After I was diagnosed about 5 years ago (at 38 years old), my endocrinologist in a research study told me that elevation has unpredictable effects on BG (some people go up, some go down, some remain steady), and researchers don’t have a good handle on why. I camp regularly in the high Sierra Nevada and other ranges in eastern California, and my research (I’m an archaeologist, if you can’t tell by my user name) often takes me to elevations above 11,000 feet in the White Mountains of California. I typically run low once I’m above about 7,000-8,000 ft elevation. I run a separate profile on my Tandem pump. My bolus settings remain the same as home (about 250 ft elevation), but I run my basal at about 50%. Some of this certainly has to do with activity–hiking, or even just moving around at low oxygen levels and increased exertion. One sleepless night after several days at high altitude my numbers kept crashing. I ended up stopping my basal altogether, my numbers flattened in the morning, and I ended up feeling “normal” for a day with no need for any basal insulin!
I’ll also add that I’ve had issues with BG meters at higher elevation. If you use OneTouch (and I’m sure some of the others), my recollection is that the manual includes fine print that readings can be affected by elevation. As such, I tend to trust my Dexcom over my meter at least for the first day or two, after which the BG meter seems to acclimate.
One other suggestion I read somewhere, possibly on this forum: if you’re using a pump at a high elevation, pop the cartridge out and put it back in. I do this when I am at higher elevations, and again when I return to lower elevations. I assume this has to do with changes in air pressure, and it seems to take care of some of the rollercoaster numbers.
Hope this helps, and the biggest (obvious?) advise is to carry lots of sugar with you!