In the last article I explained the three primary markers we use to track blood sugar: fasting blood glucose (FBG), oral glucose tolerance test (OGTT) and hemoglobin A1c (A1c). We also looked at what the medical establishment considers as “normal” for these markers. The table below summarizes those values.
| Marker | Normal | Pre-diabetes | Diabetes |
| Fasting blood glucose (mg/dL) | <99 | 100-125 | >126 |
| OGGT / post-meal (mg/dL after 2 hours) | <140 | 140-199 | >200 |
| Hemoglobin A1c (%) | <6 | 6-6.4 | >6.4 |
In this article, we’re going to look at just how “normal” those normal levels are – according to the scientific literature. We’ll also consider which of these three markers is most important in preventing diabesity, diabetes, and cardiovascular disease. But before we do that, I’d like to make an important point: context is everything.
As I mentioned in Part 1 of this article series, there are potential problems with how well these tests are utilized to diagnose diabetes. This is an area that needs further study, but continuous glucose monitoring for the general population may be a better marker.
In my work with patients, I never use any single marker alone to determine whether someone has a blood sugar issue. I run a full blood panel that includes fasting glucose, fasting insulin, A1c, fructosamine, uric acid and triglycerides (along with other lipids), and I also have them do post-meal testing at home over a period of 3 days with a range of foods.
If they have a few post-meal spikes and all other markers or normal, I’m not concerned. If their fasting BG, A1c and fructosamine are all elevated, and they’re having spikes, then I’m concerned and I will investigate further.
On a similar note, I’ve written that A1c is not a reliable marker for individuals because of context: there are many non-blood sugar-related conditions that can make A1c appear high or low. So if someone is normal on all of the other blood sugar markers, but has high A1c, I’m usually not concerned.
With all of that said, let’s take a look at some of the research.
Fasting Blood Sugar
According to continuous glucose monitoring studies of healthy people, a normal fasting blood sugar is 89 mg/dL or less. Many normal people have fasting blood sugar in the mid-to-high 70s.
While most doctors will tell you that anything under 100 mg/dL is normal, it may not be. In this study, people with FBG levels above 95 had more than 3x the risk of developing future diabetes than people with FBG levels below 90. This study showed progressively increasing risk of heart disease in men with FBG levels above 85 mg/dL, as compared to those with FBG levels of 81 mg/dL or lower.
Far more important than a single fasting blood glucose reading is the number of hours a day our blood sugar spends elevated over the level known to cause complications, which is roughly 140 mg/dl (7.7 mmol/L). I’ll discuss this in more detail in the OGGT section.
One caveat here is that very low-carb diets will produce elevated fasting blood glucose levels. Why? Because low-carb diets induce insulin resistance. Restricting carbohydrates produces a natural drop in insulin levels, which in turn activates hormone sensitive lipase. Fat tissue is then broken down, and non-esterified fatty acids (a.k.a. “free fatty acids” or NEFA) are released into the bloodstream. These NEFA are taken up by the muscles, which use them as fuel. And since the muscle’s needs for fuel has been met, it decreases sensitivity to insulin. You can read more about this at Hyperlipid.
So, if you eat a low-carb diet and have borderline high FBG (i.e. 90-105), it may not be cause for concern. Your post-meal blood sugars and A1c levels are more important.
Hemoglobin A1c
In spite of what the American Diabetes Association (ADA) tells us, a truly normal A1c is between 4.6% and 5.3%.
But while A1c is a good way to measure blood sugar in large population studies, it’s not as accurate for individuals. An A1c of 5.1% maps to an average blood sugar of about 100 mg/dL. But some people’s A1c results are always a little higher than their FBG and OGTT numbers would predict, and other people’s are always a little lower.
This is probably due to the fact that several factors can influence red blood cells.
A number of studies show that A1c levels below the diabetic range are associated with cardiovascular disease. This study showed that A1c levels lower than 5% had the lowest rates of cardiovascular disease (CVD) and that a 1% increase (to 6%) significantly increased CVD risk. Another study showed an even tighter correlation between A1c and CVD, indicating a linear increase in CVD as A1c rose above 4.6% – a level that corresponds to a fasting blood glucose of just 86 mg/dL. Finally, this study showed that the risk of heart disease in people without diabetes doubles for every percentage point increase above 4.6%.
Studies also consistently show that A1c levels considered “normal” by the ADA fail to predict future diabetes. This study found that using the ADA criteria of an A1c of 6% as normal missed 70% of individuals with diabetes, 71-84% with dysglycemia, and 82-94% with pre-diabetes. How’s that for accuracy?
What we’ve learned so far, then, is that the fasting blood glucose and A1c levels recommended by the ADA are not reliable cut-offs for predicting or preventing future diabetes and heart disease. This is problematic, to say the least, because the A1c and FBG are the only glucose tests the vast majority of people get from their doctors.
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OGTT / Post-Meal Blood Sugars
If you recall, the oral glucose tolerance test (OGTT) measures how our blood sugar responds to drinking a challenge solution of 75 grams of glucose. I don’t recommend this test, because A) it’s not realistic (no one ever drinks 75 grams of pure glucose), and B) it can produce horrible side effects for people with poor glucose control.
However, there’s another more realistic and convenient way to achieve a similar measurement, and that is simply using a glucometer to test your blood sugar one and two hours after you eat a meal. This is called post-prandial (post-meal) blood sugar testing. As we go through this section, the numbers I use apply to both OGTT and post-meal testing.
As the table at the beginning of this article indicates, the ADA considers OGTT of between 140 – 199 two hours after the challenge to be pre-diabetic, and levels above 200 to be diabetic.
But once again, continuous glucose monitoring studies suggest that the ADA levels are far too high. Most people’s blood sugar drops below 120 mg/dL two hours after a meal, and many healthy people drop below 100 mg/dL or return to baseline.
A continuous glucose monitoring study showed that sensor glucose concentrations were between 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, respectively.
On the other hand, some studies suggest that even healthy people with no known blood sugar problems can experience post-meal spikes above 140 mg/dL at one hour. As I said in the beginning of the article, context is everything and all of the markers for blood sugar must be interpreted together.
If post-meal blood sugars do rise above 140 mg/dL and stay there for a significant period of time, the consequences are severe. Prolonged exposure to blood sugars above 140 mg/dL causes irreversible beta cell loss (the beta cells produce insulin) and nerve damage. Diabetic retinopathy is an extremely common (and serious) diabetic complication. Cancer rates increase as post-meal blood sugars rise above 160 mg/dL. This study showed stroke risk increased by 25% for every 18 mg/dL rise in post-meal blood sugars. Finally, 1-hour OGTT readings above 155 mg/dL correlate strongly with increased CVD risk.
What does it all mean?
Let’s take a look again at what the ADA thinks is “normal” blood sugar:
| Marker | Normal | Pre-diabetes | Diabetes |
| Fasting blood glucose (mg/dL) | <99 | 100-125 | >126 |
| OGGT / post-meal (mg/dL after 2 hours) | <140 | 140-199 | >200 |
| Hemoglobin A1c (%) | <6 | 6-6.4 | >6.4 |
But as we’ve seen in this article, these levels depend highly on context and whether all markers are elevated, or just a few of them.
If you’re interested in health and longevity – instead of just slowing the onset of serious disease by a few years – you might consider shooting for these targets. But remember to interpret the numbers together, and also remember that blood sugar is highly variable. If you wake up one morning and have a fasting blood sugar of 95, but your A1c and post-meal numbers are still normal, that’s usually no cause for concern. Likewise, if you see a one-hour post-meal spike of 145 mg/dL, but all of your other numbers are normal, that is also usually no cause for concern.
| Marker | Ideal |
| Fasting blood glucose (mg/dL) | <86* |
| OGGT / post-meal (mg/dL after 2 hours) | <120 |
| Hemoglobin A1c (%) | <5.3 |
*If you’re following a low-carb diet, fasting blood sugars in the 90s and even low 100s may not be a problem, provided your A1c and post-meal blood sugars are within the normal range.
And the good news is that this can be done cheaply, safely and conveniently at home, without a doctor’s order and without subjecting yourself to the brutality of an OGTT.
I’ll describe exactly how to do this in the next article.
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How would all this apply to children? would the numbers be the same as adults? If not, could you share what optimal levels would be for kids? I would love to show my kids what happens to them after they junk out on high sugar foods!
Hi Chris,
Just spent five days collecting BG levels. Found this article after as many days of searching–most searches return information for diabetic rather than non-diabetic scenarios so it took awhile.
I’ve gone on a keto diet on-and-off over the last 10 years and always feel dramatically better on one, but eventually grow weary and give it up. Idea is that the glucometer will help motivate me to stay mostly low-glycemic even when keto grows old.
As expected, my levels are below ADA pre-diabetic, but are not idea by the standards of the studies cited above. Will be interesting to see if I can knock the 10-20 mg/dl off the baseline over time.
The thing I find interesting is that my BG level goes up quite a bit after each of my three weekly weight lifting sessions. I work out pretty hard and consistently, always going as close to maximum intensity as reasonably possible. Today, three days into keto, my BG was 95 before a lower-body session and a tad lower at 93 at the end of the session. Twenty minutes later it went to 120 mg/dl and gradually drifted back down to 105 over two hours. Haven’t eaten since hours before the session. Before switching to keto BG peaked at 130 after lifting. On cardio days (500 kcal in 35 minutes rowing) BG followed a similar pattern on high-carb and was close to flat on keto, rising 10 mg/dl at the end of the workout and staying there.
I’ve read that an increase in BG post-workout is to be expected since the liver brings glucose out of storage for muscles to use, but can’t find much information on what levels would be reasonable to expect for an normal to mildly glucose intolerant person.
Do you have any information regarding this? Any interesting studies?
Thanks,
David
It’s actually not strange at all to see normal FBG and impaired post-meal blood sugars. There are studies showing that people with normal FBG and impaired OGTT are at higher risk for developing diabetes later on. I don’t say this to scare you, but to clarify why post-meal #s are a better indicator of glucose tolerance and insulin sensitivity than FBG.
It may be that your numbers come down further after more time. However, it’s possible that you have LADA or some other process affecting insulin output or sensitivity. The low-tech response is to simply avoid whatever raises your blood sugar above 140 mg/dL if you continue to experience this.
Reem
How long have you been eating higher carb? Glucose tolerance TEMPORARILY decreases on LC and will be higher at first. I once got a BS of 170 when I first moved to a higher carb WOL. Now I get a MAX BS of 120 for the same amount of carbs.
So, give it time. Also, be aware that protein increases BS. You might be very surprised to find that a potato with some butter will cause less of a spike than a steak with potato. Strange, but true.
Hey chris, me again 😉
I followed your advice and continued to test. I’ve been eating 30-50g carbs for several weeks now (in the form of white rice or winter squash).
Yesterday, i tested my FBG and it was 77
Today I ate 40g carb in the form of white rice, and 1 hour later it was 171 mg/dl!!! i just tested it now (1.5 hrs later) and it is 158.
I don’t understand, i wouldve thought my body would be used to the carbs by now?? However it may be worth mentioning, that the past few days i have been recovering from jetlag and my sleeping cycle has been really messed up (going to sleep really late, sleeping 12 hours or more..) so maybe this has disrupted by BS balance?
It still doesn’t make sense though that my FBG is normal, yet my response to carbs is insanely abnormal.
BTW, I’m 20 yrs old, and have never been overweight. I’ve been eating LC for a few years now (and before that a high carb moderate fat diet, and before that a junk food, high carb high sugar high gluten and everything bad diet)
I appreciate your thoughts!!
I definitely have digestive issues (IBS-C) that I have been working on for a while. It was IBS-D before I eliminated gluten. Also had a positive ANA test, so possible autoimmune issues, although lupas was ruled out. My new years resolution might be to book an appointment with you.
Oops. My response got submitted before I was finished.
One potential question mark is that your 2-hour reading was higher than your 1-hour on two days. That can indicate a compromised or delayed insulin response, or in some cases, slow digestion. Still, the important thing is that you’re below 140 at one-hour and below 120 at two-hours which you are in all cases except the white rice day.
Looks pretty normal except for the white rice. Definitely want to avoid that.
Hey Chris, I did my glucometer testing. Here are my results for premeal, 1,2 and 3 hours after:
Day 1: 87, 96, 100, 87
Day 2: 89, 101, 114, 94
Day 3: 92, 151, 141, 80
I had white rice on day 3, which apparently I should not eat. Anything else I should gather from this?
Bryan, have you found out any further information regarding your fructosamine levels? I’m essentially [well-sourced] Paleo + raw whole milk and my levels came in at 2.08 – higher than yours. My FBG was 60 and triglycerides were 25. My fasting window is essentially from 7pm – 11am every day… Thoughts?
Fasting insulin is an inaccurate marker, especially in the mid-to-later stages of insulin resistance. I think post-meal blood sugars are more useful in measuring insulin sensitivity and glucose tolerance.
Hi Chris,
is very low fasting insulin (below the range) something worth considering or it is just good?
2.073 mcIU/ml (you mentioned <5 mcIU/ml) or 14.4 pmol/l less than 16.5 lower end of the lab range
at the same time glucose was 5.2 mmol/l or 93.69 mg/dl a bit high
I was eating low carb which could explain the higher glucose but previously in the last 2 years since I'm low carbing my FBG has been low 4.5, 3.9, 4.1. or under 81.
Thanks.
Okay. Since it’s getting towards the top of the range, probably good to test the post-meals.
It was measured in mmol/L. The report says that 1.2-2.1 is “normal”
Assuming fructosamine was measured in umol/dL, 1.9 is excellent and suggests you don’t have elevated blood sugars. But the glucometer is most accurate, so still a good idea to do that.
Thanks, Chris. I am going to pick up a glucose monitor this week. BTW, fructosamine was 1.9 in that same blood work.
Bryan: test your post-meal blood sugars. A1c isn’t particularly reliable in that it can be influenced by a number of different factors. If your post-meal #s are normal, I wouldn’t worry about the A1c – especially in light of your FBG. You could also run fructosamine, which is another measure of average blood sugar that isnt affected by hemoglobin variation.
Chris, I just got some blood results back:
FBG: 78
A1C: 5.4
HDL 63
Trig: 65
The A1C # seems high. My diet has been Paleo+raw dairy since April 2010. However, I was borderline anemic on a few tests during late summer. Think there’s anything to worry about with that A1C number?
Reem:
When your body has become accustomed to burning fat for fuel, it becomes naturally insulin resistant. However, this usually reverses after 3-4 days of a higher carb diet. I would keep testing and see if it doesn’t resolve.
It’s possible, however, that there are other mechanisms causing poor glucose tolerance that need to be explored. If your metabolism is damaged from previous poor eating habits or from autoimmune disease, your carbohydrate tolerance may remain low.
Another possibility, if you’re not doing this already, is to add high-intensity strength training to your regiment. This is an excellent way to restore insulin sensitivity and improve glucose tolerance. I’ll be writing an article on this soon.
I eat very low carb, except once a day i eat about 30-40g carbs in the form of white rice or sweet potato (this is to spare protein – i don’t want to have to eat an extra 50g protein a day just for gluconeogenesis).
My fasting BG is about 83 mg/dl , but i just checked my BG about an hour after I ate some white rice, and it was 147 mg/dl!!!
I’m pretty concerned about this! I’ve been eating this way for about 3 weeks now.(before, i was practically zero carb)..Will i always have peripheral insulin resistance, or will my body get used to this amount of carbs i eat per day?
Yikes! Lila, I read your post and the first thing I thought was that you might be a KPD. I really try not to post too much on other’s blogs so I wanted to see what Chris had to say.
First of all, you’re one of the few people who I’ve heard of that have been officially dubbed “KPD”, that is a miracle in itself.
What made me think you were KPD? Your fast acting doesn’t work. Your body is producing counter regulatory hormones strong enough to counter a fast acting insulin. You go low hours after the fast acting is long gone, which means that your body is making insulin. You have a very strong response to carbs and that A1c is hanging near 6. Welcome to the “goofy” diabetes.
This is where KPD differs from regular type 2. Everything is still there and it’s working, it’s just all miss timed. It’s as if some control element is broken so the pieces no longer work together. The continual spikes, however, are going to keep messing up the system though.
I am willing to talk with you about this. You can find me on “Diabetes Forums” as Rekarb or on Tudiabetes with the same name.