I also use 3.9 to 8 mmol/L as my target range. There's very little information out there on what constitutes normal, 100% healthy blood glucose levels as large-scale studies using CGMs in non-diabetics have not been completed yet. The only studies I know of were small-scale and did not follow the participants in the long-term to see who went on to develop health problems and who didn't.
This study is one I find easiest to interpret -
Link - 80 people wearing CGMs for 12 weeks with an average HbA1c of 5.2% - 33 mmol/mol. There were some prediabetics in the group though the average HbA1c shows that the majority had no known issues.
"Participants reaching at least 7.8 mmol/l spent a median of 31 min/day (range 0–412 min) above this IGT (Impaired Glucose Tolerance)
level. A quarter of the total cohort experienced glucose levels above this threshold for at least 75 min/day, and three individuals (3.8%) remained in this range for 5 h or more per day. These three individuals had HbA1c levels in the normal range (5.4–5.7%) and mean FPG between 4.9 and 6.5 mmol/l. The eight individuals who spent more than 2 h above the IGT cut-off had higher mean HbA1c (5.7%), FPG (6.1 mmol/l), age (55 years) and BMI (women 29 kg/m2, men 30 kg/m2) than those who did not. The proportion of non-white participants and women (six of eight for both) was higher than in the whole study population. Seven persons (9%) reached 11.1 mmol/l, with two individuals spending more than 1 h/24 h above this diabetic level."
"We found that nearly all individuals without diabetes exceeded the IGT threshold of 7.8 mmol/l (140 mg/dl) at some point during the day and spent a median of 26 min (range 0 min to 6 h 52 min) per day above this level. We also found that one in ten individuals reached diabetic levels (11.1 mmol/l, 200 mg/dl). These findings suggest that glucose levels in persons without diabetes frequently reach IGT range concentrations and that a considerable proportion reach diabetic levels. Previous smaller studies have suggested similar patterns, albeit in more homogeneous populations. A study in 32 individuals with confirmed normal glucose tolerance found that seven participants (22%) reached glucose concentrations above 11.1 mmol/l (200 mg/dl) during an average of 28 days of CGM and that participants spent on average 42 min/day at glucose concentrations above 7.8 mmol/l (140 mg/dl)"
Those figures suggest that somewhere around 7.8 mmol/L is a natural threshold that a few people never cross at all but which is the typical healthy maximum for most people, while a significant number of people with normal HbA1c results can 'spike' to levels quite a bit higher than that. Other studies like that one show similar results and so I aim to stay below 8 mmol/L most of the time while not worrying too much about going a little above that level after maybe two or three meals per week so long as it's only a little over and for a short time. If I go above 10 mmol/L at any point after a meal I don't eat that meal again. It's probably not very harmful in a healthy person to hit that level on occasion but as it takes me much longer to come back down from that level that it does for a healthy person I suspect that it's not a healthy thing for me to do regularly.
Bear in mind that CGMs can be a little bit inaccurate on the peaks of the spikes. I find the Libre 2s pretty much always overshoot when blood glucose levels are changing rapidly - the true height of the peak may be lower that it appears on the graph. Whenever I use a new sensor I do quite a few finger-prick tests when levels are flat before eating to see how well the CGM is calibrated, and sometimes I'll do a few tests at around 1 hour after eating a meal that has more carbs than my average to gauge how closely the spike on the graph matches the finger-prick reading. It's a pain but I use those graphs to plan my eating for months at a time so I feel it's worth the effort to try to figure out how high my BG levels get after any 'borderline' meal I intend to eat quite regularly.
