Now this finding has been confirmed and quantified in a meta study that looked at records of "44,623 participants aged 20 to 79 years with gradable retinal photographs" which examined the correlations between signs of retinopathy and the subjects' fasting, 2 hour glucose tolerance test, and A1c results.
Glycemic Thresholds for Diabetes-Specific Retinopathy: Implications for Diagnostic Criteria for Diabetes:The DETECT-2 Collaboration Writing group. Stephen Colagiuri et al. Diabetes Care Published online before print October 26, 2010, doi: 10.2337/dc10-1206
The conclusion of the study was this:
A narrow threshold range for diabetes-specific retinopathy was identified for FPG and HbA1c but not for 2-h PG. The combined analyses suggest that the current diabetes diagnostic level for FPG could be lowered to 6.5 mmol/L [117 mg/dl] and that an HbA1c of 6.5% is a suitable alternative diagnostic criterion.The metastudy found that "glycemic thresholds for diabetes-specific retinopathy were observed over the range 6.4-6.8 mmol/L [115 - 122 mg/dl] for F[asting]P[lasma]G[lucose] 9.8-10.6 mmol/L [176.4 - 191 mg/dl] for 2-h PG, and 6.3-6.7% for HbA1c.
From this we can safely conclude that "diabetic" retinopathy is indeed occurring at levels significantly below those established by the American Diabetes Association as defining diabetes.
Based on this, you should consider yourself at risk for retinal damage if you have fasting blood sugar over 115 mg/dl (6.4 mmol/L) , a 2 hour glucose tolerance test reading over 176 mg/dl [9.8 mmol/L) or an A1c over 6.3%.
However, you need to assess this information in light of the fact that retinopathy is a relatively late diabetic complication. Diabetic neuropathy--the nerve damage that leads to impotence, amputation, and autonomic dysfunction which raises blood pressure etc.--starts to become more common when 2 hour glucose tolerance test values go over 140 mg/dl, though there does not appear to be a direct correlation with A1c or fasting plasma glucose at the lower end of the range.
Heart disease incidence correlates with post-meal readings over 155 mg/dl and rises in a straight line from 4.7% A1cs becoming quite significant over 6%.
The good news is that though these values correlate with significant retinopathy in populations who follow traditional medical advice, keeping your blood sugar values under these thresholds after diagnosis using the strategy you will read HERE will keep you from developing it if you don't already have it, and even if you do, long term will give you a much better outcome.
You can read more about Diabetic Retinopathy HERE.
NOTE: In case you wonder why the ADA diagnostic criteria are so much higher than the levels at which diabetic retinopathy occurs, the answer is that the ADA set their diagnostic criteria years ago using on data from a few small non-European populations (Pima Native Americans and Pacific Islanders) whose diabetes is related to different genetic profiles and follows a different pattern from those common in European populations.
They did this on purpose out of a misguided desire to avoid diagnosing people with diabetes for as long as possible and have fought hard in the intervening decades to keep these flawed diagnostic criteria even though they have all been found woefully inadequate by a ton of research.
The whole sad history of how the ADA has worked for decades to ensure that you will have developed diabetic complications long before you are diagnosed with diabetes can be read HERE.