Thank you for answering questions to help us better understand - and Resolve10 is thinking pretty much what I was thinking; I am confused a little - insulin resistance is not typically associated with low bodyfat percentages. Being 5'10'', 175 @ 13% bodyfat places you in a pretty good spot, IMO and insulin resistance would not normally be a concern.
So, this is the typical dogma that we like to believe - fat = diabetes. But it turns out, and I thank you for helping me learn here, "lean diabetes" is actually a fairly common 'thing'.
The reason I am saying shakes are counter-intuitive here is that you normally see someone who is becoming diabetic because of caloric overload; and shakes make caloric overload all that much easier to obtain. Plus, they usually have items in them that can hit the blood stream quickly and cause an insulin spike.
If you don't mind sharing - did the Dr. do an A1C? Fasting Glucose Levels? Have you done any glucose monitoring yourself?
How about thyroid levels, has he tested at least TSH and free T4? What about cortisol levels?
Anyone in your family ever been diagnosed with Cushing's Syndrome?
How is your blood pressure?
Also, I am just checking for what could be my own possible confusion - but are you like, really, super, uber-active? 3,500 calories at maintenance is a lot for a guy your size unless you are doing quite a bit of work. I'm not saying it isn't true - just want to understand the background because I may be a little confused (and jealous).