First, I'm going to quote a chunk relevant to this thread that I originally posted elsewhere:
Next, from my opening post in this thread:
I also underwent a GlycoMark test, which seemed to reinforce a finding of IGT. Quoting myself again,
And not only is GlycoMark a problematic surrogate for IGT: as Dean and others have noted, HbA1c is a Poor Measure of Glucose Control in Healthy Folks, or at any rate with slow RBC turnover (indicated by low bilirubin, which many CR people have) and/or low iron levels (ditto).
Happily, I can now report that I recently went in for a new OGTT — and came out handling that big chug o' sugar just fine.
Because they had used a somewhat frequently-measured 2 h OGTT at WUSTL, I arranged to have a close approximationi done with LEF: I got ahold of the blood lab and "special ordered" their Glucose Tolerance Test (4 specimens) (which normally measures glucose at baseline, 1 hour, 2 hours and 3 hours after the drink has been administered), and asked them to instead set it up to sample at baseline, 30 mins, 1 h, and 2 h.  Without getting into the numbers, my results were highly satisfactory.
Now, does that mean all of my concern between my first highish HbA1c in 2011 and this last test was a total artifact, and my glucose tolerance has been excellent all along? I can't say for sure, because several important things have changed, bringing me back to where I was back in 2006 at WUSTL.
First, for several of those intervening tests, my weight and level of CR was at its most severe, whereas I intentionally gained back 10 lb between a test in early 2016 and this last test,returning the weight I was at in 2006 at WUSTL. As Dean notes above, the CR-IGT group at WUSTL had significantly lower BMIs than the CR-NGT, and this could plausibly explain IGT as simply being a function of not having enough of muscle (and other metabolically-active tissue) to suck up a 75 g bolus of glucose. To the extent that this explains an individual's poor OGTT, his might actually be benign: it would neither mean that one's beta-cells were dysfunctional, nor that one had some kind of "hidden" insulin resistance not caught up by HOMA-IR (which mostly measures hepatic insulin resistance): one just didn't have anywhere to put 75 g of glucose, which is fine if you never expose yourself to that kind of shock except for testing purposes.
I'm not convinced that this is the explanation, however, for several reasons. First, even with all that "extra" weight, I am still extremely goddamned skinny (as I was in 2006), and slimmer by a substantial margin than my pre-CR setpoint. Second, I continued to have HbA1c nudging into borderline-prediabetic even during a previous period of being up at my current peak between June 2012 and early 2014— but that previous weight gain was due to an injury that kept me from running for an extended period of time, to which I did not compensate with reduced energy intake, so it's confounded by lack of energy burning, likely sluggish metabolism, and likely lower % muscle in my lower body as compared to 2006 or today. And those high HbA1cs were coming in on a low-GI, Zonish diet: it's hard to believe that even my skinniest frame couldn't suck up the carb in those mixed meals, and you would expect in general that metabolically-active tissue would track energy (including carb) intake.
On the other hand, throughout that previous, involuntarily higher-weight period, my protein intake continued to be on the lower end of my historical range, albeit not at its lowest, and IGF-1 continued to be extremely low at almost every testing. After having gotten back to running, my weight gradually came down to its nadir, and unsurprisingly my IGF-1 remained low and HbA1c highish. Growing concerned about the very low IGF-1 both for its own sake and because of its possible link to what I at least thought to be CR-associated IGT, I initially increased my protein intake without increasing my energy intake, and to my surprise (and concern) my IGF-1 did not respond in my subsequent test, nor in a followup one.
It was at that point that I decided I had to take in some more energy, not just more protein. And after having raised my energy intake and "bulked up" (ha!) to the same weight as 2006 — but now with that extra protein and being physically active again, unlike in the involuntary weight-loss period — my IGF-1 and IGF-1:IGFBP3 climbed up to (finally!) just about exactly where I'd want them to be: similar to the vegans in Fontana's protein-CR-IGF-1 study, which is both substantially lower than those values had been in 2006, but also significantly higher than they had been in nearly every test from 2011 until this last test.
And, my HbA1c also looks normal again, despite my bilirubin still being very low, my very-low-normal ferritin, normal iron, and normal non-ferritin iron functional status markers.
So, was my glucose tolerance fine all along? Or have I regained my very tight control thanks to more muscle mass? Or because of higher IGF-1? Dunno.
But I'm certainly glad to have all these numbers finally where I want them to be — and I'd' strongly encourage people not to speculate, nor (like me) to procrastinate about getting an OGTT if you have any reason to suspect you might be a CR-IGT candidate, but to get yourself tested if you can.