Didn't know how to answer this poll, so didn't vote. But in the interest of documenting our symptoms for future reference, maybe even to be puzzled over by a researcher, here's my experience.
Have had constipation at nuisance level as long as I can recall. My ME probably started about 40 years ago (undiagnosed then) so really can't remember what came first.
I've had three bouts of ME with 2 lengthy almost-remissions inbetween. Constipation remained at same level throughout 'active ME' periods and remissions until about 2 years prior to the onset of my third and current bout of ME. It continued to worsen after onset and various other GI symptoms appeared, probably down-stream to the constipation.
The constipation was eventually diagnosed via transit studies as idiopathic slow transit (very slow, as in 1 bowel movement every 2-3 weeks) and the other symptoms via breath analysis as SIBO (more specifically overgrowth of methanogenic bacteria).
The SIBO resolved once I found a reliable way to keep the bowels moving with medicine. The slow transit persists but is masked by the medicine most of the time. However during PEM my gut simply stops moving anything forward leading to all sorts of GI symptoms. Occasionally it switches the opposite way to a sort of dumping syndrome when within 1/2 an hour of a meal your insides very painfully turn inside out to empty themselves violently.
At this stage I have 2 theories (with lots of holes in them):
1) Microbiome theory
I was born with a microbiome that had a tendency to produce methane. There's an association between methane and constipation, which could potentially explain the longstanding constipation. For some reason the microbiome got disturbed, possibly after a course of antibiotics, and started producing ever more methane causing ever more constipation, eventually causing SIBO.
The only connection with ME in this scenario would be that the worsening of constipation and advent of SIBO roughly coincided with the onset of my third bout of ME which is significantly more severe than the previous ones. This theory doesn't offer any obvious explanation of worsening GI symptoms during PEM though.
2) Small-fibre neuropathy theory
The slow transit is due to some autonomic small-fibre neuropathy problem. SFN has been connected to ME and it's notable that, apart from the nuisance constipation, the other symptoms that persisted at low levels throughout my remissions were autonomic ones like OI. For some reason the SFN worsened prior to onset of ME bout #3. This then caused increasing constipation which in turn caused SIBO. In this theory all GI symptoms look to be secondary to SFN. I don't really understand by which mechanism SFN could worsen during PEM; I can only observe that all my gut and autonomic symptoms do get worse then.
I'll jump in here and echo some of your experiences for the record.
My idiopathic slow transit colon (diagnosed much later) developed from about age 16 (I'm now 69) and has worsened gradually and evenly over time. It was accompanied by increasing brain fog type symptoms and a growing intolerance of swings, roller coasters and even the old Ford wallowing through dips in the road, even as early as 14 years old. I've never thought of myself as having OI but I have to admit I often feel dizzy after I bend over and feel sick after standing still for a short time. Whatever it is, it's variable and not a great nuisance in the scheme of things.
I've had a few SIBO tests and they've been negative.
The ME symptoms of lack of energy, PEM and pain became a bother about 15 years after the constipation and brain fog, but they were there from the start, although mild, when I reflect on it. I was never able to contemplate a double degree, for example, which I would have liked to do, or ever work more than 4 days a week. That's now down to 2 hours a week. Like everything else, the ME symptoms have slowly and inexorably become worse over the years.
It's probably quite unrelated, but I started to develop osteoarthritis in all joints at the same young age.
I have (presumably) small fibre neuropathy affecting my legs mainly, and especially feet. In my late teens and early twenties I recall the occasional night of painful, burning feet, but it disappeared for a few decades only to re-emerge with a vengeance in my 50s.
I had an abnormal gut microbiome test result in the mid 1990s but have since read that the results from Bioscreen in Newcastle, Aus (before they moved to Melbourne - ?) were not at all reliable so I'm none the wiser and don't intend to have any more tests for anything until more is known.
. I would answer the first option from what you describe (IBS being GI related and present before ME onset).
