There’s a small set of people who have a genetic defect (GCK-MODY) which gives them high blood sugar levels but they have no diabetic side effects. This paper looks at this group (Ali J. Chakera, Anna M. Steele, Anna L. Gloyn, Maggie H. Shepherd, Beverley Shields, Sian Ellard, Andrew T. Hattersley. Recognition and Management of Individuals With Hyperglycemia Because of a Heterozygous Glucokinase Mutation. Diabetes Care Jul 2015, 38 (7) 1383-1392).
Even after 50 years of mild hyperglycemia, people with GCK-MODY do not develop significant microvascular complications, and the prevalence of macrovascular complications is probably similar to that in the general population.
They don’t treat these people since they don’t respond to medications.
Treatment is not recommended outside pregnancy because glucose-lowering therapy is ineffective in people with GCK-MODY and there is a lack of long-term complications.
These people don’t get extra high levels of blood sugar but they are in the range diagnosed for pre-diabetic up to diabetic.
People with GCK-MODY have an HbA1c between 5.8 and 7.6%.
It seems to me that this might be evidence of something other than the blood sugar being the cause of damage in diabetics. Could it be the higher Insulin levels (hyperinsulinemia)? The paper presents some possibilities.
This is likely to be due to a number of factors: the hyperglycemia is mild, stable, and under homeostatic regulation; the hyperglycemia is often lower than the threshold above which the risk of diabetes complications increases; and people with GCK-MODY do not have the additional burden of the metabolic syndrome, with weight, lipid profile, and blood pressure being comparable with the general population
What is their C-peptide level, I wonder? See “Role of C-Peptide in the regulation of microvascular blood flow.” Forst T et al. Exp Diabetes Res. 2008 (!!!) (doi:10.1155/2008/176245) [https://www.ncbi.nlm.nih.gov/pubmed/18670621]