One of the most common questions asked by people with diabetes is: "What are normal blood sugar levels?" Unfortunately, there's no simple answer to this question, because it depends on how you define normal.
It's like deciding when someone is rich or poor, tall or short, thin or fat, or young or old. Most people would agree that a very skinny person was thin and a very fat person was fat. But how about the sizes in between? When does underweight become normal and when does normal become overweight? It's all a matter of definitions and cutoff points set by one group or another.
The definitions of normal, prediabetes, and diabetes are usually made by august committees of diabetes experts, and they change from time to time. For example, not too long ago, it was decided that you're diabetic if your fasting BG level is 126 mg/dL [to convert to mMol/L, divide by 18] or higher, instead of the previous cutoff of 140.
There are some guidelines about "nondiabetic" BG levels, for instance, "Nondiabetics never go over 140 mg/dL, no matter what they eat, and return to premeal levels in 2 or 3 hours" or "Nondiabetic fasting levels are 70 to 100 mg/dL."
Textbooks that plot BG levels during the day in diabetic and nondiabetic individuals often show the nondiabetic levels as ranging from about 80 mg/dL before meals to close to 100 after some meals and about 120 after the largest meal of the day.
However, real life doesn't always correspond to the textbooks. Some nondiabetic people, especially young, fit persons, keep their BG levels much more stable than that.
Many people with diabetes have used their home meters to measure their nondiabetic spouses and friends. Some find that the nondiabetic family members stay in the 80s no matter what they eat. Others find that some go up to well over 140 after carby meals. Were the ones who stayed in the 80s all day the true normals and the ones who went up to 120 or 160 really prediabetic or diabetic?
And how do nondiabetic BG levels fluctuate during the day when people are eating "normally" rather than in an institutional study setting or taking a glucose tolerance test?
European researchers have tried to answer this last question by studying 24 young (19 to 35 years) subjects they considered nondiabetic (fasting BG levels close to 80 mg/dL; body mass index from 19 to 25; A1c levels from 4.6 to 5.4; oral glucose tolerance tests well within normal limits, although at 1 hour some went up to only 110 and others went slightly over 160). They fitted them up with two different continuous glucose monitors and plotted their BG levels throughout the day on both an in-hospital fixed meal schedule and a "freeliving" meal schedule with fixed meal times but regular home food.
If you have broadband access and want to listen to the whole slide presentation yourself, it's available at
Otherwise, the important results are shown at
In this study, the mean BG level was about 80 mg/dL before meals and went up to 110 or 120 after meals. But the individual curves show that some people had premeal levels closer to 60 mg/dL and went up only to the 80s or 90s after meals, whereas others had premeal levels in the 90s and went over 160, especially after breakfast.
So what does all this mean? There are two different ways of interpreting this study:
- These people are all nondiabetic, and therefore you can go up to more than 160 mg/dL after carby meals and still consider yourself nondiabetic, even though the average after a meal would be closer to 100 or120.
- The people with the best BG control were truly nondiabetic. The others, the ones with A1cs of 5.4, BMIs at the top of the range, are heading toward diabetes.
In other words, if you accept an official committee's pronouncement of what diabetic limits are and then you test people who fall within this definition, you'll get certain results. On the other hand, if you say the committee's cutoff points are too high, for instance if you think really normal A1cs are under 5 and really normal postprandial BGs are always under 120, then you would have excluded some of the people in this study and you would have gotten other results.
So we're basically back to where we started. What are normal BG levels? That depends on how you define normal.
But if we accept the researchers' definitions of normal, the study is a good illustration of the daily fluctuations in BG levels throughout the day in people allowed to eat what they wanted as long as they limited their food intake to three times a day.
The results of this study show that (and remember, there were only 24 people in the study, and only 21 completed the entire study)
In most cases the peak BG level was at about 45 minutes.
Even those with the highest BG levels (peaking over 160) dropped below 120 by about 2 hours. The mean level was below 100 by 2 hours.
It took 4.5 to 5 hours to return to the starting point.
Even those with the lowest BG levels went up after meals.
The meals were eaten at home, and their composition was not given in the talk. But the study was done in Bavaria, Germany, where potatoes are a regular part of most meals, so all the meals likely included significant carbohydrate. The researchers were surprised at how high the BG levels in some of the subjects went, especially after breakfast. They'd believed the textbooks.
Distinguishing diabetic from nondiabetic will always involve definitions and cutoff points. But the more we see studies like this, the better understanding we will have of the "normal" goals we would like to reach.