The fundamental idea of eating for Irritable Bowel Syndrome is to avoid the IBS bad foods that trigger or irritate a spastic colon via the gastrocolic reflex that occurs when food enters the stomach, and to eat the soluble fiber safe foods that soothe and regulate the colon. This will relieve and prevent BOTH constipation and diarrhea, as well as the painful spasms and cramps, gas, nausea, and bloating of Irritable Bowel Syndrome.
The most difficult foods for the body to digest (in addition to insoluble fiber) are fats and certain animal products. As a result, they are the most powerful IBS trigger foods - for BOTH constipation and diarrhea - and you must strictly limit or, preferably, eliminate most of these IBS bad foods from your diet altogether. Will this require an enormous change in the way you eat? Probably. But it is a change for the better, and we will walk together through the steps needed to make this change as easily and deliciously as possible.
Fat is quite simply the single greatest digestive tract stimulant, and thus the most common IBS trigger food. When food enters your stomach, it triggers the gastrocolic reflex, which cues your colon to start contracting. (Have you ever had an IBS attack within minutes after eating the wrong thing, even though common sense tells you that the food was nowhere near your colon at that time? The gastrocolic reflex is why.)
Fat will trigger this reflex more powerfully than any other category of food. While this is normally something that would simply result in a a bowel movement, it is paradoxically likely to cause problems for people with IBS constipation OR diarrhea. This is because people with IBS do not have a normal gastrocolic reflex response, and their colons tend to spasm irregularly and often violently. In people prone to constipation, if those spasms are too strong they'll actually "seize up" the colon in a type of charley horse, and motility can halt altogether.
This will cause severe pain from the ongoing cramping of the colon, and it will also result in or worsen constipation, because motility has shut down and fecal matter is not moving through. Only when the muscles finally relax will you begin to feel a little better, but it can take quite a while after suffering such violent spasms for normal colonic motility to return.
In a corollary way high fat trigger foods also typically worsen IBS diarrhea, as the muscle spasms triggered by fats result in gut contractions so fast and hard they rush matter through the colon, without enough time for water to be absorbed. This causes pain from the spasms and diarrhea from the hypermotility. So, though it sounds odd, high fat foods are triggers for BOTH diarrhea AND constipation if you have IBS, as they are each a possible result of the gastrocolic reflex gone awry.
When it comes to reducing or eliminating the high fat trigger foods from your diet, I sympathize tremendously with you. At first glance the IBS diet changes can seem overwhelming and just too difficult, as by nature most of us are resistant to any great transformations of our lives. It is almost always easier to not alter a habit, simply because inertia takes less effort than action.
However, I really cannot stress enough that the changes in diet required for IBS do NOT equal deprivation. You will not be expected to simply give up all the foods you love, and offered a tasteless starvation diet in return. These changes are in fact a terrific opportunity for a better life, as you can easily learn how to eat safely for IBS without giving up an ounce of flavor, fun, favorite restaurants, or delicious home cooking. It is simply a matter of substitution, of replacing IBS trigger foods with safe choices. Remember that the only thing you're really giving up is the constant worry and dread of attacks, as well as the pain and agony they cause.