting in inflammation. The exact reason why some people trigger the disastrous effects of IgG, rather than gentle IgA, is not quite clear, but evidence suggests that the system for producing IgA rather than IgG somehow becomes disrupted. Some individuals also have increased l permeability of the intestines, referred to as the leaky guy syndrome. This allows the absorption of abnormally large quantities of partially digested food molecules, causing an overload of the system and an over-reaction of the immune system resulting to damage.  

How can one identify the agent that one is sensitive to?
Allergy tests can be done by your doctor, but they have many shortcomings. Not all types of allergic reactions can be identified with blood or skin tests, and allergy tests will also fail to identify food intolerance. An excellent and inexpensive way of identifying both allergy and intolerance is through the process of systematic elimination of certain food, namely with an Elimination Diet.

Which foods commonly cause allergic symptoms?
With food intolerance it is almost always the most commonly eaten foods which are the source of the problem. In Britain and other Western countries, these are usually bread (wheat) and milk, which are usually eaten a few times a day. In the USA, wheat and milk are equally significant, but intolerance to peanuts and maize are more commonly seen than in the UK, where these two products are less frequently consumed. In many Eastern countries soya and rice are responsible for the majority of symptoms, because of their much higher rate of consumption. Intolerance to soya in the western world, however, has also increased significantly since popularity has increased its consumption.

Another problem is cross-reactivity. This implies that some people, who are allergic to one food item, may be sensitive to other food items because of certain chemical properties shared between the products. The similarity can be between food items closely related and belonging to the same plant family, or between food items not related at all. (Researchers do not understand this phenomenon completely)

The Elimination Diet (ED)
The golden rule of all elimination diets (ED’s) is that one cannot attempt them half-heartedly. The ED is a diagnostic and scientific experiment, and, like all experiments, the result will be accurate only if a proper system was meticulously adhered to. Unlike breaking a diet, exposing yourself to even small quantities of an allergenic food substance will affect the final interpretation and all your efforts may eventually prove to be in vain.

All ED’s contain two phases:
During the exclusion phase, exposure to a range of possible culprits must be avoided. These items must be selected after careful consideration, and as a rule usually consists of food items that you consume on a daily basis, such as bread or milk. Symptoms should disappear if all offending foods have been identified, which may entail a certain amount of luck. If symptoms persist, experiment by excluding other food items or food groups as well. Most people have multiple food allergies, and patience and persistence is required during this phase.

The reintroduction phase can be attempted once suspected allergic or intolerant symptoms have disappeared. During this phase, the previously excluded food substances are reintroduced, one by one and over a period of time, until symptoms reappear. This observation will enable one to establish which food item / items are responsible.  

To proceed with an Elimination Diet, follow the next 4 steps.

Step 1: Stage 1 Exclusion
The purpose of step 1 is to establish healthy eating habits. Eat lots of vegetables and fruit, as well as salads, on a daily basis. Start by planning your diet carefully, and by keeping notes.

Exclude the following from your diet:

1. All food items that may have a drug-like reaction on the body. These include coffee / coffee flavoured items, tea, cola-drinks, cocoa, chocolate / chocolate flavoured items, alcohol, sugary food items, saccharin / other artificial sweeteners
2. All histamine containing food items (see list above).
3. All types of food additives, such as colourings, preservatives, antioxidants, flavour enhancers, artificial flavourings, thickeners, emulsifiers, stabilizers and other additives (Especially agents identified by an E-number.)
4. All take-aways and fast foods.
5. Anything that makes may damage the intestines and make it more permeable, such as curries, raw pineapples, papaya, aspirin and anti-inflammatory drugs.

Stage 1 requires a careful study of all the food items that you commonly consume, as well as all condiments, sauces, spreads and drinks. Adhere to stage 1 for at least one month.

Possible outcomes:
Feeling worse.
This may be due to caffeine withdrawal, and if so, all symptoms should disappear within two weeks. Other causes such as an ailment unrelated to your ED, may also be to blame. Consult your doctor for a proper diagnosis.

Feeling the same.
Proceed to stage 2, whilst continuing with your stage 1 eating habits.

Feeling slightly better.
a) If you are not yet satisfied with your results, proceed directly to stage 2.
b) If you are satisfied with your result and not keen to exclude other foods, you may stop here.

Step 2: Reintroduction

1. Begin your reintroduction phase in a systematic and logical manner, by reintroducing one previously excluded test-food item per week.
2. If no symptoms appear, exclude the item again (an explanation for this will be given later), and continue with the process of experimentation.
3. Immediately and permanently exclude from your diet any food item that produces symptoms, and proceed to the next item on your list as soon as the symptoms have subsided.
4. Once all food items have been tested, you may reintroduce them all at the same time, except those which caused noticeable reactions.

Possible outcomes:                 
Feeling a lot better.
You fortunately got off very lightly. Begin your reintroduction phase in a systematic and logical manner, by reintroducing one test-food item per week (see above), until the offending substance can be isolated.

Feeling a lot better, but then worse again.
Symptoms of food intolerance sometimes take a while to develop, and can therefore make interpretation difficult. Having avoided a product / products which you are sensitive to for a while, may imply that your symptoms have been “cured”. Be careful. Symptoms may take a while before they reappear. (This is why the principle of introducing test foods after one full week applies.) Retrace your steps until the offending item can be identified.

Step 3: Stage 2 Exclusion
Exclude all food items as listed in stage 1, as well as all food items other than those contained on the following list. You should not start your diet without planning what you are going to eat, or before buying enough food to last you a while. (Bad preparation will lead to poor compliance). Cook your meals in advance, and freeze them for later use.

Eat any of the following:
Meat: lamb, turkey, ostrich, duck, goose, rabbit, pork,
Vegetables: all, including potatoes.
Grains: only rice. (If you frequently eat rice, it has to be excluded as well)
Fruit: all fruit except citrus fruit, pineapple and papaya.
Legumes: all, beans, chickpeas and lentils.
Nuts: all nuts except those that you eat frequently.

Exclude from your diet:
All other food items not listed above.

Possible outcomes:
Feeling worse.
This often happens during the first few days of the exclusion phase, and is generally considered a good sign. Withdrawal symptoms are frequently seen in some food sensitive patients, and the presence of these symptoms may indicate that the correct items have been successfully excluded. Be patient, these symptoms will soon pass.  

Feeling worse could obviously also be related to another medical condition, such as the early stages of a viral illness. Consult you doctor for a proper diagnosis.

Feeling the same.
Proceed to stage 3.

Feeling a lot better.
Reintroduce one food item per week according to the re-introduction principles.

Feeling better, then worse again.
This is a bad sign, and may imply that you have become sensitised to one of the new food items that you are consuming. You will need some expert advice, and we suggest that you consult a doctor or dietician who specialise in food allergies.

Feeling much better, but with one or two lingering symptoms.
This implies that you have not yet successfully identified all the agents that are responsible for your symptoms.
If you are satisfied with your result and not keen to exclude certain foods, you may stop here. Go to the reintroduction phase.  
If you are not satisfied with your results, proceed to stage 3.

Step 4: Stage 3 Exclusion
Only a minority of patients will have to try the rigorous Stage 3 Exclusion and it usually implies that they have multiple food allergies, or that perhaps their symptoms are not being caused by food allergy. Stage 3 Exclusion will require even more planning than Stage 2 Exclusion. The aim is to come up with a list of at least 12 food items that are nutritious, obtainable and affordable, and which you have never eaten in any quantity, or with any regularity, before. The list should include a variety of different items, such as fruit, vegetables, meat / fish and some starchy foods. Adhere to stage 1 principles during the Stage 3 Exclusion diet and experiment with the exclusion and reintroduction principles until symptoms can be related to certain foods.

[Read more about Egg Allergy]
[Read more about Fish & Shellfish Allergy]
[Read more about Fruit Allergy]
[Read more about Milk Allergy]
[Read more about Wheat Allergy]