A study of CGMS measurements taken in 74 normal people aged between 9 and 65 years old over a period of 3 to 7 days was published in June of 2010. It found the following:
Sensor glucose concentrations were 71-120 mg/dl for 91% of the day. Sensor values were less than or equal to 60 or >140 mg/dl for only 0.2% and 0.4% of the day, respectivelyOverall only 5.6% of sensor readings were were over 140 mg/dl. (7.7 mmol/L) and these higher readings were more frequent in people under 25 years old. Only 4.4% of the readings of those over 45 were over 120 mg/dl.
Only .4% of all readings were over 140 mg/dl. But most significantly, this group was screened to ensure they had all off the following: A1Cs less than 6.0%, fasting blood glucose 70 to 99 mg/dl, 2-h oral glucose tolerance test (OGTT) levels below 140 mg/dl and no antibodies characteristic of autoimmune diabetes. After all these tests, all 17 people over age 45 who met the screening criteria had NO CGMS readings over 140 mg/dl at all.
This is probably because by the age of 45 people with the underlying genetic conditions that lead to diabetes, whose blood sugars would have been normal at younger ages, but who would have been getting higher than true normal readings after meals, would have progressed to where they failed the screening test. So it is a good bet that the people in the 45 and older age group in this study are truly, physiologically normal. With that in mind we are safe saying that normal people do not go over 140 mg/dl ever and are only rarely (4.4% of the time in this study) over 120 mg/dl--no matter what they eat.
Variation of Interstitial Glucose Measurements Assessed by Continuous Glucose Monitors in Healthy, Nondiabetic Individuals/ Juvenile Diabetes Research Foundation Continuous Glucose Monitoring Study Group. Diabetes Care June 2010 vol. 33 no. 6 1297-1299. doi: 10.2337/dc09-1971