I decided to open a new topic because of my personal experience and the observation that I have known many people with IBS (irritable bowel syndrome) how it may relate to aging and I noted a post by @Lori “Your Rapamycin Benefits?: My IBS has vanished from my first dose of 4mg. It is now week 4 and for the first time in 15 years I have a normally functioning bowel.”
After looking at several papers I conclude that there is still much not known about it and it is frequently misdiagnosed.
IBS is categorized into subtypes based on the most common symptoms experienced:
- IBS with predominant constipation (IBS-C): Characterized by hard, lumpy stools.
- IBS with predominant diarrhea (IBS-D): Characterized by frequent, loose, watery stools.
- IBS with mixed bowel habits (IBS-M): A combination of both constipation and diarrhea.
- IBS with unclassified bowel habits (IBS-U): Symptoms vary and cannot be categorized into the other three subgroups.
My strange case (to me) of IBS. Not too long ago I started to experience regular bouts of diarrhea. To shorten the story, I went to my PP, and he referred me to a gastroenterologist.
The gastroenterologist, of course, examined me and suggested I should have a colonoscopy, which I did. Fortunately, it came back clean.
I was prescribed a medication for IBS. It was quite ineffective. My PP suggested that I control the diarrhea with loperamide, which worked but tended to produce constipation.
I wasn’t pleased with the results I was getting. So I researched it a little further and started keeping a small journal of my meals and the effect they had on the diarrhea.
I almost immediately noticed that I had diarrhea in the evening or the next day after a particularly high-fat meal. For quite some time I have been on a quasi-ketogenic diet, consuming both polyunsaturated and animal fats. So I went on a low-fat diet and my diarrhea problems went away only to be replaced by constipation. I used to have a cast-iron digestive system that would tolerate most anything. Unfortunately, I failed to recognize what old age does to the system.
Snippets from my inquiries:
“General trend: In healthy aging, most people can still digest fats reasonably well, but reserve capacity declines in later decades. Reviews of elderly GI function show reduced enzyme secretion and slower motility, even in people without overt disease.”
“Bottom line: Many people tolerate fats well into old age, but late 60s–80s is the range where clinically meaningful intolerance becomes more common.”
"What your change in symptoms is telling us.
Your sequence:
Before: Bloating + frequent diarrhea
After cutting fat sharply: Diarrhea gone → now mild constipation
This suggests:
Fat was a strong trigger for your loose stools (very common in IBS-D).
When you removed that trigger almost completely, your bowel motility may have swung the other way, revealing a tendency toward IBS-C / slower transit underneath.
In older adults, the colon is often a bit “sluggish” by default; fat acts like a stimulant. Take away too much fat and you lose that stimulus."
Aim for moderate fat, not ultra-low or very high
Instead of:
Old pattern: Large, greasy, high-fat meals → diarrhea
New pattern: Extremely low fat → constipation
The goal is often:
Middle lane: Small to moderate amounts of fat with meals, spread across the day
Examples of what “middle” looks like (conceptually):
3 meals with modest amounts of fat (e.g., a small pat of butter, a teaspoon or two of olive oil, a bit of avocado)
Enough fat for normal absorption of fat-soluble vitamins (A, D, E, K) and healthy calories,
But not so much that it overwhelms your bile/IBS system and turns everything to water.
Note: Not one doctor, my PP, or my gastroenterologist ever asked what my diet was or suggested I reduce my fat intake.
Did I ever have IBS, or am I just too old to tolerate much fat in my diet?
A lower-fat diet plus more carbs and drinking more water seems to have everything under control, at least for now.