What is normal blood glucose?
In children and adults, ideally, blood sugar levels are tightly-regulated to stay in a narrow range that’s optimal for physiological function. What are these levels, and how much of a glucose excursion is “normal”?
Notably, there are some transient exceptions to the accepted “normal range”. In particular, women tend to exhibit lower blood glucose levels during pregnancy. Also, a phenomenon called stress hyperglycemia (such as due to trauma or acute illness) can temporarily cause abnormally high blood glucose levels. Finally, certain medications, like steroids can also cause hyperglycemia, even in healthy individuals.
A 2016 review describes that
“Just after birth, for the first few hours of life in a normal term neonate appropriate for gestational age, blood glucose levels can range between 1.4 mmol/L and 6.2 mmol/L [~25-112 mg/dL] but by about 72 h of age fasting blood glucose levels reach normal infant, child and adult values (3.5–5.5 mmol/L) [~63-99 mg/dL]. Normal blood glucose levels are maintained within this narrow range by factors which control glucose production and glucose utilization.”
Based on the diagnostic criteria for pre-diabetes and diabetes, as issued by the American Diabetes Association, normal blood glucose levels generally look as follows:
- A normal fasting blood glucose level is under 100 mg/dL
- A normal 2-hr post-glucose excursion (or post-prandial) level is less than 140 mg/dL (this is based on the criteria for passing the oral glucose tolerance test, OGTT)
To what extent do healthy individuals exceed these values?
Until recently, not many studies focused on assessing blood glucose patterns in individuals without diabetes. However, with the advent of continuous glucose monitoring (CGM) technology, more studies are examining what day-to-day blood glucose patterns really look like.
Here are some interesting data from studies looking at glucose profiles in healthy individuals using CGM technology:
- A 2007 investigation was conducted on 24 patients (50% female) who wore a CGM over the course of two days. The individuals all consumed the same meals containing 50 g of carbohydrate four times per day. The data showed that the average 24-hr glucose level was ~89 mg/dL +/- 6 mg/dL. The average highest post-prandial level observed was ~132 mg/dL +/- 17 mg/dL and occurred after breakfast. The average post-lunch and post-dinner peaks were lower (averaged 118 mg/dL and 123 mg/dL, respectively). Notably, “meals with a higher fiber, protein, and fat content induced a smaller increase and a slower decrease of postprandial glucose concentrations.”
- A 2008 research study involving 62 patients who wore a CGM for approximately 30 days showed that the average glucose level was 102 mg/dL +/- 7 mg/dL, and on average, did not exceed 117 mg/dL for all patients who were verified to have a normal glucose tolerance test result prior to enrolling in the study. Also, “glucose variability, as expressed by the interquartile range, was 21 ± 4 mg/dL (range, 14–29 mg/dL).”
- A 2010 analysis involving 80 patients without diabetes who wore a CGM over the course of 12 weeks showed that “93% of participants reached glucose concentrations above the impaired glucose tolerance (IGT) threshold of 7.8 mmol/l [~140 mg/dL] and spent a median of 26 min/day above this level during continuous glucose monitoring. Eight individuals (10%) spent more than 2 h in the IGT range. They had higher HbA1c, fasting plasma glucose (FPG), age and BMI than those who did not. Seven participants (9%) reached glucose concentrations above 11.1 mmol/l [~200 mg/dL] during monitoring.”
Overall, this research indicates that individuals who are verified as having normal glucose tolerance rarely exceed ~120 mg/dL. However, in studies where patients were not verified to have normal glucose tolerance via the OGTT, many spent a considerable proportion of time above the “normal” threshold.
The experts noted that:
“Since chronic glucose exposure is considered to be one of the main mediators of long-term outcomes, including microvascular and cardiovascular disease, our findings demonstrate that exposure to moderately elevated glucose levels remains under-appreciated when individuals are classified on the basis of isolated glucose measurements. If no adverse outcomes attend these periods of hyperglycemia, our findings suggest that it is common for persons who are regarded as ‘normoglycemic’ by accepted current definitions to experience transient hyperglycemia during everyday circumstances.”
Notably, some studies have indicated that certain health risk factors increase with blood glucose levels even within the normal A1c range. These findings are further reinforced by the recent study showing that A1c testing may miss most of the cases of impaired glucose tolerance, as well as prediabetes and diabetes diagnoses.
The extent to which transiently elevated blood glucose levels affect individual health and how high and for how long they must be elevated to cause problems is not fully understood yet and likely depends on a combination of factors, including genetic and lifestyle factors.