It all seems very clear. Food allergy is an immunological reaction to a specific food protein, while food intolerance is the sensitivity of the gut to a range of foods that contain simple or complex sugars. Clear water! Nothing could be more straightforward. But nothing in medicine is ever straightforward.
In the vast majority of cases, food intolerance is the interaction of two concepts: a gut that is abnormally sensitive and foods that stimulate it. People who do not have the Irritable Bowel Syndrome or any other disease of the gut can tolerate wheat, milk, fruit, onions, pulses and even sprouts with impunity. And the sensitive gut can tolerate more bland foods; chicken, white fish, potatoes, rice and many others without undue discomfort.
A certain blurring of boundaries, however, occurs when we start to consider what may make the gut sensitive in the first place. The most common risk factor is a previous attack of gastroenteritis, but gut sensitisation and IBS only occurs in about 10% of people after an attack of gastroenteritis and they tend to be those who were depressed, anxious or had something going on their lives at the time of their infection1. In other words there is an interaction with the other most common risk factor, psychological upset or trauma.
Post infectious IBS involves an increase in colonic permeability (a leaky gut) and an excitation of the immune system with a mild inflammation2. A leaky gut would allow food allergens (intact proteins) to invade the gut wall and set up an allergic reaction, maintaining a state of sensitivity. This explains why some people develop food allergy for the first time after suffering severe gastroenteritis. Studies of people with post infectious IBS have shown a mild inflammation of the gut wall with immunoreactive mast cells, eosinophils and a rise in the immunological messengers, the cytokines2. Mastocytosis and immunological activation can also occur in patients with ‘IBS-diarrhoea’, who do not have a clear history of gastroenteritis. So rather in the same way that being exposed to the sun for too long may leave the skin red and sensitive to the touch of our clothing, the inflamed gut may remain irritated to anything that stimulates it, either the stresses that cause peristalsis or spasms or the poorly absorbed carbohydrates that distend the gut with fluid or gas. Gastroenteritis and local allergy may also damage the intestinal lining cells, critically reducing ‘brush border’ enzymes that digest sugars as they are being absorbed. This may explain lactose intolerance occurring after gastroenteritis or food allergy.
Cutting down fermentable sugars works in the short term, but it could leave a person at risk to nutritional deficiency. This is why dietitians stress that the low FODMAP diet is a learning exercise and people should reintroduce the foods they can tolerate as soon as possible3. Unfortunately, it can take a long time for such sensitization to wear away. Not only may continued stress ignite it through connections between the brain and gut immune system, but also exposure to allergens could maintain a state of gastrointestinal allergy before this has a chance to heal. In a paper published this year, Fitcher-Ravens and her colleagues used confocal electron microscopy to demonstrate that exposure to certain food allergens, milk, wheat, soy could open the gaps between the intestinal cells and increase intra-epithelial lymphocytes in people with IBS and suspected food intolerance within minutes of exposure4.
If only it were that simple! An attack of gastroenteritis or the administration of antibiotics to treat it can deplete the colonic microbiome, the trillions of bacteria that occupy the colon, reducing its diversity and stability. Among many other functions, colonic bacteria play an essential role in restoring and maintaining the integrity of the colonic epithelium and keeping the immune system in check5. Another paper published this year has shown that depletion of the microbiome by antibiotics can cause food allergy6. The low FODMAP diet, applied too rigorously might do the same thing by reducing the nutrient supply to the microbiome. Two published papers have shown FODMAP exclusion can deplete the Bifidobacteria spp that play such an important role in maintaining colonic integrity7.
If food intolerance and gastrointestinal food allergy co-exist, it is possible that the cycle of microbial depletion and immunological sensitization might be halted by a judicious combination of prebiotic foods that are not rapidly fermented and a probiotic preparation that contains Bifidobacteriacae.