Food intolerances are quite prevalent, affecting approximately 15-20% of the population. They lead to digestion difficulties linked to specific food products. It is important to note the difference between food intolerance, which usually includes the digestive system excluding the immune system and food allergies, which are a result of immune system activation.

Food intolerances are shown to be more frequent in individuals suffering from digestive system disorders such as irritable bowel syndrome (IBS). IBS network states that the majority with IBS also experience food intolerances.

An individual affected by food intolerance often experiences discomfort soon after ingesting food products. The range of symptoms is vast and commonly involves the digestive system itself. The most frequent symptoms linked to food intolerance involve bloating, stomach pain, diarrhoea, migraine, headaches, runny nose, excess gas and malaise.

The symptoms linked to food intolerance can take some time before becoming apparent. The onset can present even up to several hours after eating and the symptoms can persist for numerous hours or even days. It is quite complex when it comes to differentiation between food intolerance and a food allergy as signs and symptoms of both conditions overlap.

Food intolerance vs. food allergy

Both conditions share similar symptoms. However, symptoms related to food intolerance take longer to appear then food allergy symptoms. Food allergy is a result of an immune system reaction towards a specific product while food intolerance includes the digestive system rather than an immune system. With regards to food intolerance, the amount of food ingested is directly linked to the severity of the reaction. On the contrary the food allergy can be triggered even with a minor allergen amount and lead to a severe, life-threatening response.  Food allergies are commonly mediated by the IgE antibodies and food intolerance is mediated by IgG class of antibodies.

Food intolerances happen when the body is unable to digest food products. It can be a result of absence or malfunction of certain digestive enzymes and extreme sensitivity related to certain chemicals. Food commonly linked with food intolerances involve milk, gluten, food colouring and preservatives, sulphites, substances including caffeine and fructose.

Body response to food

Overreaction of the immune system in response to a food allergen by production of antibodies called Immunoglobulin E (IgE) indicates an allergic reaction. These antibodies travel to cells, which are responsible for the release of chemicals such as histamines which contribute to inflammation. Below is a list of various negative body's reactions to certain food products:

Immunoglobulin E (IgE)-mediated (Allergy): This is considered a "real" food allergy. The immune system over produces antibodies crucial for the fight against the threat. However, in this case the threat is a peanut or a glass of milk instead of a virus or bacteria. This type of an allergic reaction can lead to anaphylaxis and it can begin within a few minutes after food ingestion.

Non-IgE mediated (Allergy): In these conditions, the immune system is involved but the IgE antibodies are not being produced and therefore will not show on the test. Oral allergy syndrome is included in these type of reactions as well as FPIES (Food Protein Induced Enterocolitis Syndrome) diagnosed among infants as a response to milk, soya and grains.

Mixed IgE mediated (Allergy): These conditions can present as IgE- or not IgE-mediated reaction. One of the examples of a mixed IgE-mediated allergy includes Eosinophilic Esophagitis (EOE). EOE is also referred to as "the asthma of the oesophagus" and can lead to food obstruction and dysphasia - a feeling of food being stuck in the throat. The treatment includes an elimination diet which excludes most of the common foods such as wheat, milk, soy, nuts, eggs, fish and shellfish. Medications such as steroids should be avoided. One or more biopsies each month are recommended for monitoring of treatment regression.

IgG & IgE antibodies: IgE antibodies are found in the skin, lungs and mucosal membranes. They lead to body protective reactions against foreign substances including pollen, fungus spores as well as animal dander. Type I - immune reactions are the best well known and studied among all the food allergies and they involve the classical food allergy, immediate onset, IgE-mediated and atopic food allergies. They are experienced by approximately 2-5% of the population.

IgG antibodies are found within all the body fluids. They are the smallest most common antibodies making up to even 80% of all the antibodies. These antibodies are extremely important when fighting bacterial and viral infections. Type III immune reactions (delayed onset food allergies) are much more commonly involved in food allergies than Type I reactions. They were reported by 45-60% of the population. Delayed food allergy reactions incorporate the immune system. However, the IgG antibodies bind directly to the food as it enters the bloodstream instead of attaching to mast cells like in the Type I allergies. Hence, the symptoms of Type III reactions are delayed in onset developing anywhere from a few hours, even up to a few days.

Lactose intolerance

Digestive enzymes are being used by the body in order to break down foods. Therefore, if one lacks certain enzymes, they may be less capable of digesting certain food products. Lactose is a sugar found within cow's milk. Individuals suffering from lactose intolerance do not have enough lactase, which is an enzyme responsible for lactose breakdown into smaller molecules which can then be easily absorbed by the intestine. If lactose persists within the digestive tract it can lead to spasms, stomach-ache, bloating, diarrhoea and gas.

Fructose intolerance

Fructose is a sugar which can be found in fruits, certain vegetables and honey. Fructose intolerance can present as a metabolic disorder resulting from absence of an enzyme (aldolase B). In these cases, it is known as hereditary fructose intolerance (HFI). Individuals with HFI, ingestion of fructose (fruit sugar) as well as sucrose (table sugar) leads to severe hypoglycaemia (low blood sugar levels) and accumulation of toxic substances in the liver.

However, fructose malabsorption in which the body is lacking a protein allowing sugar absorption to form the intestine is much more prevalent. In this case fructose ferments in the gut bring about the sensation of fullness, bloating, cramps together with diarrhoea.

Gluten intolerance

Gluten is a protein which can be found within cereals involving wheat, barley and rye. An individual who has gluten intolerance experiences unpleasant symptoms including pain, bloating or nausea after ingestion of products containing gluten.

Gluten intolerance can also manifest with nondigestive symptoms including brain fog, headaches, joint pains, depression, anxiety, fatigue and malaise.

Gluten intolerance differs from celiac disease, which is an autoimmune system response to gluten as well as wheat allergy which is an allergic reaction to wheat products. Nevertheless, symptoms of these conditions can be very similar. Gluten intolerance is also known as nonceliac gluten sensitivity.

Symptoms related both to celiac disease as well as gluten intolerance improve after gluten is eliminated from the daily diet but will return once the products are reintroduced.

Salicylate intolerance

Salicylates can be found within various plant foods including fruits, vegetables, herbs and spices. Moreover, they are a part of artificial flavourings and preservatives found in toothpaste, chewing gum and candies. The majority can tolerate a moderate number of salicylates as part of a diet, however, some individuals present a decreased tolerance. Symptoms can show as hives, rashes, diarrhoea, fatigue, stomach-ache, runny nose and wheezing.

Food additives and intolerance

Many individuals show concern with regards to intolerances related to food additives, however, the Asthma and Allergy Foundation of America studies show that only a few additives may lead to problems and those affect only a few. Additives are commonly used by food producers in order to enhance flavouring, make food products look more appealing and increase their expiry dates. Food additives include antioxidants, artificial colourings, artificial flavourings, emulsifiers, preservatives, sweeteners together with flavour enhancers.

A few out of the thousands of additives commonly used by the food industry are proven to cause health issues. The following are responsible for leading to adverse reactions among some individuals:

• Nitrates - commonly occur within processed meats and the symptoms include headaches and hives.
• Monosodium glutamate (MSG) - this is a flavour enhancer which can lead to headaches, sensation of chest pressure, nausea and diarrhoea.
• Sulphites - common sources include wine, dried fruits, fresh shrimps, jams and jelly products. Individuals with an intolerance can experience chest tightness, diarrhoea, hives and anaphylaxis (severe, potentially life-threatening allergic reaction).

Conclusion

Food allergies and food intolerances share a lot of common symptoms yet there are also a number of crucial differences one should be aware of. Food allergies are brought about by a reaction of the immune system while intolerances are linked to a response of the digestive system. Additionally, an intolerance is not life-threatening but a food allergy can be. Reactions happening as a result of allergies happen quickly and can be triggered by minimal amounts of the offending food while intolerances usually need greater amounts and entail hours or even days to manifest. Food intolerance is commonly associated with symptoms such as abdominal pain which will not lead to severe reactions such as anaphylaxis.

Finally, there are tests available for intolerances such as lactose or fructose intolerances, but they are mainly tackled by a process of elimination guided by a certified dietician. Allergy tests are available but unfortunately many are unreliable including the IgE based. The result interpretation should be guided by a certified professional such as a gastroenterologist.

Prof. Renald Blundell is a biochemist and biotechnologist with a special interest in Natural and Alternative Medicine. He is a professor at the Faculty of Medicine and Surgery, University of Malta

Andrea Weronika Gieleta is a registered nurse and is currently a medical student at the University of Malta