IBS symptoms are life-disrupting at the very least and it is understandable that people afflicted by this condition wonder why they happen when they happen.
For example, you have breakfast and, within minutes, you need to run to the bathroom and experience abdominal cramps and diarrhea. You conclude the symptoms were triggered by one or more of the foods in your breakfast. Correct? Not exactly.
Let's take a closer look at how the GI system works. After you eat, it may take (according to research studies) from 12 to 48 hours for the food to move all the way to the end of the GI tract (mouth to colon). When you are trying to figure out which food(s) may have triggered your symptoms, you need to look further back to the meals in that time period.
You may then wonder: why do you sometimes experience symptoms right after eating? This is more likely due to the effect of hormones and nerve regulators that are stimulated after a meal. For example, there are hormones that stimulate the production of digestive enzymes. Also, when the food enters the GI tract, it stretches the intestinal walls and triggers nerves that stimulate muscles which push food through the GI tract.
As digestion may take hours to days to be complete, there are always food particles (and possibly FODMAPs) in your GI tract from a meal you ate earlier that day or even the day before. The symptoms you experience, therefore, are delayed symptoms: they have nothing to do with the meal you just ate but are likely due to foods in a previous meal.
The bottom line? It is quite tricky for anyone to interpret the origin of their symptoms. The best way to track them is to keep a food and symptom diary, where you write down what you eat and the timing of your symptoms. As you do this day in and day out, you may notice some interesting patterns. If you are still confused, write a detailed list of foods (including the timing of your symptoms) for at least 2 days and ask an expert dietitian to help you figure out what foods may trigger them.