‘The pain and the bloating come on as soon as I’ve eaten and then the diarrhoea starts.’

 So is this food intolerance?  As pointed out in a recent post by Jaci Barrett from the Monash Group, entitled The Timing of Symptoms, the symptoms of food intolerance caused by FODMAP foods are generated when unabsorbed carbohydrate reaches the colon and is fermented by colonic bacteria.  This normally occurs at least two hours and usually about four hours after ingestion of the meal.  So we would not expect symptoms to be generated immediately after a meal containing onions or pulses, lactose, wheat or any other FODMAP food.

But, as Ms Barrett points out,  digestion in the small intestine takes many hours,  it may still contain material from the previous meal.   Therefore, as a new meal is consumed,  stimulating the vagus nerve and hormones such as cholecystokinin, any contents remaining in the ileum at the end of the small intestine will be propelled into the colon where they will be fermented.  So symptoms may be experienced upon eating that have nothing to do with the current meal and everything to do with the previous meal.  This is why it is important to consider not only the food  you are currently eating when you are trying to pinpoint your symptoms but also what you had for your previous meal.  This, I believe, is more likely to be responsible for the double peak in breath hydrogen that some investigators have sought to explain by Small Intestinal Bacterial Overgrowth.

Of course, if a patient does have has SIBO, which may occur in asmany as 30% of people with IBS (especially those taking acid blockers and probiotics), symptoms may well occur fairly soon after ingestion, but sufficient carbohydrate has to empty from the stomach and be fermented in the small intestine, and this may take an hour or more.

But there are other causes for immediate symptoms.  You might have an allergy to a particular food.   Acute hypersensitivity to a specific food ingredient comes on straight away, but it is usually associated with other symptoms, perhaps swelling of the lips or tongue, maybe some faintness, perhaps a rash.  And it is very specific.  Nuts, strawberries, shellfish are common culprits.  But acute hypersensitivity to food ingredients occur in less than 2% of adults.  Although celiac disease is a delayed hypersensitivity to gluten, symptoms may come on very rapidly after consuming gluten.   

In people with a sensitive gut, just the eating of food, the filling of the stomach and the release of cholecystokinin and stimulation of the vagus nerve as the meal enters the small intestine is enough to trigger a gastrocolic reflex, which can cause pain and trigger defaecation.   .

But perhaps the most likely cause for immediate reactions to food is not so much the biological effect of the food but our fear of it.  Perhaps it occurs after a particular meal because that meal conveys the context and memory of some shocking or distressing event.  Sometimes it is the idea that a particular meal may cause symptoms.  If somebody is gluten sensitive, just the thought that a meal out may  contain a trace of gluten, is enough to provoke an attack.  When any of us are ill, we are very suggestible.  Just the thought of eating may be enough to bring on symptoms and if we fear we may have eaten  something that could upset us, it undoubtedly will.

It can be revealing to note when attacks of IBS come on.  Do they, for example, come on Monday mornings just as you are about to go back to work or to school?  Why do they clear or flare up when you go on holiday? What is it about a visit from your mother or other key relative or friend that brings on the symptoms?   It is not always the food that brings on attacks but the context.