I've been an avid cyclist from High School on (with a few short breaks) I'm now 50 years old. 100 miles/week +/- a few has been my traditional norm.
As I've aged, I've had trouble with heart (afib) , thyroid (failed) , kidneys (30-50% function) , extreme fatigue and more recently pancreas (low insulin output) and massive inflammation, especially mid section. My diet has been exceptionally careful, healthy, low glycemic for a decade, and healthy before that. No alcohol, ever.
I was trending downward in my health, but more recently I had some sort of "event" after eating a veggie burrito at a quality mex place, I bloated up like a balloon, intestines swollen nearly shut, and pancreas nearly shut down. As one might expect, blood sugar rose from 70-100 to a fasting 160! But low enough during the day 60-120 due to diet and forced regular exercise. I felt awful, like I'd been poisoned, very short of breath and , but worse than that, unable to function normally. Found myself off the side of the road wondering how I got there, AND doing incredibly stupid things/not remembering. (possibly due to very low insulin levels)
My endocrinologist suggested a change to a high protein diet, and a gym membership, lifting weights at as high an intensity as I can stand. Back off on the cardio. I'm not particularly strong, so this was an experience in itself.
It worked! For 3 months nothing happened, and I still felt awful. Then for just one day, I felt human again. Waking/fasting BG 89, whoa, that's a big change from the 169 the day before. Then a month later, just as suddenly, I felt good. I had gained a bit of strength, but all the other problems were gone. Mostly due to the gym. It's been 6 months, and I feel like a normal person!