I spent several months researching just this question and after reading an immense amount of journal research, I put what I found on my web site at http://www.phlaunt.com/diabetes
Check out the “Research connecting organ damage to blood sugars” page and you’ll see why the endo tells you to stay under 140 mg/dl. There is compelling evidence that staying above that level for sustained periods of time correlates with developing neuropathy. They are also finding people blood sugars over 140 mg/dl at 2 hours on a GTT developing retinopathy. So it’s a good idea to shoot for under 140 as much as possible.
If you have the kind of Type 1 where you get hypos even if you keep your carbs at a reasonable level a higher target might be necessary, but if that’s your case, it would still be worth trying to monkey around with carb intake and insulin ratios, timing, and changing insulins to see if you could get better control of the hypo problem. I realize not everyone can. But for those who can, it does look like staying under 140 mg/dl as much as possible should really lower the chances of complications.
Normal people appear to go up to 120 and then pretty much go right back down, BTW, based on studies with CGMS.
My targets for several years have been:
1 hour after eating 140 Maximum but I try to get lower
2 hours after eating 120 Maximum
3 hours after eating under 100
Ideally I try to get under 100 at 2 hours and in the 80s as much of the day as possible.
I don’t always make it to my targets, (understatement!) but just shooting for them is very helpful and has translated into an A1c that has bounced around between 5.7 and 5.5% for the last 3 years.